Drivers and Impacts of Changing Subpolar North Atlantic Surface Temperature and Salinity

Two aspects of Subpolar North Atlantic variability are explored using observations and model analysis. The first aspect is the autumn-winter seasonal reduction of sea surface temperature (SST). In a climate change simulation with the HadGEM3-GC3.1-HM model, a strong increase in the magnitude of the seasonal temperature reduction (STR) is found in sea-ice affected regions and the subpolar gyre. Similar results are obtained from an observational analysis using the HadISST dataset. In both cases, the STR has increased in magnitude by up to 0.3 ºC per decade over 1951-2020. The primary driver for the increased STR is a greater sensitivity of SST to heat loss due to increased surface stratification brought about predominantly by warming of the northern ocean regions. The increase in STR, leads to a greater winter meridional SST gradient, with potential consequences for increasing winter storminess. The second aspect is an investigation of the atmospheric impacts of surface salinity anomalies through modification of mixed layer properties and the surface heat exchange. For this analysis, the seasonal evolution of two 20-member ensembles of HadGEM3-GC3.1-HM have been undertaken with and without an imposed initial winter salinity anomaly in the western Subpolar North Atlantic that is similar in magnitude to the Great Salinity Anomaly. The evolution of the perturbed model runs will be examined with a focus on the consequences for European spring-summer climate conditions.