Chapter 3: Global Circulation

3.5 Monsoons »
3.5.3 Evolution of the Asian Monsoon System

The Asian monsoon is regionally varied. The earliest onset is over the southern Bay of Bengal in late April, over the Indo-Chinese peninsula and south India in early May, and then it progresses north and northwestward into the continent reaching Japan by late June to July (Fig. 3.34a). By the end of the peak season over Japan, the monsoon is already retreating over India (Fig. 3.34b). Given the regional evolution, the Asian monsoon can be divided into two separate but interactive monsoon sub-systems: the Indian Summer Monsoon or South Asian monsoon and the East Asian monsoon (Fig. 3.34).33 The latter can be sub-divided into the East Asian and Western North Pacific monsoon.

(a) Mean onset date and (b) peak pentad of the Asian summer monsoon rainy season; (c) division of the Asian monsoon (Adapted from Wang and Lin 2002).
Fig. 3.34. (a) Mean onset date and (b) peak pentad of the Asian summer monsoon rainy season; (c) division of the Asian monsoon. (Adapted from Wang and Lin 2002)33
Climatological pentad (5-day) mean precipitation rate (mm/day) averaged over (a) the Indian sector (70°E-95°E) and (b) the western Pacific sector (115°E-140°E). The data used are derived from Xie and Arkin (1996)  for the period of 1979-2000. (From Wang et al. 2005)
Fig. 3.35. Climatological pentad (5-day) mean precipitation rate (mm day-1) averaged over (a) the Indian sector (70°E-95°E) and (b) the western Pacific sector (115°E-140°E). The data used are derived from Xie and Arkin (1996)34 for the period of 1979-2000. Adapted from Chang et al. (2005)35

The South Asian monsoon is the northern branch of the seasonal migration of the east-west oriented precipitation belt (the ITCZ) (Fig. 3.35). The precipitation belt migrates from the southern to the Northern Hemisphere in the boreal summer and vice versa in the boreal winter (austral summer).36 The northern most extent of the tropical rain belt in the South Asian monsoon is about 20°N and it retreats to about 5°S during the winter. Two locations are favorable for the rain band: over the heated subcontinent (in keeping with the early theories) and the warm eastern equatorial Indian Ocean. This oceanic cloud band can aid or suppress the main monsoon rains. If the oceanic precipitation is intense, it leads to subsidence over land.36 Strong surface temperature gradients (Fig. 3.36a) lead to large-scale pressure gradients and cross-equatorial winds between the south and north Indian Ocean. Over India, the mean surface pressure at 20°N ranges from 1016 hPa in the winter to 1002 hPa at the peak of the summer monsoon.37 The Somali Jet is the result of the strong temperature and pressure gradients and channeling of the cross-equatorial flow by the East African Mountains (Fig. 3.36b).

Mean wind stress at the ocean surface showing the low-level Somali Jet which results from the strong cross equatorial pressure gradient and the high terrain of East Africa.Cross section along 5°N showing the magnitude and areal extent of the Somali jet core (data from the Japanese 25-year Reanalysis, 1979-2004).
Fig. 3.36. (a) Mean wind stress at the ocean surface showing the low-level Somali Jet which results from the strong cross equatorial pressure gradient and the high terrain of East Africa. (b) Cross section along 5°N showing the magnitude and areal extent of the jet core (data from the Japanese 25-year Reanalysis, 1979-2004, digital elevation from NOAA NGDC).

The annual monsoon cycle is regulated by heat transported across the equator by the atmosphere and the ocean. Oceanic heat is transported southward during the summer monsoon and northward during the winter monsoon over the Indian Ocean (Fig. 3.37). The southward movement of heat tends to cool the South Indian Ocean while the northward transport warms the North Indian Ocean southward heat flux in the summer tends to cool the North Indian Ocean. The coupled ocean-atmosphere interaction reduces the SST gradients and imposes a strong negative feedback on the system thereby regulating the seasonal extremes of the monsoon. While the north-south gradient dominates, an east-west SST gradient is also present (Fig. 3.36a).

Schematic of regulation of the seasonal cycle of the Indian Ocean for (a) the boreal summer (June- September) and (b) the boreal winter (December-February). Curved solid lines indicate near-surface winds forced by the large-scale pressure gradient associated with the cross-equatorial heating gradient denoted by ?warm? and ?cool?. Small blue arrows denote Ekman transport and the direction of the associated heat flux.
Fig. 3.37. Schematic of regulation of the seasonal cycle of the Indian Ocean for (a) the boreal summer (June- September) and (b) the boreal winter (December-February). Curved solid lines indicate near-surface winds forced by the large-scale pressure gradient associated with the cross-equatorial heating gradient denoted by "WARM" and "COOL". Grey arrows denote Ekman transport and the direction of the associated heat flux. Adapted from Loschnigg and Webster (2000)38

The annual cycles of the monsoons over India and East Asia are different primarily because the atmospheric response to heating is affected by land-ocean distribution and topography. The strong north-south gradient between the warm land and the cool ocean, enhanced by the heating of the elevated Tibetan Plateau, creates a strong monsoon over India. Over East Asia, the situation is more complex. The forcing comes from both the north-south gradients between cool Australia and warm western north Pacific and east-west gradient between the heated Asian landmass and the cooler Pacific. The result is a weaker monsoon circulation and bands of precipitation along the tropical monsoon circulation and subtropical frontal zones, which we will now examine in more detail.

Conceptual model of the (a) mid-May and (b) mid-June monsoon onset over East Asia. Closed solid (dashed) curves mark the subtropical high before (after) onset. Light (dark) shades signify an increase (decrease) in clouds and/or evaporation. Dashed arrows show the direction of the extension of low-level westerlies (from Wu 2002).
Fig. 3.38. Conceptual model of the (a) mid-May and (b) mid-June monsoon onset over East Asia. Closed solid (dashed) curves mark the subtropical high before (after) onset. Light (dark) shades signify an increase (decrease) in clouds and/or evaporation. Dashed arrows show the direction of the extension of low-level westerlies. (Wu 2002)39

For the East Asian monsoon, the area of clouds and evaporation increases during mid-June relative to the mid-May pattern and the longitudinal extent of the southwesterly winds shifts east into the Pacific (Fig. 3.38a). The Pacific High shifts east and north after the mid-May onset, expands, and strengthens (Fig. 3.38a). A large, thermal trough replaces the subtropical ridge over the continent. Air flows into the equatorial trough, which leads to cloudiness in the ITCZ. The Monsoon Trough extends northwestward from the equatorial trough into the continent. Other features are similar to the South Asian monsoon, the TEJ in the upper troposphere and the cross-equatorial flow at low-levels. To the north at the upper levels is a weakened Subtropical High (Fig. 3.38).

Mei-yu/Baiu Front

Mean annual cycle of surface pressure over India (20N) and south Indian Ocean (20S).Conceptual model of the Meiyu-Baiu frontal cloud zone (Ninomiya, 2004 ). Note the hatched areas representing a family of cloud systems along the front, the 850 hPa low-level jet, and the mid-tropospheric maximum wind tracks (short-waves develop along this track and enhance instability and ascent).
Fig. 3.39. (a) General synoptic pattern at the onset of the East Asian monsoon. Inset map shows the relative locations of the Mei-yu/Baiu, and Changma fronts. (b) Conceptual model of the Mei-yu/Baiu frontal cloud zone. (Chang et al. 2005)35 Note the hatched areas representing a family of cloud systems along the front, the 850 hPa low-level jet, and the mid-tropospheric maximum wind tracks (short-waves develop along this track and enhance instability and ascent).

A prominent, signature feature of the East Asian spring and early summer is the Mei-Yu/Baiu front, a semi-permanent, quasi-stationary, weak frontal zone that extends from eastern China east-northeastward into the Pacific (Fig. 3.39). To the north, in Japan, the frontal zone is called the Baiu front, and, in Korea, the Changma front (their relative locations are shown in the inset map on Fig. 3.39a). The Mei-yu and Baiu fronts begin in mid May and continue through early to mid summer while shifting northward. The Mei-yu front is the focal point for persistent heavy precipitation produced by mesoscale convective systems (MCSs) that form and track eastward along the front (Fig. 3.40). Instability, strong rising motion, and persistent deep convection are associated with a low-level jet that brings warm moist air from the South China and Bay of Bengal, low-level warm-air advection, and upper-level divergence, strongest in the right forward quadrant of the Subtropical Jet (Fig. 3.41). Most of the heavy rain is south and east of the front, an area of high relative humidity (Fig. 3.41b). The Baiu front is more typically midlatitude in structure. Weak cyclonic disturbances move along the Baiu front at 3-day intervals. They bring stratus, fog, and light rain to northern edge of the front and thunderstorms and heavy rain along and immediately south of the front.

