In this study, the western North Pacific–East Asian (WNP–EA) rainfall anomaly induced by the strong El Niño 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 20°N), which is dominated by southwest–northeast tilting frontal systems, positive rainfall anomalies from the fall before the El Niño 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 Niño. Over the southern part of the WNP–EA region (south of 20°N), 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.
The annual cycle of precipitation over the western North Pacific and East Asian (WNP-EA) sector has five major periods: spring, the first and second wet periods, fall, and winter. In this study, processes that induce precipitation in each period are examined from a large-scale point of view. The wet phase over this sector has two distinct periods, which are dominated by the Asian summer monsoon circulation induced by the land-ocean contrast of net energy into the atmospheric column (F net ). In the first wet period, the pre-mei-yu/mei-yu rainband is directly associated with a moisture flux convergence caused by the southwesterly Asian summer monsoon flow and the southeasterly trade winds, and indirectly associated with a dynamic feedback induced by this horizontal moisture convergence. The tropical convection, in the meantime, is associated with a rising motion that is induced by positive F net . In the second wet period, the WNP summer monsoon gyre dominates the rainfall of this region, which is partially associated with warmer local sea surface temperature (SST) via positive F net . The land-sea contrast of F net and the atmosphere-ocean interaction also play an important role in establishing the monsoon gyre. The dry phase over the WNP-EA region is the winter period in which precipitation is associated with winter storm activities and large-scale lifting associated with a pressure surge. In the two transition phases, due to a difference in heat capacity, the atmosphere and ocean have distinct impacts on precipitation, albeit similar solar insolations during the two periods. In the spring period, the atmospheric condition is favorable for convection, while the ocean surface is relatively colder, so the horizontal moisture advection associated with the westward extent of the Pacific subtropical high, which is different from a typical winter frontal system, is a major source for the spring rain. In the fall period, however, the atmospheric conditions dominated by the Asian winter monsoon circulation suppress convection, while relatively warmer SST still maintains tropical convection over the southern part of the WNP-EA region. Over the northern part of the WNP-EA region, the fall precipitation is associated with frontal systems, similar to those in winter.
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