Preferred solar signal and its transfer in the Asian–Pacific subtropical jet region

Abstract

Solar impact on the tropospheric subtropical jet (SJ) has been identified previously from a zonally averaged perspective. The SJ was observed to be weaker in the high solar activity winters. However, some regional features of solar-induced SJ variations might remain unrecognized. Here it is found that the regional solar signal in wintertime Asian–Pacific zonal wind at 200 hPa, which exhibits a tripolar banded structure, greatly resembles the second internal mode of zonal wind within the same sector. Significant response of the Asian–Pacific SJ (APSJ) to increased solar forcing in boreal winter exclusively marks its center region, showing a deceleration in westerlies. Further exploration suggests two possible top–down routes to interpret this particular manifestation of solar signal in APSJ center, a tropical route and a middle–high latitude route. Regarding the tropical route, during the cold season, driven by the solar-associated reduction in Brewer–Dobson circulation, ozone concentration in tropical lower stratosphere increases notably and merely within the zonal range of APSJ center. This heats the air here and the tropical tropospheric regional upwelling is thereby suppressed. Consequently, a significant weakened APSJ center is produced via local Hadley cell. Regarding the middle–high latitude route, in early winter, solar-related pronounced westerly anomalies in the mid-latitude stratosphere only appear in the longitudinal range of APSJ center. Meanwhile, the upward propagating planetary waves from the troposphere could be reflected back downward by this intensified stratospheric westerlies. As winter progresses, through wave mean flow interactions, a resultant weakened APSJ center markedly presents in the middle of winter.