Mean annual cycle of surface pressure over India (20N) and south Indian Ocean (20S).
Fig. 3.40. The 10-yr (1998–2007) mean rainfall accumulation (mm) during (a) the period of 11 May–24 Jun; (b) the Mei-yu, defined as days from 11 May to 24 Jun with rainbands. Note the major contribution of the Mei-yu to the total. (Adapted from Xu et al. 2009)40
Schematic diagram showing the flow structure of an observed Meiyu front (from Y. L. Chen et al. 1994). The thin solid line depicts the direct (D) circulation while the thin dashed line depicts the indirect (I) circulation. The heavy solid line shows the frontal position. The character J denotes the jet positions. The thick heavy line represents the tropopause boundary. Regions with relative humidity greater than 70% are shaded.
Fig. 3.41. (a) Schematic of 3-D structure of large MCS along Mei-Yu/Baiu front. (used permission from H. Yamada)41 (b) Schematic diagram showing the flow structure of an observed Mei-yu front. (Chen et al. 1994)42 The thin solid line depicts the direct (D) circulation while the thin dashed line depicts the indirect (I) circulation. The heavy solid line shows the frontal position. The character J denotes the jet positions. The thick heavy line represents the tropopause boundary. Regions with relative humidity greater than 70% are shaded.

West Pacific Monsoon trough

Schematic of the western North Pacific tropical cyclogenesis region partitioned into a monsoon trough zone and the near-equatorial ITCZ, meeting at a confluence zone (following Briegel and Frank 1997).
Fig. 3.42. Schematic of the western North Pacific tropical cyclogenesis region partitioned into a monsoon trough zone and the near-equatorial ITCZ, meeting at a confluence zone. (following Briegel and Frank 1997)43

The Western North Pacific monsoon region has the highest frequency of tropical cyclones on Earth, mainly because the tropical western Pacific is the warmest part of the tropical ocean. Tropical cyclogenesis is most common in the monsoon trough (Fig. 3.42, Chapter 8, Section 8.3.2Chapter 8, Section 8.3.2)44 and the location of the monsoon trough has major influence on the distribution of tropical cyclone activity in this region.45 Large-scale cyclonic vorticity in the monsoon trough is derived from the low-level equatorial westerly or southwesterly winds and the subtropical easterly trade winds (Fig. 3.42). Tropical cyclone formation is favored near the eastern end of the monsoon trough and in the confluence zone between the monsoon westerlies and the easterly trade winds (Fig. 3.42). Sometimes a reverse-oriented monsoon trough forms,45 one that extends northeastward from the South China Sea, in the direction of the Mei-yu/Baiu front (Fig. 3.39a). Under those conditions, tropical cyclone activity in the monsoon region is reduced.45 On rare occasions, tropical cyclones formed when the monsoon trough organizes into a gyre.46 While infrequent, once formed gyres can last 2-3 weeks and generate several vortices, the seedlings for tropical cyclones.46 Learn more about tropical cyclones in the monsoon trough in the chapter on tropical cycloneschapter on tropical cyclones.

East Indian Ocean Summer Monsoon systems

The eastern Indian Ocean has its characteristic summer monsoon thunderstorm system, such as the Sumatras. The Sumatras are eastward moving, short-lived squall lines that form over the Straits of Malacca in the low-level convergence between land breezes from Sumatra and Malaya.47 They form at night, during predawn hours, and attain lengths of 200-300 km while moving east to Malaysia and Singapore. Sumatras are critical to the regional rainfall; they produce heavy rain and occur about 3-4 times per month.

The Asian Winter Monsoon

The winter monsoon is stronger over East Asia than over the Indian Subcontinent because of the contrast between the very cold Asian landmass and the warm North Pacific Ocean (the warm Kuroshio Current transports heat from the equator). In addition, the cross-equatorial flow to the Australian-Indonesia monsoon is enhanced by the difference between hot Australia and the relatively cool north Pacific. The north-south contrast is reduced over the Indian subcontinent because the Tibetan Plateau blocks the cold Siberia air mass.

Differences of TRMM PR rainfall and QuikSCAT winds between boreal winter and boreal summer (DJF minus JJA). Warm colors are the boreal summer monsoon regime and cool colors are the boreal winter monsoon regime (Chang et al. 2005)
Fig. 3.43. Differences of TRMM PR rainfall and QuikSCAT winds between boreal winter and boreal summer (DJF minus JJA). Warm colors are the boreal summer monsoon regime and cool colors are the boreal winter monsoon regime. (Chang et al. 2005)48

Figure 3.43 shows the difference in TRMM precipitation radar data and QuikSCAT winds between December-January-February (DJF) and June-July-August (JJA) over tropical East Asia. Yellow to red areas get more rain during June-August while green to blue areas get more rain during December-February. During the boreal winter, rainfall occurs well north of the equator (e.g., east of the Philippines and Indochina) but during boreal summer, rainfall is mostly confined to the Northern Hemisphere. The difference can be partly explained by stronger boreal winter monsoon winds that blow directly onshore while few coastal regions face the southwesterly monsoon winds.

The Maritime Continent has its most active convection during the winter monsoon. Cold surges from continental Asia serve as the destabilizing mechanism for widespread, prolonged deep convection. In addition, Borneo vortices, synoptic-scale, low-level cyclonic features49 that, while mostly quasi-stationary can migrate within the southern South China Sea. The region west of Borneo is a common location for mesoscale convective complexes.50 Northeast monsoon winds and the sea breeze interact to produce strong low-level convergence offshore that favors organized mesoscale convection.51

