Impact of Atlantic SST and high frequency atmospheric variability on the 1993 and 2008 Midwest floods: Regional climate model simulations of extreme climate events

The role of sea surface temperature anomalies (SSTAs) in the Atlantic and eastern Pacific and high-frequency atmospheric variability from the Pacific during the 1993 and 2008 Midwest floods is investigated with a regional climate model. The SSTAs insignificantly modulate Midwest rainfall during the 1993 flood, but enhance precipitation during the 2008 peak flood by strengthening the southern portion of the Great Plains low-level jet, enhancing moisture transport from the Gulf of Mexico into the Midwest. This work suggests that while North Atlantic SST strongly controls Midwest decadal drought and pluvial periods, it plays a minimal or secondary role in modulating extreme flood events lasting weeks to months. A negative Pacific/North American (PNA) teleconnection marked the peak of both floods, suggesting a link between extreme Midwest warm season rainfall and high-frequency PNA variations. Simulations that apply a 10-day low-pass filter to the western lateral boundary condition indicate that interactions between the eddy and time-mean flow played a significant, but counterintuitive role, during the 1993 flood. Although above normal Pacific cyclone activity was observed to trigger heavy Midwest precipitation, the synoptic eddies also indirectly influenced rainfall by modifying the time-mean circulation. Simulations show that eddies from the Pacific dampened the positive rainfall anomalies by weakening vertically integrated moisture transport and upper level divergence anomalies over the Midwest. Copyright Springer Science+Business Media Dordrecht 2015