January 18, 2018

Arctic Oscillation (AO)The Arctic Oscillation (AO) is a climate index of the state of the atmospheric circulation over the Arctic. It consists of a positive phase, featuring below average geopotential heights , which are also referred to as negative geopotential height anomalies , and a negative phase in which the opposite is true. In the negative phase, the polar low pressure system (also known as the polar vortex) over the Arctic is weaker, which results in weaker upper level winds (the westerlies). The result of the weaker westerlies is that cold, Arctic air is able to push farther south into the U.S., while the storm track also remains farther south. The opposite is true when the AO is positive: the polar circulation is stronger which forces cold air and storms to remain farther north. The Arctic Oscillation often shares phase with the North Atlantic Oscillation (NAO) (discussed below), and its phases directly correlate with the phases of the NAO concerning implications on weather across the U.S.
AO PositiveAO Negative

Images in right column From: National Geographic Magazine, March 2000; Sources: Doug Martinson, Wieslaw Maslowski, David Thompson, and John M. Wallace

North Atlantic Oscillation (NAO)

The North Atlantic Oscillation (NAO) consists of two pressure centers in the North Atlantic: one is an area of low pressure typically located near Iceland, and the other an area of high pressure over the Azores (an island chain located in the eastern Atlantic Ocean). It is important to note that these two locations are most commonly used to measure the NAO, but studies have found that the pressure centers move around on a seasonal basis, and other locations have also been used for measuring this index. Fluctuations in the strength of these features significantly alters the alignment of the jet stream, especially over the eastern U.S., and ultimately affects temperature and precipitation distributions in this area. It is also important to note that the AO and NAO are two separate indices that are ultimately describing the same phenomenon of varying pressure gradients in the northern latitudes and the resultant effects on temperature and storm tracks across the continent.

Positive NAO

NAO_POS_PressureDuring a positive NAO there is a strengthening of the Icelandic low and Azores high. This strengthening results in an increased pressure gradient over the North Atlantic, which cause the westerlies to increase in strength. The increased westerlies allow cold air to drain off the North American continent rather than letting it build up and move south.

  • Above average geopotential heights are observed over the eastern U.S., which correlates to above average temperatures
  • The eastern U.S. often sees a wetter pattern with stronger storms during the winter season in this phase due to increased upper level winds
  • Recent studies at the SCO indicate a decreased potential for wintry weather in NC due to the lack of cold air availability and above average temperatures associated with a positive NAO in this region

 500mb positive NAO

Negative NAO

NAO_POS_PressureA negative NAO indicates weakening of both the Icelandic low and Azores high, which decreases the pressure gradient across the North Atlantic. This decreased pressure gradient results in a slackening of the westerlies. The decrease in the westerlies allows cold air to build up over Canada, and this combined with below average heights (troughing) over the eastern U.S. gives the cold air a greater chance to move south and affect the eastern United States.

  • Below average geopotential heights are often observed over the eastern U.S. during the negative phase of the NAO, which correlates to below average temperatures
  • The eastern U.S. typically receives colder, drier air masses during the winter season in this phase
  • Recent studies at the SCO indicate an increased potential for wintry weather in NC due to the position and availability of cold air, and a more favorable upper level pattern conducive to coastal storm tracks

 

500mb Height Anomalies During a Positive NAO Surface Temperature Anomalies During a Positive NAO

A stronger Icelandic low and Azores high translates into lower than average 500mb heights near Iceland, and above average heights near the Azores Islands. In turn, above normal temperatures are seen over the eastern U.S. as the stronger westerlies transport colder air away from North America.

 

When the Icelandic low and Azores high are weaker, above average 500mb heights are located near Iceland, and below average heights near the Azores Islands. This results in a phenomenon referred to as high latitude blocking, which allows cold air to drain from Alaska and Canada into the U.S. and become entrenched, resulting in cold air outbreaks and below normal temperatures.

500mb Height Anomalies During a Negative NAO Surface Temperature Anomalies During a Negative NAO

A. Positional Effects

Favorable positioning of the geopotential height anomalies within the negative phase of the NAO appears to enhance snowfall potentialin the eastern U.S. When negative NAO anomalies are positioned such that an area of high pressure is located near Greenland and a polar vortex (area of low pressure) is somewhere near 50°N 50°W, we find an increased potential for winter weather along the U.S. east coast.

