The Influences of the Multi-Scale Sea Surface Temperature Anomalies in the North Pacific on the Jet Stream in Winter

Using Climate Forecast System Reanalysis (CFSR) data and numerical simulations, the impacts of the multi-scale sea surface temperature (SST) anomalies in the North Pacific on the boreal winter atmospheric circulations are investigated. The basin-scale SST anomaly as the Pacific Decadal Oscillation (PDO) pattern, a narrow meridional band of frontal-scale smoothed SST anomaly in the subtropical front zone (STFZ) and the spatial dispersed eddy-scale SST anomalies within the STFZ are the three types of forcings. The results of statistical methods find that all three oceanic forcings may correspond to the winter North Pacific jet changing with the similar pattern. Furthermore, several atmospheric general circulation model simulations are used to reveal the differences and detail processes of the three forcings. The basin-scale cold PDO-pattern SST anomaly first causes negative turbulent heat flux anomalies, atmospheric cooling, and wind deceleration in the lower atmosphere. Subsequently, the cooling temperature with an amplified southern lower temperature gradient and baroclinity brings a lagging middle warming because of the enhanced atmospheric eddy heat transport. The poleward and upward development of baroclinic fluctuations eventually causes the acceleration of the upper jet. The smoothed frontal- and eddy-scales SST anomalies in the STFZ cause comparable anomalous jet as the basin-scale by changing the upward baroclinic energy and Eliassen-Palm fluxes. The forcing effects of multi-scales SST anomalies coexist simultaneously in the mid-latitude North Pacific, which can cause similar anomalous upper atmospheric circulations. This is probably why it is tricky to define the certain oceanic forcing to the specific observed atmospheric circulation variation.