El Nino Impacts on Precipitation in the Western North Pacific-East Asian Sector

In this study, the western North Pacific-East Asian (WNP-EA) rainfall anomaly induced by the strong El Nino in 1982-83, 1991-92, and 1997-98, and its association with the mean state, are examined. Over the northern part of the WNP-EA region (north of 208N), which is dominated by southwest-northeast tilting frontal systems, positive rainfall anomalies from the fall before the El Nino peak phase (year 0) to the first wet period after the peak phase (year 1) are affected by low- and midlevel horizontal moisture convergence anomalies induced by low-level anticyclonic circulation anomalies over the WNP region that are associated with El Nino. Over the southern part of the WNP-EA region (south of 208N), which is dominated by tropical convection, positive precipitation anomalies in the first and second wet periods of year 0 and negative precipitation anomalies from the fall of year 0 to the second wet period of year 1 are associated with the variation of the net energy into the atmosphere, which is mainly contributed to by local evaporation anomalies. The mechanisms for inducing the precipitation anomalies over both northern and southern parts of the WNP-EA region are similar to the mechanisms of the mean precipitation in each rainy period, but the detailed processes for the southern WNP-EA precipitation anomalies are more complicated, particularly in summer. In the first wet periods of years 0 and 1 and the fall of year 0, the precipitation anomalies are induced by evaporation anomalies that are contributed to by similar effects of sea surface temperature (SST) and wind speed anomalies. In the second wet period of years 0 and 1, on the other hand, near-surface wind speed anomalies affect precipitation via the process of evaporation. These wind speed anomalies are associated with the concurrence of the low-level circulation anomalies over the WNP region and the Asian summer monsoon trough. The SST anomalies are merely a response to evaporation and downward solar radiation anomalies. The dependence of the rainfall anomalies on the mean state, that is, similar causes for the rainfall mean and anomalies in each rainy period, implies that the mean state plays a key role in simulating the interannual variation over the WNP-EA region.