North Atlantic Oscillation

The North Atlantic Oscillation Index is based on the surface sea-level pressure difference between the Subtropical (Azores) High and the Subpolar Low. The positive phase of the NAO reflects below-normal heights and pressure across the high latitudes of the North Atlantic and above-normal heights and pressure over the central North Atlantic, the eastern United States and western Europe. The negative phase reflects an opposite pattern of height and pressure anomalies over these regions. Both phases of the NAO are associated with basin-wide changes in the intensity and location of the North Atlantic jet stream and storm track, and in large-scale modulations of the normal patterns of zonal and meridional heat and moisture transport, which in turn results in changes in temperature and precipitation patterns often extending from eastern North America to western and central Europe.

Strong positive phases of the NAO tend to be associated with above-normal temperatures in the eastern United States and across northern Europe and below-normal temperatures in Greenland and oftentimes across southern Europe and the Middle East. They are also associated with above-normal precipitation over northern Europe and Scandinavia and below-normal precipitation over southern and central Europe. Opposite patterns of temperature and precipitation anomalies are typically observed during strong negative phases of the NAO. During particularly prolonged periods dominated by one particular phase of the NAO, abnormal height and temperature patterns are also often seen extending well into central Russia and north-central Siberia. The NAO exhibits considerable interseasonal and interannual variability, and prolonged periods (several months) of both positive and negative phases of the pattern are common.

The NAO index is obtained by projecting the NAO loading pattern to the daily anomaly 500 millibar height field over 0-90°N. The NAO loading pattern has been chosen as the first mode of a Rotated Empirical Orthogonal Function (EOF) analysis using monthly mean 500 millibar height anomaly data from 1950 to 2000 over 0-90°N latitude.

(For more information on this index, please refer to the Climate Prediction Center).

El Niño / La Niña

El Niño is a phenomenon in the equatorial Pacific Ocean characterized by an average positive sea surface temperature departure from normal (for the 1971-2000 base period) greater than or equal in magnitude to 0.5C, for three months or more, in a region defined by 120°W-170°W and 5°N-5°S (commonly referred to as Niño 3.4). Conversely, La Niña is a phenomenon in the equatorial Pacific Ocean characterized by an average negative sea surface temperature departure from normal (for the 1971-2000 base period) greater than or equal in magnitude to 0.5°C, for three months or more, in the same region.

The onset of El Niño occurs when the 3-month running average rises above +0.5°C in the Niño 3.4 region. Conversely, La Niña begins when the 3-month running average falls below -0.5°C.

Sea surface temperature anomalies were calculated using the reconstructed sea surface temperature data set developed by Dick Reynolds and Tom Smith at the National Climatic Data Center.

DISCUSSION:

Historically, scientists have classified the intensity of El Niño based on sea surface temperature (SST) anomalies exceeding a pre-selected threshold in a certain region of the equatorial Pacific. The most commonly used region is the Niño 3.4 region (120°W-170°W, 5°N-5°S), and the most commonly used threshold is a positive SST departure from normal greater than or equal to +0.5°C. Since this region encompasses the western half of the equatorial cold tongue region, it provides a good measure of important changes in SST and SST gradients that result in changes in the pattern of deep tropical convection and atmospheric circulation. The criteria, that is often used to classify El Niño episodes, is that five-month running mean SST anomalies exceed the threshold for at least five months. Since it is recognized that El Niño-related impacts may develop over periods as short as a season, more recently it has been suggested that a three-month average be used and that the duration correspondingly be shortened to a season.

Studies have shown that a necessary condition for the development and persistence of deep convection (enhanced cloudiness and precipitation) in the Tropics is that the local SST be 28°C or greater. Once the pattern of deep convection has been altered due to anomalous SSTs, the tropical and subtropical atmospheric circulation adjusts to the new pattern of tropical heating, resulting in anomalous patterns of precipitation and temperature that extend well beyond the region of the equatorial Pacific. A SST anomaly of +0.5°C in the Niño 3.4 region is sufficient to reach this threshold from late March to mid-June. During the remainder of the year a larger SST anomaly, up to +1.5°C in November-December-January, is required in order to reach the threshold to support persistent deep convection in that region.

SST values in the Niño 3.4 region may not be the best choice for determining La Niña episodes but, for consistency, the index has been defined by negative anomalies in this area. A better choice might be the Niño 4 region (160°E-150°W, 5°N-5°S), since that region normally has SSTs at or above the threshold for deep convection throughout the year. A SST anomaly of –0.5°C in that region would be sufficient to bring water temperatures below the 28°C threshold, which would result in a significant westward shift in the pattern of deep convection in the tropical Pacific.

(The above discussion provided by Vernon Kousky at the Climate Prediction Center. For more information on El Niño and La Niña, please see NOAA's El Niño information website.)

