Extreme Convective Events in the Lofoten Basin

Abstract

Strong oceanic convection plays a crucial role in the variability of the global climate. In this study, we investigate extreme convection in the Lofoten Basin (LB) of the Norwegian Sea using Argo profiles. Mixed layer depth (MLD) is the main metric characterizing convection intensity. MLD exceeds 1000 m in March–April and December 2010 in the Lofoten Basin Eddy (LBE), whereas the mean MLD is about 200 m and seldom exceeds 400 m in the basin. We connect the extreme convection events with water volume formed at mid-depth of the central LB, between 1000 m depth and the isosteric surface s07. Analytical assessments of final mixing depth demonstrate perfect correspondence to measured values of MLD, indicating variations in the buoyancy flux and stratification as the main reasons for MLD variability in the basin. This variability is easily explained as a result of heat release to the atmosphere over the basin. Atmospheric conditions during extreme convection events are described. Northerly winds are as common as dominating southwesterly winds during the months with extreme convective events. We analyze 32 cases of extreme convective events with MLD exceeding 350 m and reveal that composite maps of sea level pressure (SLP) and surface heat flux match well with the North Atlantic Oscillation (NAO)−/East Atlantic Pattern (EAP)− pattern of atmospheric variability in the Northern Atlantic, with the negative NAO pattern prevailing in the climate during winter–spring. The heat release is the major trigger of extreme convection. We establish the stronger heat release associated with the extreme convection events in the LB.