Can Polar Lows Lead to a Warming of the Ocean Surface?

The main objectives of this research has been to investigate the effect of intense polar lows on the upper ocean. Our hypothesis is that vertical mixing induced by surface stresses may lead to entrainment of waters from a warm and saline core beneath the sea-surface. If the time scale of the entrainment and subsequent surface warming is sufficiently short, a positive feedback on the cyclone intensity is possible. As will be demonstrated, in the seas where polar lows often developed, the North-Atlantic Current (NAC) sub- ducts under colder and less saline waters and Arctic water flows on top of warmer and more saline waters. The result of this is the frequent presence of a temperature inversion in winter-time hydrographic sections in areas influenced by the NAC. By temperature inversion, we will here refer to temperatures increasing with depth in upper ocean. For sufficiently strong wind events, turbulent entrainment of this sub-surface warm core may lead to a rapid surface warming. One of our main findings is that the surface warming of more than 1°C may take place within a few hours. The result is based on model runs with initial temperature and salinity profiles from CTD-observations. Observational evidence of surface temperatures that support the hypothesis are found from microwave satellite observations of polar low events. In the cases presented here, increased sea-surface temperatures between 1°C and 2°C were observed in the wake of polar lows. The impact of increased SST on the cyclone intensity has been investigated using Carnot theory and by performing simulations with an axis-symmetric cyclone model coupled to a model for turbulent mixing in the upper ocean. For realistic temperature differences between the atmosphere and ocean, a one degree increase in SST yields an additional pressure drop of about 2-3 hPa. Entraintment of waters from a sub-surface warm core due to mixing by storms leads to a cooling of the ocean. As warm waters from deeper layers become exposed to the atmosphere and gradually cools, heat is lost from the ocean. Hence, frequent storm events during winter may strengthen the thermohaline circulation. In the absence of storm mixing the fresher water masses overlaying the NAC act as insulation to heat exchange between the NAC and the atmosphere. Through the cooling and transformation of Atlantic waters, polar lows and intense storm in the Nordic Seas may thus have an impact on the climate.