Trend in Northern Hemisphere Winter Atmospheric Circulation during the Last Half of the Twentieth Century

Abstract

During the last half century, the trend in the Northern Hemisphere tropospheric circulation has been associated with a deepening of both the Aleutian and Icelandic lows, a pattern akin to the ?Cold Ocean Warm Land? (COWL) pattern. A simplified dynamical model is used to show that the observed simultaneous deepening trend in both the Aleutian and Icelandic lows can be largely explained as a hemispheric planetary wave response to tropical diabatic forcing in the Indo?Pacific region. In the model, the extratropical storm tracks play a role in modulating the wave train, tending to enhance (weaken) the anomalous Icelandic (Aleutian) low in the North Atlantic (North Pacific) sector. The model results also suggest two ways in which the circulation trend over the North Atlantic sector could have been influenced by tropical forcing: one a direct, linear planetary wave response from the eastern tropical Pacific and the other an indirect response of the North Atlantic storm track to tropical forcing over the western Indo?Pacific region. The possible role of tropical SST warming and anthropogenic forcing is also discussed.