Effects of Large-scale Orography on the Coupled Atmosphere-Ocean System in the Tropical Indian and Pacific Oceans in Boreal Summer

Abstract

We investigated the effects of large-scale orography on the tropical coupled atmosphere-ocean system over the Indian and Pacific Oceans in northern summer, using the Meteorological Research Institute coupled atmosphere-ocean General Circulation Model (GCM). Six different experiments were conducted with mountain heights of 100%, 80%, 60%, 40%, 20%, and 0% of the standard mountain height. The results show that a pool of warm sea surface temperatures (SSTs) appears in the western Pacific as orography increases, although SST in the tropical Pacific decreases as a whole. In addition, easterly winds at low levels over the equatorial Pacific strengthen as mountains rise. The enhanced easterlies alter surface heat flux and ocean dynamics, changing the water temperature field in the upper Pacific Ocean. Water temperatures between the surface and 300 m in the western Pacific increase as upwelling is suppressed and the thermocline deepens. Water temperatures in the eastern Pacific decrease and the thermocline rises. Therefore, the east-west gradient of water temperature in the Pacific is enhanced for cases with mountain heights of 80% and 100% of the standard mountain height. In the equatorial Indian Ocean, the east-west gradient of ocean heat content weakens as mountain heights increase, in connection with the evolution of the Asian summer monsoon. An increase in diabatic heating over South Asia as mountain heights increase causes sea level pressure (SLP) to decline over the Indian Ocean, and enhances upper atmospheric divergence over the eastern hemisphere. Consequently, the east-west circulation over the Indian and Pacific Oceans strengthens as mountains become taller. The east-west circulation may also be enhanced by changes in convective activity associated with SST changes. The coupled general circu- lation model (GCM) results show that uplift of large-scale orography, particularly the Tibetan Plateau, significantly affects the tropical atmospheric and oceanic climate, by changing the east-west circulation and altering the evolution of the Asian summer monsoon.