An ocean-atmosphere interaction mechanism for the active break cycle of the Asian summer monsoon

作者：Joseph P V^*; Sabin T P

来源：Climate Dynamics, 2008, 30(6): 553-566.

DOI：10.1007/s00382-007-0305-2

A typical active-break cycle of the Asian summer monsoon is taken as beginning with maximum SST (pentad 0) over the north Bay of Bengal when the oceans to its west and east from longitude 40 degrees-160 degrees E, and between latitudes 10 degrees and 25 degrees N (area A) also has maximum SST. During this pentad the recently found "Cold Pool" of the Bay of Bengal (between latitudes 3 degrees N and 10 degrees N) has its minimum SST. An area of convection takes genesis over the Bay of Bengal immediately after pentad 0 in the zone of large SST gradient north of the Cold Pool and it pulls the monsoon Low Level Jetstream (LLJ) through peninsular India. Convection and the LLJ westerlies then spread to the western Pacific Ocean during pentads 1-4 taken as the active phase of the monsoon during which convection and LLJ have grown in a positive feed back process. The cyclonic vorticity to the north of the LLJ axis is hypothesized to act as a flywheel maintaining the convection during the long active phase against the dissipating effect of atmospheric stabilization by each short spell of deep convection. By the end of pentad 4 the SST over area A has cooled and the convection weakens there, when the LLJ turns clockwise over the Arabian Sea and flows close to the equator in the Indian ocean. A band of convection develops at pentad 5 between the equator and latitude 10 degrees S over the Indian ocean and it is nourished by the cyclonic vorticity of the LLJ now near the equator and the moisture supply through it. This is taken as the break monsoon phase lasting for about three to four pentads beginning from pentad 5 of a composite active-break cycle of 40 day duration. With reduced wind and convection over the area A during the break phase, solar radiation and light winds make the SST there warm rapidly and a new active-break cycle begins. SST, convection, LLJ and the net heat flux at the ocean surface have important roles in this new way of looking at the active-break cycle as a coupled ocean-atmosphere phenomenon.

• 出版日期2008-5