Dynamical Study Of The Western Hemisphere Circulation Pattern

A newly identified low-frequency atmospheric regime—the Western Hemisphere(WH)circulation pattern—provides a unique example of a mode of variability that accounts for variance over the whole North Atlantic-North America-North Pacific sector.The role of synoptic waves in the formation and maintenance of the WH pattern is investigated using the ERA reanalysis datasets.Persistent WH events are characterized by the propagation of quasi-stationary Rossby waves across the North Pacific-North America—North Atlantic regions and by associated storm track anomalies.The eddy-induced low-frequency height anomalies maintain the anomalous low-frequency ridge over the Gulf of Alaska,which induces more equatorward propagation of synoptic waves on its downstream side.The eddy forcing favors the strengthening of the mid-latitude jet and the deepening of the mid-high latitude trough over the North Atlantic,whereas the deepening of the trough over eastern North America mostly arises from the quasi-stationary waves propagating from the North Pacific.A case study for the 2013/14 winter is examined to illustrate the downstream development of synoptic waves.The results confirm the contributions of downstream eddy development to the maintenance of the WH pattern.In order to better understand the different physical mechanisms in association with the tropospheric precursors of sudden stratospheric warmings(SSWs),classifying the tropospheric precursor patterns of SSWs could be helpful.Based on 37 major SSWs during 1958-2014 winters and ERA reanalysis datasets,the self-organizing maps method is used to classify the tropospheric precursor patterns of SSWs.The cluster analysis indicates that one of the precursor patterns appears as the negative-signed Western Hemisphere(WH)circulation pattern,which was recently identified by Bao and Wallace(2015).The precursor pattern with the negative-signed WH pattern can explain its linkage to the subpolar North Pacific low,Atlantic blocking and the western Pacific teleconnection pattern.Other clusters confirm the Northern Europe blocking and the Gulf of Alaska blocking as the precursors of SSWs.Linear interference with the climatological planetary waves provide a simple interpretation for the precursors.