The Boreal Spring Stratospheric Final Warming And Its Relationship With The South Asian Summer Monsoon Onset

By using the multiple reanalysis datasets, dates of the boreal spring stratospheric final warming (SFW) events are defined as the time when the zonal-mean zonal wind at the central latitudes of the westerly polar jet drops below zero and never recovers until the subsequent autumn. Then, the paper analyzes the circulation characteristics and planetary-wave activities during the SFW onset and discusses the relationship between the occurrence of winter stratospheric sudden warming events (SSW) and the seasonal timing of SFW events. Also, the linkage between the SFW onset date (SFWOD) and the South Asian summer monsoon (SASM) onset are revealed. Based on these results, the simulations of the SFW events by the Coupled Model Intercomparison Project phase-5(CMIP5) models are evaluated. The main conclusions are as follows:The SFW events occur successively from the mid to the lower stratosphere and averagely from the mid to late April with a temporal lag of about13days from10hPa to50hPa. The earliest SFW occurs in mid March whereas the latest SFW happens in late May, showing a clear interannual variability of the time of SFW. Accompanying the SFW onset, the stratospheric circulation transits from a winter dynamical regime to a summertime state, and the maximum negative tendency of zonal wind and the strongest convergence of planetary-wave are observed. Composite results show that the early (late) SFW events in boreal spring correspond to a quicker (slower) transition of the stratospheric circulation. Meanwhile, the planetary wave activities are relatively stronger (weaker) associating with an out-of-(in-phase) circumpolar circulation anomaly before and after the SFW events in the stratosphere. All these results indicate that, the earlier breakdown of the stratospheric polar vortex, as for the winter SSW events is driven mainly by wave forcing; and in contrast, the later breakdown of the stratospheric polar vortex exhibits more characteristics of its seasonal evolution.Composite analyses of the major-SSW years, minor-SSW years and non-SSW years show that the intensity of planetary-wave activity in the extratropical stratosphere tends to be stronger (weaker) and the westerly polar jet is anomalously weaker (stronger) in major-SSW (non-SSW) winter. However, in the following spring, the planetary-wave activity is weaker (stronger) accompanied with an anomalously stronger (weaker) stratospheric polar vortex. While in minor-SSW years, the stratospheric polar vortex and the westerly polar jet exhibits relatively gentle variations in spring, very close to climatology. These results indicate that the occurrence and intensity of SSW events in midwinter may be related to the SFW onset in spring.Statistical results shows that though the occurrence (absence) of major SSW events in midwinter may not always be followed by late (early) SFWs in spring, there is a much higher (lower) probability of late SFWs than early SFWs in spring after major-SSW (non-SSW) winters. Further, early spring SFWs tend to be preceded by non-SSW winters, while late spring SFWs that on average take place up until early May are mostly preceded by major SSW events in midwinter. In contrast, the frequency of early and late SFW events is basically the same following the winter with or without minor SSW events, which indicate that there is no significant relationship between the occurrence of minor SSW events and the SFW onset.The analyses of the tropospheric systems before the stratospheric warming event occurrence in the anomalous SFW years show that the strengthening of the western Eurasian high can acts as a tropospheric precursor to the seasonal timing of SFWs. It appears during early to mid-January in late-SFW years but does not appear until late February to mid-March in early SFW years, and always precedes the strengthening of planetary-wave activity in the stratosphere.As two significant symbols of the boreal circulations in spring transitional season, SFWOD has an intimate relationship with the SASM onset. When the SFW events break out late (early), the stratospheric polar vortex is still persistent (has already disappeared) during the period from April to early May, and the upper and lower troposphere exhibits a positive (negative) phase of the Northern Annular Mode and the Arctic Oscillation, respectively, which will consequently motivate an anticyclonic (cyclonic) circulation anomaly over the land to the North of the Arabia sea. Meanwhile, the increased (decreased) westerly wind and ascending motion is observed in the SASM region in lower troposphere, causing an early (late) SASM onset. Besides, the relationship between SFWOD and SASM onset has an interdecadal change, which shows a significant negative correlation since late1970s and more significantly in recent years.The dynamic process during the SFW onset in the simulations of CMIP5models is obviously insufficient. Specifically, the SFWODs in most models are generally later and have smaller standard deviations than observation. Besides, the zonal-mean zonal wind always shows a gentle decrease during the onset of early SFW events. While in the late-SFW years of models, the zonal-mean zonal wind evolution from winter to spring has no reflection of the rapid decrease process from observation in association with winter SSW occurrence, manifested as a seasonal variation.