Impact of Northwest Atlantic SST Anomalies on the Circulation over the Ural Mountains during Early Winter

Abstract

Both the observed background circulation and the northwest Atlantic sea surface temperature anomalies (SSTA) associated with the circulation anomaly over the Ural Mountains during early winter (October-December) are investigated, and it is shown that a positive height anomaly over the Urals is remotely linked to a positive SSTA by an upper wave-train-like anomaly chain across the North Atlantic and coastal Europe. To investigate whether and how the SSTA affects the circulation over the Urals, large-ensemble atmospheric general circulation model (GCM) experiments are conducted, and the results show that the SSTA forces a similar wave-train-like anomaly chain, resulting in a positive geopotential height anomaly over the Urals. The mechanism that maintains the response is diagnosed by investigating the roles of anomalous diabatic heating, and transient vorticity forcing, via a linear baroclinic model (LBM). The results suggest that the two upstream anomalies in the chain are largely maintained by anomalous transient vorticity forcing, although it is modulated by anomalous diabatic heating. In contrast, the Ural response is largely maintained by anomalous diabatic heating. To mimic the initial mechanism of the response, an idealized heating representing the initial SSTA-induced heating is prescribed. The LBM response to the idealized heating is obtained, and then transient feedback to the heating-induced anomalous flow is simulated, via a linear storm track model (STM). The LBM responses to the anomalous transient vorticity forcing resulting from the idealized heating resembles the GCM simulation upstream, but is not significant over the Urals. This suggests further that the Ural response is triggered, and maintained, by anomalous diabatic heating.