Interdecadal Modulation of the Impact of ENSO on Drought over the United States

Composites based on observations and model outputs from the CLIVAR drought experiments were used to examine the decadal modulation of the impact of El Nino Southern Oscillation (ENSO) on drought over the United States. Drought here is classified based on the 6-month Standardized Precipitation Index and soil moisture anomalies from the North American Land Data Assimilation VIC model

The impact of ENSO on drought is not stationary. It is modulated both by the trend mode and the Atlantic multi -decadal Oscillations (AMO). For warm (cold) ENSO winter and spring, composites show warming (cooling) over the Northwest and cooling (warming) over the Southeast and the Gulf States. However, the patterns and magnitudes of composites change over time. For precipitation, the largest ENSO impact occurs in winter. While the ENSO impact strengthened over the Southwest, California and the Gulf States for the recent periods, the impact over the Ohio Valley reached a peak in 1930-1970 and has been weakening in recent years.

The direct influence of the AMO on drought is small. The major influence of the AMO is to modulate the impact of ENSO on drought. The influence is large when the sea surface temperature anomalies(SSTAs) in the tropical Pacific and in the North Atlantic are opposite in phase. A cold (warm)event in a positive (negative) AMO phase amplifies the impact of the cold (warm) ENSO on drought. The ENSO influence on drought is much weaker when the SSTAs in the tropical Pacific and in the North Atlantic are in phase.