  • Atmospheric features in a “west-based” negative NAO disrupt the polar jet stream, causing it to buckle and move southward into the Southeast
  • Allows cold, Arctic air to be transported south and increases the likelihood of interaction between the northern and southern jet streams
  • Jet stream interactions often result in a combining of energy (phasing), which leads to rapid, intense surface cyclogenesis over the southern U.S.

Current AO & NAO Conditions

Sources and Additional Information

AO: http://nsidc.org/arcticmet/patterns/arctic_oscillation.html

NAO: http://www.cpc.noaa.gov/data/teledoc/nao.shtml

NAO: http://www.cep.rutgers.edu/~oman/NAO.htm

 

el-nino-la-ninaThe El Niño-Southern Oscillation (ENSO) is a naturally occurring phenomenon that involves fluctuating ocean temperatures in the equatorial Pacific. The warmer waters essentially slosh, or oscillate, back and forth across the Pacific, much like water in a bath tub. For North America and much of the globe, the phenomenon is known as a dominant force causing variations in regional climate patterns. The pattern generally fluctuates between two states: warmer than normal central and eastern equatorial Pacific SSTs (El Niño) and cooler than normal central and eastern equatorial Pacific SSTs (La Niña).

Often, sea surface temperatures (SSTs) are used to identify this oscillation, but it is important to understand that changes in sub-surface ocean temperatures are the first to respond to an oncoming change in the ENSO phase. For instance, when ENSO is transitioning into a warm phase the sub-surface temperatures begin to warm above average, while a shallow layer of near average temperature remains at the surface. Eventually, the surface ocean temperatures will respond to the warming of the sub-surface temperatures, and a warm phase of the ENSO cycle ensues. The same cycle occurs, only opposite, for the cool phase of ENSO. When temperatures in the ENSO region of the Pacific are near average it is known as ENSO neutral, meaning that the oscillation is neither in a warm nor cool phase. Typically, atmospheric patterns during ENSO neutral are controlled more by other climate patterns (NAO, PNA) that vary on shorter timescales; these are examined on the following pages.

El Niño (Warm Phase)

1997_Nino_SSTThe warm phase of the ENSO cycle features warmer than normal SSTs across the central and eastern equatorial Pacific

along with:

  • Weaker low-level atmospheric winds along the equator
  • Enhanced convection across the entire equatorial Pacific
  • Effects are strongest during northern hemisphere winter due to the fact that ocean temperatures worldwide are at their warmest. This increased ocean warmth enhances convection, which then alters the jet stream such that it becomes more active over parts of the U.S. during El Niño winters. This results in enhanced precipitation across the southern U.S., including NC
  • In the southeast, winter temperatures are often cooler than normal
  • During hurricane season (June to November), the jet stream is aligned in such a way that the vertical wind shear is increased over the Caribbean and Atlantic. The increased wind shear helps to prevent tropical disturbances from developing into hurricanes

 

Typical El Nino Effects

La Niña (Cool Phase)

1988_Nina_SSTThis phase of the ENSO cycle features cooler than normal SSTs across the central and eastern equatorial Pacific along with:

  • Stronger low-level atmospheric winds along the equator
  • Decreased convection across the entire equatorial Pacific results in a more suppressed southern jet stream. Consequently, the southern U.S., including NC, sees less precipitation
  • In the U.S., winter temperatures are often warmer than normal in the southeast, and cooler than normal in the Northwest
  • During hurricane season (June to November), upper level winds are much lighter, and therefore more favorable for hurricane development in the Caribbean and Atlantic

Typical La Nina Effects

 

The State Climate Office has also studied the relationship between severe weather, such as tornadoes, and El Niño / La Niña. In North Carolina, it appears that there are more tornadoes during La Niña events.

Current ENSO Conditions

Sources and Additional Information

ENSO FAQ

http://www.cpc.noaa.gov/products/analysis_monitoring/impacts/warm_impacts.shtml

The Pacific Decadal Oscillation (PDO) is a pattern of Pacific climate variability similar to ENSO in character, but which varies over a much longer time scale. The PDO can remain in the same phase for 20 to 30 years, while ENSO cycles typically only last 6 to 18 months. The PDO, like ENSO, consists of a warm and cool phase which alters upper level atmospheric winds. Shifts in the PDO phase can have significant implications for global climate, affecting Pacific and Atlantic hurricane activity, droughts and flooding around the Pacific basin, the productivity of marine ecosystems, and global land temperature patterns. Experts also believe the PDO can intensify or diminish the impacts of ENSO according to its phase. If both ENSO and the PDO are in the same phase, it is believed that El Niño/La Nina impacts may be magnified. Conversely, if ENSO and the PDO are out of phase, it has been proposed that they may offset one another, preventing “true” ENSO impacts from occurring.

