North American droughts in ERA-40 global and NCEP North American Regional Reanalyses: A Palmer drought severity index perspective

The Palmer drought severity index (PDSI) monitors meteorological and surface hydrological parameters to represent the severity of drought conditions. PDSI datasets are developed for the NCEP North American Regional Reanalysis (NARR) and the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) to facilitate North American drought research with these datasets. The drought index calculation, in particular, allows diagnostic assessment of the relative contributions of various surface water balance terms in generation of drought conditions by selectively holding these terms to their climatological value in PDSI computations. The length of the diagnosed PDSI permits analysis of sub-decadal time-scale variability, such as ENSO, whose influence on North American drought evolution is investigated. ENSO's considerable drought impact is potentially predictable, especially in the southern half of the United States.