Introducion

Climate Patterns as discussed here are atmospheric and oceanic oscillations meaning they switch back and forth between two states of condition such as higher and lower pressure or warmer and colder water. The patterns affect weather over large areas of the globe including locations far away from where they occur. The connection between these oscillation events and the weather that results in a different part of the world is called a Teleconnection.


Climate Pattern Definitions

El Niño

El 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 Niña

La Niña is generally the opposite condition 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). For reference, the latitude of Duluth International Airport in Duluth, MN is 46.84 degrees north. The positive phase is defined by lower than mean pressure in the arctic and higher than mean pressure in the mid latitudes. The negative phase is marked by higher than mean pressure in the Arctic and lower than mean pressure in the mid latitudes. A strong pressure gradient and stronger polar vortex circulation exists with the positive phase. A weak pressure gradient and weaker polar vortex circulation exists with the negative phase. The pattern extends through the depth of the troposphere, as noted in the National Snow and Ice Data Center's Cryosphere Glossary, and thus affects the strength and latitudinal position of the jet stream. Generally expect a stronger, more zonal, and more northern Jet with the positive phase. 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 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 affects 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 and Southeast United States is also colder but is 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 affects of the positive and negative phases on Minnesota and Wisconsin since they are an 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 on 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.


Other Climate Patterns

North Atlantic Oscillation (NAO)

See references section below for more information.


Impacts on Snow Season, Especially for Minnesota and Wisconsin

From the start, the point must be made that most depictions of El Niño and La Niña affects on winter climate focus on or within the December to March time period. The snow season for the Upper Mississippi Valley and Western Great Lakes starts BEFORE December and ends AFTER March. 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 correlations to weather are stronger in some other parts of the United States than in Minnesota and Wisconsin. The Southern third of United States, for example has a strong tendency to be wetter than average due to El Niño events and drier than average due to La Niña events. The Pacific Northwest typically experiences the opposite result. Ultimately, the climate patterns will affect the structure and location of jet streams in the middle to upper Troposphere which will in turn determine the location, intensity, and frequency of both cold air outbreaks and tracks of storm systems.

The jet stream due to El Niño frequently has a wide split with a strong southern jet, 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 series of two or more strong storms will move northeast from the Central or Northern Gulf of Mexico into the Southeast United States, then up the Middle Atlantic Coast. Sometimes the northern branch of the jet stream will sink far enough south along the East Coast to supply enough cold air to produce major snow and ice storms in the southeast. In Minnesota and Wisconsin, temperatures are warmer than average. Departures from average precipitation (and snowfall) are less defined.

The jet stream due to La Niña is more consolidated over the United States with a stronger jet stream out of Canada and sometimes from the Pacific Ocean into the Pacific Northwest. For Minnesota and Wisconsin, snowfall potential is increased due to the likely increased number of storms and 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 and example, an image 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 type of situation the affects of La Niña would likely be amplified.

Concerning the Arctic Oscillation, since the positive phase keeps the core of the jet stream dominantly north of the United States, Minnesota, Wisconsin, and the rest of the United States can typically expect fewer storms, less cold air, and less chances for snow. The negative phase results in a more variable and wavy jet that produces more storms and allows arctic air to move farther south into the United States more often.

Some Recent High and Low Snow Total Years for Duluth, MN and Presence of El Niño or La Niña

Since the year 2000, three snow seasons for Duluth, MN have exceeded 100 inches officially at the National Weather Service. They are 2003-2004, 2012-2013, and 2013-2014. Two snow seasons, 2011-2012 and 2014-2015, only reached near 50 inches. Based on examination of the Oceanic Niño Index the corresponding El Niño and La Niña conditions are listed as follows.

          High Snow Total Years
2003-2004   Near Neutral
2012-2013   Near Neutral
2013-2014   Near Neutral

          Low Snow Total Years
2011-2012   Moderate to Strong La Niña
2014-2015   Weak El Niño

Note that other indexes are also available such as the Multivariate ENSO Index (MEI) which also supports the occurrence of the moderate to strong 2011-2012 La Niña

Recent winters exceeding 100 inches of snow in Duluth lacked a well defined El Niño and 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.

Of the recent very low snow seasons, the one that may stand out is Winter 2011-2012 as the moderate La Niña would suggest the odds favored above average snowfall. The Arctic Oscillation, as indicated by a chart of 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 to the north and kept cold air outbreaks to a minimum. For two weeks, end of February and start of March, the weather pattern broke down, allowing more cold air and storms into the region. More than 25+ inches of snow fell in the two week period and accounted for half of the seasons snowfall.

Just for the record, Winter 2017-2018 was accompanied by a weak to moderate La Niña with 91.9 inches of snowfall recorded at the National Weather Service in Duluth. Most of Minnesota and Wisconsin had a very active winter, especially the second half. The highlight was a big storm that dumped a large chunk of Minnesota and Wisconsin with 10 to 20 inches of snow, with local amounts 24 to around 30 inches in Northeast Wisconsin including Green Bay. The snow along the corridor from Minneapolis-St. Paul to Green Bay was convective at times with some thunder occurring. There was also an early season snowstorm on October 26 to 27 that dropped 6 to 12 inches of wet snow in the higher elevations of Duluth also some higher amounts in the higher elevations to the south an east of Superior, WI. Slightly warmer temperatures and more rain mixing with the snow kept accumulations in the lower elevations much less.


References

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

Oceanic Niño Index

El Niño/La Niña Years and Intensities Based on Oceanic Niño Index

Multivariate ENSO Index (MEI)

References for Arctic Oscillation

National Snow & Ice Data Center - Definition from the Cryosphere Glossary

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

a chart of the Arctic Oscillation Index from the National Weather Service's Climate Prediction Center

References for Pacific Decadal Oscillation

NASA Earth Observatory - La Niña and Pacific Decadal Oscillation Cool the Pacific

References for Pacific North American Pattern (PNA)

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

Pacific/North American Pattern Climate Prediction Center

Pacific/North American Pattern North Carolina Climate Office

Global Patterns: Pacific/North American North Carolina Climate Office

References for North Atlantic Oscillation (NAO)

Climate Variability: North Atlantic Oscillation, LuAnn Dahlman, Climate Prediction Center

North Atlantic Oscillation Climate Prediction Center

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

References for Weather Implicaions

El Niño and La Niña Related Weather Features Over North America

How 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.