Читать книгу Tropical Marine Ecology - Daniel M. Alongi - Страница 18

The Asian monsoon system reaches from the western Arabian Sea through East Asia and North Australia (Figure 2.8). This system is composed of the Indian (or South Asian) and East Asian subsystems, roughly divided at about 105°E. Both subsystems are linked to varying degrees by regions of strong sensible heating (Indo‐Asian landmass) and strong latent heat export (the western Pacific Warm Pool and the southern subtropical Indian Ocean). However, they also have significant differences determined by the contrasting sea‐land distributions.

FIGURE 2.8 Global tracks and intensity of all tropical storms, 1856–2006.

Source: Image retrieved via public access from NASA Earth Observatory. http://earthobservatory.nasa.gov/IOTD/view.php?id=7079. (accessed 25 June 2019). © NASA.

The Indian system is characterised by land in the north and ocean in the south, whereas the East Asian system has land in the north and south, a maritime continent in the west and open ocean to the east. Summer heating of the Asian continent, especially around the Tibetan Plateau, generates low‐pressure cells and thus summer rains in South and East Asia. In the winter, a reversed high‐pressure system is established, with dry, cool to cold winds blowing out of Asia. Waters of the northern Indian Ocean and South China Sea are subjected to seasonal changes in SSTs and salinity patterns and current strength and direction.

These dynamics lead to seasonal upwelling in the NW Arabian Sea. The monsoon is a result of northward seasonal migration of the east–west‐oriented precipitation belt (Tropical Convergence Zone) from the Southern Hemisphere in winter to the Northern Hemisphere in summer. The largest northward movement of the rain belt takes place over the Indian monsoon region where it moves from a mean position of about 5°S in winter to about 20°N in summer. Equatorial easterlies in the upper atmosphere are weak and confined between 5°N and 10°S, while the subtropical westerlies intrude up to 10°N during winter. The subtropical westerlies recede to north of 30°N during summer and a strong easterly jet characterises the equatorial upper atmosphere. Seasonal rainfall is highly variable with prolonged spells of dry and wet conditions often lasting for two–three weeks. Periods of wet conditions are generally associated with an increase in cyclonic activity and decrease in surface pressure over the central Indian monsoon trough region (northern Bay of Bengal to West India) and strengthening of the low‐level westerly jet over the Arabian Sea.

The East Asian monsoon features powerful breaks of cold air from the Eurasian continent during winter. During summer, the Tibetan Plateau acts as an elevated heat source and fosters an imposing subtropical monsoon front extending from the Bay of Bengal and the South China Sea all the way to the North Pacific, bringing abundant rainfall to East Asia, and transporting water and energy from the tropics to the mid‐latitudes. In contrast to warm trends elsewhere on earth, an interdecadal surface cooling has occurred in some regions of East Asia in the past few decades. Such interdecadal changes have been attributed to tropical ocean forcing. For example, in the late 1970s, summer rainfall anomalies in China were consistent with an intensification and southward extension of the western Pacific subtropical high over the subtropical regions of East Asia. Changes in SSTs over the Indian Ocean and tropical western Pacific may have partly accounted for these interdecadal changes.