Using an ocean general circulation model (OGCM) forced by daily mean wind stresses and heat fluxes derived from the bulk formulation with the NCEP/NCAR reanalysis of 1973-1995, we examined interannual atmosphere-ocean variations in the tropical western North Pacific related to the Asian summer monsoon-ENSO coupling.
The OGCM simulation was successful in reproducing the east-west gradient of summertime SST anomalies between the South China Sea and the tropical western Pacific east of the Philippines that is linked with anomalous tropical convection in that vicinity. A heat budget analysis shows that a longitudinal asymmetry of surface latent heat flux anomalies is crucially responsible for the reinforcement and persistence of the east-west gradient of SST anomalies between the two regions.
It is also found that the transition from premonsoon regime to monsoon regime in the tropical Indian Ocean affects the interannual atmosphere-ocean variations in the tropical western North Pacific during the period from the late 1970s to the early 1990s. In the spring before strong Asian summer monsoon, an equatorially asymmetric air-sea coupled mode tends to appear in the tropical Indian Ocean. Simultaneously with the beginning of the strong monsoon regime, the northern Indian Ocean and South China Sea are covered by cool SST anomalies due to enhanced wind speed and evaporation, whereas warm anomalies relevant to a cold episode of ENSO are still maintained in the warm pool region east of the Philippines. The latitudinally asymmetric anomalies of tropical convection and SST become dissipated and convection over the warm pool region of the western North Pacific becomes localized and enhanced with the establishment of east-west gradient of SST anomalies. Due to enhanced convective heating, equatorially asymmetric atmospheric Rossby waves are excited to the west of anomalous convection, which induce low-level westerly anomalies. This dynamic process further facilitates the localization of intense convection through the change in surface latent heat flux and SST. While such a positive atmosphere-ocean feedback system persists in boreal summer, the Pacific and Japan (PJ) teleconnection pattern in response to enhanced convection prevails and brings about unusually hot summers in recent years especially in the vicinity of Japan.