Researchers have found evidence for just two full PDO cycles in the past century: cold PDO regimes prevailed from 1890-1924 and again from 1947-1976, while warm PDO regimes dominated from 1925-1946 and from 1977 through the mid-1990’s.

Warm PDOThe broad area of above average water temperatures off the coast of North America from Alaska to the equator is a classic feature of the warm phase of the Pacific Decadal Oscillation (PDO). The warm waters wrap in a horseshoe shape around a core of cooler-than-average water. Impacts from the PDO depend in part on how it is aligned with the ENSO cycle; if the cycles are in opposite phases, then effects will be weakened. However, when both the PDO and ENSO are in the warm phase, meaning ENSO would be in the El Niño phase, expected impacts on the southeast include:

  • Below average winter temperatures
  • Above average winter precipitation
Sea Surface Temperature Anomalies During a Warm PDO Phase
Cold PDOOpposite of the warm PDO, the expansive area of below average water temperatures off the coast of North America from Alaska to the equator signals the cold phase of the PDO. The area of warmer-than-average sea surface temperatures in the central Pacific are surrounded by below average temperatures near the North American continent. Expected impacts from a cold PDO and ENSO (La Nina) phase on the southeast include:

  • Above average winter temperatures
  • Below average winter precipitation
Sea Surface Temperature Anomalies During a Cold PDO Phase

Current PDO Conditions

http://www1.ncdc.noaa.gov/pub/data/cmb/teleconnections/pdo-f-pg.gif

Sources and Additional Information

http://jisao.washington.edu/pdo

http://www.john-daly.com/theodor/pdotrend.htm

The Pacific/North American teleconnection pattern (PNA) is one of the most recognized, influential climate patterns in the Northern Hemisphere mid-latitudes beyond the tropics. It consists of anomalies in the geopotential height fields (typically at 700 or 500mb) observed over the western and eastern United States. It is important to note that the PNA has been found to be strongly influenced by the El Niño-Southern Oscillation (ENSO) phenomenon. The positive phase of the PNA pattern tends to be associated with Pacific warm episodes (El Niño), and the negative phase tends to be associated with Pacific cold episodes (La Niña).
Positive PNAThe positive phase consists of above normal geopotential heights over the western U.S. and below normal geopotential heights over the eastern U.S. This correlates to ridging over the western U.S., and deep troughing over the east. The net result of the height field pattern in this phase is that it forces cold air residing in Canada to plunge southeastward, which results in below normal temperatures over the eastern U.S. and above normal temperatures over the western U.S.

  • Research at the SCO indicates that a positive PNA, especially during an El Niño year, produces an above average number of winter weather events in NC

Positive PNA - Height and Temperature

In the positive phase, above average geopotential heights are seen over the western U.S., and below average geopotential heights are seen across the eastern U.S. This results in warm air moving much farther north than normal over the western U.S., while cold, Canadian air is forced southward over the eastern U.S. resulting in below normal temperatures.

Negative PNAThe negative phase features troughing and below normal geopotential heights over the western U.S. and ridging with above normal geopotential heights over the eastern U.S. The result is below average temperatures for the western U.S., and above average temperatures over the eastern U.S.

  • Research at the SCO indicates that a negative PNA typically results in a reduced potential for winterweather in NC

Negative PNA - Height and Temperature

The negative phase of the PNA pattern features below average geopotential heights over the western U.S., and above average geopotential heights across the eastern U.S. This results in deep troughing over the western U.S., which allows cold air from western Canada to drain southward into this region. In the eastern U.S., warm, moist air from the Gulf of Mexico and the Atlantic Ocean is able to travel northward, often resulting in above normal temperatures and more humid conditions.

Current PNA Conditions

Sources and Additional Information

http://www.cpc.noaa.gov/data/teledoc/pna.shtml