In Situ Measurements of Surface Winds, Waves, and Sea State in Polar Lows Over the North Atlantic

International audiencePolar low (PL) storms are an important feature of the wintertime subsynoptic-scale atmospheric circulation of middle- and higher-latitude ocean areas. They can generate hazardous conditions impacting coastal and marine activities like fishing, transport, and oil extraction. However, there are few studies available of individual PL systems based on high-resolution maritime surface data. Accordingly, the meteorological impacts of 29 PLs have been investigated for the 14 winters 1999-2013, using in situ measurements at eight stations in the Norwegian and North Seas. On average, the highest wind speed and significant wave height (SWH) occur following the minimum in sea level pressure of the PL, respectively, 1 and 3 hr after its passage. The strongest wind speed averages 17.1 m/s, and the highest peak SWH is 6.3 m, but these can reach 31 m/s and 11 m, respectively. PL characteristics of system horizontal extent, propagation speed, and the larger-scale atmospheric circulation environment explain the large intercase differences. Large, multiple, and fast-moving PLs within a meridional circulation environment appear to generate stronger near-surface winds and higher waves than do small, single, and slow-moving PLs within a zonal circulation. Multiple systems may have the largest impacts (e.g., SWH > 8 m), although a larger sample size is required to confirm this possibility. The impacts of PLs on sea surface temperature (SST) are quite small and are difficult to interpret separate from the background SST variation. The observed SST decrease may be mainly caused by the cold air outbreak within which the PL is embedded; indeed, a positive SST minus air temperature anomaly is found during the 24 hr preceding the passage of PL vortices, indicating enhanced low-level atmospheric instability