Drought Continuum in the Southwest

What is the background risk of a drought lasting 1, 5, 10, ....35 years?

In a recently published project, I worked with Sloan Coats examining the background risk of drought in the American Southwest.

First, we compared instrumentally-based drought metrics (PDSI, station-based GPCCv7 precipitation, etc.) to paleoclimate-derived (NADAv2a PDSI from tree rings) and climate model (NCAR CESM, CMIP5) drought frequency in the Southwest in the 20th Century.

The well-observed 20th century is a short time period, and our observations during this time period may not capture one of the rare 20-year or 30-year long droughts that have been observed in the paleoclimate record. So, we used paleoclimate (NADAv2a) data and climate model data (NCAR CESM LME, CMIP5) to examine the background risk of drought in the pre-climate change world of the last millennium (850-1850CE).

The figure below shows the average risk of a drought of a given length (per 100-year time period). All of our data sources agreed that short droughts are much more common than long droughts, and droughts lasting 10-30 years occur once every few hundred years.

Are short (subdecadal) and long (decadal-multidecadal) droughts caused by the same ocean-atmosphere conditions?

We also used the NCAR CESM LME and CMIP5 past1000 data to examine the ocean-atmosphere dynamics of droughts lasting a few years to multiple decades. We wanted to know if the same ocean-atmosphere conditions lead to droughts on all timescales, from interannual to multiple decades, in Southwestern North America.

We found that droughts lasting decades in the Southwest tend to coincide with a cool eastern tropical Pacific, as well as a cool PDO-like pattern (the blue horseshoe-shaped pattern in the North Pacific ocean in the figure below). However, not all droughts lasting decades coincide with these ocean surface temperature patterns.

Furthermore, although many CMIP5 models agree that this pattern tends to coincide with extended droughts in the Southwest, not all models agree on the details of this pattern.

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