Variation of the North Pacific Trough and Its Links with the Asian-Pacific-American Summer Climate Anomalies

Abstract:

One of the most significant climate characteristics over the North Pacific is the mid-ocean trough, which peaks at 200- and 150- hPa level in July and August. We define an index of the North Pacific trough (NPT), and investigate the relationship between the NPT and the Asian-Pacific-American summer climate anomalies. In the extratropics, an intensified NPT features a low-level anticyclone and positive air temperature anomalies over East Asia and the North Pacific, associated with local descending motion. The stationary disturbances induced by divergent winds propagate eastward to North America and the Atlantic, leading to anomalous high (low) pressure and hot/dry (cool/wet) summer in Midwest (Southeast) of the US. In the meantime, hot and dry conditions appear over Japan, Korea, the Yangtze River basin, and the North Pacific. In the tropics, the NPT index shows a strong relationship with the central Pacific El Niño. At the early stage of the NPT, positive sea surface temperature anomalies (SSTAs) develop in the western Pacific and favor local convection, while negative SSTAs in the central and eastern Pacific suppress the regional convection. Anomalous westerlies in the equatorial upper troposphere benefit the development of the NPT. Moreover, an enhanced NPT provides an upper-level outflow over the lower tropospheric convergence, leading to strong convective activity and increased rainfall over the East China Sea and the Philippine Sea. Furthermore, the strongest intrusion of perturbation kinetic energy from the extratropics to the tropics occurs along the NPT. Thus, it is also interesting to understand the role that the NPT plays in the interaction between the two hemispheres.