The Presence of Africa and Limited Soil Moisture Contribute to Future Drying of South America. Issue 21 (12th November 2019)

• Record Type: Journal Article
• Authors: Pietschnig, M.; Lambert, F. H.; Saint‐Lu, M.; Vallis, G. K.
• Is Part Of: Geophysical research letters. Volume 46:Issue 21(2019)
• Page Start: 12445
• Page End: 12453
• Publication Date: 2019-11-12
• Subjects: tropical precipitation change -- global warming -- Amazon basin -- simple scaling -- atmospheric circulation -- Matsuno‐Gill; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2019GL084441
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Over oceans, precipitation generally increases with warming in regions where preindustrial precipitation minus evaporation is positive. This simple "wet‐get‐wetter" principle does not hold over land. The Amazon region and Equatorial Africa currently receive ample rainfall, but the former is projected to dry out whereas precipitation is expected to increase over the latter. Our experiments with an idealized Atmospheric General Circulation Model and realistic continents show that land surface evaporation must be limited in order to obtain drying over the Amazon basin. Our simulations with rectangular, flat continents reveal that large parts of South America would receive more rainfall with warming in the absence of Africa. We suggest that this is due to a warming‐induced Matsuno‐Gill‐type circulation anomaly over Africa. We propose a new simple scaling that diagnoses precipitation change from surface relative humidity change and provides further evidence for the importance of circulation changes for future rainfall. Plain Language Summary: Comprehensive climate models project different precipitation responses to increasing CO2 levels over land compared to oceans. Maritime precipitation is projected to increase where preindustrial precipitation exceeds evaporation (such as the equatorial regions) and decrease in the subtropical oceans (where evaporation rates exceed rainfall rates). However, continental precipitation change cannot be summarized in such a simplified manner. Equatorial Africa and the Amazon basin both currently see much more rainfall than evaporation, but the Amazon region is projected to dry out in the future, whereas Central Africa will probably receive more rainfall. We find that the drying of the Amazon basin is partly caused by the increase in convection—and thus rainfall—over Equatorial Africa. The ascent of air over Africa leads to subsidence to the west of the continent, resulting in unfavorable conditions for precipitation over tropical South America and the Atlantic Ocean. Key Points: Limited evaporation from land surfaces poses a major constraint on continental precipitation change, particularly in the Amazon basin Warming‐induced circulation changes over Africa lead to a decrease in precipitation over South America A new simple scaling of precipitation change provides evidence for the impact of circulation anomalies on future rainfall