Winter severity in the Great Lakes region: a tale of two oscillations

The effects of Pacific Decadal Oscillation (PDO) and the El Nino-Southern Oscillation (ENSO) on Laurentian Great Lakes regional winter air temperature and, more generally, surface-air temperatures (SAT) over North America are examined. The relationship between ENSO and winter severity in the Great Lakes is highly nonlinear and depends on the strength of El Nino events. Significant positive correlation between the winter severity and ENSO indices noted in earlier works is limited to strong El Nino events and is associated with an expansion of positive temperature anomalies in NW North America inland toward the Great Lakes. Although both ENSO and PDO are positively correlated with the Pacific-North American (PNA) teleconnection index, a closer look at the large-scale atmospheric circulation associated with the 2 oscillations reveals a substantial difference. During warm PDO phases (not coincident with strong El Nino events), atmospheric circulation resembles the classical PNA pattern, with a strong Aleutian Low at the surface and amplified ridges and troughs in the mid-troposphere. During strong El Nino events the Aleutian Low is also stronger than normal, but shifted eastward, to the Gulf of Alaska. Over North America, an upper atmospheric ridge on the west is not accompanied with a deep trough on the east as in the classical PNA pattern. As a result, outbreaks of cold Arctic air over the eastern US are rare and winters in the Great Lakes region are abnormally mild.