Stratification and Mixed Layer Depth around Iceland, characterization and inter-annual variability

Abstract. The ocean around Iceland witnesses some of the most important transformations of water masses that drive the global ocean circulation. Here, we analyze 29 years (1990–2019) of quarterly hydrographic sections data collected around Iceland. The hydrographic properties around Iceland show important spatial variability. Based on temperature, salinity, and stratification structure, we classified the Icelandic waters in three distinct regions: the South, the North and Northeast regions. The warm and salty Atlantic Waters that dominate the south show the deepest winter mixed layers (~500 m) while the North and Northeast show shallower depths (~100 m). Based on the decomposition of total stratification into temperature and salinity contributions, we find that, in the South, the subsurface stratification is mainly dominated by temperature, in the Northwest salinity dominates, while in the North, the seasonality of the North Icelandic Irminger Current and East Icelandic Current alternate the temperature and salinity contribution to stratification. The interannual variability of the mixed layer and of its thermohaline properties is also large around Iceland. Mixed layer waters were generally colder in the 90’s, then warmed until approximately 2015, and became colder again from 2015 to 2018. In the Northeast, a clear multidecadal mixed layer warming trend clearly emerges from the interannual variability as the Atlantic Water progresses northeastward, which is responsible for transforming locally, the upper stratification from salinity dominated into temperature dominated, allowing for the formation of deeper mixed layers. This is associated with the “Atlantification” of the Arctic. Elsewhere, we observe density-compensated changes in mixed layer temperature and salinity, without clear trends. This study provides an unprecedented and detailed description of the seasonal to multi-decadal variability of the mixed layer depth and stratification around Iceland, and their link with the changing North Atlantic under global warming.