The combined role of ENSO and the NAO on winter precipitation and summer drought in the southwestern United States

In this study, summer Palmer Drought Severity Index (PDSI) data from the United States Climate Division dataset for the period 1895-2006 are analyzed to determine the concomitant role of antecedent winter El-Nino Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) conditions on drought severity across the southwestern United States. While ENSO climate impacts in the region are well established, no major research study has yet identified a significant NAO influence within the western United States. However, the results obtained here suggest that negative phases of the NAO tend to exacerbate the dry winter and drought prone summer conditions of La Niña throughout the region. Conversely, the data show that wet-season precipitation is significantly (p < 0.05) greater when El Niño events occur during positive phases of the NAO than when El Niño occurs in conjunction with negative phases of the NAO. For example, Arizona statewide precipitation during El Niño/NAO+ winters averages 56% above normal, but just 9% above normal when El Niño occurs in conjunction with negative phases of the NAO. This modulation of El Niño winter precipitation by the NAO also occurs in California, Colorado, New Mexico, Oklahoma, Texas, and southern Nevada. Additionally, modulation of La Niña winter precipitation by the NAO occurs throughout Texas and Oklahoma. For instance, Oklahoma statewide precipitation during La Niña/NAO- winters averages 20% below normal, while winters featuring La Niña/NAO+ conditions result in slightly above normal precipitation.

Due to the modulation of antecedent winter precipitation by ENSO and the NAO, summer drought conditions in the southwestern United States also show significant variation by combined ENSO/NAO phase. For example, the average statewide summer PDSI value in Arizona following El Niño/NAO+ winters is 2.70, which is considered moderately wet by PDSI classification. By comparison, the average statewide PDSI value is just 0.22 following El Niño/NAO- winters. Similar results were found throughout the southwestern United States.

An analysis of monthly and seasonal 500 hPa geopotential heights from the NCEP/NCAR reanalysis project for the period 1949-2006 suggests that significantly different ridge/trough patterns develop over North America depending on the combined phase of ENSO and the NAO. Specifically, winters with El Niño/NAO+ conditions feature anomalous ridging over the Great Lakes region while negative height anomalies occur from the Aleutians to the west coast of North America. This contrasts with the large-scale flow observed in El Niño/NAO- winters, which is characterized by relatively higher 500 hPa heights over the western third of North America. Accordingly, the results of this study suggest that the NAO may be an important modulating feature on the ENSP climate impacts in the southwestern United States.