Fram Strait Marine Cold Air Outbreaks and associated surface heat fluxes in the ERA5 & CARRA reanalyses

The rapidly transforming Svalbard and Fram Strait region is characterised by strong air-sea exchanges and represents a major gateway of oceanic and atmospheric transport between the Arctic and lower latitudes. In winter, Marine Cold Air Outbreaks (MCAOs) extract large amounts of energy from the ocean in the form of surface sensible and latent heat fluxes. We investigate how the spatiotemporal variability in Fram Strait MCAOs affects the heat fluxes in ERA5 and the novel Arctic reanalysis CARRA over ocean, sea-ice and land during November-March 1991-2020.

We find that the daily mean heat fluxes are strongly correlated with the MCAO index and that wind speed only plays a large role for the heat fluxes when the MCAO index is positive. The sensible heat flux from the surface to the atmosphere reaches greater values in CARRA than in ERA5 while the opposite is true for the latent heat flux. The difference between the reanalyses scale with the magnitude of the heat fluxes, leading to large disagreement over ice-free ocean, where the fluxes have their highest values. When accounting for the differences in magnitude, we find the largest disagreement between the reanalyses over sea ice. 

In addition, we find that although sea ice loss drives positive ocean-to-atmosphere heat flux trends around much of Svalbard, negative trends in the monthly mean heat fluxes are seen in Fram Strait during the winter, especially in January. These negative trends reflect the decline in the surface-atmosphere potential temperature difference which forms the basis for the MCAO index. 

Finally, we examine the vertical structure of the atmosphere during MCAOs and find anomalously northerly winds, low temperature and low specific humidity throughout the troposphere. The specific humidity anomalies are strongest at low altitudes over the ice-free ocean in southern Fram Strait, while the temperature anomalies reach their maximum in the vicinity of the ice edge. Over the ice-free ocean, where the heat fluxes warm the air from below, the strongest temperature anomalies are found around the altitude of the 800 hPa level.