Principal Roles of the Atlantic Multidecadal Oscillation in Variations of Circulation and Precipitation in North America

Statistical analyses of data of the past have suggested above (below)-average summertime precipitation in central and western United States and Mexico in the years of cold (warm) SST anomaly in the North Atlantic associated with the Atlantic Multidecadal Oscillation (AMO). GCM studies have also indicated a tendency of having more (less) precipitation in North America corresponding to specified cold (warm) SST anomalies in the North Atlantic. While these results point to a strong effect of the AMO on North American precipitation variation at the multidecadal timescale, it remains unclear if such effect may have been a statistical coincidence of the data, including data from extended model runs, instead of showing any true physical connection. This question is addressed in this study. We use three sets of idealized GCM simulations: the aquaplanet, the continents without orography, and continents with orography. Each set has three experiments with a) average, b) above- and c) below-average SST in the North Atlantic Ocean. Major results show that warm or cold SST anomaly in the North Atlantic has significant and direct impact on the circulation and precipitation in North America. The atmospheric response to warm SST anomaly is fairly similar among the three sets of experiments. In the lower troposphere, the response has significant positive geopotential anomaly and shows negative precipitation anomaly in North America. However, the response to cold SST anomaly changes considerably among the sets of experiments. These results show strong physical connections of the AMO SST and atmospheric circulation and precipitation anomalies in North America. Moreover, the rather stable atmospheric response to the warm SST among the sets of experiments may explain the observed largely consistent response of precipitation to the warm SST anomaly. The varying responses of the precipitation to the cold SST anomaly indicate a strong sensitivity of the atmosphere to other forcings during the cold SST anomaly in the North Atlantic. This sensitivity could explain the varying and less stable response of the atmosphere and the more mosaic response pattern in precipitation to the cold SST anomaly during the AMO.