Understanding the annual cycle of equatorial Pacific as

a result of ocean-atmosphere-land interactions

Xiouhua FU and Bin W ANG

International Pacific Research Center and Department of Meteorology, School of Ocean and Earth Science and

Technology, University of Hawaii at Manoa, Honolulu, H196822, USA

Abstract -An intermediate coupled model over the giobal tropics has been used to study the roles of air- sea-Iand coupling on the annual cycie of tropical Pacific. Both the air-sea coupling and the adjacent contI- nental monsoons have considerable impacts on the SST annual cycle of tropical Pacific Ocean.

.The adjacent continental monsoons won' t produce a westward propagating zonal wind along the equa- torial Pacific. It is the air-sea coupling responsible for the equatoriai westward propagation of the annual cycles of SST and zonai wind. The dominant feedback is between the zonal heat advection-SST -zonal

winds.

The air-sea coupling forced with the seasonally varying solar radiation in the tropical Pacific alone can- not adequately reproduce the annual cycle of the tropical Pacific SST. In the regional Pacific air-sea coupling system, the equatorial Pacific is systematically warmer than that in the control run, especially in the west- ern Pacific. A quite weak and phase-Iagged SST annual cycle in the eastern equatorial Pacific and a fictl- tlous annual cycle in the western Pacific are produced. The annual marches of the ITCZ/Cold Tongue are not fullya self-sustained system too. The onset of rainy season in the ITCZ (from March to May) is primarily due to the warming of the water under ITCZ. The diabatic heating associated with ITCZ rainfall enhances

both the southerly and westerly perturbations near the equator. The former causes the decrease of SST in the Cold Tongue; a positive coupiing is established between ITCZ/Cold Tongue as suggested by Mitchell and Wallace ( 1992). However, the westerly perturbation associated with the enhanced ITCZ rainfall favors the warming of the Cold Tongue. This negative feedback will weaken the meridional coupling.

The Asian-Australian monsoons can influence the climatological mean of the tropical Pac!fic SST and the equatorial thermocline slope through changing the mean strength of trades in the western pacific. They also produce a semiannual cycle of surface wind speed in the western equatorial Pacific, which is important for the SST semiannual cycle in this region, but have little influence on the SST annual cycle in the eastern equatorial Pacific. The American continental monsoons affect primarily the eastern Pacific SST annuai cycle, but not the climatological mean. The Columbian and Central and North American continental monsoons have little influence on the annual cycle of the SST in the Cold Tongue. However, the South American monsoons exert profound impacts on the annual variations of the southeast trades in the east Pacific. This process is shown to be an important external forcing of the SST annual cycle in the eastern equatorial Pacific.