Effects Of Pacific SST Anomalies On The Stratospheric Polar Vortex In Early Spring In The Northern Hemisphere

The stratospheric Arctic vortex(SAV)anomalies are important indicators of extreme cold weathers in the Northern Hemisphere during winter and spring,as well as Arctic stratospheric ozone depletion events.By analyzing the ERA5 reanalysis data,the interannual variability of the stratospheric Arctic polar vortex in early spring is found to be associated with the second mode of the sea surface temperature(SST)anomalies in the North Pacific(the main spatial type is the Victoria mode)and the tropical Pacific(the main spatial type is the SST anomalies in the west tropical Pacific)in early spring in 1979-2020.The SAV tends to be enhanced both during the warm phase of the North Pacific or the west tropical Pacific.The specific progresses of the North Pacific and west tropical Pacific SST anomalies affecting the SAV is revealed by the CAM5 numerical simulations.In addition,the modulation effect of Quasi-Biennial Oscillation(QBO)on the SAV affected by SST anomalies in the North Pacific is further analyzed by WACCM5 numerical simulations.The main conclusions are as follows:(1)During the warming phase in the North Pacific crucial region in winter and spring,the exit zone of the tropospheric upper level jet over the North Pacific moves westwards,the precipitable water increases over the northwest Pacific while decreases over the northeast Pacific.Furthermore,the latent heating anomalies over the North Pacific induce an anomalous Rossby wave.The anomalous Rossby wave reduces the stability of the trough-ridge positions of wave 1 and wave 3 and increases their phase differences between the meridional wind waves and temperature waves.As a result,less wave activity fluxes of wave 1 and wave 3 are propagated to the stratosphere in middle-high latitude.Although the eddy heat flux of wave 2intensifies,the total eddy heat flux is anomalous negatively and strengthens the SAV in early spring.(2)The SST warming in the western equatorial Pacific in winter and spring can intensify local deep convective precipitation.Then the anomalous latent heating induces a Rossby wave(high pressure anomalies in the upper troposphere)to its northwest side through the Matsuno-Gill atmospheric response.The Rossby wave adjusts the strength and position of the strongest trough-ridge system of the Northern Hemisphere in the North Pacific along the great circle path,leading to a decrease of the amplitude of wave 1 of the meridional wind and an increase of the phase difference of the wave 3 between the meridional wind and temperature,which results in less wave activity fluxes of the wave 1 and wave 3 propagating to the stratosphere.While more wave activity fluxes of wave 2 propagate to the stratosphere,which partially offsets the upward propagation of total wave activity fluxes.The anomalies of the total eddy heat fluxes are negative,promoting an enhanced SAV in early spring.(3)During the easterly phases of the QBO,the North Pacific SST warming in winter and spring weakens the strongest trough-ridge system of the Northern Hemisphere in the North Pacific and increases the phase difference of the wave 1 between the meridional wind and temperature.As a result,wave 1 dominates the reduction of poleward eddy heat fluxes and enhances the SAV.During the westerly phases of the QBO,the North Pacific SST warming in winter and spring strengthens the strongest trough-ridge system of the Northern Hemisphere in the North Pacific and makes the phase differences of the wave 2 and wave 3 between the meridional wind and temperature reduced and the amplitude of wave 3 of the meridional wind and temperature increased.Finally,more poleward eddy heat fluxes of wave 2 and wave 3jointly dominate the anomaly of planetary wave activity fluxes and weaken the SAV.