The El Niņo/Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO)/Arctic Oscillation (AO) represent the leading factors of winter climate variability in the Northern Hemisphere. Their individual impacts on the hydroclimatology of eastern United States (USA) have been documented. Recent literature also suggests the presence of a physical connection between the tropical Pacific and the annular mode. This research aims to quantify the impact of the AO on ENSO precipitation signatures across the eastern US in winter (DJFM). The study uses instrumental records of standardized winter precipitation as described by the Standardized Precipitation Index (SPI) between 1899 and 2007 across 260 climate divisions. The SPI is computed from the monthly precipitation values using the gamma distribution. Time series of monthly Niņo-3.4 SST index is used to extract El Niņo, La Niņa, and ENSO-neutral conditions; the AO index winter time series is used to classify the record into AO negative, AO positive, and AO neutral winters from 1899 through 2007. 500 hPa geopotential height fields extracted for each winter month between 1949 and 2007 are used to construct various composite maps based on each of the nine ENSO|AO combinations to better understand the physical mechanisms behind the observed relationships.

Results show that extreme phases of the AO significantly modulate the ENSO-precipitation relationships across several regions of the study area. Without the influence of the ENSO, positive phases of the AO significantly alter winter SPI regimes in only very limited areas of the eastern USA including North Dakota and Minnesota, where conditions are unseasonably drier, and throughout northern Oklahoma and southern Kansas, where wetter-than-expected conditions prevail at this time. During AO-negative winters, unseasonably wet conditions are present throughout Florida, with dry conditions observed throughout a region that extends from southeast Oklahoma through Arkansas into the Tennessee River and lower Ohio River Valley regions.

Winter SPI regimes associated with the El Niņo are significantly different during the extreme phases of the AO. Without the influence of the AO, El Niņo winters are wet throughout Florida, the Great Plains region and in Texas. The presence of the negative phase of the AO coincides with a drying of a large area spanning towards the northeast from eastern Kansas to the Great Lakes via Missouri, Illinois, Indiana, Ohio, Kentucky, and eastern Pennsylvania. Also modified at this time are precipitation patterns across southern Florida, where the typically wet conditions associated with El Niņo become even wetter when the AO is in its negative phase. When the AO becomes positive, the wet conditions typically observed across Florida spread into the interior and along the eastern seaboard to Virginia, the most significantly affected states being Alabama, Georgia, and South Carolina. The positive phases of the AO alter the expected synoptic conditions over the Pacific North American (PNA) region during El Niņo winters. Mid-tropospheric height anomalies drop over the high latitudes and over Eurasia, with significant declines in pressures occurring over Greenland and central Eurasia. Geopotential heights rise throughout the mid-latitudes, significantly over Europe and just off the northeastern coast of the USA. The presence of a well-developed high pressure system over the North American continent, including the eastern USA and the western Atlantic basin, promotes the influx of moist air into the study area from the east. Mid-tropospheric height anomalies decline (rise) through the mid (high)-latitudes of the northern hemisphere during El Niņo years when the AO is negative. Significant rise in geopotential heights spans an area around the polar cap from the Beaufort and Siberian Seas to the Greenland Sea and Labrador. The circulation is marked by an intensified Aleutian Low that extends west past the dateline and a considerably weakened Bermuda High over the Atlantic basin. The high-pressure cell typically situated over the interior of Canada at this time is displaced to the north and east, reaching maximum intensity over eastern Canada and Greenland. The intensification of the atmospheric circulation over the North Pacific can be attributed to the anomalously weak polar vortex and increased meridional flow in conjunction with the expected circulation anomalies associated with El Niņo.

When the AO is in its neutral phase, La Niņa winters are typically marked by wet conditions throughout the southern portions of the Ohio Valley, in the Tennessee Valley, around the northern Great Lakes of Wisconsin, Michigan, Minnesota, and in North Dakota. At this time, dry conditions dominate throughout Texas, along the Gulf coast, and in Florida. Outside of Florida, the SPI values return to seasonal values once the AO becomes positive, and the areas with significant effects on SPI regimes become small in extent and randomly scattered. Precipitation conditions also return to seasonal values throughout the study areas once the AO becomes negative. The mid (high)-latitudes are dominated by positive (negative) geopotential height anomalies during La Niņa winters when the AO is in its positive phase, enhancing the expected atmospheric flow over the North Pacific and southeastern USA. The positive height anomalies typically present over the central North Pacific and over the southeastern USA fall as the Aleutian Low regains strength and the influence of the Bermuda High weakens when the AO becomes negative. The resulting synoptic pattern closely represents that of a typical winter circulation pattern over the PNA region and the study area at this time.

The results of this study suggest that winter ENSO-related precipitation patterns are significantly impacted by the phases of the AO. This information underlines the important role that the state of the Arctic and the north Atlantic can play in shaping the observed ENSO-precipitation relationships across eastern US.