Interannual Variability of the Warm Arctic-Cold North American Pattern

Abstract

This study examines the evolution of the interannual warm Arctic-cold continents (WACC) pattern over the North American sector, which refers to the warm Arctic-cold North American pattern (WACNA), and explores its driving mechanism. WACNA features a pair of opposite surface air temperature anomalies centered over the Chukchi- Bering Seas and the North American Great Plains. A negative phase of the warm Arctic-cold Eurasia (WACE) pattern tends to lead a positive phase of the WACNA pattern by about 25 days. Negative Asian-Bering-North American (ABNA)- and Pacific-North American (PNA)-like atmospheric circulation patterns also appear upstream and precede a positive WACNA by about 25 days, gradually develop, reach their peaks when both circulation patterns lead theWACNA by 5 days, and weaken afterward. The negative ABNA-like pattern can be driven by the Siberian snow decline that is related to a negative WACE pattern and its featured Eurasian warming, whereas the negative PNA-like pattern is influenced by negative SST anomalies over the tropical central-eastern Pacific Ocean that resemble the tropical ENSO variability. The surface signatures of both patterns highlight a horseshoe-shaped high pressure anomaly straddling over the Gulf of Alaska, Alaska, and northwestern Canada. The anomalous warm advection from the North Pacific and cold advection from the Arctic that follow the circulation anomalies, as well as sea ice declines over the Chukchi-Bering Seas and growth over Hudson Bay, lead to the formation of the positiveWACNA pattern. Processes with circulation anomalies of opposite signs will likewise lead to the negative WACNA pattern.