Abstract
Abstract. Major sudden stratospheric warmings (SSWs) are extreme wintertime circulation events of the Arctic stratosphere that are accompanied by a breakdown of the polar vortex and are considered an important source of predictability of tropospheric weather on subseasonal to seasonal timescales over the Northern Hemisphere midlatitudes and high latitudes. However, SSWs themselves are difficult to predict, with a predictability limit of around 1 to 2 weeks. The predictability limit for determining the type of event, i.e., wave-1 or wave-2 events, is even shorter. Here we analyze the dynamics of the vortex breakdown and look for early signs of the vortex deceleration process at lead times beyond the current predictability limit of SSWs. To this end, we employ a mode decomposition analysis to study the potential vorticity (PV) equation on the 850 K isentropic surface by decomposing each term in the PV equation using the empirical orthogonal functions of the PV. The first principal component (PC) is an indicator of the strength of the polar vortex and starts to increase from around 25 d before the onset of SSWs, indicating a deceleration of the polar vortex. A budget analysis based on the mode decomposition is then used to characterize the contribution of the linear and nonlinear PV advection terms to the rate of change (tendency) of the first PC. The linear PV advection term is the main contributor to the PC tendency at 25 to 15 d before the onset of SSW events for both wave-1 and wave-2 events. The nonlinear PV advection term becomes important between 15 and 1 d before the onset of wave-2 events, while the linear PV advection term continues to be the main contributor for wave-1 events. By linking the PV advection to the PV flux, we find that the linear PV flux is important for both types of SSWs from 25 to 15 d prior to the events but with different wave-2 spatial patterns, while the nonlinear PV flux displays a wave-3 wave pattern, which finally leads to a split of the polar vortex. Early signs of SSW events arise before the 1- to 2-week prediction limit currently observed in state-of-the-art prediction systems, while signs for the type of event arise at least 1 week before the event onset.
Highlights
Major sudden stratospheric warmings (SSWs) (Baldwin et al, 2021) are extreme wintertime circulation events of the Arctic stratosphere that are accompanied by a breakdown of the polar vortex which consists of strong circumpolar westerly winds in the polar stratosphere that form in fall and decay in spring
When these planetary waves reach a critical level in the stratosphere, they break and deposit easterly momentum into the mean flow, resulting in a deceleration of the mean flow, which can eventually lead to a breakdown of the polar vortex, and an SSW event if the winds reverse to easterlies (Charlton and Polvani, 2007)
We observed that the signals that are characteristic of SSWs emerge as early as 20–25 d before the onset of SSWs. Given that these results hint that SSWs are potentially predictable at longer lead times, i.e., beyond the current predictability limit of 1–2 weeks, we provide a physical interpretation of these signals that we identified through the mode decomposition analysis
Summary
Major sudden stratospheric warmings (SSWs) (Baldwin et al, 2021) are extreme wintertime circulation events of the Arctic stratosphere that are accompanied by a breakdown of the polar vortex which consists of strong circumpolar westerly winds in the polar stratosphere that form in fall and decay in spring. Given the fact that the development of the two types of SSW events is considered to be different (Matthewman et al, 2009; Albers and Birner, 2014), the dynamical processes that lead to the breakdown of the polar vortex should be distinct between displacement (wave-1) and split (wave-2) events and should be distinguishable from normal winter days (without SSWs). By projecting the other variables from the PV equation (i.e., the zonal and meridional wind) onto the EOF basis from PV, one can analyze the contribution of each term of the equation to the changes in the PC time series in order to identify the dynamical processes that are the most relevant for the weakening of the polar vortex This approach was proposed by Aikawa et al (2019) and called mode decomposition analysis.
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