Climate Patterns and Oscillations

Atmospheric and oceanic climate patterns affect the weather over large areas of the globe including locations far away from where the patterns occur. The relationship between these patterns and the weather that results is called a teleconnection.

Some Definitions

El Niño

El Niño is a warming of the sea surface to above mean temperatures in the central and eastern Pacific Ocean along and near the equator. Trade winds from the east weaken or reverse direction causing water to warm due to reduced upwelling.

La Niña

La Niña is generally the opposite condition to El Niño. The central and eastern Pacific Ocean surface near the equator cools to below mean temperatures. Trade winds from the east increase in strength causing water to cool due to increased upwelling.

El Niño Southern Oscillation (ENSO)

The El Niño Southern Oscillation is a cycle that switches back and forth between El Niño and La Niña conditions. 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 can influence weather 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 degrees north). For reference, the latitude of the Duluth International Airport in Duluth, MN is 46.84 degrees north. The pressure pattern extends through the depth of the troposphere, as noted by the definition in the National Snow and Ice Data Center's Cryosphere Glossary, and thus affects the strength and latitudinal position of the jet stream. The positive phase is a strong pressure gradient produced by lower than mean pressure in the Arctic associated with a strong polar vortex compared to higher than mean pressure in the mid-latitudes. The negative phase is a weaker pressure gradient resulting from higher than mean pressure in the Arctic associated with a weak polar vortex compared to lower than mean pressure in the mid-latitudes. The positive phase produces a stronger, more zonal jet stream that tends to keep storms farther north and restrict cold air from penetrating into the United States. The negative phase results in a jet that is more wavy and variable in position and strength. Cold air can move south more often and more storms can occur farther south. The Arctic Oscillation switches phases every few weeks to few months but one phase or the other can dominate for several years 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. The alternating phases of the oscillation last longer than those of the El Niño Southern Oscillation. The negative phase, also called the cool phase, is indicated by warmer than average water temperatures in most of the Pacific Ocean in the Northern Hemisphere but surrounded on the edge (like a horseshoe?) with cooler than average water temperatures off the coast of Central and North America with the cooler water bending west into the Gulf of Alaska. The positive phase (also called the warm phase) is basically the reverse. Each phase can last several years to several decades.

Pacific North American Pattern (PNA)

The Pattern involves a series of troughs and ridges that span the region from Hawaii to south of the Aleutian Island chain of Alaska, then east into and across North America including the United States. The shifting position and strength of the troughs and ridges affect the strength and position of jet stream flow across the central pacific and into North America including the United States. The strength and location of the jet stream affect the number and strength of storms that move across the United States. The articles listed in the references section give a thorough description of pattern specifics. In general for the United States, the positive phase features an upper-level ridge over the western third of the country with the jet stream carving a deep trough over the central and east. The negative phase is marked by a less amplified ridge and trough and their positions are shifted to the west. The positive phase tends to occur with an El Niño in progress. The negative phase tends to occur with a La Niña. Phases last for several months at a time but the PNA can be dominantly in one phase or the other for one or a few years.

The positive phase increases the odds of colder and drier winter conditions in the middle to upper Midwest, with similar conditions extending east to the Middle Atlantic and Northeast. The deep South Central United States and the Southeast United States are also colder but also wetter than the average, increasing the risk of snow, sleet, and freezing rain. The Pacific Northwest can be either wetter or drier depending upon the positioning of the jet stream extending into that region from the Pacific Ocean. The negative phase is associated with increased precipitation in the winter along the West Coast and overall the western third of the country is colder. The central and east part of the country is generally milder, but with the storm track farther north, wetter conditions are more frequent from the Middle and Upper Mississippi Valley east to the Middle Atlantic and Northeast Coast. Not sure about the effects of the positive and negative phases on Minnesota and Wisconsin since they are on the edges of the temperature and precipitation anomalies. Odds appear to favor warmer with a positive PNA and colder with a negative PNA. Less confidence in the precipitation. The influence of the PNA on weather in the United States is much weaker during the warmer half of the year than in the colder half.

North Atlantic Oscillation (NAO)

The North Atlantic Oscillation (NAO) is an oscillation between increased and decreased strength of the pressure gradient between low pressure centered near Iceland, or more generally from Iceland to the southern part of Greenland, and high pressure farther south over the North Atlantic. The oscillation is in the positive phase when the pressure gradient is stronger than average, meaning lower than average pressure (stronger low pressure system) near Iceland and higher than average pressure to the south (stronger high pressure system). The oscillation is in the negative phase when the pressure gradient is weaker, meaning higher than average pressure near Iceland (weaker low pressure system) and lower than average pressure to the south (weaker high pressure system).