1. Halley, E., 1686: An historical account of the trade winds, and monsoons, observable in the seas between the Tropicks, with an attempt to assign the physical cause of the said Winds. Philos. Trans. R. Soc. London, 16, 153-168.
2. Hadley, G., 1735: Concerning the cause of the general trade winds. Phil. Trans. Roy. Soc. London, 39, 58-63.
3. Ferrel, W., 1856: An essay on the winds and currents of the ocean. Nashville Journal of Medicine and Surgery, 4, 7-19.
4. Held, I. M., A. Y. Hou, 1980: Nonlinear axially symmetric circulations in a nearly inviscid atmosphere. J. Atmos. Sci., 37, 515-533.
5. Riehl, H., J. Malkus, 1958: On the heat balance in the equatorial trough zone. Geophysica, 6, 503-538.
6. Rossby, C. G., 1941: The scientific basis of modern meteorology. U.S. Yearbook of Agriculture. Climate and Man, 656-661.
7. James, I. N., 1994: Introduction to circulating atmospheres. Cambridge University Press, 422.
8. Fu, Q., C. M. Johanson, J. M. Wallace, and T. Reichler, 2006: Enhanced mid-latitude tropospheric warming in satellite measurements. Science (Wash.), 312, 1179.
9. Seidel, D. J., W. J. Randel, 2007: Recent widening of the tropical belt: Evidence from tropopause observations. J. Geophys. Res.(D Atmos.), 112.
10. Hu, Y., Q. Fu, 2007: Observed poleward expansion of the Hadley circulation since 1979. Atmos. Chem. Phys., 7, 5229-5236.
11. Krishnamurti, T., H. Bhalme, 1976: Oscillations of a monsoon system, Pt. 1, Observational aspects. J. Atmos. Sci., 33, 1937-1954.
12. Sadler, J. C., 1975: Upper tropospheric circulation over the global Tropics. [Available online at]
13. Gadgil, S., J. Srinivasan, 1990: Low-frequency variation of tropical convergence zones. Meteorol. Atmos. Phys., 44, 119-132.
14. World Meteorological Organization, G., 1985: Atmospheric ozone: assessment of our understanding of the processes controlling its present distribution and change. Its Global Ozone Research and Monitoring Project, Report 1985. 16, 1095.
15. Brewer, A. W., 1949: Evidence for a world circulation provided by the measurements of helium and water vapour distribution in the stratosphere. Quart. J. Roy. Meteor. Soc., 75, 351-363.
16. Dobson, G. M. B., 1956: Origin and distribution of polyatomic molecules in the atmosphere. Proc. Roy. Soc. London, A236, 187-193.
17. Gettelman, A., P. Forster, 2002: A climatology of the tropical tropopause layer. J. Meteorol. Soc. Japan, 80, 911-924.
18. Stohl, A., H. Wernli, P. James, M. Bourqui, C. Forster, M. Liniger, P. Seibert, and M. Sprenger, 2003: A new perspective of stratosphere-troposphere exchange. Bull. Amer. Meteor. Soc., 84, 1565-1573.
19. Reed, R. J., W. J. Campbell, L. A. Rasmussen, and D. G. Rogers, 1961: Evidence of a downward-propagating annual wind reversal in the equatorial stratosphere. J. Geophys. Res., 66, 813-818.
20. Veryard, R. G., R. A. Ebdon, 1961: Fluctuations in tropical stratospheric winds. The Meteorological Magazine, 90, 125-143., 90, 125-143.
21. Andrews, D. G., J. R. Holton, and C. B. Leovy, 1987: Middle atmosphere dynamics. Incorporated, 489.
22. Baldwin, M. P., L. J. Gray, T. J. Dunkerton, K. Hamilton, P. H. Haynes, W. J. Randel, J. R. Holton, M. J. Alexander, I. Hirota, T. Horinouchi, D. B. A. Jones, J. S. Kinnersley, C. Marquardt, K. Sato, and M. Takahashi, 2001: The quasi-biennial oscillation. Rev. Geophys., 39, 179-229.
23. Ekman, V. W., 1905: On the influence of the Earth's rotation on ocean currents. Arkiv for Matematik. Astronomi och Fysik, 2, 1-53.
24. Honjo, S., R. Weller, 1997: Monsoon winds and carbon cycles in the Arabian Sea: One of the most significant natural phenomena that influences the everyday life of more than 60 percent of the world's population. Oceanus, 40.
25. Matsuno, T., 1966: Quasi-geostrophic motions in the equatorial area. J. Meteor. Soc. Japan, 44, 25-43.
26. Gill, A. E., 1980: Some simple solutions for heat-induced tropical circulation. Quart. J. Roy. Meteor. Soc, 106, 447-462.
27. Ramage, C., 1971: Monsoon Meteorology. International Geophysics Series, Vol. 15, Academic Press, San Diego, Calif., 296 pp.
28. Webster, P., 1987: The elementary monsoon. Monsoons, J. S. Fein and P. L. Stephens, Eds., John Wiley & Sons, 3-32.
29. Sikka, D., S. Gadgil, 1980: On the maximum cloud zone and the ITCZ over Indian longitudes during the southwest monsoon. Mon. Wea. Rev., 108, 1840-1853.
30. --1978: Large-scale rainfall over India during the summer monsoon and its relation to the lower and upper tropospheric vorticity. Indian J. .Meteor., Hydro. & Geophys., 29, 219-231.
31. Gadgil, S., J. Srinivasan, 2011: Seasonal prediction of the Indian monsoon. Curr. Sci., 100, 343-353.
32. Meehl, G. A., 1987: Tropics and their role in the global climate system. Geograph. J., London, 153, 21-36.
33. Wang, B., L. Ho, 2002: Rainy Season of the Asian-Pacific Summer Monsoon. J. Climate, 15, 386-398.
34. Xie, P., P. A. Arkin, 1996: Analyses of global monthly precipitation using gauge observations, satellite estimates, and numerical model predictions. J. Climate, 9, 840-858.
35. Chang, C.-P., B. Wang, and N.-C. Lau, Eds., 2005: The Global Monsoon System: Research and Forecast Report of the International Committee of the Third International Workshop on Monsoons (IWM-III). Vol. WMO/TD No. 1266 (TMRP Report No. 70), Secretariat of the World Meteorological Organization, 542 pp.
36. Gadgil, S., 2003: The Indian Monsoon and its variability. Annu. Rev. Earth Planet. Sci., 31, 429-467.
37. Webster, P. J. and J. T. Fasullo, 2003: Monsoon: Dynamical Theory. Encyclopedia of Atmospheric Sciences, J. Holton and J. Curry, Eds., 1st ed. Academic Press, 1370-1386.
38. Loschnigg, J. and P. J. Webster, 2000: A coupled ocean-atmosphere system of SST modulation for the Indian Ocean. J. Climate, 13, 3342-3360.
39. Wu, R., 2002: Processes for the northeastward advance of the summer monsoon over the western north Pacific. J. Meteorol. Soc. Japan, 80, 67-83.
40. Xu, W., E. J. Zipser, and C. Liu, 2009: Rainfall characteristics and convective properties of Mei-Yu precipitation systems over south China, Taiwan, and the south China Sea. Part I: TRMM observations. Mon. Wea. Rev., 137, 4261-4275.
41. Yamada, H., B. Geng, K. Reddy, H. Uyeda, and Y. Fujiyoshi, 2003: Three-dimensional structure of a mesoscale convective system in a Baiu- frontal depression generated in the downstream region of the Yangtze River. J. Meteorol. Soc. Japan, 81, 1243-1271.
42. Chen, Y., X. A. Chen, and Y. Zhang, 1994: A diagnostic study of the low-level jet during TAMEX IOP 5. Mon. Wea. Rev., 122, 2257-2284.
43. Briegel, L. M., W. M. Frank, 1997: Large-scale influences on tropical cyclogenesis in the western North Pacific. Mon. Wea. Rev., 125, 1397-1413.
44. Chen, T., S. Wang, and M. Yen, 2006: Interannual variation of the tropical cyclone activity over the western north Pacific. J. Climate, 19, 5709-5720.
45. Elsberry, R. L., 2004: Monsoon-related tropical cyclones in East Asia. The East Asian Monsoon, C. P. Chang, Ed., World Scientific Series on Meteorology of East Asia, Vol. 2, 463-498.
46. Lander, M. A., 1994: Description of a monsoon gyre and its effects on the tropical cyclones in the western north Pacific during August 1991. Wea. Forecasting, 9, 640-654.
47. McGregor, G. R., S. Nieuwolt, 1998: Tropical Climatology: An Introduction to the Climates of the Low Latitudes. 2 ed. ed. Wiley and Sons Ltd (United Kingdom), 339 pp.
48. Chang, C., Z. Wang, J. McBride, and C. Liu, 2005: Annual cycle of southeast Asia-maritime continent rainfall and the asymmetric monsoon transition. J. Climate, 18, 287-301.
49. Cheang, B. K., 1977: Synoptic feature and structure of some equatorial vortices over the South China Sea in the Malaysian region during the winter monsoon of December 1973. Pure Appl. Geophys., 115, 1303-1333.
50. Johnson, R. H., P. E. Ciesielski, and T. D. Keenan, 2004: Oceanic East Asian monsoon convection: Results from the 1998 SCSMEX. East Asian Monsoon, C.-. Chang, Ed., 436-459.
51. Houze, R. A.,Jr, 1981: Winter monsoon convection in the vicinity of North Borneo, Pt. 1, Structure and time variation of the clouds and precipitation. Mon. Wea. Rev., 109, 1595-1614.
52. McBride, J., 1987: The Australian summer monsoon. Monsoon Meteorology, C.-. Chang and T. N. Krishnamurti, Eds., Oxford University Press, 203-231.
53. Drosdowsky, W., 1996: Variability of the Australian summer monsoon at Darwin: 1957-1992. J. Climate, 9, 85-96.
54. Krishnamurti, T. N., 1971: Tropical east-west circulations during northern summer. J. Atmos. Sci., 28, 1342-1347.
55. Chang, C., K. Lau, 1982: Short-term planetary-scale interactions over the Tropics and midlatitudes during northern winter, Pt. 1, Contrasts between active and inactive periods. Mon. Wea. Rev., 110, 933-946.
56. Chang, C.-P., H. C. Kuo, and C. H. Liu, 2003: Typhoon Vamei: An equatorial tropical cyclone formation. Geophys. Res. Lett., 30 (50), 1-4.
57. Holland, G. J., 1984: On the climatology and structure of tropical cyclones in the Australian/Southwest Pacific region, Pt. 2, Hurricanes. Australian Meteorological Magazine, Canberra, 32, 17-31.
58. Hendon, H. H., B. Liebmann, 1990: The intraseasonal (30-50 day) oscillation of the Australian summer monsoon. J. Atmos. Sci., 47, 2909-2923.
59. Hendon, H. H., C. Zhang, and J. D. Glick, 1999: Interannual variation of the Madden-Julian oscillation during austral summer. J. Climate, 12, 2538-2550.
60. Drobinski, P., B. Sultan, and S. Janicot, 2004: Role of the Hoggar Mountain in West African monsoon onset. Geophys. Res. Lett., 32.
61. Sultan, B., S. Janicot, 2003: The west African monsoon dynamics. Part II: The 'Preonset' and 'Onset' of the summer monsoon. J. Climate, 16, 3407-3427.
62. Lele, M., P. J. Lamb, 2010: Variability of the Intertropical Front (ITF) and rainfall over the west African Sudan-Sahel Zone. J. Climate, 23, 3984-4004.
63. Burpee, R. W., 1972: The origin and structure of easterly waves in the lower troposphere of North Africa. J. Atmos. Sci., 29, 77-90.
64. Cook, K. H., 1999: Generation of the African easterly jet and its role in determining West African precipitation. J. Climate, 12, 1165-11
65. Parker, D., C. Thorncroft, R. Burton, and A. Diongue-Niang, 2005: Analysis of the African easterly jet, using aircraft observations from the JET2000 experiment. Quart. J. Roy. Met. Soc., 131, 1461-1482.
66. Riehl, H., 1945: Waves in the easterlies and the polar front in the tropics. Vol. Misc. Rep. No. 17, Department of Meteorology, University of Chicago, 79 pp.
67. Burpee, R. W., 1974: Characteristics of the North African easterly waves during the summers of 1968 and 1969. J. Atmos. Sci., 31, 1556-1570.
68. Reed, R. J., D. C. Norquist, and E. E. Recker, 1977: The structure and properties of African wave disturbances as observed during phase III of GATE. Mon. Wea. Rev., 105, 317-333.
69. Thorncroft, C., K. Hodges, 2001: African easterly wave variability and its relationship to Atlantic tropical cyclone activity. J. Climate, 14, 1166-1179.
70. Berry, G., C. Thorncroft, 2005: Case study of an intense Africaneasterly wave. Mon. Wea. Rev., 133, 752-766.
71. Lin, Y., K. Robertson, and C. Hill, 2005: Origin and propagation of a disturbance associated with an African easterly wave as a precursor of Hurricane Alberto (2000). Mon. Wea. Rev., 133, 3276-3298.
72. Zhou, J., K. Lau, 1998: Does a monsoon climate exist over South America? J. Climate, 11, 1020-1040.
73. Mechoso, C. R., A. W. Robertson, C. F. Ropelewski, and A. M. Grimm, 2005: The American Monsoon Systems: An Introduction. The Global Monsoon System: Research and Forecast Report of the International Committee of the Third International Workshop on Monsoons (IWM-III), C.-P. Chang, B. Wang and N.-C. Lau, Eds., Secretariat of the World Meteorological Organization, 197-206.
74. Ropelewski, C. F., D. Gutzler, R. W. Higgins, and C. R. Mechoso, 2005: The North American Monsoon system. The Global Monsoon System: Research and Forecast Report of the International Committee of the Third International Workshop on Monsoons (IWM-III), C.-P. Chang, B. Wang and N.-C. Lau, Eds., Secretariat of the World Meteorological Organization, 207-218.
75. Shukla, J., 1987: Interannual variability of monsoon. Monsoons, J. S. Fein and P. L. Stephens, Eds., John Wiley & Sons, 3-32.
76. Mooley, D., J. Shukla, 1987: Variability and forecasting of the summer monsoon rainfall over India. Chang, Chih-Pei; Krishnamurti, Tiruvalam Natarajan, Monsoon meteorology, Oxford, Eng., Oxford University Press, Inc., 1987, , 26-59.
77. Sikka, D. R., 1999: Monsoon drought. India-Joint COLA/CARE Technical report, Vol. 2, Center for Ocean-Land-Atmosphere studies, University of Maryland, .
78. Webster, P. J., A. M. Moore, J. P. Loschnigg, and R. R. Leben, 1999: Coupled ocean-atmosphere dynamics in the Indian Ocean during 1997-98. Nature, 401, 356-360.
79. Wang, B., R. Wu, and K. Lau, 2001: Interannual variability of the Asian summer monsoon: Contrasts between the Indian and the western north Pacific-east Asian monsoons. J. Climate, 14, 4073-4090.
80. Meehl, G. A., 1994: Influence of the land surface in the Asian summer monsoon: external conditions versus internal feedbacks. J. Climate, 7, 1033-1049.
81. Loschnigg, J., G. Meehl, P. Webster, J. Arblaster, and G. Compo, 2003: The Asian monsoon, the tropospheric biennial oscillation, and the Indian Ocean zonal mode in the NCAR CSM. J. Climate, 16, 1617-1642.
82. Pillai, P. A., K. Mohankumar, 2007: Tropospheric biennial oscillation of the Indian summer monsoon with and without the El Niño-Southern Oscillation. Int. J. Climatol., 27, 2095-2101.
83. Nicholls, N., 1984: The Southern Oscillation and Indonesia sea surface temperature. Mon. Wea. Rev., 112, 424-432.
84. Chang, C., T. Li, 2000: A theory for the tropical tropospheric biennial oscillation. J. Atmos. Sci., 57, 2209-2224.
85. Meehl, G., J. Arblaster, 2002: The tropospheric biennial oscillation and Asian-Australian monsoon rainfall. J. Climate, 15, 722-744.
86. Wu, R., B. Kirtman, 2004: The tropospheric biennial oscillation of the Monsoon-ENSO system in an interactive ensemble coupled GCM. J. Climate, 17, 1623-1640.
87. Meehl, G., J. Arblaster, and J. Loschnigg, 2003: Coupled ocean-atmosphere dynamical processes in the tropical Indian and Pacific oceans and the TBO. J. Climate, 16, 2138-2158.
88. Kumar, K. K., B. Rajagopalan, and M. A. Cane, 1999: On the weakening relationship between the Indian monsoon and ENSO. Science (Wash.), 284, 2156-2159.
89. Gadgil, S., P. Vinayachandran, P. Francis, and S. Gadgil, 2004: Extremes of the Indian summer monsoon rainfall, ENSO and equatorial Indian Ocean oscillation. Geophys. Res. Lett., 31.
90. Wang, B. and T. Li, 2004: East Asian winter monsoon-ENSO interactions. In The East Asian Monsoon, C. P. Chang, Ed., World Scientific Series on Meteorology of East Asia, Vol. 2, 177-212.
91. McBride, J., M. Haylock, and N. Nicholls, 2003: Relationships between the maritime continent heat source and the El Niño-Southern Oscillation phenomenon. J. Climate, 16, 2905-2914.
92. Janicot, S., S. Trzaska, and I. Poccard, 2001: Summer Sahel-ENSO teleconnection and decadal time scale SST variations. Clim. Dyn., 18, 303-320.
93. Ward, M., 1998: Diagnosis and short-lead time prediction of summer rainfall in tropical North Africa at interannual and multidecadal timescales. J. Climate, 11, 3167-3191.
94. Saji, N., B. Goswami, P. Vinayachandran, and T. Yamagata, 1999: A dipole mode in the tropical Indian Ocean. Nature, 401, 360-363.
95. Li, T., B. Wang, C. Chang, and Y. Zhang, 2003: A theory for the Indian Ocean dipole-zonal mode. J. Atmos. Sci., 60, 2119-2135.
96. Hu, Z., M. Latif, E. Roeckner, and L. Bengtsson, 2000: Intensified Asian summer monsoon and its variability in a coupled model forced by increasing greenhouse gas concentrations. Geophys. Res. Lett., 27, 2681-2684.
97. Gadgil, S., M. Rajeevan, and P. Francis, 2007: Monsoon variability: Links to major oscillations over the equatorial Pacific and Indian oceans. Curr. Sci., 93, 182-194.
98. Giannini, A., R. Saravanan, and P. Chang, 2003: Oceanic forcing of Sahel rainfall on interannual to interdecadal time scales. Science (Wash.), 302, 1027-1030.
99. Folland, C., T. Palmer, and D. Parker, 1986: Sahel rainfall and worldwide sea temperatures, 1901-85. Nature, 320, 602-607.
100. Lamb, P. J., R. A. Peppler, 1992: Further case studies of tropical Atlantic surface atmospheric and oceanic patterns associated with sub-Saharan drought. J. Climate, 5, 476-488.
101. Druyan, L. M., T. M. Hall, 1996: The sensitivity of African wave disturbances to remote forcing. J. Appl. Meteor., 35, 1100-1110.
102. Cadet, D., 1986: Fluctuations of precipitable water over the Indian Ocean during the 1979 summer monsoon. Tellus, Series A, Dynamic Meteorology and Oceanography, Stockholm, 38, 170-177.
103. Rajeevan, M., S. Gadgil, and J. Bhate, 2010: Active and break spells of the Indian summer monsoon. J. Earth Syst. Sci., 119, 229-247.
104. Goswami, B., P. Xavier, 2003: Potential predictability and extended range prediction of Indian summer monsoon breaks. Geophys. Res. Lett., 30.
105. Wang, B., T. Li, Y. Ding, R. Zhang, and H. Wang, 2005: East Asian-western north Pacific monsoon: A distinctive component of the Asian-Australian monsoon system. Global Monsoon Systems: Research and Forecast Report of the International Committee of the Third International Workshop on Monsoons (IWM-III), 2-6 November 2004, Hangzhou, China, C.-P. Chang, B. Wang and N.-C. Lau, Eds., Secretariat of the World Meteorological Organization, 72-94.
106. Wheeler, M. C., J. L. McBride, 2005: Australian-Indonesian Monsoon Region. Intraseasonal Variability in the Atmosphere-Ocean Climate System, W. K. M. Lau and D. E. Waliser, Eds., Praxis Springer, 125-173.
107. Madden, R., P. R. Julian, 1972: Description of global scale circulation cells in the Tropics with 40–50 day period. J. Atmos. Sci., 29, 1109-1123.
108. Matthews, A. J., 2000: Propagation mechanisms for the Madden-Julian Oscillation. Quart. J. Roy. Meteor. Soc., 126, 2637-2651.
109. Wheeler, M. C., H. H. Hendon, 2004: An all-season real-time multivariate MJO index: Development of an index for monitoring and prediction. Mon. Wea. Rev., 132, 1917-1932.
110. Wheeler, M. C., K. M. Weickmann, 2001: Real-time monitoring and prediction of modes of coherent synoptic to intraseasonal tropical variability. Mon. Wea. Rev., 129, 2677-2694.
111. Chen, B., M. Yanai, 2000: Comparison of the Madden-Julian oscillation (MJO) during the TOGA COARE IOP with a 15-year climatology. J. Geophys. Res., 105, 2139-2149.
112. Hsu, H., M. Lee, 2005: Topographic effects on the eastward propagation and initiation of the Madden-Julian Oscillation. J. Climate, 18, 795-809.
113. Shapiro, M. A., A. J. Thorpe, 2004: THORPEX International Science Plan. Vol. WMO/TD-No.1246 WWRP/ THORPEX, No.2, World Meteorological Organization, 51 pp.
114. Slingo, J., D. Rowell, K. Sperber, and F. Nortley, 1999: On the predictability of the interannual behaviour of the Madden-Julian Oscillation and its relationship with El Niño. Quart. J. Roy. Meteor. Soc., 125, 583-609.
115. Lavender, S. L., A. J. Matthews, 2009: Response of the west African monsoon to the Madden-Julian Oscillation. J. Climate, 22, 4097-4116.
116. Janicot, S., B. Sultan, F. Mounier, N. M. Hall, S. Leroux, and G. N. Kiladis, 2009: Dynamics of the west African monsoon. Part IV: Analysis of 25-90-day variability of convection and the role of the Indian monsoon. J. Climate, 22, 1541-1565.
117. Paegle, J. N., L. A. Byerle, and K. C. Mo, 2000: Intraseasonal modulation of South American summer precipitation. Mon. Wea. Rev., 128, 837-850.
118. Chen, J., B. Carlson, and A. Del Genio, 2002: Evidence for strengthening of the tropical general circulation in the 1990s. Science (Wash.), 295, 838-840.
119. Wielicki, B., T. Wong, R. Allan, A. Slingo, J. Kiehl, B. Soden, C. Gordon, A. Miller, S. Yang, D. Randall, F. Robertson, J. Susskind, and H. Jacobowitz, 2002: Evidence for large decadal variability in the tropical mean radiative energy budget. Science (Wash.), 295, 841-844.
120. Meehl, G., J. Arblaster, 2003: Mechanisms for projected future changes in south Asian monsoon precipitation. Clim. Dyn., 21, 659-675.
121. Ashfaq, M., Y. Shi, W. Tung, R. J. Trapp, X. Gao, J. S. Pal, and N. S. Diffenbaugh, 2009: Suppression of south Asian summer monsoon precipitation in the 21st century. Geophys. Res. Lett., 36.
122. Stensrud, D. J., 1996: Importance of low-level jets to climate: a review. J. Climate, 9, 1698-1711.
123. Vera, C., J. Baez, M. Douglas, C. Emmanuel, J. Marengo, J. Meitin, M. Nicolini, J. Nogues-Paegle, J. Paegle, O. Penalba, P. Salio, C. Saulo, M. Silva Dias, P. Silva Dias, and E. Zipser, 2006: The South American low-level jet experiment. Bull. Amer. Meteor. Soc., 87, 63-77.
124. Munoz, E., A. Busalacchi, S. Nigam, and A. Ruiz-Barradas, 2008: Winter and summer structure of the Caribbean low-level jet. J. Climate, 21, 1260-1276
125. Cook, K. H., E. K. Vizy, 2010: Hydrodynamics of the Caribbean low-level jet and its relationship to precipitation. J. Climate, 23, 1477-1494.


