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The El Nino-Southern Oscillation (ENSO) and the Arctic Oscillation (AO), respectively, dominate global and Northern Hemispheric climate variability on interannual timescales. Statistical and composite analyses of historical climate records between 1899 and 2007 reveal a strong influence of the AO on eastern USA winter precipitation and atmospheric circulation over the Pacific/North American (PNA) region during ENSO years. Records of the standardized precipitation index (SPI) and anomalies in the 500 hPa geopotential height field show the AO to exert greatest effect on El Nino-related climate. The presence of a strong northwardly displaced polar front jet and a well- developed Bermuda high during months when the AO is positive promotes the influx of warm and moist air from the western Atlantic into the study area, significantly enhancing winter El Nino- related precipitation in the southeast USA. Negative phases of the AO that coincide with El Ninos deepen the lows over the North Pacific and diminish the influence of the Bermuda high on the east- ern USA. This modification enhances the influx of northerly air into the study area, bringing unsea- sonably dry winters to the Midwest and the Ohio Valley. The presence of the negative phase of the AO returns the wintertime circulation and SPI regimes to near-normal values throughout the study area during La Nina winters, whereas no appreciable effects on La Nina-related circulation and SPI regimes are noted when AO becomes positive. The interaction between ENSO and the AO helps to account for the inter-ENSO variability of climate teleconnections across the PNA region, possibly enhancing our ability to provide better winter climate forecasts across the eastern United States during ENSO episodes.

Effect of the Arctic Oscillation on precipitation in the eastern USA during ENSO winters