Climate Patterns Introduction

Climate Patterns as discussed here are atmospheric and oceanic oscillations meaning they switch back and forth between two conditions such as higher and lower pressure or warmer and colder water. The patterns affect the weather over large areas of the globe including locations far away from where they occur. The relation between any of these patterns and the resulting weather 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 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 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 exist with the positive phase. A weak pressure gradient and weaker polar vortex circulation exist 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 phases every several weeks to several months but one or the other can dominate 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. 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.


Other Climate Patterns

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

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 the 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 but the tendency is to be drier.

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 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 fewer 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 snowfall seasons and low snowfall seasons for Duluth, MN in relation to El Niño and La Niña

Since the year 2001, four snow seasons in Duluth, MN have exceeded 100 inches as recorded by the National Weather Service. They are 2003-2004, 2012-2013, 2013-2014, and 2018-2019. Two snow seasons with particularly low snow totals were 2011-2012 and 2014-2015 which only reached near 50 inches. Based on examination of the Oceanic Niño Index the corresponding El Niño and La Niña conditions are listed.

          High Snow Seasons
2003-2004   Near Neutral
2012-2013   Near Neutral
2013-2014   Near Neutral
2018-2019   Weak El Niño

          Low Snow Seasons
2011-2012   Moderate La Niña
2014-2015   Weak El Niño

Note that other indexes are also available such as the Multivariate ENSO Index (MEI) which generally supports the assessment of the Oceanic Niño Index.

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. 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 two recent very 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.

Analysis of some other seasons affected by El Niño and La Niña

Winter 2017-2018 was accompanied by a weak to moderate La Niña with 91.9 inches of snowfall officially measured. Most of Minnesota and Wisconsin had a very active winter, especially during the second half of the season. The highlight was a big storm that hit a large chunk of Minnesota and Wisconsin with 10 to 20 inches of snow. Some local amounts of 24 to around 30 inches fell 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 reported. A storm earlier in the season on October 26 to 27 dropped 6 to 12 inches of wet snow in the higher elevations of Duluth and some higher elevated locations south and 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 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 Arctic Oscillation

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

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 Pacific Decadal Oscillation

NASA Earth Observatory

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

References for Pacific North American Pattern (PNA)

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

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 North Atlantic Oscillation (NAO)

Climate Variability: North Atlantic Oscillation - LuAnn Dahlman, NOAA Climate.gov

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, Climate.gov

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.