Absolute angular momentum
For the atmosphere, the absolute angular momentum, per unit mass of air, is the sum of the angular momentum relative to the earth and the angular momentum due to the rotation of the earth.
Absolute vorticity
See Vorticity.
Anything that retains incident electromagnetic radiation due its physical composition.
The process by which incident radiant energy is retained by a material due to the material's physical composition.
Absorption band
A portion of the electromagnetic spectrum where radiation is absorbed and emitted by atmospheric gases such as water vapor, carbon dioxide, and ozone.
African easterly wave
A trough or cyclonic curvature maximum in the trade-wind easterlies. The wave may reach maximum amplitude in the lower middle troposphere.
The clumping together of ice crystals after they collide.
The deviation of a quantity over a specified period from the normal value for the same region. For example, El Niño is identified by sea surface temperature anomalies.
Atlantic Multidecadal Oscillation (AMO)
A natural oscillation of the North Atlantic SST between warm and cool phases. The SST difference between these warm and cool phases is about 0.5°C and the period of the oscillation is roughly 20-40 years (the period is variable, but is a few decades long). Evidence suggests that the AMO has been active for at least the last 1,000 years.
Any process in which the intensity of radiation decreases due to scattering or absorption.
Atmospheric Window
A portion of the electromagnetic spectrum where radiation passes through the atmosphere without absorption by atmospheric gases such as water vapor, carbon dioxide, and ozone.
Available potential energy (APE)
The portion of the total potential energy available for adiabatic conversion to kinetic energy. The total potential energy is a combination of the APE and the potential energy representing the mass distribution needed to balance the mean atmospheric motions.

