The role of the Atlantic Multidecadal Oscillation on medieval drought in North America: Synthesizing results from proxy data and climate models

During medieval times (900–1300AD; henceforth MT), persistent drought (‘megadrought’) was a ubiquitous feature for much of North America. To better understand the mechanisms for these droughts, relationships between specific sea surface temperatures (SSTs) and drought in North America have been synthesized from previous studies using modern observations, proxy paleo-data, and simulations from multiple climate models. Particular focus is on the role of the Atlantic Multidecadal Oscillation (AMO), a roughly 60–80year fluctuation in the North Atlantic between relatively warm (‘warm phase’) and cool (‘cold phase’) SSTs. Present-day relationships, for which instrumental observations can be used, indicate that persistent droughts in the U.S. Great Plains and Southwest are closely related to AMO. During AMO warm (cold) phases, most of North America is dry (wet). The MT drought is closely related to AMO-like warm phases in the North Atlantic. However, the MT drought is just one of numerous droughts on centennial timescales affected by the AMO that new results from the proxy record indicate impacted North America during the Holocene. The influence of North Atlantic SST on modern North American drought is examined using simulations made by global climate models. When forced by warm North Atlantic SST anomalies, all models captured significant drying over North America, despite some regional differences. Overall, the ensemble of the 5 models could well reproduce the statistical relationship between the dry/wet fluctuations in the North America and North Atlantic SST anomalies. Using one particular climate model, we further demonstrated that warm North Atlantic SST anomalies might have played a major role in the MT drought over much of North America. The MT drought could be simulated either by perpetual La Niña-like conditions in the eastern Pacific or warm phase of the AMO in the North Atlantic. Best match to proxy reconstructions was obtained, however, when both La Niña and warm phase AMO conditions were imposed. During the warm phase of AMO, the subtropical high is displaced north and east of its mean location, reducing moisture transport into the US except along the mid-Atlantic coast. During the cool phase, the subtropical high strengthens and pushes westward, allowing for more moisture transport into the central and western US. In summary, the AMO modulates the large-scale circulation to be more (less) conducive to precipitation over the central and western U.S. during cold (warm) phases. Its effects must be considered along with other, shorter time-scale factors such as ENSO, or PDO, as well as local effects that affect land surface–atmosphere interactions, such as soil moisture.