An Imminent Transition to Drier Conditions over the Continental U.S.?

Drought is a very damaging natural disaster in the contiguous United States, which has experienced relatively wet conditions from 1977-1998. Since 1999, however, the U.S., especially the West, has experienced frequent severe droughts, intense heat-waves, and large wild-fires. Does this represent a climate shift towards drier conditions over the U.S. for the foreseeable future? Previous studies have reported the potential drying over the Southwest U.S. under greenhouse gas-induced global warming and associated with cold sea surface temperatures (SSTs) in the tropical Pacific since the late 1990s. Here I further demonstrate that precipitation in the West (especially Southwest) U.S. follows closely with the Inter-decadal Pacific Oscillation (IPO) in the tropical Pacific since the 1920s, and the IPO is largely responsible for the wetting trend from the 1950s to the 1990s over much of the continental U.S. The tropical Pacific Ocean has, however, abruptly switched into a cold phase since around 1999 that may last for two more decades and cause below-normal precipitation over the central and West U.S. Combined with the drying trend from global warming over most of the U.S. as predicted by the latest climate models, these two factors strongly suggest that the current dry conditions over the central and West U.S. may persist and even worsen in the coming decades.