Polar lows influence ocean circulation

The strong winds and large ocean-atmosphere temperature differences associated with polar lows result in significant local impacts on the surface of the ocean. Indeed in some cases polar lows can generate heat fluxes of a similar magnitude to those seen in category one hurricanes. Althougth the sensitivity of the intensification of polar lows to sea surface conditions has been studied extensively, up to now the extent to which polar lows themselves influence large-scale ocean circulation has not been clearly demonstrated (“large-scale” here refers to the spatial scale of seas such as the Norwegian Sea). This is a difficult problem to study because ocean circulation varies on longer time scales and responds slowly to the cumulative effect of many polar lows. However, a study published in Nature Geosciences this month by Alan Condron and Ian Renfrew (C&R) is a major advance in understanding the effect of polar lows on ocean circulation. In particular their study demonstrates that polar lows probably have a significant influence on the strength of ocean currents in the northern North Atlantic. Their approach was to run a state-of-the-art ocean model both with and without the effects of small-scale (less than approximately 500 km) atmospheric cyclones (see Figure below). C&R note that they don’t exclusively assess polar lows in their study, but the combined effect of all detected small-scale cyclones. It is likely however that polar lows are the most important subset of these small-scale cyclones in generating the ocean response, although it would be interesting to confirm this in a future study since the wind drag of the other small-scale cyclones may be important.

Example case (b) with and (a) without the surface wind associated with a polar low. In more detail: (a) The near-surface wind speed without the polar low resolved, (b) as in (a) but with the addition of an estimated wind field from the polar low and (c) best estimates of the actual wind field. For details of the methodology see supplementary material of Condron and Renfrew, from which Figure S1 is reproduced here by permission from Macmillan Publishers Ltd: Nature Geoscience, copyright (2013).

C&R demonstrate that small-scale cyclones have a significant effect on simulated northeast Atlantic Ocean circulation. Most significantly, the current that flows westward around Greenland and then southward down the Labrador Sea (the North Atlantic subpolar gyre) increases in strength by 5.5% when small-scale cyclones are taken into account. As a result the northward transport of heat to northern Europe and North America is increased. This effect is missing in most global climate models, which do not have a sufficiently high spatial resolution to capture small-scale cyclones. This motivates the use of higher-resolution climate models to capture this and other important processes in seasonal and climate forecasting.

Full reference

Condron, A. and I. A. Renfrew (2013). The impact of polar mesoscale storms on northeast Atlantic Ocean circulation. Nature Geoscience, 6, 34-37, doi:10.1038/ngeo1661.