Recently Amplified Interannual Variability of the Great Lakes Ice Cover in Response to Changing Teleconnections

Abstract The interannual variability of the annual maximum ice cover (AMIC) of the Great Lakes is strongly influenced by large-scale atmospheric circulations that drive regional weather patterns. Based on statistical analyses from 1980 to 2020, we identify a reduced number of accumulated freezing degree days across the winter months in recent decades, a step-change decrease of AMIC after the winter of 1997/98, and an increased interannual variability of AMIC since 1993. Our analysis shows that AMIC is significantly correlated with El Niño–Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the Pacific–North American pattern (PNA) before the winter of 1997/98. After that, the AMIC is significantly correlated with the tropical–Northern Hemisphere pattern (TNH) and eastern Pacific oscillation (EPO). Singular value decomposition of the 500-hPa geopotential height and surface air temperature shows a dipole pattern over the northeast Pacific and North America, demonstrating the ridge–trough system. This dipole pattern shifts northward to the northern Rocky Mountains, placing the Great Lakes region in the trough after 1997/98. This shift coincides with the increased interannual variability of the EPO index, as well as the change in the sea surface temperature (SST) over the northeast Pacific, where the second mode of the empirical orthogonal function (EOF) on SST shows a warm blob-like feature manifested over the Gulf of Alaska. The regression of wave activity flux onto the SST EOF shows that the source of upward and eastward propagation of a stationary Rossby wave shifts to the west coast of North America, likely moving the ridge–trough system eastward after the winter of 1997/98. Significance Statement We found that the Great Lakes annual maximum ice cover decreased after one of the strongest El Niño–Southern Oscillation events in 1997/98 and that its year-to-year fluctuations increased in the recent 20 years. After the winter of 1997/98, the Great Lakes annual maximum ice cover started to correlate with the warm sea surface temperature anomaly in the northeast Pacific, which appears to disrupt the polar vortex far up in the stratosphere, and the polar vortex ultimately shifts eastward the ridge–trough system over North America. When the shifted system develops enough, it can encase the Great Lakes region in the Arctic air and thereby cause larger year-to-year fluctuations. This connection was found to be characteristic in the recent few decades, after the winter of 1997/98.