Impact of Seesaw Spring Soil Moisture Anomalies in the Middle Latitudes on the General Circulation in Summer and Its Mechanism

In this paper, the effects of spring soil moisture (SM) anomalies in the mid-latitudes on the atmospheric circulation in summer over the Northern Hemisphere (NH) are investigated. The results show that there are two regions of maximum interannual variability of the spring SM in the mid-latitudes, which are located in central North America (CNA) and Europe and central Asia (ECA). In addition, the interannual variation of spring SM anomalies between CNA and ECA exhibits a seesaw pattern. The CNA-ECA seesaw pattern of the spring SM anomalies leads to the surface heat anomalies having opposite phases in CNA and ECA from spring to summer, which subsequently cause the opposite phase of baroclinicity anomalies in spring. The anomalous meridional temperature advections in spring cause the baroclinicity anomalies to have the same phase around CNA and ECA in summer. Corresponded with the same phase of baroclinicity anomalies, the anomalous centers of the stationary Rossby wave train (RWT) and Rossby wave source (RWS) have the same phase in CNA and ECA in summer. Through analysis of the vorticity budgets, the maintenance mechanism of the RWT in summer is considered as a positive feedback that anomalous meridional winds characterized by a RWT, transport the mean absolute vorticity and subsequently lead to an anomalous RWS, which in turn maintains the stationary RWT. Numerical experiments further demonstrate the effects of CNA-ECA seesaw pattern of spring SM anomalies on stationary RWT and RWS in summer. The interannual variation of soil moisture (SM) anomalies in mid-latitudes exhibits a seesaw pattern in central North America (CNA) and Europe and central Asia (ECA). When spring SM anomalies are negative in CNA and positive in ECA, positive (negative) sensible heat flux anomalies are observed in CNA (ECA) while negative (positive) latent heat flux anomalies are observed in CNA (ECA) from spring to summer, or vice versa. Influenced by surface heat anomalies, baroclinicity anomalies exhibit a seesaw pattern in CNA and ECA in spring, and have same phase in CNA and ECA in summer due to anomalous meridional temperature advections. The baroclinicity anomalies in CNA and ECA in summer contribute to the same variation of wave-flow interactions. The Rossby wave train (RWT) induced and maintained by CNA-ECA seesaw pattern of spring SM anomalies in spring can be maintained by its meridional transportation of the mean absolute vorticity in summer. Apparently, spring SM anomalies in the middle latitudes can persistently influence the large-scale atmospheric circulations over the Northern Hemisphere by maintaining RWT. Seesaw pattern of spring soil moisture anomalies in central North America (CNA) and Europe and central Asia (ECA) Baroclinicity anomalies change from opposite to same phase in CNA and ECA during the transition from spring to summer Rossby wave train induced by spring CNA-ECA seesaw pattern is maintained by its meridional mean absolute vorticity transportation in summer