The Atlantic Jet Response to Stratospheric Events: A Regime Perspective

Abstract

The tropospheric response to Sudden Stratospheric Warmings (SSWs) is associated with an equatorward shift in the midlatitude jet and associated storm tracks, while Strong Polar Vortex (SPV) events elicit a contrasting response. Recent analyses of the North Atlantic jet using probability density functions of a jet latitude index have identified three preferred jet latitudes, raising the question of whether the tropospheric response to SSWs and SPVs results from a change in relative frequencies of these preferred jet regimes rather than a systematic jet shift. We explore this question using atmospheric reanalysis data from 1979 to 2018 (26 SSWs and 33 SPVs), and a 202‐years integration of the Whole Atmosphere Community Climate Model (92 SSWs and 68 SPVs). Following SSWs, the northern jet regime becomes less common and the central and southern regimes become more common. These changes occur almost immediately following “split” vortex events, but are more delayed following “displacement” events. In contrast, the northern regime becomes more frequent and the southern regime less frequent following SPV events. Following SSWs, composites of 500‐hPa geopotential heights, surface air temperatures, and precipitation most closely resemble composites of the southern jet regime, and are generally opposite in sign to the composites of the northern jet regime. These comparisons are reversed following SPVs. Thus, one possible interpretation is that the two southernmost regimes appear to be favored following SSWs, while the southernmost regime becomes less common following SPVs.

Modulation of Atmospheric Rivers by the Arctic Stratospheric Polar Vortex

“…The zone of highest AR frequency in the Atlantic exhibits a marked meridional tilt, extending approximately from the Carolinas through the British Isles to Scandinavia, with increased zonal extent. This is consistent with the structure of the “northern” jet regime (Woollings et al., 2010), which is favored during strong vortex events (Goss et al., 2021). The combination of poleward shift and increased zonal extent yields anomalies of up to 70% over Scandinavia (Figure 1f), accordant with the tendency for ARs to occur most often in this region during the positive NAO (Benedict et al., 2019).…”

“…This can then be contrasted with the regression map of DJFM‐mean U100 with DJFM‐total precipitation (Figure 5d), with the residual (Figures 5c and 5d) shown in Figure 5e. We note that Figures 5c and 5d resemble the patterns of precipitation variability associated with stratospheric vortex variability shown in previous studies (Büeler et al., 2020; Douville, 2009; Goss et al., 2021).…”

“…In the Atlantic, the zone of highest Atlantic AR frequency (>3 days per 4 weeks) exhibits less meridional tilt compared with climatology, extending approximately along 30–35°N from Florida to Iberia, with a peak in the central subtropical Atlantic southwest of the Azores. This distribution is consistent with the structure of the “southern” eddy‐driven jet regime (Woollings et al., 2010), which is favored following SSWs (Goss et al., 2021; Maycock et al., 2020). During a weak vortex, ARs are anomalously common in the Mediterranean (50% more common than climatology over southern Iberia), with a peak over Turkey exceeding 3 days per 4 weeks, consistent with increased precipitation and anomalous warmth in the Mediterranean following SSWs (Ayarzagüena et al., 2018; Domeisen & Butler, 2020; Monnin et al., 2021).…”

“…The tropospheric impacts of variability in stratospheric vortex strength include time‐mean latitudinal shifts in the eddy‐driven jets and mid‐latitude storm tracks (Kidston et al., 2015; Kushner & Polvani, 2004; Maycock et al., 2020), shifts to the distribution and persistence of the AO/NAO concomitant with the stratospheric NAM anomaly (Ambaum & Hoskins, 2002; Baldwin & Dunkerton, 2001; Domeisen, 2019; Thompson et al., 2002), and modulation of recurrent flow regimes (Beerli & Grams, 2019; Charlton‐Perez et al., 2018; Goss et al., 2021; Lee et al., 2019). Such tropospheric anomalies following the onset of anomalous vortex states are associated with distinct, and often extreme, mid‐to‐high latitude temperature and precipitation patterns (e.g., Douville, 2009; Goss et al., 2021; J. Huang et al., 2021; Knight et al., 2017; Kolstad et al., 2010; Zhang et al., 2022). The persistence of the stratospheric anomalies and the robust average tropospheric response can therefore yield “windows of opportunity” for actionable extended‐range forecasts (Mariotti et al., 2020), which could be extended to include ARs.…”

Stratospheric influence on the winter North Atlantic storm track in subseasonal reforecasts

“…For roughly one third of SSW events, the tropospheric response is associated with a poleward shift of the tropospheric jet in the North Atlantic (Afargan-Gerstman and Domeisen, 2020). On the other hand, a strengthening of the stratospheric polar vortex, which can result in so-called strong polar vortex (SPV) events when the stratospheric wind speed increases above a certain threshold, is generally associated with a poleward shift of the North Atlantic jet (Kidston et al, 2015;Goss et al, 2021). In the North Atlantic, such changes in the strength of the stratospheric polar vortex coincide with a latitudinal displacement of the storm track position: weakening of the polar vortex is associated with a southward shift of the storm track, compared with a poleward shift following a strong vortex (Baldwin and Dunkerton, 2001).…”