The effect of an arctic polynya on the Northern Hemisphere mean circulation and eddy regime: a numerical experiment

The feedback of an arctic polynya, which is a large ice-free zone within the sea ice, on the hemispheric climate is studied with the ECMWF T21 GCM. For this purpose a control and an anomaly integration, in which a polynya was introduced in the Kara Sea, are compared. As the GCM, like the real atmosphere, shows a high level of low frequency variability, the mean response to the changed boundary conditions is obscured by internal noise. The necessary significance analyses are thus performed to enhance the signal-tonoise ratio within the framework of an a priori chosen guess pattern and a multivariate test statistic. The sensible and latent heat fluxes increased above the polynya, which resulted in a warming of the lower troposphere above and near the polynya. No statistically significant local or global sea-level pressure changes are associated with this heating. However we find a significant change of hemispheric extent of the geopotential fields at 300 hPa, if we use as guess patterns the eigenmodes of the barotropic vorticity equation. The different mean flow field is accompanied by significant changes of the synoptic transient eddy field. We find a significant variation in the barotropic and baroclinic forcing of the mean flow by the eddies, a change in the location and intensity of the storm tracks and in the conversion between eddy available and eddy kinetic energy. The additional heat flux from the polynya results in a reduction of the meridional heat flux by the synoptic eddies on the western Atlantic.