Multiple Causes of the North American Abnormal Winter 1976–77

Abstract

The severe 1976–77 winter over eastern North America and the drought in the west are related to contemporary and antecedent atmospheric, oceanic and cryospheric factors. Although greatly amplified, the atmospheric flow pattern was in phase with the normal winter pattern so that the seasonal forcing by mountains, coastlines, etc., did not oppose the anomalous pattern. The autumn pattern of southerly air flow over the eastern North Pacific reduced heat losses from the ocean, induced advection of warmer waters, and reduced coastal upwelling. Meanwhile, the cold sea surface temperatures generated in spring and summer over the central North Pacific persisted. These cold waters seem to have been generated by the persistently strong Aleutian low which, following Bjerknes’ hypothesis, was associated with a persistent El Niño which began several months before and lasted through the abnormal winter. The atmospheric baroclinicity from the strong sea surface temperature gradient near 140°W strengthened fronts and cyclones. This gradient increased the upper level southerly flow and steered storms far north of their usual path. Vorticity redistribution from this wind system reinforced the west coast ridge and the eastern trough with recurrent outbreaks of arctic air and attendant snow. The high albedo and refrigerating effect of the snowy surface enhanced the baroclinicity along the eastern seaboard leading to stronger east coast storms with replenished snow and reinforcement of the mean trough at a wavelength compatible with the western ridge and perhaps leading to arctic blocking. The “signals” of the oncoming winter's patterns were clear enough by November to permit a reasonably successful long-range forecast.