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

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. Plain Language Summary Extreme events in the stratosphere, so-called sudden stratospheric warming (SSW) events, have long-lasting impacts on the weather over the North Atlantic that can be characterized by variability in the North Atlantic Oscillation (NAO). While daily observations of the NAO date back to 1850, estimates of SSW event frequency only go back to the 1950s. This study explores to what extent the frequency of SSW events can be estimated from surface NAO variability as a first step toward reconstructing SSW frequency before the 1950s. It is found that characteristic effects in the surface NAO that typically occur after SSW events are only associated with SSW events in about a quarter of all cases. Nevertheless, an index can be generated entirely based on surface information that can to a good extent reproduce the SSW frequency for the period for which information for the stratosphere is available. Extending the index back to 1850 indicates that the frequency of SSW events exhibited decadal variability, as observed in the second half of the twentieth century. It is also found that the early 1990s, a period where very few SSW events were observed, was likely the longest absence of SSW events since 1850.