Extreme Variability in Irminger Sea Winter Heat Loss Revealed by Ocean Observatories Initiative Mooring and the ERA5 Reanalysis

Ground-breaking measurements from the ocean observatories initiative Irminger Sea surface mooring (60 degrees N, 39 degrees 30W) are presented that provide the first in situ characterization of multiwinter surface heat exchange at a high latitude North Atlantic site. They reveal strong variability (December 2014 net heat loss nearly 50% greater than December 2015) due primarily to variations in frequency of intense short timescale (1-3days) forcing. Combining the observations with the new high resolution European Centre for Medium Range Weather Forecasts Reanalysis 5 (ERA5) atmospheric reanalysis, the main source of multiwinter variability is shown to be changes in the frequency of Greenland tip jets (present on 15days in December 2014 and 3days in December 2015) that can result in hourly mean heat loss exceeding 800W/m(2). Furthermore, a new picture for atmospheric mode influence on Irminger Sea heat loss is developed whereby strongly positive North Atlantic Oscillation conditions favor increased losses only when not outweighed by the East Atlantic Pattern. Plain Language Summary The ocean loses heat to the atmosphere in the far northern Atlantic. This is important as heat loss influences how much deep water is formed and the strength of the Atlantic circulation. However, the amount of heat lost is poorly known because measurements are difficult to obtain in the icy, high wind conditions of the subpolar seas. New measurements from a state-of-the-art mooring in the Irminger Sea east of Greenland are presented here. They are the first multiwinter measurements obtained at such high latitudes and reveal strong variability in ocean heat loss. This variability is due to changes between winters in the number of intense heat loss events. The events are caused by the mountainous Greenland terrain which focuses winds into narrow, very strong jets over the ocean. We develop a new picture which explains how changing atmospheric circulation influences the number of events and hence the ocean heat loss.