Abstract
The role of the oceanic water cycle in the record-breaking 2015 warm-season precipitation in the US is analyzed. The extreme precipitation started in the Southern US in the spring and propagated northward to the Midwest and the Great Lakes in the summer of 2015. This seasonal evolution of precipitation anomalies represents a typical mode of variability of US warm-season precipitation. Analysis of the atmospheric moisture flux suggests that such a rainfall mode is associated with moisture export from the subtropical North Atlantic. In the spring, excessive precipitation in the Southern US is attributable to increased moisture flux from the northwestern portion of the subtropical North Atlantic. The North Atlantic moisture flux interacts with local soil moisture which enables the US Midwest to draw more moisture from the Gulf of Mexico in the summer. Further analysis shows that the relationship between the rainfall mode and the North Atlantic water cycle has become more significant in recent decades, indicating an increased likelihood of extremes like the 2015 case. Indeed, two record-high warm-season precipitation events, the 1993 and 2008 cases, both occurred in the more recent decades of the 66 year analysis period. The export of water from the North Atlantic leaves a marked surface salinity signature. The salinity signature appeared in the spring preceding all three extreme precipitation events analyzed in this study, i.e. a saltier-than-normal subtropical North Atlantic in spring followed by extreme Midwest precipitation in summer. Compared to the various sea surface temperature anomaly patterns among the 1993, 2008, and 2015 cases, the spatial distribution of salinity anomalies was much more consistent during these extreme flood years. Thus, our study suggests that preseason salinity patterns can be used for improved seasonal prediction of extreme precipitation in the Midwest.
Highlights
In the spring of 2015, an extreme rainfall event affected the Southern US and triggered record-breaking floods
The precipitation data used in this study are from the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC) US Unified Precipitation for 1950–2006 and from the Real-Time US Daily
The moisture flux calculated from the six datasets is averaged over their overlapping periods (Table 1) to minimize the uncertainties introduced by the choice of reanalysis datasets gence (MFD) is (Li et al 2013)
Summary
In the spring of 2015, an extreme rainfall event affected the Southern US and triggered record-breaking floods. According to the NOAA National Centers for Environmental Information (2016), the year 2015 was the second wettest year on record for the Midwest This 2015 extreme event has been attributed to the coexistence of an intensified Great Plains Low-Level Jet (GPLLJ) and an anomalous upper tropospheric trough over the Southwest US (Wang et al 2015). Our recent study has shown that the springtime water cycle in the subtropical North Atlantic significantly influences the moisture abundance for synchronized precipitation in the Southern US and modulates summer precipitation in the US Midwest (Li et al 2016b). 3, atmospheric moisture flux in the 2015 extreme event is presented, and the contributions of the subtropical North Atlantic water cycle to the rainfall extreme are quantified.
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