External and Internal Summer Atmospheric Variability in the Western North Pacific and East Asia

Abstract

The interannual variation of summer climate in the western North Pacific and East Asia (WNP/EA) has been investigated in this study using an experiment forced by global observed sea surface temperatures (SSTs). The ensemble integrations enable us to separate externally forced variability from internal variability. It is found that the lower-tropospheric circulation anomaly over the WNP is dominated by the external variability forced by SSTs, while the anomaly of the East Asian jet (EAJ), is dominated by the atmospheric internal variability. The external variability in the WNP/EA sector is mainly reflected by the first leading empirical orthogonal function (EOF) mode of lower-tropospheric zonal wind, and partially by the second mode. The first mode is characterized by a cyclonic/anticyclonic circulation anomaly over the tropical WNP, reflecting changes in the WNP subtropical high. This mode is associated with precipitation and SST anomalies in the tropics. The second mode is characterized by a wave-like pattern of zonal wind in the meridional direction, with zonally-oriented cells over the WNP. This second mode is associated with very weak SST anomalies, and more like a mode of the internal variability. The internal mode is found to be well organized in the WNP/EA sector. The features associated with this mode include the meridional displacement, and intensity variation of EAJ, and precipitation anomaly over the WNP.