Abstract
In early spring the stratospheric zonal circulation reverses from westerly to easterly. The transition, called Stratospheric Final Warming (SFW), may be smooth and late, mainly controlled by the solar radiative heating of the polar region, or early and abrupt with rapid increase of polar temperature and deceleration of the zonal wind, forced by the planetary wave activity. Here we present a study, based on 71 years meteorological reanalysis data. Two composites of radiative and dynamical SFWs have been built. There is a very significant difference in the evolution during the year of polar temperature and 60°N zonal wind between the two composites. The state of the polar vortex on given month is anticorrelated with its state 2 to 3 months earlier. Early winter is anticorrelated with mid-winter and mid-winter with late winter/early spring. The summer stratosphere keeps a memory of its state in April–May after the SFW until late June.
Highlights
In early spring the stratospheric zonal circulation reverses from westerly to easterly
● The Stratospheric Final Warming (SFW) date is defined as the day of the final reversal of the zonal wind at 60°N, 10 hPa smoothed over 5 days
The date of the SFW varies between day 66 (5 March 2016) and day 131 (11 May 1981) and the SFW temperature anomaly varies between −3.8 K and +24.7 K (Fig. 1a)
Summary
In early spring the stratospheric zonal circulation reverses from westerly to easterly. The transition, called Stratospheric Final Warming (SFW), may be smooth and late, mainly controlled by the solar radiative heating of the polar region, or early and abrupt with rapid increase of polar temperature and deceleration of the zonal wind, forced by the planetary wave activity. The temperature of the Arctic stratosphere during the spring is controlled by the intensity of midwinter planetary wave activity, a strong (weak) midwinter forcing leading to a warmer (colder) spring Artic temperature[19] This suggests that there is a preconditioning of the stratosphere to favour the occurrence of SSW events and that the state of the stratosphere in a given month in winter is depending on its state during the previous months[20,21]. The transition from westerly to easterly circulation, called Stratospheric Final Warming (SFW), may be late and very smooth, guided mainly by radiative processes, or very abrupt, preceded by an amplification of planetary waves and a strong temperature peak similar to a mid-winter SSW5,6. This study is based on 71 years of ERA5 reanalysis recently extended backward to 195029
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