Mixed layer depth in winter convection in the Lofoten Basin in the Norwegian Sea and assessment methods

Abstract

We calculate the depth of winter convection in the Lofoten Basin in the Norwegian Sea using the oceanic reanalysis GLORYS12V1 data for the period 1993 to 2018. Two independent methods are used to estimate the depth of the mixed layer depth (MLD). We call the first method as the Kara method and the second one as the Montegut method. We build the monthly average maps of the MLD for the period from December to April. The maximum values of the MLD are observed in the area of the Lofoten Vortex. The MLD is maximal in March reaching 400-500 m, and 200 to 400 m in the other months. The MLD tends to increase in the northern and northwestern parts of the study area. We show the estimates of the MLD obtained by the Montegut method to be underestimated in comparison with the estimates by the Kara method. We estimate coefficients of the linear trend for monthly averaged MLD values from December to April for the period 1993 to 2018. We demonstrate in the interannual variability that the winter convection decreases in December, January, and February at the end of the study period, but it increases in March and April. This means a shift in the periods of maximum development of winter convection to a later date. This shift may be due to the processes of global warming. There is a significant intra-monthly variability when the values of the MLD can differ by 1.5-2 times during a month. Since the methods by Kara and Montegut are based both on empirical criteria, the estimates of MLD in the Lofoten Basin differ from each other. However, the empirical approaches for MLD estimates make it impossible to determine the advantages of one method relative to another