Stratospheric Sudden Warming And Its Impact On East Asian Winter Monsoon

Based on the ERA-40 daily reanalysis data from ECMWF, composition analysis was used to study three types of SSW(stratospheric sudden warming) events from 1957 to 2002, including 20 vortex displacement major warming, 11 vortex splitting major warming and 21 minor warming events. The changes of stratosphere of these three were studied, and the anomalies of lower stratospheric planetary wave-number 1 and wave-number 2 during the sudden warming period were also interviewed in detail. Secondly, the authors composited the anomalies of the warming events which can propagate down to the troposphere to analyze the changes of the EAWM, then further discuss the role of planetary wave during the propagation period. Finally, the authors made some attempts to find the conditions of the anomalous propagation to the troposphere. Firstly, we examine the corresponding AO indices of the all SSW events including the none-propagation events. Then we picked out one propagation event and one none-propagation event to study separately. The results are as follows:(1) The vortex displacement major warming offsets its low temperature center and the vortex and reverses wind field of high latitude from westerly to easterly wind. The vortex splitting type splits its low temperature center and the vortex and reverses wind field of high latitude, as well. Minor warming events only offsets its low temperature center, the vortex and wind field of high latitude remain stable.(2) The amplitude of wave-number 1 anomalously increases before the sudden warming and the amplitude of wave-number 1 reaches its climax when the warming occurs. The vortex displacement events and minor warming events remain its climax of amplitude of wave-number 1 for approximately 6-8 days and the vortex splitting type begins to decrease. As to the wave-number 2, the vortex displacement type and minor warming events are alike, the amplitudes increase before the sudden warming, and then decrease when the warming began, however, the splitting type increases its amplitude after the warming began. The EP flux of wave-number 1 and wave-number 2 were also calculated and analyzed. The minor warming type and vortex displacement type have strong upward wave-number 1 EP flux, and the vortex displacement type is stronger, however, the upward wave-number 2 EP flux of these two types is much weaker. For the vortex splitting type, the upward wave-number 2 EP flux is much stronger.(3) When the SSW occurs, the Arctic Oscillation(AO) and the North Pacific Oscillation(NPO) are both in the negative phase, and that the tropospheric circulation is quite wave-like. The Siberian high and the Aleutian low are both strengthened, leading to an increased gradient between the Asian continent and the North Pacific. Hence, a strong EAWM is observed with widespread cooling over inland and coastal East Asia. After the peak of the SSW, in contrast, the tropospheric circulation is quite zonally symmetric with negative phases of AO and NPO. The mid-tropospheric East Asian trough deepens and shifts eastward. This configuration facilitates warming over the East Asian inland and cooling over the coastal East Asian centered over Japan.(4) The activities of planetary waves during the lifecycle of the SSW were analyzed. The anomalous propagation and the attendant altered amplitude of the planetary waves can well explain the observed circulation and the EAWM, although the mechanism of downward propagation of the SSW signal from the stratosphere to the troposphere remains a challenging issue.(5) The SSW event that could propagate down to the troposphere often corresponds to the high absolute value of a negative AO index, which is the negative phase of the AO. In the early and late period of descending events, the vertical distribution shows a quasi-barotropic structure, which is conducive to the propagation of abnormal geopotential height downward to the troposphere. However, the event that didn’t descend to the lower levels appears to be baroclinic in the vertical distribution, which is not so conducive to the propagation of anomalies. The case studies of propagation and none-propagation events showed that the wave-number 2 is more significant during the propagation period.