Mapping extreme precipitation 'hotspots' across the midlatitudes

The most severe thunderstorms - those that produce extreme precipitation - occur over subtropical and midlatitude regions. Atmospheric conditions conducive to such intense thunderstorms commonly involve the coupling of a low-level jet (LLJ) with synoptic short waves. The short-wave activity is frequently modulated by the circumglobal teleconnection (CGT) – in which the meridional gradients of the jet streams act as a guide for short Rossby waves. Previous research has linked either the CGT or the LLJ with extreme precipitation events, but has not linked the two circulation features together. In this study, a circulation-based index was developed by combining (a) the degree of the CGT and LLJ coupling, (b) the extent to which this CGT-LLJ coupling connects to regional precipitation, and (c) the spatial correspondence with the CGT (short-wave) trending pattern over the most recent 32 years (1979-2010). Four modern-era global reanalyses, in conjunction with four gridded precipitation data, were utilized in order to minimize the possibility of spurious trends. The results are suggestive of a link between several recent extreme precipitation events and the CGT/LLJ trends, including those leading to the 2008 Midwest flood in U.S., the 2011 tornado outbreaks in southeastern U.S., the 2010 Queensland flood in northeastern Australia and the 2010 Pakistan flood. Moreover, an analysis of three CMIP5 models from the historical experiments points to the role of greenhouse gases in forming the CGT trends during the warm season.