20th Century North American and European climate change: A forced response to tropical ocean warming

Surface temperatures across the Northern Hemisphere (NH) continents have risen by the end of the 20th Century to levels that are unprecedented in the instrumental record. These trends have been accompanied by equally dramatic changes in atmospheric circulation, notably a preference for the Pacific/North American (PNA) and North Atlantic Oscillation (NAO) modes of variability to reside in their positive phases during recent decades. Hurrell (1996) has previously demonstrated that much of the warming across Eurasia and North America can be directly attributed to such atmospheric circulation changes, though their cause has remained an open question.

We present results from climate model simulations of the 20th Century that indicate the changes in NH circulation patterns can be explained as a direct response to a profound warming of tropical sea surface temperatures. This oceanic warming, which has recently been shown to also penetrate to depths of 3000 m (Levitus et al. 2000), has taken place in all of the major ocean basins, and appears not to be a simple residual of the El Nino/Southern Oscillation phenomenon. Two sets of experiments are described, one in which an atmospheric GCM is forced with the monthly evolving global SSTs during 1950-1999, and a second in which only the evolving tropical SSTs are prescribed as boundary forcings. Both sets of ensemble runs realistically capture the trend in the PNA and the NAO circulation patterns observed during the last half century. Our presentation focuses on the origin of the trend in the NAO, for which we offer a new theory that emphasizes the tropical ocean's role in abrupt climate change over the Atlantic/Eurasian region.