Multi-year Droughts and Pluvials over the Southwestern U.S. and Associated Circulations

Abstract:

The southwestern U.S. is highly vulnerable to climate variability leading to extremes, such as droughts and pluvials, because of its geographic location and climatologic characteristics. These can include multi-year and multi-decadal extremes in precipitation, as occurred in the 1950s drought. Multi-year droughts and pluvials can have severe consequences on water resources management, which makes them events of interest to a major water utility in the region, Denver Water. Denver Water managers are especially interested in 36-month droughts for planning water management, both for maintaining supply and adequate operating revenue. Our analysis is part of an effort to understand the basis for past droughts and pluvials and how they might change in the future.

Several indices, based on their sensitivity to drought development at different time scales, data availability and drought types, have been developed for detecting and monitoring drought and pluvial severity. Among them are the Palmer drought severity index, standardized precipitation index (SPI), and standardized precipitation evapotranspiration index (SPEI). This study uses 36-month SPEI and SPI to analyze drought and pluvial episodes over the southwestern U.S., with a focus on the region that supplies water for Denver Water. Aims of this study include: 1) characteristics of multi-year droughts and pluvials (e.g., onset, duration, cessation, intensity, and frequency), 2) the contrasts between drought and pluvial characteristics, and 3) atmospheric large-scale features associated with the initiation and maintenance of extended drought and pluvial periods. We find that droughts and pluvials are not simply mirror images of each other in their characteristics, so that one is not simply the opposite phase of the other. This result is particularly true of the large-scale circulation patterns associated with droughts and pluvials. We diagnose how the circulation behavior sustains the very wet and very dry episodes revealed by the SPEI and SPI indices.