Effects of the Pacific Decadal Oscillation and global warming on drought in the US Southwest

Droughts are among the most expensive weather related disasters in the US. In the semi-arid regions of the US Southwest, where average annual rainfall is already very low, multiyear droughts can have large economic, societal and ecological impacts. The US Southwest relies on annual precipitation maxima during winter and the North American Monsoon (NAM), both of which undergo considerable interannual variability associated with large-scale climate patterns, in particular ENSO, the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO). The region is also part of the subtropical belt projected to become more arid in a warming climate. These impacts have not been combined and compared with projections of long-term variations due to natural climate patterns. This study addresses this need by deriving future projections of rainfall departures for Arizona and New Mexico with the PDO and AMO and combining these with projected global warming impacts. Depending on the precipitation dataset used, the impacts for the ongoing negative PDO phase are projected to be between 1-1.6 times as large as the multi-model means projection of precipitation minus evaporation during 2020-2040 in the IPCC A1B Scenario. The projected precipitation impacts of a combined negative PDO and positive AMO phase are between 1-2 times as large as the A1B Scenario projection. The study also advances earlier work by addressing problems in detecting the effect of the PDO on precipitation. Given the different mechanisms with which the PDO affects precipitation during winter and the NAM season, precipitation impacts are here investigated on a monthly scale. The impacts of the PDO also vary with other climate patterns. This can be partly addressed by investigating precipitation departures in dependence on other patterns. It is further found that the long-term effect of the PDO can be more clearly separated from short-term variability by considering return periods of multi-year drought measures rather than return periods of simple drought measures.