The result of the positive phase is a strong zonal wind flow through a deep layer of the atmosphere that keeps the jet stream farther north with fewer instances of the jet dipping or buckling to the south. For the Eastern United States, this means fewer penetrations of cold air and fewer strong winter storms. The result of the negative phase is a more southern and wavy jet stream that produces more storms and allows more cold air outbreaks. Although the influence of the NAO seems less defined for the rest of the United States, the NAO does tend to mimic the phase of the Arctic Oscillation (AO) which in turn influences much of the rest of the country. During a positive AO phase, a strong jet stream tends to stay far enough north to prevent cold air from penetrating into the United States and limits the overall storminess. The negative phase of the AO results in a jet stream that is farther south and produces more storms. The jet also allows arctic air outbreaks to be more frequent and more severe.

Impacts on Snow Season, especially for Minnesota and Wisconsin

An illustration from the NOAA Climate Prediction Center's website shows typical jet stream patterns and resulting winter climate anomalies resulting from the more well defined El Niño and La Niña events. Correlations to precipitation and temperature are general since every El Niño and La Niña is different in terms of strength and persistence. El Niño and La Niña affects on weather are stronger in some parts of the United States than in Minnesota and Wisconsin. The southern third of the United States, for example, has a strong tendency to be wetter than average due to El Niño and drier than average due to La Niña. The Pacific Northwest typically experiences the opposite result. El Niño and La Niña affect the structure and location of jet stream winds in the middle to upper Troposphere which in turn determine the location, intensity, and frequency of cold air outbreaks and tracks of low storm systems.

The jet stream due to El Niño tends to develop a wide split with a strong southern branch steering frequent storms across the southern United States. A northern branch is located far to the north in Canada or the far northern United States. Not unusual at some point in time during the winter to see a series of big "bowling ball" type storms smash into Southern California. Likewise, a few strong storms may form in the central or north part of the Gulf of Mexico, then move into the Southeast United States, then up the East Coast. Sometimes the northern branch of the jet stream will sink far enough south to supply cold air to produce major snow and ice storms in the Southeast.

In Minnesota and Wisconsin, temperatures tend to be a little warmer than average. Sorry, but still going to snow. Departures from average precipitation are less defined but the tendency is to be drier. Again, don't forget, the snow season in this part of the country extends will beyond either side of the typically designated winter months.

The Polar jet stream due to La Niña tends to be more consolidated over the United States, especially the northern half of the country. The jet flows southeast out of southwest Canada into the United States or can flow from the Pacific Ocean into the Pacific Northwest of the United States. The flow then extends east across the country. For Minnesota and Wisconsin, snowfall potential can increase due to the increased number of storms embedded in the upper flow and the increased frequency and strength of cold air outbreaks.

The overlapping occurrence of other climate influences such as phases of the Arctic Oscillation and Pacific Decadal Oscillation can enhance or dampen the weather patterns resulting from El Niño and La Niña. As an example, a graphic from NASA Earth Observatory website shows the superposition of cooler than average water of both La Niña and the negative (cool) phase of the Pacific Decadal Oscillation. In this circumstance, the effects of La Niña would likely be amplified.

Concerning the Arctic Oscillation, since the positive phase tends to keep the core of the jet stream north into Canada and brushing the far northern United States, most of the United States, sometimes including Minnesota and Wisconsin, can expect fewer storms, less cold air, and thus less snow. Be forewarned that since the Arctic Oscillation can switch phases as often as every few weeks, even a brief break from a dominantly positive phase can open the door for a few big storms.

Some recent high and low snowfall seasons for Duluth, MN in relation to El Niño and La Niña

Since 2001-2002, five snow seasons in Duluth, MN have exceeded 100 inches as recorded by the National Weather Service. Three snow seasons failed to exceed 50 inches. Using the Oceanic Niño Index as a guide, the data for each season includes a description of the ENSO conditions that influenced the season. A stronger El Niño is one that generally peaks around or above an index of 2.0. A stronger La Niña is indicated by index values around or below -2.0. Other indexes are also available such as the Multivariate ENSO Index (MEI). The index appears to generally support the assessment of the Oceanic Niño Index for this particular analysis.

          High Snow Seasons
2003-2004   109.9   Near Neutral
2012-2013   129.4   Near Neutral
2013-2014   131.0   Near Neutral
2018-2019   106.8   Weak El Niño
2022-2023   140.1   Weakening La Niña especially after December

          Low Snow Seasons
2011-2012   49.7   Moderate La Niña
2014-2015   49.2   Weak El Niño
2023-2024   38.7   Moderate to almost strong El Niño

Winters exceeding 100 inches of snow lacked a well defined El Niño or La Niña. Winter 2003-2004 was sandwiched between the moderate 2002-2003 El Niño and the weaker 2004-2005 El Niño. Winter 2012-2013 was preceded by the moderate 2011-2012 La Niña. Winter 2013-2014 was followed by the weak 2014-2015 El Niño. Winter 2018-2019 had a discernible El Niño but a weakly negative phase (cool phase) of the Pacific Decadal Oscillation (PDO) was also present during the season through March. The cooling influence of the PDO may have acted against the warming to dampened the strength of the El Niño.

