Characteristics Of East Asian Mid-latitude Cyclone Activity And Its Linkage With East Asian Summer Monsoon

Based on the 4 times daily ERA-Interim mean sea level pressure (MSLP) reanalysis data for the period of 1979-2013, extratropical cyclone track datasets in the northern hemisphere are identified by using the objective identification and tracking method, then the mid-latitude cyclone activity in East Asia (EAMC) is calculated. Extended empirical orthogonal function (EEOF) analysis is applied to obtain the second mode of East Asian Summer Monsoon (EASM) by separating the vector field on 850hPa pressure level. Finally, the possible mechanisms that EAMC impacts on EASM are explored through statistical methods including regression. Moreover, the cause of the anomalous cyclone activities in recent years are analyzed briefly. The main conclusions are as follows:(1) The objective identification and tracking method determines cyclone center according to the local minimum of MSLP and maximum of vorticity, and estimates the possible position at the next time on the basis of steering wind and its own speed. The cyclone track is identified by the matching probability between the cyclone center location and estimated position. The spatial pattern of cyclone frequency and intensity are similar with synoptic filter variance of geopotential, which indicates the objective method can describe the cyclone activities well in northern hemisphere.(2) The center of Mongolia is not only the most frequent area, but also the most variable area. It is reasonable to consider Mongolia as the critical region of cyclone activity. There is a significant weakening and decadal change in the mid-latitude cyclone activity over East Asia during summer from 1979 to 2013; in particular, the decadal transition occurs in early 1990s, and the cyclone activity significantly weakened thereafter. Further analysis indicates that there is a significant positive correlation between the mid-latitude summer cyclone activity over East Asia and EASM; when the mid-latitude summer cyclone activity over East Asia is strong (weak), EASM is intensified (weakened). Over the past 20 years, there has been a close relationship between the weakening of the mid-latitude summer cyclone activity in East Asia and the weakening of EASM; after 1993, the cyclone activity and EASM both become weaker.(3) Through feedback of the synoptic-scale atmospheric transient forcing on the mean flow, the poleward vorticity flux transportation related to abnormally strong cyclone activity can induce significant negative anomalies in the sea level pressure and the geopotential heights near the Lake Baikal, which in turn cause abnormal cyclonic circulation in this area. As a result, the southerly anomaly related to anomalous cyclone activity to the southeast of the abnormal cyclonic circulation near the Lake Baikal is conducive to the significant enhancement of EASM. Weak cyclone activity anomaly results in nearly opposite situation and usually weakens EASM.(4) Inhomogeneous warming over Eurasian continent is confirmed based on four surface air temperature (SAT) datasets, the strongest warming area is located to the southwest of Lake Baikal. There is a close relationship between the weakening of cyclonic activity after early 1990s and the non-uniform surface warming of the Eurasian continent. Thermal anomaly over Eurasian continent induces less EAMC activity by changing meridional temperature gradient as well as the low level baroclinicity.