Role of an Equatorially Asymmetric Mode in the Tropical Indian Ocean in the Asian Summer Monsoon-ENSO Coupling
Ryuichi Kawamura (1), Tomonori Matsuura (2) and Satoshi Iizuka (2)
(1) Department of Earth Sciences, Toyama University, Toyama, Japan
(2) National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Ibaraki, Japan
Using an ocean general circulation model forced by daily mean winds derived from the NCEP/NCAR reanalysis, we examined possible roles of the tropical Indian Ocean in the Asian summer monsoon-ENSO coupling. A dominant precursory signal of anomalous monsoon circulation relevant to ENSO is the presence of latitudinal asymmetric anomalies of tropical convection and SST about the equator in the preceding spring. The model results show that the equatorial asymmetry of surface latent heat flux plays a vital role in generating that of SST in the preceding spring, implying that a positive wind-evaporation-SST (WES) feedback proposed by Xie and Philander is crucially responsible for generation and maintenance of these equatorial asymmetries.
In the spring before strong monsoon, land-atmosphere interaction is also activated over the Asian continent while the WES feedback regime with warm (cool) SST anomalies north (south) of the equator persists over the tropical Indian Ocean. A upper-tropospheric anomalous anticyclone, accompanied by large-scale descending atmospheric motion, emerges over central Asia as a result of a Rossby wave response to enhanced convective heating over the northern tropical Indian Ocean, resulting in decreased rainfall, less soil moisture and increased surface temperature in that vicinity. When climatological mean low-level flow over the northern tropical Indian Ocean changes from easterly to westerly due to seasonal reversal of land-ocean temperature gradient from spring to early summer, the equatorial asymmetric anomalies become dissipated; whereas cool SST anomalies develop rapidly over the northern tropical Indian Ocean, hence intensifying land-ocean thermal contrast together with the increased land-surface temperature. Enhanced convection migrates northward and a strong monsoon regime commences in South Asia. The reverse situation is quite true for weak monsoon years.
Associated with anomalous Walker circulation in response to ENSO, anomalous convection is indicated off the equator over the warm pool region of the western North Pacific in the preceding winter, thereby exciting equatorially asymmetric atmospheric Rossby waves. The westward propagation of convection-induced waves can trigger a WES mode in the tropical Indian Ocean. It is anticipated that the WES mode in the tropical Indian Ocean serves as a bridge between the ENSO prevailing in the preceding winter and anomalous summer monsoon.
