Oregon Sea Grant

The impacts of El Niño and South Oscillation (ENSO) and Arctic Oscillation (AO) or North Atlantic Oscillation (NAO) on Great Lakes ice cover were investigated using ice observations for winters 1963-2008 and National Centers for Environmental Prediction (NCEP) reanalysis data. Signatures of ENSO and AO/NAO were found in Great Lakes ice cover. However, the impacts are nonlinear and asymmetric. Strong El Niño events are often associated with least ice cover on the Great Lakes, while the impacts of weak El Niño and La Niña events (of all intensities) on the Great Lakes are marginally significant. Negative AO/NAO events are often associated with severe ice cover, while positive AO/NAO events often lead to lower ice cover. The strong El Niño and negative AO/NAO events account for about 50% of the least and severe ice cover winters on the Great Lakes, respectively. The interference of the effects of ENSO and AO/NAO over the Great Lakes makes the relationships complicated. This may be an important cause of nonlinear and asymmetric responses of the regional climate and Great Lakes ice to ENSO and AO/NAO. Based on the cross composite analysis, it is found that during the simultaneous occurrence of El Niño (La Niña) and +AO (-AO) events, Great Lakes ice cover tends to be least (severe).