Dependence of sudden stratospheric warming type‐transition on preceding North Atlantic Oscillation conditions

Abstract

Most sudden stratospheric warming (SSW) events initiate with their centers being displaced from the pole. Some retain their displaced form until termination but some split into two vortices during their course. Here, we show that existence of a transition during the course of the SSW life cycle can be attributable to the condition of North Atlantic Oscillation (NAO) preceding before onset: Positive NAO favors SSW of displacement type with no transition while negative NAO favors the displacement–split type. We show that, in positive NAO precondition, vertical flux of wave activity immediately before onset is mostly contributed only by wavenumber 1 component, which contrasts with the relatively stronger contribution of wavenumber 2 in negative NAO precondition. Whole Atmosphere Community Climate Model (WACCM) simulation results are also consistent with the observational findings. Therefore, NAO can be regarded as a useful precursor for determining the type of forthcoming SSW events.

Comparison of stratospheric evolution during the major sudden stratospheric warming events in 2018 and 2019

“…The enhancement of tropospheric wave 3 may have been responsible for the splitting vortices in the 2019 SSW (e.g., Rao J et al, 2019b). It is interesting that we observed all the transitions of the polar vortices over the Atlantic region, whereas negative phases of the NAO indices, as suggested by Choi et al (2020), may not always be suitable to explain the splitting vortices. The stratospheric evolution of the geopotential heights indicates that the recombination vortex in the 2018 SSW may have been due to the Euro‐Atlantic anticyclone moving into the Pacific region through the polar region, whereas the splitting vortices in the 2019 SSW may have been related to the anticyclones moving from the Aleutian High toward the Atlantic region via the polar region.…”

“…According to the postwarming vortices shown in Figure 4, the recombination and splitting vortices were all observed over the Atlantic region (0°–60°W). Choi et al (2020) suggested that the negative NAO phase pattern may have contributed to the enhanced wave 2 component and favored displacement–split‐type SSW events. However, the amplitudes of wave 2 were extremely low at the central date of the 2019 SSW, when the displacement–split vortex occurred.…”

Observational Evidence of Distinguishable Weather Patterns for Three Types of Sudden Stratospheric Warming During Northern Winter

“…However, for DS and SS types ( Figures 1H,I), the North Atlantic response is relatively uncertain. According to Choi et al (2020), for DS type, the negative NAO phase can be considered a precursor rather than a response. Afargan-Gerstman and Domeisen (2020) found that the anomalous weather patterns in the Pacific may contribute to the sign of the Atlantic response by synoptic storm propagation from the East Pacific toward the Atlantic.…”

“…They showed that the results based on the traditional two-type classification (Charlton and Polvani, 2007) could weaken the distinct features between DS and SS types, and suggested that DS-type SSW events should be objectively defined to improve the understanding of SSW-related phenomena. In their extended study, differences in tropospheric precursor patterns over the North Atlantic between DD-and DS-type SSW were identified (Choi et al, 2020).…”

Influence of tropical convective enhancement in Pacific on the trend of stratospheric sudden warmings in Northern Hemisphere

“…When the cloudiness in the box region is above (below) 4.5 okta, the values of the tropospheric heat source are above (below) zero, which means the troposphere in the box region is a heat source (sink). Figure 5d Previous studies have mentioned that the extratropical SST anomalies (Jadin et al 2010;Woo et al 2015;Li et al 2018), NAO (Choi et al 2020) and solar activity (Shindell et al 1999) would modulate SSWs. SST variations in the extratropical Pacific such as PDO (Mantua et al 1997), which is the first mode of North Pacific SST anomalies, and Victoria mode, which is the second mode of North Pacific SST anomalies are reported to be capable to modulate the interannual variations of SSWs (Bond et al 2003;Woo et al 2015;Li et al 2018;Hu et al 2018).…”