Of the low snow seasons, the most interesting is Winter 2011-2012. The presence of a moderate La Niña would suggest the odds favored normal to greater than average snowfall. The Arctic Oscillation, as indicated by the Arctic Oscillation Index from the National Weather Service's Climate Prediction Center, was persistently in a strong positive phase and appears to be responsible for keeping the jet stream too far north. The north position of the jet stream kept most storms north of Minnesota and Wisconsin and kept cold air outbreaks to a minimum. For two weeks, from late February into early March, the weather pattern broke down, allowing more cold air and storms into the region. More than 25+ inches of snow fell during the period, accounting for half of the season's snowfall.

Additional Climate Influences to Consider

Atlantic Multidecadal Oscillation (AMO)

The "Frequently Asked Questions" page on the website of the NOAA Atlantic Oceanographic and Meteorological Laboratory (AOML) gives a summary of the phenomenon. A few of the highlights are as follows.

”The AMO is an ongoing series of long-duration changes in the sea surface temperature of the North Atlantic Ocean, with cool and warm phases that may last for 20-40 years at a time and a difference of about 1°F between extremes. These changes are natural and have been occurring for at least the last 1,000 years.” Each phase lasting 20 to 40 years means the complete cycle would be 40 to 80 years. ”Most of the Atlantic between the equator and Greenland changes in unison. Some area of the North Pacific also seem to be affected.”

”Models of the ocean and atmosphere that interact with each other indicate that the AMO cycle involves changes in the south-to-north circulation and overturning of water and heat in the Atlantic Ocean. This is the same circulation that we think weakens during ice ages, but in the case of the AMO the changes in circulation are much more subtle than those of the ice ages. The warm Gulf Stream current off the east coast of the United States is part of the Atlantic overturning circulation. When the overturning circulation decreases, the North Atlantic temperatures become cooler.”

”It is associated with changes in the frequency of North American droughts...” “Recent research suggests that the AMO is related to the past occurrence of major droughts in the Midwest and the Southwest. When the AMO is in its warm phase, these droughts tend to be more frequent and/or severe (prolonged?). Vice-versa for negative AMO. Two of the most severe droughts of the 20th century occurred during the positive AMO between 1925 and 1965: The Dustbowl of the 1930s and the 1950s drought. Florida and the Pacific Northwest tend to be the opposite - warm AMO, more rainfall.”


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

El Niño Southern Oscillation (ENSO) - NOAA Climate Prediction Center

"El Niño and La Niña-Related Winter Features Over North America" - NOAA Climate Prediction Center

"What are El Niño and La Niña?" - NOAA National Ocean Service

"How El Niño and La Niña affect the winter jet stream and U.S. climate" - NOAA El Niño and La Niña Page

List of ENSO Indices - National Weather Service Fort Worth/Dallas, TX

Oceanic Niño Index - Climate Prediction Center of the National Weather Service

Multivariate ENSO Index (MEI) - NOAA Physical Sciences Laboratory

References for the Arctic Oscillation

Arctic Oscillation and Climate Variability -

All About Arctic Climatology and Meteorology - National Snow & Ice Data Center

Graph of the Arctic Oscillation Index (3-month running mean) - Climate Prediction Center of the National Weather Service

Daily Arctic Oscillation Index for recent months - Climate Prediction Center of the National Weather Service

References for the Pacific Decadal Oscillation

Pacific Decadal Oscillation (PDO) - NOAA National Centers for Environmental Information

References for the Pacific North American Pattern (PNA)

Climate Variability: Pacific - North American Teleconnection Pattern - LuAnn Dahlman, NOAA

Pacific/North American Pattern - Climate Prediction Center of the National Weather Service

Pacific/North American Pattern - North Carolina Climate Office

Global Patterns: Pacific/North American - North Carolina Climate Office

References for the North Atlantic Oscillation (NAO)

Climate Variability: North Atlantic Oscillation - LuAnn Dahlman, NOAA

North Atlantic Oscillation - Climate Prediction Center of the National Weather Service

Global Patterns: Arctic & North Atlantic Oscillations - North Carolina Climate Office

References for Impacts on Snow Season

How El Niño and La Niña affect the winter jet stream and U.S. climate - Rebecca Lindsey,

The Arctic Oscillation, winter storms, and sea ice - National Snow, Ice, and Data Center

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.

References for the Atlantic Multidecadal Oscillation (AMO)

Atlantic Multidecadal Oscillation (AMO) - NOAA Atlantic Oceanographic and Meteorological Laboratory (AOML)

Snowfall Data for Duluth, MN

National Weather Service Forecast Office in Duluth, MN