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That portion of radiation scattered back toward the source.
Dependence on the horizontal temperature contrast between warm and cold air masses., In a baroclinic atmosphere, the geostrophic wind varies with height in direction as well as speed and its shear is a function of the horizontal temperature gradient (the thermal wind equation).
The atmosphere has the same horizontal structure at all levels in the vertical. This is equivalent to the absence of horizontal temperature gradients.
Barotropic-Baroclinic Instability
Barotropic and baroclinic instability analyses are used to explain the growth of a small perturbation to the flow. A perturbation growing due to baroclinic instability draws its energy from the available potential energy (APE). A perturbation growing due to barotropic instability draws its energy from the kinetic energy of the background flow. A perturbation growing through both APE and mean kinetic energy conversion to kinetic energy of the growing system (intensifying the system) is developing through combined barotropic baroclinic instability.
Best track
As defined by the National Hurricane Center, it is a subjectively-smoothed representation of a tropical cyclone's location and intensity over its lifetime. The best track contains the cyclone's latitude, longitude, maximum sustained surface winds, and minimum sea-level pressure at 6-hourly intervals. Best track positions and intensities, which are based on a post-storm assessment of all available data, may differ from values contained in storm advisories. They also generally will not reflect the erratic motion implied by connecting individual center positions fixed during operations.
Beta (β) effect
Denotes how fluid motion is affected by spatial changes of the Coriolis parameter, for example, due to the earth's curvature. The term takes its name from the symbol β representing the meridional gradient of the Coriolis parameter at a fixed latitude. The asymmetric flows resulting from the interaction of the vortex with the changing Coriolis parameter is known as the β-gyres.
Beta (β) plane
An approximation of the Coriolis parameter in which f = f0 + βy, where β is a constant. The Coriolis parameter is assumed to vary linearly in the north-south direction. The term takes its name from the symbol β representing the meridional gradient of the Coriolis parameter at a fixed latitude.
An object that absorbs all incident radiation and emits the maximum amount of energy at all wavelengths.
Blended precipitation estimate
An estimate that is derived by combining low earth-orbiting microwave measurements, which have high resolution but low frequency, with the more frequently available geostationary IR.
Bow echo
An organized mesoscale convective system, so named because of its characteristic bow shape on radar reflectivity displays. Bow echoes are typically 20–200 km long and last for 3–6 hours. They are associated with severe weather, especially high, straight-line surface winds, which are the result of a strong rear-inflow jet descending to the surface.
Brightness temperature
The Planck temperature associated with the radiance for a given wavelength.

