Reply on RC3

Abstract

We develop 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 construct gridded daily-mean temperature anomalies 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 to monitor SSW warming on a daily basis and also a trailing-phase TEA to monitor potential upper stratospheric cooling in the wake of the warming phase. Based on the main-phase TEAs, three key metrics, including Main-Phase Strength (MPS), Duration (MPD) and Area (MPA), are estimated and used for the detection and classification of SSW events, enabling minor, major, and extreme event categories. An informative 42 winters’ SSW climatology 1980–2020 was developed, including the three key metrics as well as onsets date, maximum-warming-anomaly location and strength 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 and observational data that cover the polar region and winter timeframes of interest. Within the 42 winters, 40 SSW events were detected, yielding a frequency of about 0.95/year. In the 1990s, where recent studies showed gaps, we detected several events. About 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 also found a statistically significant increase in the duration of SSW main-phase warmings, by about 4 days from the 1980s to the 2010s, raising the average duration by 40 % from about 10 to 14 days and inducing an SSW strength increase by near 30 million km2 days (about 30 %) from about 105 to 135 million km2 days. The results can be used as a reference for further long-term 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.