Stratification and mixed layer depth around Iceland: characterization and inter-annual variability

Abstract

The ocean around Iceland is a key region where major water masses and currents interact, influencing the global ocean circulation. Here, we analyze 29 years (1990-2019) of quarterly hydrographic section 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 (similar to 500 m) while the north and northeast show shallower depths (similar to 100 m). Based on the decomposition of total stratification into temperature and salinity contributions, we find that the subsurface stratification is mainly controlled by temperature in the south and by salinity in the northwest, while in the north, the North Icelandic Irminger Current and East Icelandic Current alternate seasonally, shifting the region between temperature-dominated and salinity-dominated 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 1990's, then warmed until approximately 2015, and became colder again from 2015 to 2018. In the northeast, a multidecadal mixed layer warming trend 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. This is associated with the "Atlantification" of the Arctic. Within the mixed layer south of Iceland, density has continuously decreased since the mid 1990's. Elsewhere, we observe density-compensated changes in mixed layer temperature and salinity, without clear long 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, showing links between this regional variability and changing North Atlantic under global warming.