The El Niņo-Southern Oscillation (ENSO) is the dominant source of global climate variability on interannual timescales. The oceanic and atmospheric anomalies present throughout the equatorial Pacific basin in winter excite the Pacific/North American (PNA) teleconnection pattern that propagates energy from the equatorial region towards North America, modifying the expected weather patterns of the region. The Arctic Oscillation (AO), known to be closely related to the North Atlantic Oscillation (NAO) or the Northern Annular Mode (NAM) is the major contributor to weather and climate variability across the Northern Hemisphere (NH) at intraseasonal to interdecadal timescales. Its state reflects the strength of the polar vortex in the stratosphere and westerly wind flow north of 45° in the troposphere such that its positive phase is associated with strong polar vortex and anomalously strong westerly winds, with complementary conditions dominating during the negative phase. The impacts of the AO and especially the ENSO on the expected weather patterns of the NH are now well understood. Recently, a number of studies have also begun to document the existence of the relationships between the AO and the ENSO. This study aims to build on this literature by examining the interaction between the two phenomena with a focus on understanding the influence of the AO on ENSO-related precipitation patterns across the eastern US during the extended winter seasons (DJFM).

In its methodology, the study applies various statistical and composite analyses of monthly and seasonal records of standardized precipitation for the eastern US for various AO|ENSO combinations. To explore the physical mechanism of the observed precipitation regimes the study also analyzes composite maps of NH sea level pressure (SLP) and 500 hPa geopotential height anomalies during various states of the AO and the ENSO. The time period of study consists of 107 years, between 1900 and 2006. It is hypothesized that during ENSO events the state of the atmospheric circulation over the NH as defined by the state of the AO influences the structure of the teleconnection pattern that will emanate from the equatorial Pacific into the NH. Such modifications will in turn change the strength and position of the jet streams and storm tracks, influencing the expected surface precipitation regimes across the region.

The results show that the ENSO-related teleconnection pattern is significantly different during various phases of the AO across the PNA sector with this modification impacting the expected precipitation regimes throughout eastern US as well. The interaction between the AO and the ENSO is most evident in the atmospheric circulation patterns over the North Pacific, the western Atlantic and over the eastern US. Overall, El Niņo-related teleconnection and related precipitation regimes are more impacted by the extreme phases of AO than are weather conditions during La Niņa events.

Wetter-than-expected conditions are observed during El Niņo events along the southeast coast of the US when the AO is in its positive phase. These conditions coincide with weakened Aleutian low and higher-than-expected SLPs over the northeast US and the north Atlantic. At this time, the upper air flow is dominated by strong westerlies, with an influx of warm and moist air from the south east into the study area. When the AO is negative, El Niņo winters are drier than expected throughout large sections of the central Midwest, the Ohio Valley, and across the southern Great Plains regions, while southern Florida is significantly wetter than during El Niņo years when the AO is neutral. The synoptic conditions across the North American region resemble an enhanced PNA flow dominated by an influx of northwesterly winds into the US from Canada that bring cold and dry air into the mid section of the study area, and wet conditions to the south.

The presence of either extreme phase of the AO during La Niņa winters acts to subdue the expected anomalies in surface precipitation and atmospheric circulation over the study area. Seasonal conditions dominate throughout the eastern US during both the positive and negative phases of the AO. The atmospheric circulation at the mid-latitudes resembles the expected pattern of mean winter circulation over the PNA region when the AO is negative, with no appreciable changes in the expected atmospheric circulation observed over the eastern US when the AO is positive.

The results of this study display evidence of significant interaction between two principal modes of extratropical variability, the ENSO and the AO. The state of the AO influences the ENSO-related teleconnection patterns and associated winter precipitation regimes of the eastern US. This knowledge contributes to our understanding of the well-known inter-ENSO variability of remote teleconnections across the PNA region, and in turn can have a positive impact on our ability to provide better winter climate forecasts across the eastern US during ENSO episodes, especially once the state of the AO can be forecasted with appreciable skill with a lead time of one or more seasons.