The Association Between El Niño Intensity and North American Winter Weather

The El Niño-Southern Oscillation (ENSO) is the periodic anomalous warming of the central and eastern tropical Pacific Ocean coupled with alterations in tropical circulations that has various impacts on global circulation patterns. On average, for North America, El Niño events feature wetter and cooler conditions in the southern and western US with warmer and drier conditions across the northern US. and Great Lakes. Currently, this paradigm is used for seasonal forecasts involving El Niño events, but it does not take into account the strength of the El Niño conditions. This ‘one-size-fits-all’ approach to ENSO forecasting can result in inaccurate forecasts and potentially poor planning, resulting in adverse socioeconomic impacts.

The goal of this project is to understand how the impacts of ENSO events vary based on the strength of the event. Although El Niño has many effects on certain regions, we focused our attention on North American climate variability during the winter months of December through February due to the timing of El Niño’s peak intensity.  We investigated geopotential heights, surface air temperature, wind speed, precipitation rate and sea surface temperature (SST) for strong and weak El Niño events over the last century to quantify these impacts. The results clearly demonstrate that the intensity of a positive ENSO event significantly affects winter weather patterns over North America. For example, while we typically consider the Southern US to be wet during El Niño events, this is true for only strong events – for weak events, anomalously low precipitation is observed over the Southern US. Furthermore, weak/strong El Niño events feature below/above average wintertime temperatures in the Eastern US. The potential cause of these differences in North American sensible weather patterns is the difference in the induced Pacific wavetrain from the tropical Pacific during strong and weak events. Strong events feature a more north-south ridge/trough pattern in the northern Pacific, with a strong projection onto the Aleutian Low, while weak events feature an east-west pattern over the same area. We hypothesize that this change impacts the jet stream across North America and may also be tied to the actual SST structure of the strong vs. weak El Niño events. Indeed, strong El Niño episodes feature warmer than average water centered in the eastern Pacific, whereas weak episodes feature anomalously warm waters more in the central Pacific. A discussion incorporating these differences, based on the intensity of ENSO events, into seasonal forecasts is also presented.