Abstract
Abstract. Marine cold air outbreaks (MCAOs) in the northeastern North Atlantic occur due to the advection of extremely cold air over an ice-free ocean. MCAOs are associated with a range of severe weather phenomena, such as polar lows, strong surface winds and intense cooling of the ocean surface. Given these extreme impacts, the identification of precursors of MCAOs is crucial for improved long-range prediction of associated impacts on Arctic infrastructure and human lives. MCAO frequency has been linked to the strength of the stratospheric polar vortex, but the study of connections to the occurrence of extreme stratospheric events, known as sudden stratospheric warmings (SSWs), has been limited to cold extremes over land. Here, the influence of SSW events on MCAOs over the North Atlantic ocean is studied using reanalysis datasets. Overall, SSW events are found to be associated with more frequent MCAOs in the Barents Sea and the Norwegian Sea compared to climatology and less frequent MCAOs in the Labrador Sea. In particular, SSW events project onto an anomalous dipole pattern of geopotential height 500 hPa, which consists of a ridge anomaly over Greenland and a trough anomaly over Scandinavia. By affecting the variability of the large-scale circulation patterns in the North Atlantic, SSW events contribute to the strong northerly flow over the Barents and Norwegian seas and thereby increase the likelihood of MCAOs in these regions. In contrast, the positive geopotential height anomaly over Greenland reduces the probability of MCAOs in the Labrador Sea after SSW events. As SSW events tend to have a long-term influence on surface weather, these results are expected to benefit the predictability of MCAOs in the Nordic Seas for winters with SSW events.
Highlights
Marine cold air outbreaks (MCAOs) in the North Atlantic are characterized by large-scale advection of cold polar air masses over open ocean
This study focuses on the influence of the stratosphere on the occurrence of marine cold air outbreaks in the North Atlantic and their connection to the large-scale circulation patterns over the North Atlantic and Europe
We investigate how the frequency and the magnitude of such MCAOs in the Barents Sea, the Norwegian Sea and the Labrador Sea are affected by the large-scale conditions after the onset of extreme events in the stratosphere, known as stratospheric warmings (SSWs) events
Summary
Marine cold air outbreaks (MCAOs) in the North Atlantic are characterized by large-scale advection of cold polar air masses over open ocean. While the frequency of cold extremes over land has been shown to increase in response to a weak stratospheric polar vortex, Papritz and Grams (2018) found indications that for winters with a weak polar vortex, MCAOs are less frequent in certain regions, namely the Greenland and Iceland seas due to a weakening of the storm track in the Norwegian Sea, whereas in the Barents Sea a modest increase of MCAO frequency was found. This suggests a spatially complex, not yet fully understood linkage between stratospheric variability and MCAO formation. Our results aim to shed light on the precursors and occurrence of MCAOs over the Barents and Norwegian seas, as well as for the Labrador Sea, which is expected to benefit long-range predictability of their extreme impacts
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