Monitoring sudden stratospheric warmings under climate change since 1980 based on reanalysis data verified by radio occultation

We developed a new approach to monitor Sudden Stratospheric Warming (SSW) events under climate change since 1980 based on reanalysis data, verified by radio occultation data. We constructed gridded daily-mean temperature anomalies from the input fields at different vertical resolution (basic case full resolution; cross-check with reanalysis at 10 stratospheric standard pressure levels or 10 hPa and 50 hPa level only) and employed the concept of Threshold Exceedance Areas (TEAs), the geographic areas wherein the anomalies exceed predefined thresholds (such as 30 K) to monitor the phenomena.

We derived main-phase TEAs, representing combined middle and lower stratospheric warming, to monitor SSWs on a daily sampling basis. Based on the main-phase TEAs, three key metrics, including main-phase duration, area, and strength are estimated and used for the detection and classification of SSW events. An SSW is defined to be detected if the main-phase warming lasts at least 6 days. According to the strength, SSW events are classified into minor, major and extreme. An informative 42 winters’ SSW climatology 1980-2021 was developed, including the three key metrics as well as onset date, maximum-warming-anomaly location and other valuable SSW characterization information.

Detection and validation against previous studies underpins that the new method is robust for SSW detection and monitoring and that it can be applied to any quality-assured reanalysis, model, and observational temperature data that cover the polar region and winter timeframes of interest, either using high vertical resolution input data (preferable basic case), coarser standard-pressure-levels resolution or (at least) 10 hPa and 50 hPa pressure level data. Within the 42 winters, 43 SSW events were detected for the basic case, yielding a frequency of about one event per year. In the 1990s, where previous studies showed gaps, we detected several events. Over 95 % of event onset dates occurred in deep winter (Dec-Jan-Feb timeframe; about 50 % in January) and three quarters have their onset location over Northern Eurasia and the adjacent polar ocean.

Regarding long-term change, we found a statistically significant increase in the duration of SSW main-phase warmings, by about 5 days over the climate change period from the 1980s to the 2010s, raising the average duration by near 50 % from about 10 to 15 days and inducing an SSW strength increase by about 40 million km² days, from about 100 to 140 million km² days. The results are robust (consistent within uncertainties) across using different input data resolution. They can hence be used as a reference for further climate change-related studies and be a valuable basis for studying SSW impacts and links to other weather and climate phenomena, such as changes in polar vortex dynamics and in mid-latitude extreme weather.