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Location of the vertical axis of a tropical cyclone, usually defined by the location of minimum wind or minimum pressure. The cyclone center position can vary with altitude.
Cloud track winds
Winds derived from tracking movement of cloud elements using IR and water vapor images from geostationary satellites.
Conditional Instability of the Second Kind (CISK)
A theory for tropical cyclone development that relates boundary layer moisture convergence (driven by Ekman pumping) to the potential for tropical cyclone intensification. As the storm intensifies, the moisture convergence must increase, providing a feedback to the system. As with WISHE, CISK relies on the presence of an incipient disturbance.
Coordinated Universal Time (UTC)
Same as Zulu (Z) and Greenwich Mean Time (GMT).
Coriolis parameter, f
A measure that is twice the local vertical component of the angular velocity of a spherical planet, 2Ω sinφ, where Ω is the angular speed of the planet and φ is the latitude.
The formation of a cyclone.
An closed circulation of low pressure, rotating counter-clockwise in the Northern Hemisphere and clockwise in the SH.
Cyclone Phase Space (CPS)
A concise, three-parameter summary of the structure of a storm. It can be used to describe the structure of any synoptic or meso-synoptic cyclone.

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The process by which molecules are changed from the vapor phase directly to the solid phase, such as from water vapor to ice.
Doppler Effect
The apparent shift in the frequency and wavelength of a wave perceived by an observer moving relative to the source of the wave.
Doppler radar
Radar that uses the Doppler effect to detect radial velocity of targets based on the phase shift between the transmitted pulse and the received backscatter.
Dvorak Technique
a classification scheme for estimating the intensity of TCs from enhanced IR and visible satellite imagery. It is the primary method of estimating intensity everywhere, except the North Atlantic and North Pacific where aircraft reconnaissance is routine.

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Eddy angular momentum flux (EAMF)
Flux (net transport) of angular momentum into a circle centered on the storm. If EAMF is positive, the flow inside the circle will become more cyclonic; negative EAMF render the system less cyclonic (more anticyclonic). See Box 8-6 for a definition and discussion of angular momentum in tropical cyclones.
Ekman layer
Thin horizontal layer of water at top of the ocean that is affected by wind.  That layer has a force balance between pressure gradient force, Coriolis force and frictional drag.
Ekman pumping
The force balance determining the vector wind is modified by friction at the Earth's surface. The addition of friction changes the force balance to slow the winds and change their direction: winds now flow into a low and out of a high pressure system. Winds flowing into a low because of friction are forced upwards and out of the boundary layer. This process is known as Ekman pumping.
El Niño-Southern Oscillation (ENSO)
An oscillation of the ocean-atmosphere system in the tropical Pacific which affects global  weather and climate. El Niño, the warm phase of ENSO, is a quasi-periodic (2-7 years) warming of ocean surface waters in the equatorial and eastern tropical Pacific and an eastward shift in convection from the western Pacific climatological maximum. Changes occur in the tropical trade easterlies, vertical wind shear,  and ocean height. Cool ocean temperature anomalies are observed in the tropical western Pacific extending eastward into the subtropics of both hemispheres. "La Niña" refers to the less intense, anomalous  cool phase of ENSO. The Southern Oscillation refers to the atmospheric pressure difference between Darwin and Tahiti that is correlated with El Niño.
Electromagnetic (EM)
Energy carried by electric and magnetic waves.
The process by which a material generates electromagnetic radiation due to its temperature and composition.
The emitting efficiency of an object compared to an ideal emitter (or blackbody). A blackbody has an emissivity of one.
Anything that radiates measurable electromagnetic radiation.
Empirical Orthogonal Function (EOF)
See Principal Component Analysis.
The capacity to do work or transfer heat. Measured in SI units as Joules.
The integration of unsaturated environmental air into the turbulent cloud-scale circulation. The antonym of entrainment is detrainment.
Explosive Deepening
A decrease in the minimum sea-level pressure of a tropical cyclone of 2.5 hPa hr-1 for at least 12 hours or 5 hPa hr-1 for at least six hours.

A term used to indicate that a cyclone has lost its “tropical” characteristics. The term implies both poleward displacement of the cyclone and the conversion of the cyclone’s primary energy source from the release of latent heat of condensation to baroclinic processes.

It is important to note that cyclones can become extratropical and still retain winds of hurricane or tropical storm force. Given that these dangerous winds can persist after the cyclone is classified as extratropical, the Canadian Hurricane Centre (for example) follows them as “Former hurricane XXX.”

Extratropical Transition (ET)
The evolution of a poleward-moving initially tropical cyclone resulting in an extratropical cyclone. In the process of this evolution the energy source of the storm shifts from latent heat release to baroclinic development.
Eye (of tropical cyclone)
The approximately circular area of light winds at the center of a tropical cyclone. It is surrounded entirely or partially by clouds in the eyewall.
Eyewall / Wall Cloud
The full or partial ring of thunderstorms that surround the eye of a tropical cyclone. The strongest sustained winds in a tropical cyclone occur in the eyewall.

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Field of View (FOV)
Generally associated with the ground resolution from the detector standard viewing location, field of view is the solid angle through which a detector observes radiation.
Fraction of Photosynthetically Active Radiation (FPAR)
An index that measures how much sunlight the leaves are absorbing.
The number of recurrences of a periodic phenomenon per unit time. The frequency, v, of electromagnetic energy is usually specified in Hertz (Hz), which represents one cycle per second.
Fujiwhara Effect
The mutual advection of two or more nearby tropical cyclones about each other. This results in cyclonic rotation of the storms about each other.

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Gale Force Wind
A sustained surface wind in the range 17 m s-1 (39 mph, 63 km hr‑1 or 34 knot) to 24 m s-1 (54 mph, 87 km hr‑1 or 47 knot) inclusive, and not directly associated with a tropical cyclone.
Geostationary or Geosynchronous orbit
An orbit whose rotation period equals that of the Earth. The altitude of a geostationary orbit is approximately 35,800 km. Its orbit keeps it above a single point on the equator.
Geostationary Operational Environmental Satellite (operated by NOAA).
GOES Precipitation Index
An estimate of precipitation that uses 235K as the IR temperature with the best correlation to average precipitation for areas spanning 50-250 km over 3-24 hours.
Global Positioning System, a network of defense satellites established in 1993. Each satellite broadcasts a digital radio signal that includes its own position and the time, accurate to one billionth of a second. GPS receivers use the signals to calculate their position to with a few hundred feet.
GPS radio occultation
The technique by which satellite receivers intercept signals from GPS and infer the deviations in the signal's path caused by temperature and moisture gradients.
Gravity waves
Oscillations usually of high frequency and short horizontal scale, relative to synoptic- scale motions, which arise in a stably stratified fluid when parcels are displaced vertically. Gravity is the restoring force.
Greenwich Mean Time (GMT)
Mean solar time of the meridian at Greenwich, England, used as the basis for standard time throughout most of the world. Also referred to as Zulu (Z) and Coordinated Universal Time (UTC).

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Hadley Cells
Circulation cells in which air rises in the ITCZ, sinks into the subtropical highs, and returns to the equatorial low along the trade winds. George Hadley proposed a model (1735) of the global atmospheric circulation with rising motion at the equator, where there is surplus heating, and sinking motion at the poles, where there is net cooling. Hadley's model did not account for the Coriolis effect, which leads to average westerly motion in the mid-latitudes. The Hadley model does explain the circulation within 30 degrees of the equator.
Horizontal Convective Rolls
Lines of overturning motion with axes parallel to the local surface. These rolls result from a convective instability (high density over low density – often corresponding to cool air over warm) and can mix strong winds from above down towards the surface.
A tropical cyclone in which the maximum sustained surface wind (using the local time averaging convention) is at least 33 m s-1 (74 mph, 119 km hr-1 or 64 knot). The term "hurricane" is used for in the Northern Atlantic and Northeast Pacific; "tropical cyclone" east of the International Dateline to the Greenwich Meridian; and "typhoon" in the Pacific north of the Equator and west of the International Dateline.

