SST cooling in the western North Pacific and its association with regional climate variations during summer

Seasonal and interannual variations of SST cooling in the western North Pacific (WNP) and its association with the WNP summer monsoon are investigated using satellite, in situ measurements, and regional atmospheric model. In July-August, strong northeastward surface wind jets develop over the tropical WNP when the WNP monsoon wind passes between the Mindanao Island, Halmahera Island, and New Guinea Island. The monsoon winds enhance evaporation from the shallow ocean on the continental shelf in the Indonesian archipelago, and advect the resultant cold water into the tropical WNP. Moreover, a couple of cyclonic and anticyclonic ocean eddies, the Mindanao and Halmahera eddies, produces strong northeastward current in the upper-ocean, which plays a crucial role in spreading the cold SST into the WNP. This ocean current separates into northward and southward flows west of Palau Island, which forms a fan-shaped cold SST area. The cold SST contributes to strengthen pressure gradient around 10N, 130-140E, resulting in local acceleration of surface wind. Therefore, surface wind convergence also intensifies north of the cold SST area, along with increase in preciptation. In September, on the other hand, this cold SST area disappears together with weakening of the northeastward wind jets and ocean current. Interannual variance of summer SST has a local maximum over the climatological cold SST area along with local maximum of surface wind variance. A cold SST index is constructed for July-August, which displays strong phase-locked correlation with meridional sea level pressure gradient over the Indonesian archipelago and precipitation anomalies north of the climatological cold SST area. These statistical analyses indicate that land-atmosphere-ocean interactions involved in summer monsoon has significant influence on the regional climate over the WNP.