The Fall 2019 Southeast U. S. Flash Drought: An Extreme Event Driven by an Extreme Positive Indian Ocean Dipole

Abstract:

Much of the southeast United States experienced record dry conditions during September of 2019. In this study we employed MERRA-2 and the GEOS-5 AGCM to diagnose the underlying causes of the drought’s onset, maintenance, and demise. The basic approach involves performing a series of AGCM simulations in which the model is constrained to remain close to MERRA-2 over pre-specified areas that are external to the drought region. Our results show that the drought had little if any local precursors but was in fact driven by factors far removed from the drought region. In particular, the drought began as a result of anomalous heating in the central/western tropical Pacific that resulted in low level anti-cyclonic flow and a tendency for descending motion over much of the U.S. southeast. An anomalous ridge associated with a Rossby wave train (emanating from the Indian Ocean region) is found to be the main source of the most intense temperature and precipitation anomalies that develop over the southeast during the last week of September. A second Rossby wave train (emanating from the same region) is responsible for the substantial rain that fell during the second half of October to end the drought. Links to a waning El Nino and a near record positive Indian Ocean dipole indicate this event was potentially predictable several weeks in advance.