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Inertial period
The time taken to complete one rotation. In the tropical cyclone this is calculated by dividing the circumference at the radius of interest (commonly, the radius of maximum winds) by the wind speed at that radius.
Infrared (IR)
Electromagnetic energy within the wavelength interval generally defined from 0.7 to 100 microns.
The energy per unit time incident upon a unit area of a given surface, measured in SI units as Wattsm-2.
The incoming solar radiation that reaches the earth and its atmosphere.
The peak sustained surface wind in the region immediately surrounding the storm center, or the minimum central pressure measured in the eye.
Intertropical Convergence Zone (ITCZ)
The zone where the northeast and southeast trade winds converge. It is marked by low pressure, rising motion, and thunderstorms, which occur with strong surface heating. Its latitudinal position shifts in response to the solar maximum and heating response of the surface. It is recognized in satellite images as a band of thunderstorms across the tropics. It is often, but not always, co-located with the zone of low pressure known as the "Equatorial Trough".
Varying on time scales shorter than one season.

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SI unit of energy equal to 0.2389 calories.

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Kelvin waves
At the equator, eastward propagating waves with negligible meridional velocity component and Gaussian latitudinal structure in zonal velocity, geopotential, and temperature, symmetric about the equator.

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The intersection of the surface center of a tropical cyclone with a coastline. Because the strongest winds in a tropical cyclone are not located precisely at the center, it is possible for the strongest winds to be experienced over land even if landfall does not occur.
Leaf Area Index (LAI)
The ratio of green leaf area to the total surface area occupied by vegetation.
Longwave (LW)
Electromagnetic energy lying in the wavelength interval generally defined from 4.0 microns to an indefinite upper limit.
Low earth orbit (LEO)
An orbit that is located at an altitude generally between 200 and 1000 km.
Low earth orbit satellite
A satellite that has a low earth orbit. Most have paths crossing the poles and can provide synchronous observations (e.g., the NOAA series or Defense Meteorological Satellite Program systems). The TRMM is an LEO satellite that orbits between ±35º latitude.

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Madden-Julian Oscillation (MJO)
Tropical rainfall exhibits strong variability on time scales shorter than the seasonal. These fluctuations in tropical rainfall often undergo a 30-60 day cycle that is referred to as the Madden-Julian Oscillation or intraseasonal oscillation. The MJO is a naturally occurring component of the Earth's coupled ocean-atmosphere system that significantly affects the atmospheric circulation throughout the global tropics and subtropics.
Maritime Continent
The region of Southeast Asia that comprises many islands, peninsulas, and shallow seas (including countries such as Indonesia, Malaysia, Papua New Guinea, and the Phillipines and covers approximately 12°S to 8°N, 95°E to 150°E).
North-south, crossing latitudes; by convention the meridional wind from the south is positive.
Spatial scale of 100-1000 km and temporal scale of hours to a day; between synoptic and convective scale. Tropical clouds are most often organized into mesoscale systems.
Mesoscale convective complex (MCC)
A large, quasi-circular mesoscale convective system that produces heavy rainfall and severe weather. In some MCCs, a mid-tropospheric vortex forms and remains after the deep convection has dissipated.
Mixed Rossby-Gravity (MRG) Wave
A divergent Rossby wave, resulting from conservation of potential vorticity and buoyancy forcing. These waves propagated westward along the equator. Meridional velocity is symmetric about the equator. Zonal wind, temperature, and geopotential area antisymmetric about the equator.
Of or pertaining to a single wavelength, or in practice, perhaps a very narrow spectral interval.
A term whose roots are from the Arabic for "season", it is a seasonal wind reversal. The monsoon has inflow to a surface heat low and an offshore flow from high pressure during the winter when the land cools relative to the ocean. The Indian monsoon is the most prominent but it has been recognized that that monsoon region extends from Southeast Asia to West Africa. The summer monsoon is a vital source of moisture; its arrival, duration, and amount of precipitation modulates the economies of these regions.
Monsoon Gyre
A closed, symmetric circulation at 850 hPa with horizontal extent of 25° latitude that persists for at least two weeks. The circulation is accompanied by abundant convective precipitation around the south-southeast rim of the gyre.
Monsoon Region
Refers to the combination of features including a monsoon trough, confluence zone, and the ITCZ.

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The satellite viewing angle directly downward (viewing zenith angle = 0 degrees). Also used to refer to the sub-satellite point location.

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Ocean conveyor belt
The name given to summarize the pattern of global ocean currents. The surface ocean currents generally transport warm salty water polewards, out of the tropics. The water cools as it moves polewards, becoming increasingly dense (remember that salty water is more dense than fresh water). This water sinks in the North Atlantic and also in the Southern Ocean near Antarctica. The deep water currents transport the water around the globe until it rises to the surface again, once more part of the surface ocean currents.
A physical description of a material which attenuates electromagnetic radiation.
Optical depth
A measure of the cumulative attenuation of a beam of radiation as a result of its travel through the atmosphere.

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Pacific Decadal Oscillation (PDO)
The PDO is a basin-scale pattern of Pacific climate variability; PDO climate anomalies are most visible in the North Pacific and North American regions, with secondary features in the tropics. The phases of the PDO persist for 20-to-30 years. Causes for the PDO have not yet been explained.
Planck's Law
An expression for the variation of monochromatic radiance as a function of wavelength for a blackbody at a given temperature.
Planetary Boundary Layer (PBL)
The layer of the atmosphere that extends upward from the surface to heights of 100 to 3000 m. The boundary layer is directly influenced by surface forcing such as friction, heating, and evapotranspiration.
Polar orbit
An orbit whose path crosses the polar regions. This type of orbit is located at an altitude generally between 200 and 1000 km, and can provide sun-synchronous observations.
Polar Orbiting Environmental Satellite (POES)
A satellite which has a polar orbit, such as the NOAA series or Defense Meteorological Satellite Program systems.
Potential evapotranspiration
A measure of the maximum possible water loss from an area under a specified set of weather conditions.
Potential Intensity (PI)
The largest possible intensity (maximum wind, minimum pressure) expected to be possible for a particular tropical cyclone.
Potential vorticity
A scalar measure of the balance between the vorticity and the thermal structure of the atmosphere.
Principal component analysis
A mathematical technique for identifying patterns in data by reducing multidimensional data to a smaller number of dimensions. A number of variables that are (possibly) correlated are transformed into a new coordinate system. The transformation identifies the components that account for variability in the data. The first principal component often accounts for the most of variability in the data. Also known as Empirical Orthogonal Function (EOF) analysis.

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Quasi-Biennial Oscillation (QBO)
An oscillation in the lower stratospheric zonal winds averaged around the equator. It is typically diagnosed from the zonal winds between 30-70 hPa (although it is evident as high as 10 hPa). The QBO has a varying from about 24 to 30 months. The zonal winds change by about 40 m s-1 between the maximum easterly and maximum westerly phase.

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Radar (Radio Detection And Range)
An instrument that detects objects remotely by transmitting high-frequency pulses to the atmosphere and measuring the "backscatter" or echoed pulses from that object. Weather radar transmits microwave (mm-cm) pulses; the returned signal is interpreted to determine where it is precipitating.
A measure of radiant intensity produced by a material in a given direction and per unit wavelength interval, measured in Watts/m 2 /steradian/micron. Monochromatic radiance is the most fundamental unit measured by satellite instruments.
Energy transferred by electromagnetic waves.
Radius of Maximum Winds
The distance from the center of a tropical cyclone to the location of the cyclone's maximum winds. In well-developed systems, the radius of maximum winds is generally found at the inner edge of the eyewall.
Rapid Deepening
A decrease in the minimum sea-level pressure of a tropical cyclone of 1.75 hPa hr-1 or 42 hPa for 24 hours.
The poleward motion of a tropical cyclone taking it from the mean tropical easterlies to the midlatitudes westerlies. This change in the advection of the storm results in curvature in the storm track.
The process by which incident radiation is scattered in the backward direction (backscattered).
The fraction of incident radiation reflected by a material.
Relative vorticity
See Vorticity.
Remnant Low
Used for systems no longer having the sufficient convective organization required of a tropical cyclone (e.g., the swirls of stratocumulus in the eastern North Pacific).
The process or end result of a process where physical quantities such as water vapor, temperature, and/or pressure are extracted from measurements of total upwelling radiance to space; here involving the GOES sounder.
The formation of ice by the rapid freezing of supercooled water drops as they impinge upon an object such as an ice crystal or aeroplane wing.
Rossby Radius of Deformation
The Rossby radius is the critical scale at which rotation becomes as important as buoyancy, which allows an initial disturbance to be sustained. It is a function of the absolute vorticity, stability, and depth of the disturbance. When a disturbance is wider than LR, it will persist; systems that are smaller than LR will dissipate.
Rossby Wave
A planetary wave, resulting from conservation of potential vorticity. Gradients of potential vorticity provide a restoring mechanism to allow propagation of the waves. This text focuses on Rossby waves centered on the equator equatorial (n=1) Rossby waves.

