Contrasts between Summer Monsoons over the Bay of Bengal and the Eastern North Pacific

Abstract

Structure of the July mean local Hadley circulation is investigated by utilizing wind, surface pressure, sea surface temperature (SST), and outgoing longwave radiation (OLR) data along 90°E and 110°W. The objective is to compare two types of monsoon, one is totally driven by land-sea contrast (Asiatic summer monsoon) and the other primarily induced by SST contrast over the ocean (eastern North Pacific summer monsoon).
In July, there exists a large-scale Hadley circulation covering both hemispheres along 90°E. Superimposed upon this are distinctly separate regional-scale Hadley cells, one each over either hemisphere. The northern hemisphere Hadley cell is associated with southerly surface winds accelerating from the equator toward the monsoon trough as a low-level branch, vigorous summer monsoon rains near the head Bay of Bengal representing an updraft leg, and 200mb northerly winds as an upper-level return flow. Finally, the northern cell is completed by an ill-defined downdraft leg due to divergent southerly surface winds over the equatorial Bay of Bengal. Continentality with high mountains is the major contributor to enhancement of the summer monsoon over the Bay of Bengal, while the contribution due to SST is minimal. The southern hemisphere Hadley cell is supported by low-level southerly surges bursting out of the midlatitude anticyclone and subjected to considerable warming and air mass modification due to strong SST gradients between 20°S and 10°S. Modified air converges into the near-equatorial trough and causes winter-time rains near 2°-8°S, where an updraft leg of the southern hemisphere Hadley cell is located. The southern Hadley cell interacts with the northern counterpart via low-level cross-equatorial southerlies that are generated as a response to an overall heat contrast between the winter and summer hemispheres.
Continentality appears to be insignificant along 110°W. The eastern Pacific is characterized by a zonally oriented cold SST tongue along the equator. This exceptionally strong south-to-north SST gradient causes a prominent poleward surface pressure gradient, which tends to accelerate southerly surface winds to the north of the equator. A large amount of energy is likely to be furnished to these southerlies due to a rapid increase of SST between the equator and about 10°N. Convection becomes strongest near 12°N, two to three degrees south of the thermal equator where SST is maximum (28.3°C). No convection occurs over the tropical South Pacific because of cold SST (less than 25°C). Throughout the year, the inter-tropical convergence zone (ITCZ) remains in the tropical North Pacific. The summer monsoon occurs when the ITCZ (updraft leg of the Hadley circulation) reaches it's northern most latitude of about 14°N in July. Over the equatorial eastern Pacific, the annual variation of surface meridional winds (low-level Hadley circulation) is much more pronounced than the annual variation of surface zonal winds (low-level Walker circulation). This favors the development of the summer monsoon over the eastern North Pacific.