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.
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).
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).
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.
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).
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.
West Pacific Monsoon trough
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.
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