Predictability of downward propagation of major sudden stratospheric warmings

Abstract

Major sudden stratospheric warmings (SSWs) are striking phenomena of wintertime stratospheric circulation usually defined as a reversal of zonal mean circulation from westerlies to easterlies. SSWs often have significant impact on tropospheric circulation and cause anomalies in surface climate lasting for up to 2 months. For this reason, dynamics and predictability of SSW receive considerable attention. It is however well‐known that not all SSWs cause significant, long‐lasting impact on the troposphere. In order to explain differences in tropospheric impacts following SSWs, several reasons have been previously proposed, including differences in type of SSW (split or displacement), persistence of stratospheric anomalies, preconditioning of the tropospheric circulation, and whether or not SSW was accompanied by a planetary wave reflection in the stratosphere. Here we address the predictability of tropospheric impacts by SSWs by seeking early precursors of the impacts. We separate midwinter SSWs into two groups: those which are followed by significant, long‐lasting impacts on the tropospheric circulation (defined in terms of anomalous Northern Annular Mode) and those not followed by significant anomalies in the annular mode. We show that SSWs characterised by a more negative Northern Annular Mode index in the lower stratosphere around 150 hPa and enhanced wave activity propagation to the stratosphere during the first few days following the central date have a larger probability of being followed by tropospheric impact, both in reanalyses and in climate model runs. These anomalies play a more important role in the subsequent downward propagation of the signal to the troposphere than the type of SSW: whether it is a split or a displacement, or absorptive or reflective SSW. We propose that using these anomalies as precursors of tropospheric impacts of SSW can enhance climate predictability.

Estimating the Frequency of Sudden Stratospheric Warming Events From Surface Observations of the North Atlantic Oscillation

“…NAO events that are not preceded by stratospheric anomalies are believed to be generated through tropospheric internal variability, though some of the identified NAO events without a preceding major SSW event may potentially be associated with minor SSW events or other stratospheric forcing. In terms of their classification of a NAO surface impact, 74% of the SSW events agree with the downward impact classification developed in Karpechko et al () that also uses lower stratospheric anomalies. Unlike SSW events, NAO events exhibit a local minimum in midwinter, with more events in early and late winter, indicating a more frequent association of NAO events with SSW events in midwinter.…”

“…A range of studies has classified SSW events and their surface impacts, with respect to their geometry, that is, split and displacement events (Maycock & Hitchcock, ; Mitchell et al, ), their evolution, that is, absorbing and reflecting events (Kodera et al, ), and based on their lower stratospheric and surface response (Karpechko et al, , hereafter K17). Runde et al () find that 20% of extreme stratospheric vortex events exhibit a surface response for a classification of stratospheric extreme events that includes strong vortex events and minor sudden stratospheric warmings.…”

A thermodynamic climatology of the disturbed stratospheric polar vortex

“…The influence of these polar vortex disturbances (PVDs) on the troposphere has been well documented. A weakened polar vortex corresponds to anomalies in the northern annular mode, which propagate down to the surface and can persist for weeks (e.g., Limpasuvan et al ., ; Karpechko et al ., ). The strength of the polar vortex also correlates with the position of the mid‐latitude tropospheric jet, with a weaker vortex corresponding to an equatorwards shift in the jet (Dunn‐Sigouin and Shaw, ; Kidston et al ., ).…”

Rossby Wave Propagation into the Northern Hemisphere Stratosphere: The Role of Zonal Phase Speed

“…Anomalous wave forcing can lead to strong disruptions of the stratospheric flow, so-called sudden stratospheric warming (SSW) events, which in turn affect tropospheric variability (e.g., Baldwin & Dunkerton, 2001) and predictability (e.g., Domeisen et al, 2015;Karpechko et al, 2017). Anomalous wave forcing can lead to strong disruptions of the stratospheric flow, so-called sudden stratospheric warming (SSW) events, which in turn affect tropospheric variability (e.g., Baldwin & Dunkerton, 2001) and predictability (e.g., Domeisen et al, 2015;Karpechko et al, 2017).…”