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Abstract Southwest North America is projected by models to aridify, defined as declining summer soil moisture, under the influence of rising greenhouse gases. Here, we investigate the driving mechanisms of aridification that connect the oceans, atmosphere, and land surface across seasons. The analysis is based on atmosphere model simulations forced by imposed sea surface temperatures (SSTs). For the historical period, these are the observed ones, and the model is run to 2041 using SSTs that account for realistic and plausible evolutions of Pacific Ocean and Atlantic Ocean interannual to decadal variability imposed on estimates of radiatively forced SST change. The results emphasize the importance of changes in precipitation throughout the year for declines in summer soil moisture. In the worst-case scenario, a cool tropical Pacific and warm North Atlantic lead to reduced cool season precipitation and soil moisture. Drier soils then persist into summer such that evapotranspiration reduces and soil moisture partially recovers. In the best-case scenario, the opposite states of the oceans lead to increased cool season precipitation but higher evapotranspiration prevents this from increasing summer soil moisture. Across the scenarios, atmospheric humidity is primarily controlled by soil moisture: drier soils lead to reduced evapotranspiration, lower air humidity, and higher vapor pressure deficit (VPD). Radiatively forced change reduces fall precipitation via anomalous transient eddy moisture flux divergence. Fall drying causes soils to enter winter dry such that, even in the best-case scenario of cool season precipitation increase, soil moisture remains dry. Radiative forcing reduces summer precipitation aided by reduced evapotranspiration from drier soils. Significance StatementSouthwest North America has long been projected to undergo aridification under rising greenhouse gases. In this model-based paper, we examine how coupling across seasons between the atmosphere and land system moves the region toward reduced summer soil moisture. The results show the dominant control on summer soil moisture by precipitation throughout the year. It also shows that even in best-case scenarios when changes in decadal modes of ocean variability lead to increases in cool season precipitation, rising spring and summer evapotranspiration means this does not translate into increased summer soil moisture. The work places projections of regional aridification on a firmer basis of understanding of the ocean driving of the atmosphere and its coupling to the land system.
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Ocean-forcing of cool season precipitation drives ongoing and future decadal drought in southwestern North America
Abstract The US Southwest is in a drought crisis that has been developing over the past two decades, contributing to marked increases in burned forest areas and unprecedented efforts to reduce water consumption. Climate change has contributed to this ongoing decadal drought via warming that has increased evaporative demand and reduced snowpack and streamflows. However, on the supply side, precipitation has been low during the 21st century. Here, using simulations with an atmosphere model forced by imposed sea surface temperatures, we show that the 21st century shift to cooler tropical Pacific sea surface temperatures forced a decline in cool season precipitation that in turn drove a decline in spring to summer soil moisture in the southwest. We then project the near-term future out to 2040, accounting for plausible and realistic natural decadal variability of the Pacific and Atlantic Oceans and radiatively-forced change. The future evolution of decadal variability in the Pacific and Atlantic will strongly influence how wet or dry the southwest is in coming decades as a result of the influence on cool season precipitation. The worst-case scenario involves a continued cold state of the tropical Pacific and the development of a warm state of the Atlantic while the best case scenario would be a transition to a warm state of the tropical Pacific and the development of a cold state of the Atlantic. Radiatively-forced cool season precipitation reduction is strongest if future forced SST change continues the observed pattern of no warming in the equatorial Pacific cold tongue. Although this is a weaker influence on summer soil moisture than natural decadal variability, no combination of natural decadal variability and forced change ensures a return to winter precipitation or summer soil moisture levels as high as those in the final two decades of the 20th century.
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- PAR ID:
- 10462832
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- npj Climate and Atmospheric Science
- Volume:
- 6
- Issue:
- 1
- ISSN:
- 2397-3722
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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