A series of major sudden stratospheric warming events in winter 2023/2024

During winter months, the high-latitude stratospheric circulation is characterized by a prominent eastward flow, also known as the stratospheric polar vortex. However, this flow pattern is regularly disturbed by planetary waves (PW) originating from the troposphere, that occasionally cause either a temporary weakening or a complete reversal to a summer-like westward flow regime. Such events are called minor and major stratospheric sudden warmings (SSWs), correspondingly.  Major SSW events tend to take place approximately every two years on average.

In winter 2023/2024, the stratospheric polar vortex was weak and highly variable. Three major SSWs were detected based on the standard definition at 10 hPa in the period between January and March 2024. Two short duration events (under 9 days) and one long duration event (over 20 days) occurred in mid-January, mid-February and early March, respectively. This number of SSW events within a single winter is notably uncommon. In this study, we investigate the dynamical evolution of the polar stratosphere, the tropospheric forcing and precursors, and the surface impacts throughout the winter, with a focus on the first two short-duration events. The study is mostly based on the fifth generation ECMWF reanalysis (ERA5) data.

It has been shown in previous studies that synoptic evolution of blocking highs and anomalous lows in the upper troposphere conditions the duration of SSW events. For the short SSW event in January, we show that blocking highs developed over the Euro-Atlantic region were indeed responsible for enhanced planetary waves activity prior to the SSW central date.  Later an anomalous, westward-propagating High over the North Pacific suppressed the zonal-mean PW activity flux into the stratosphere and caused a quick termination of the event. Despite a similar development timeline, the second short-duration SSW event in February had a different tropospheric forcing. In this case, we investigate the role of snow accumulation in creating an increased meridional heat flux over Ural Mountains. Finally, a canonical long-duration Major SSW developed in early March.