El NiñoEl Niño is a warming of the sea surface in the central and eastern Pacific Ocean along and near the equator to above mean temperatures.
La NiñaLa Niña is generally the opposite conditon to El Niño. The central and eastern Pacific water surface near the equator surface cools to below mean temperatures.
El Niño Southern Oscillation (ENSO)The El Niño Southern Oscillation is a cycle of switching back and forth between El Niño and La Niña conditions with a period of near neutral conditions between the two states. Each condition can last several months to a couple of years and occur every few years. They both tend to peak in late Fall and Early Winter but affects on weather can last well beyond that time into the spring and summer.
Arctic Oscillation (AO)The Arctic Oscillation is an oscillation between strong and week pressure gradients between the Arctic and the higher mid latitudes (around 45 deg N). The positive phase is represented by higher than mean pressure in the arctic and lower that mean pressure in the mid latitudes. The negative phase is marked by weaker high pressure in the Arctic and weaker low pressure in the mid latitudes. A strong pressure gradient exists with the positive phase and a weak pressure gradient exists with the negative phase. The pattern exists through the depth of the troposphere and thus affects the strength and latitudinal position of the jet stream. The strong temperature gradient produces a more northern storm track and retards arctic air from penetrating into the United States as frequently and aggressively as it would otherwise. The pattern switches between phases every several weeks to several months but can stay dominantly in one phase or the other for a year or more.
Pacific Decadal Oscillation (PDO)The Pacific Decadal Oscillation is an oscillation in water temperature that affects most of the Pacific Ocean in the Northern Hemisphere. is a longer period fluctuation in Pacific Ocean temperatures. The positive phase is indicated by warmer than average water temperatures in most of the Pacific Ocean in the Northern Hemisphere but surrounded on the edge with cooler than average water temperatures of the coast of Central and North America and bending west into the Gulf of Alaska. Each phase can last several years to several decades.
Impacts on Winter Precipitation, Especially for Minnesota and Wisconsin
Arctic Oscillation (AO)
Pacific Decadal Oscillation (PDO)
References for El Niño, La Niña, and El Niño Southern Oscillation (ENSO)"What is the El Niño-Southern Oscillation (ENSO) in a nutshell?" - Michelle L'Heureux, May 5, 2014, NOAA Climate.gov
NOAA National Ocean Service
NOAA NWS Climate Prediction Center
NOAA El Niño and La Niña Page
Climate Prediction Center's El Niño/La Niña Home
References for Arctic OscillationNational Snow & Ice Data Center - Definition from the Cryosphere Glossary
National Snow & Ice Data Center - All About Arctic Climatology and Meteorology
References for Pacific Decadal OscillationNASA Earth Observatory - Pacific Decadal Oscillation (PDO)
References for Weather ImplicaionsHow El Niño and La Niña affect the winter jet stream and U.S. climate - Rebecca Lindsey, Climate.gov
National Snow, Ice, and Data Center - The Arctic Oscillation, winter storms, and sea ice
Smith, S. R., and J. J. O'Brien, 2001: Regional Snowfall Distributions Associated with ENSO: Implications for Seasonal Forecasting. Bull. Amer. Meteor. Soc., 82, 1179-1191.
Jillien, M. P., S. R. Smith, and J. J. O'Brien, 2003: Impacts of ENSO on Snowfall Frequencies in the United States. Wea. Forecasting, 18, 965-980.