Fram Strait Marine Cold Air Outbreaks in CARRA and ERA5: Effects on Surface Turbulent Heat Fluxes and the Vertical Structure of the Troposphere

Abstract

Marine Cold Air Outbreaks (MCAOs) have a profound influence on atmospheric conditions and the surface-atmosphere heat exchange in Fram Strait and Svalbard. Comparing the global reanalysis ERA5 to its novel Arctic counterpart CARRA for November–March 1991–2020, we investigate the surface turbulent heat fluxes and the spatial characteristics associated with MCAO conditions. We find that the sensible heat flux from the surface to the atmosphere is substantially higher in CARRA while the latent heat flux is higher in ERA5. For sensible heat flux, the differences scale with the magnitude, leading to maximum disagreement over the ice-free ocean where the heat flux is high. Accounting for the varying heat flux magnitude over different surface types, we find the largest relative disagreement over sea ice. During MCAOs, negative anomalies in temperature and specific humidity are present throughout most of the troposphere in both reanalyses. Over the studied period, sea ice decline has caused positive trends in the surface-atmosphere potential temperature difference forming the basis of the MCAO index, leading to higher heat fluxes around much of Svalbard. In ice-free Fram Strait however, both reanalyses show negative trends in the MCAO index and the heat fluxes in January, when the increase in potential temperature is larger at 850 hPa than at the surface. Finally, large differences between two individual MCAO cases are found in reanalysis heat flux variability and radiosonde-derived atmospheric conditions over Svalbard.