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Saffir-Simpson scale
A scale that links the observed damage and the effects of wind, pressure and storm surge that could lead to such damage. Initial wind damage scale was defined by Herbert Saffir and later expanded by Robert Simpson to include storm surge.
The process by which a material interacts with and redirects incident radiation (in any given direction).
A radar that infers near-surface wind velocity by sending pulses of microwave energy to the ocean surface and measuring the backscatter from small-scale waves. Scatterometry wind retrievals can be ambiguous during rain, since rain creates additional backscatter and attenuates the radar beam.
Shortwave (SW)
Electromagnetic radiation generally defined as having a wavelength shorter than 4.0 microns.
The mean radius of a tropical cyclone enclose by winds of at least 17 m s-1. Size may also be defined as the outer closed isobar of the surface pressure.
Solar declination angle
The angle between the rays of the Sun and the equatorial plane of the Earth. It is zero during an equinox and 23.5° during a solstice.
Southern Oscillation Index (SOI)
The normalized difference in sea level pressure between Darwin, Australia and Tahiti, French Polynesia.
Specific humidity
The mass of water vapor per unit mass of air (including water vapor), usually denoted by q and measured in units of grams per kilograms.
A descriptor for radiometric quantities or measurements which have a limited wavelength range.
Split window
A pair of regions of the electromagnetic spectrum which are closely located in wavelength, but have slightly different attenuation characteristics. Used to denote the 11- and 12-micron regions in which greater water vapor attenuation at 12 microns causes slightly different brightness temperatures.
Stefan-Boltzmann Law
The energy emitted per unit area (from all wavelengths and represented by the area under the blackbody curve) is proportional to the 4 th power of the absolute temperature
The unit of measure of solid angles, equal to the angle subtended at the center of a sphere.
Storm Surge
An abnormal rise in sea level accompanying a tropical cyclone or other intense storm, and whose height is the difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone. Storm surge is usually estimated by subtracting the normal or astronomic high tide from the observed storm tide.
Storm Tide
The actual level of sea water resulting from the astronomic tide combined with the storm surge.
Subtropical Cyclone

A non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones.

The most common type is an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about 100 miles or more from the center. In comparison to tropical cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the center, and typically have a less symmetric wind field and distribution of convection.

A second type of subtropical cyclone is a mesoscale low originating in or near a frontolyzing (dying frontal) zone of horizontal wind shear, with radius of maximum sustained winds generally less than about 50 km (30 miles). The entire circulation may initially have a diameter less than 160 km (100 miles). These generally short-lived systems may be either cold core or warm core.

Subtropical Depression
A subtropical cyclone in which the maximum sustained surface wind speed does not exceed 17 m s-1 (39 mph, 63 km hr‑1 or 34 knot).
Subtropical Storm
A subtropical cyclone in which the maximum sustained surface wind speed is at least 17 m s-1 (39 mph, 63 km hr‑1 or 34 knot).
Synthetic Aperture Radar (SAR)
Works like other radars except that it has very fine resolution in the azimuthal direction. It synthesizes the fine resolution normally achieved with a large antenna by combining signals from an object along a radar flight track and processing the signals as if obtained simultaneously from a single large antenna. The distance over which the signals are collected is known as the synthetic aperture.

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Trade Winds
Prevailing easterly winds flowing from the subtropical highs that affect equatorial and subtropical regions. Trade winds are mostly east to northeasterly in the Northern Hemisphere and east to southeasterly in the Southern Hemisphere. During the monsoon, easterly trades are replaced by mostly westerly winds.
The process by which incident radiation propagates forward through a material.
The process by which water vapor enters the atmosphere through the stomata in the leaves of plants.
The inversion layer separating the near-surface warm waters from the colder, deeper layers of oceans and lakes.  It is about 1km deep and is thermally stratified. In the ocean, it also separates the fresher waters near the surface from the saltier waters below.
Tropical Cyclone
A warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center. Once formed, a tropical cyclone is maintained by the extraction of heat energy from the ocean at high temperatures and heat export at the low temperatures of the upper troposphere. In this they differ from extratropical cyclones, which derive their energy from horizontal temperature contrasts in the atmosphere (baroclinic effects). Also see Hurricane.
Tropical Cyclone Season
The portion of the year having a relatively high incidence of tropical cyclones. Also known as "Hurricane Season" or "Typhoon Season".
Tropical Depression
A tropical cyclone in which the maximum sustained surface wind speed is not more than 17 ms-1 (39 mph, 63 km hr‑1 or 34 knot).
Tropical Disturbance
A discrete tropical weather system of apparently organized convection – generally 185 to 550 km (100-300 n mi) in diameter – originating in the tropics or subtropics, having a nonfrontal migratory character, and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field.
Tropical Storm
A tropical cyclone in which the maximum sustained surface wind speed ranges from 17 ms-1 (39 mph, 63 km hr‑1 or 34 knot) to 33 ms-1 (74 mph, 119 km hr-1, 64 knot).
See Tropical Cyclone and Hurricane.

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Ultraviolet (UV)
Electromagnetic radiation of shorter wavelength than visible radiation but longer than x-rays (approximately 0.03 to 0.4 microns)

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The region of the electromagnetic spectrum which is detectable to the human eye (approximately 0.4 to 0.7 microns).
The local rotation of the flow, calculated as the the curl (cross product) of the vector wind. Vorticity has units of inverse seconds (s-1).

“Relative vorticity” is the vorticity calculated for the observed winds. It is called “relative” since the winds are the flow relative to the Earth’s rotation.
The vertical component of the vorticity vector is most often used since it is much larger than the other vorticity components. This is because the horizontal winds in tropical cyclones are much greater than the vertical wind component.

“Absolute vorticity” is the vorticity calculated for the total motion of the atmosphere the combination of the observed winds and the Earth’s rotation.

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Walker Circulation
The east-west circulation cells that form along the equator in response to differential surface heating.
A warning that sustained winds exceeding the threshold for either tropical storm or tropical cyclone and associated with such a storm are expected in a specified coastal area in 24 hours or less.
An announcement for specific coastal areas that either tropical storm or tropical cyclone conditions are possible within 36 hours.
The distance a wave will travel in the time required to generate 1 cycle, denoted by λ. A length measured from the midpoint of a crest (or trough) to the midpoint of the next crest (or trough).
The reciprocal of the wavelength, denoted by κ.
Water Vapor Channel (or water vapor IR channel)
A spectral band in which the radiance is attenuated by water vapor. This usually refers to the 6.7 micron channel in this module.
Weighting function
A mathematical expression representing the relative radiance contribution provided from a given level of the atmosphere (usually a function of atmospheric pressure).
Wind-Induced Surface Heat Exchange (WISHE)
A tropical cyclone development theory based on a conceptual model of a tropical cyclone as an atmospheric Carnot engine. Consistent with its Carnot engine roots, WISHE relates (i) fluxes of heat and moisture from the ocean surface and (ii) the temperature of the tropical cyclone outflow layer to the potential for continued storm development. The fluxes increase with surface wind speed providing a feedback to the system. As with CISK, WISHE relies on the presence of an incipient disturbance.
Wind profiler
Vertically pointing radar which operates on the same principle as horizontally-scanning Doppler radar; provides best measurements of vertical air motion inside convective storms
Wien's Displacement Law
The wavelength of maximum blackbody emission is inversely proportional to its absolute temperature.

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East-west, crossing longitudes; by convention, the zonal wind from the west is positive.
Zulu (Z)
Used to represent the same clocktime at GMT and UTC. See Greenwich Mean Time (GMT), or Coordinated Universal Time (UTC)

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