Understanding the annual cycle of equatorial Pacific as a result of ocean-atmosphere-land interactions

An intermediate coupled model over the global tropics has been used to study the roles of air-sea-land coupling on the annual cycle of tropical Pacific. Both the air-sea coupling and the adjacent continental 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 on the equatorial Pacific. It is the air-sea coupling responsible for the equatorial westward propagation of the annual cycles of SST and zonal wind. The dominant feedback is between the zonal heat advection-SST-zonal wind.

The annual cycle of the tropical Pacific SST cannot be adequately reproduced by the air-sea coupling forced with the seasonally varying solar radiation in the tropical Pacific alone. Only considering the regional Pacific air-sea coupling system, the entire equatorial Pacific becomes warmer especially the western Pacific. A quite weak and phase-lagged SST annual cycle in the eastern equatorial Pacific and a fictitious annual cycle in the western Pacific are produced.

The annual marches of the ITCZ/Cold Tongue may not be a 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 atmospheric diabatic heating associated with ITCZ rainfall forces both the southerly and westerly perturbations near the equator. The former causes the decrease of SST in the Cold Tongue; a positive coupling is established between ITCZ/Cold Tongue as suggested by Mitchell and Wallace (1992). However, the westerly due to the enhanced ITCZ rainfall favors the warming of the Cold Tongue. This negative feedback will weaken the meridional coupling.

The Asian-Australian monsoons have significant influences on the mean position of the western Pacific warm-pool and the equatorial Pacific thermocline slope. 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 annual 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 eastern Pacific. This process is shown to be an important external forcing of the SST annual cycle in the eastern equatorial Pacific.