Response of the Nordic Seas to the 2–6 February 2020 marine cold air outbreak in the GLORYS12 ocean reanalysis

Abstract. Marine Cold Air Outbreaks (MCAOs) play a crucial role in wintertime water mass transformation in the Nordic Seas. However, due to the spatio-temporal variability of atmospheric forcing and lateral ocean transport, the processes by which MCAOs influence the ocean remain unclear. Using the eddy-resolving ocean and sea ice reanalysis GLORYS12, we investigate the mechanisms driving the ocean response over the Nordic Seas to the particularly intense 2–6 February 2020 MCAO event. To assess the impact of the MCAO on the ocean, we quantify the contributions of the mean surface turbulent heat flux relative to the mean change in ocean heat content during the event. The western part of the Nordic Seas (Greenland Sea and northern interior Iceland Sea) was primarily affected by the air-sea heat exchanges, with an overall mixed layer cooling by approximately 0.02 °C·day-1 during the event in the interior Greenland Sea and a deepening of more than 30 m·day-1 in some areas. In the eastern part (Norwegian Sea), on the other hand, the air-sea heat exchanges were masked by stronger lateral oceanic heat transport, with a cooling or warming of an order of magnitude higher. In the interior part of northern Iceland Sea, the mixed-layer depth increased by approximately 5 m·day-1, while it decreased near the boundary current in the western Iceland Sea by approximately 8 m·day-1 concomitantly with a shoaling of the warm Atlantic-origin water mass.