Impact of small scale Gulf Stream SST features on the North Atlantic storm track

Abstract:

An essential ingredient of the large scale circulation in the Northern Hemisphere is the North Atlantic storm track, which is of high importance for the exchange of energy between lower and higher latitudes. As the storm track is influenced by the Gulf Stream (GS) and its extension, accurate model representation of air-sea interactions over the GS sea surface temperature (SST) front is essential for the representation of large scale circulation and associated weather and climate patterns in the Northern Hemisphere.

We attempt to isolate the effect of ocean component resolution in the GS region, where SST gradients are large, on model representation of local and remote atmospheric processes. We performed a clean case-control experiment in the EC-Earth climate model with a horizontal resolution of about 25 km (T799) in which smoothing is applied to the SSTs in the western North Atlantic, while resolution of the atmospheric component is kept high. This approach adds to previous studies as the applied SST smoothing is subtler, leaving the front largely intact and smoothing only the smallest scale features to an apparent resolution of about 125 km.

We find that, in boreal winter, small scale SST features affect local turbulent mixing processes, associated with strong effects on surface con-/divergence and associated vertical motion. Differences in heat transport and local circulation affect the vertical and horizontal temperature gradients over the North Atlantic, hence modifying baroclinic stability, leading to a more meridionally tilted, northeastward extended North Atlantic storm track when GS SST resolution is high. A complementary shift in the prevailing Rossby wave structure, accompanied by higher surface pressure over both the North American and (southern) European continent implies changes to the large scale circulation that have a positive effect on surface temperature in both the US and (northern) Europe.

These findings suggest that small scale features of western boundary current ocean temperatures can be of large importance for the accurate representation of spatially and temporally remote weather and climate processes. Systematic biases in climate models (such as a too zonal storm track) may thus be partly explained and resolved by improved representation of oceans and air-sea interactions.