Arctic Oscillation

The Arctic Oscillation is a large scale mode of climate variability, also referred to as the Northern Hemisphere annular mode. The Arctic Oscillation is a climate pattern characterized by winds circulating counterclockwise around the Arctic at around 55 degrees north latitude. When the AO is in its positive phase, a ring of strong winds circulating around the North Pole acts to confine colder air across polar regions. This belt of winds becomes weaker and more distorted in the negative phase of the AO, which allows an easier southward penetration of colder, arctic airmasses and increased storminess into the mid-latitudes.

AO index is obtained by projecting the AO loading pattern to the daily anomaly 1000 millibar height field over 20°N-90°N latitude. The AO loading pattern has been chosen as the first mode of EOF analysis using monthly mean 1000 millibar height anomaly data from 1979 to 2000 over 20°N-90°N.

(For more information on this index, please refer to the Climate Prediction Center).

Pacific-North America Index

The PNA pattern is one of the most prominent modes of low-frequency variability in the Northern Hemisphere extratropics, appearing in all months except June and July. The PNA pattern reflects a quadripole pattern of 500 millibar height anomalies, with anomalies of similar sign located south of the Aleutian Islands and over the southeastern United States. Anomalies with sign opposite to the Aleutian center are located in the vicinity of Hawaii, and over the intermountain region of North America (central Canada) during the Winter and Fall (Spring).

The spatial scale of the PNA pattern is most expansive in winter. During this period, the Aleutian center spans most of the northern latitudes of the North Pacific. In Spring, the Aleutian center contracts and becomes confined primarily to the Gulf of Alaska. However, the subtropical center near Hawaii reaches maximum amplitude during the spring. The PNA pattern then disappears during June and July, but reappears in the late summer and fall. During this period, the midlatitude centers become dominant and appear as a wave pattern emanating from the eastern North Pacific. The subtropical center near Hawaii is weakest during this period.

The PNA index is obtained by projecting the PNA loading pattern to the daily anomaly 500 millibar height field over 0-90°N. The PNA loading pattern has been chosen as the second mode of a Rotated EOF analysis using monthly mean 500 millibar height anomaly data from 1950 to 2000 over 0-90°N latitude.

(For more information on this index, please refer to the Climate Prediction Center).

Southern Oscillation Index

The Southern Oscillation Index is a standardized index based on the observed sea level pressure differences between Darwin and Tahiti. The Southern Oscillation Index (SOI) is one measure of the large-scale fluctuations in air pressure occurring between the western and eastern tropical Pacific (i.e., the state of the Southern Oscillation) during El Niño and La Niña episodes. In general, smoothed time series of the SOI correspond very well with changes in ocean temperatures across the eastern tropical Pacific. The negative phase of the SOI represents below-normal air pressure at Tahiti and above-normal air pressure at Darwin. Prolonged periods of negative SOI values coincide with abnormally warm ocean waters across the eastern tropical Pacific typical of El Niño episodes. Prolonged periods of positive SOI values coincide with abnormally cold ocean waters across the eastern tropical Pacific typical of La Niña episodes. The methodology used to calculate SOI can be found here.

(For more information on this index, please refer to the Climate Prediction Center).

Outgoing Longwave Radiation

Outgoing Longwave Radiation (OLR) data at the top of the atmosphere are observed from the AVHRR instrument aboard the NOAA polar orbiting spacecraft. Data are centered across equatorial areas from 160° east to 160° west longitude. The raw data are converted into a standardized anomaly index. Positive values of OLR are indicative of suppressed convection, while negative values suggest enhanced convective activity. More convective activity in the central and eastern equatorial Pacific implies higher, colder cloud tops, which emit much less infrared radiation into space.

(For more information on this index, please refer to the Climate Prediction Center).

Pacific Decadal Oscillation

The Pacific Decadal Oscillation, or PDO, is often described as a long-lived El Niño-like pattern of Pacific climate variability (Zhang et al. 1997). As seen with the better-known El Niño/Southern Oscillation (ENSO), extremes in the PDO pattern are marked by widespread variations in Pacific Basin and North American climate. In parallel with the ENSO phenomenon, the extreme phases of the PDO have been classified as being either warm or cool, as defined by ocean temperature anomalies in the northeast and tropical Pacific Ocean. When sea surface temperatures (SST) are anomalously cool in the interior North Pacific and warm along the Pacific Coast, and when sea level pressures are below average over the North Pacific, the PDO has a positive value. When the climate anomaly patterns are reversed, with warm SST anomalies in the interior and cool SST anomalies along the North American coast, or above average sea level pressures over the North Pacific, the PDO has a negative value. (Courtesy Mantua, 1999).

The NCDC PDO index is based on NOAA's extended reconstruction of sea-surface temperatures (ERSST Version2). It is constructed by regressing the ERSST anomalies against the Mantua PDO index for their overlap period, to compute a PDO regression map for the North Pacific ERSST anomalies. The ERSST anomalies are then projected onto that map to compute the NCDC index. The NCDC PDO index closely follows the Mantua PDO index.

(For more information on this index, please refer to Dr. Nathan Mantua's PDO page.)