Megadroughts in the Indian Monsoon Region and Southwest North America and a Mechanism for Associated Multidecadal Pacific Sea Surface Temperature Anomalies

Gerald A. Meehl National Center for Atmospheric Research,* Boulder, Colorado

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Aixue Hu National Center for Atmospheric Research,* Boulder, Colorado

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Abstract

A 1360-yr control run from a global coupled climate model (the Parallel Climate Model) is analyzed. It simulates “megadroughts” in the southwestern United States and Indian monsoon regions. The megadroughts represent extreme events of naturally occurring multidecadal precipitation variations linked to the dominant pattern of multidecadal SST variability in the Indian and Pacific Oceans. Gaining insight into the occurrence of megadroughts thus requires an understanding of the mechanism that is producing this multidecadal SST variability. Analysis of the model variability shows that the mechanism involves atmosphere–ocean and tropical–midlatitude interactions, with a crucial element being wind-forced ocean Rossby waves near 20°N and 25°S in the Pacific whose transit times set the decadal time scale. At the western boundary, the Rossby waves reflect into the equatorial Pacific to affect thermocline depth. The resulting feedbacks, involving surface temperature, winds, and the strength of the subtropical cells, produce SST anomalies and associated precipitation and convective heating anomalies. These anomalies are associated with atmospheric Rossby waves and resulting anomalous atmospheric circulation patterns in the midlatitude North and South Pacific. Consequent surface wind stress anomalies extend equatorward into the Tropics and help force ocean Rossby waves near 20°N and 25°S, and so on. Though there are some common elements with various ENSO processes, this decadal mechanism is physically distinct mainly because the surface wind stress anomalies near 20°N and 25°S supplement the wave reflections at the eastern boundary to force the ocean Rossby waves that provide the decadal time scale. These wind anomalies are closely tied to the anomalous midlatitude atmospheric circulation that is a product of teleconnections from the multidecadal SST and tropical convective heating anomalies, themselves linked to precipitation anomalies in the southwestern United States and south Asia associated with megadroughts.

Corresponding author address: Dr. Gerald A. Meehl, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307. Email: meehl@ncar.ucar.edu

Abstract

A 1360-yr control run from a global coupled climate model (the Parallel Climate Model) is analyzed. It simulates “megadroughts” in the southwestern United States and Indian monsoon regions. The megadroughts represent extreme events of naturally occurring multidecadal precipitation variations linked to the dominant pattern of multidecadal SST variability in the Indian and Pacific Oceans. Gaining insight into the occurrence of megadroughts thus requires an understanding of the mechanism that is producing this multidecadal SST variability. Analysis of the model variability shows that the mechanism involves atmosphere–ocean and tropical–midlatitude interactions, with a crucial element being wind-forced ocean Rossby waves near 20°N and 25°S in the Pacific whose transit times set the decadal time scale. At the western boundary, the Rossby waves reflect into the equatorial Pacific to affect thermocline depth. The resulting feedbacks, involving surface temperature, winds, and the strength of the subtropical cells, produce SST anomalies and associated precipitation and convective heating anomalies. These anomalies are associated with atmospheric Rossby waves and resulting anomalous atmospheric circulation patterns in the midlatitude North and South Pacific. Consequent surface wind stress anomalies extend equatorward into the Tropics and help force ocean Rossby waves near 20°N and 25°S, and so on. Though there are some common elements with various ENSO processes, this decadal mechanism is physically distinct mainly because the surface wind stress anomalies near 20°N and 25°S supplement the wave reflections at the eastern boundary to force the ocean Rossby waves that provide the decadal time scale. These wind anomalies are closely tied to the anomalous midlatitude atmospheric circulation that is a product of teleconnections from the multidecadal SST and tropical convective heating anomalies, themselves linked to precipitation anomalies in the southwestern United States and south Asia associated with megadroughts.

Corresponding author address: Dr. Gerald A. Meehl, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307. Email: meehl@ncar.ucar.edu

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