Winter midlatitude cold anomalies linked to North Atlantic sea-ice and SST anomalies: The pivotal role of potential vorticity gradient

This study tries to establish a linkage between the North Atlantic sea ice (SIC) or sea surface temperature (SST) and cold anomalies over North Europe and North America through the Greenland blocking (GB) change. It is revealed that the magnitude of meridional potential vorticity (PV) gradient in the North Atlantic mid-high latitudes plays a key role for whether strong cold anomalies occur over the North America or North Europe or both, while it is related to the SIC change over Baffin Bay, Davis Strait and Labrador Sea (BDL) and North Atlantic SST anomaly.

It is found that when the PV gradient is large for strong midlatitude SST warming or positive BDL SIC anomaly, no distinct cold anomalies are seen over North America because of less westward movement and short lifetime of GB. Instead a relatively strong cooling appears over the west of Europe. Its prior large-scale atmospheric circulation corresponds to the positive phase of North Atlantic Oscillation (NAO). Moreover, strong cold anomalies can simultaneously occur over North America and North Europe, only when the PV gradient is small because of the influence of large BDL SIC decline or intense mid-high latitude SST cooling across the Gulf Stream extension. Its prior large-scale atmospheric circulation corresponds to a negative NAO. Daily composite shows that strong North American cold anomaly is distributed along the northwest-southeast direction in the presence of large BDL SIC decline, but midlatitude cold anomaly can occur over North America in the presence of strong mid-high latitude SST cooling even if the large BDL SIC decline is absent.