‘Missing' polar lows enhance deep-water formation in the Nordic Seas

The atmosphere plays a key role in forcing the large-scale ocean circulation by moderating the formation of deep water in the sub-polar North Atlantic. Every year thousands of intense storms – polar lows - cross this climatically sensitive region of ocean that are either too small, or short-lived, to be captured in meteorological reanalyses or numerical models. Here we show that by parameterizing ‘missing' polar lows we are able to reproduce the high wind speeds and heat fluxes associated with individual storms, as well as their integrated effects on the ocean, in remarkable agreement with observations. In a high resolution ocean circulation model our realistic atmospheric forcing increases the frequency and area of deep convection in the Greenland and Irminger Seas, and results in an unexpectedly large increase in the volume of deep water overflowing Denmark Strait (by up to 0.5 Sv). We conclude that polar lows play an important role in driving the large-scale ocean circulation and so must be accounted for in models in order to accurately predict near-future climate. Recent studies predict a decrease in the number of polar lows over the Northeast Atlantic in the 21st Century which, based on our work, implies a reduction in deep convection and a potential weakening of the Atlantic Meridional Overturning Circulation.