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

Abstract

Sudden stratospheric warming (SSW) events can exhibit long‐lasting surface impacts that promise improvements in medium‐range to seasonal predictability. Their surface impact is dominated by the negative phase of the North Atlantic Oscillation (NAO). Hence, the question arises if stratospheric variability, and in particular the frequency of SSW events, can in turn be estimated from surface NAO conditions. This is especially relevant for the period before frequent upper air observations became available, while daily surface observations of the NAO date back to 1850. The surface impact is here quantified by NAO characteristics that are commonly observed after SSW events: a switch from a positive to a negative NAO and an extended persistence of the negative NAO, termed NAO events. Two thirds of SSW events are found to be followed by either a persistence or switch NAO event, and a quarter of SSW events are followed by both. On the other hand, less than 25% of winter surface NAO events are preceded by a SSW event. Based on these findings, an index purely based on surface NAO observations is derived that estimates SSW frequency for the satellite era and extends it back to 1850, indicating that decadal stratospheric variability was present for the entire time series, with no significant trend. The minimum in SSW frequency in the 1990s is found to be coincident with the longest absence of NAO events since 1850, indicating that the early 1990s may constitute the longest absence of SSW events for the 150‐year record.

Stratospheric modulation of the large‐scale circulation in the Atlantic–European region and its implications for surface weather events

“…Consistent with its negative NAO projection, the GL regime behaves in the opposite way, with a doubling of its frequency during weak stratospheric polar vortex conditions (24% compared to 12% in the winter mean) and a suppression during strong stratospheric conditions (3%). The modulation of regime frequencies for the ZO, ScTr and GL regimes corroborates earlier studies that report a modulation of the NAO dependent on stratospheric conditions in winter (Baldwin and Dunkerton, 2001;Papritz and Grams, 2018;Domeisen, 2019; Figure 2b). The remaining regimes, AT, AR, EuBL, ScBL and the "no regime" category, occur during a broader range of either NAO state (Figure 2a).…”

“…Both SSWs and strong polar vortex events are associated with long‐lasting significant geopotential height anomalies in the troposphere (about 40 days, starting from the central date of the stratospheric polar vortex event), which project onto the NAO– and NAO+ phases, respectively. Recently, Domeisen () showed that the switch of the NAO phase and NAO persistence is a reasonable indicator of SSW events.…”

Observed Relationships Between Sudden Stratospheric Warmings and European Climate Extremes

“…We examined the temporal evolution of observed daily standardized NAO and AO values before and after SSW events in addition to the distributions of NAO and AO in 30‐day windows before and after SSW events (Figure ). In the second month after the SSW central date there is a slight tendency toward more negative NAO conditions (Domeisen, ) and the distribution is significantly negatively skewed, reflective of the MSLP anomalies found in the “decay” period (31–60 days postcentral SSW date) after SSWs previously (Lehtonen & Karpechko, ), but the signal is weak. The AO also tends to be negative after SSW events (Baldwin & Dunkerton, ), although with significant variability between events.…”

The Remarkably Strong Arctic Stratospheric Polar Vortex of Winter 2020: Links to Record‐Breaking Arctic Oscillation and Ozone Loss

“…These relationships are commonly expressed using metrics that describe phases of the “Northern Annular Mode” (NAM), a pattern that characterizes meridional shifts of mass into or out of the polar cap throughout the atmospheric column (note that the NAM and AO are often used interchangeably; Baldwin, 2001; Thompson & Wallace, 2000). Anomalously strong or weak polar vortex states correspond to positive or negative phases of the stratospheric NAM, respectively, and these tend to be followed in the troposphere by positive or negative AO events, which may last for weeks to months and alter patterns of surface temperatures and precipitation (Baldwin & Dunkerton, 2001; Domeisen, 2019; Dunn‐Sigouin & Shaw, 2015; Kidston et al, 2015; King et al, 2019; Limpasuvan et al, 2005; Orsolini et al, 2018; Polvani & Kushner, 2002; Tripathi et al, 2015). Downward wave coupling events can not only strengthen the polar vortex but also directly induce tropospheric circulation patterns consistent with a positive AO on short time scales (Dunn‐Sigouin & Shaw, 2015; Shaw & Perlwitz, 2013).…”