Teleconnections

• Understanding the nature of teleconnections and changes in their behaviour is central to understanding regional climate change.

• In the NH, one-point correlation maps illustrate the Pacific-North American (PNA) pattern and the NAO, but in the SH, wave structures do not emerge as readily owing to the dominance of the SAM.

• Although teleconnections are best defined over a grid, simple indices based on a few key station locations remain attractive as the series can often be carried back in time long before complete gridded fields were available; the disadvantage is increased noise from the reduced spatial sampling.

Defining the Circulation Indices

• Southern Oscillation Index (SOI). The MSLP anomaly difference of Tahiti minus Darwin,

• North Atlantic Oscillation (NAO) Index. The difference of normalised MSLP anomalies between Lisbon, Portugal and Stykkisholmur,

• Northern Annular Mode (NAM) Index. The amplitude of the pattern defined by the leading empirical orthogonal function of winter monthly mean NH MSLP anomalies poleward of 20°N. The NAM has also been known as the Arctic Oscillation (AO), and is closely related to the NAO.

• Southern Annular Mode (SAM) Index. The difference in average MSLP between SH middle and high latitudes (usually 45°S and 65°S).

• Pacific-North American pattern (PNA) Index. The mean of normalised 500 hPa height anomalies at 20°N, 160°W and 55°N, 115°W minus those at 45°N, 165°W and 30°N, 85°W (Wallace and Gutzler, 1981).

• Pacific Decadal Oscillation (PDO) Index . defined as the pattern and time series of the first empirical orthogonal function of SST over the North Pacific north of 20°N

• North Pacific Index (NPI). The NPI is the average MSLP anomaly in the Aleutian Low over the Gulf of Alaska (30°N–65°N, 160°E–140°W)

Teleconnections

• Quadrelli and Wallace (2004) found that many patterns of NH interannual variability can be reconstructed as linear combinations of the first two Empirical Orthogonal Functions (EOFs) of sea level pressure (approximately the NAM and the PNA).

• Teleconnection patterns tend to be most prominent in the winter (especially in the NH), when the mean circulation is strongest.

El Niño-Southern Oscillation

• El Niño involves warming of tropical Pacific surface waters from near the International Date Line to the west coast of South America, weakening the usually strong SST gradient across the equatorial Pacific, with associated changes in ocean circulation. Its closely linked atmospheric counterpart, the Southern Oscillation (SO), involves changes in trade winds, tropical circulation and precipitation.

• The El Niño-Southern Oscillation has global impacts, manifested most strongly in the northern winter months (November–March).

El Niño-Southern Oscillation

El Niño-Southern Oscillation

• Strong ENSO events occurred from the late 19th century through the first 25 years of the 20th century and again after about 1950, but there were few events of note from 1925 to 1950 with the exception of the major 1939–1941 event.

• The 1976–1977 climate shift was associated with marked changes in El Niño evolution, a shift to generally above-normal SSTs in the eastern and central equatorial Pacific and a tendency towards more prolonged and stronger El Niños.

• Such decadal atmospheric and oceanic variations are more pronounced in the North Pacific and across North America than in the tropics but are also present in the South Pacific, with evidence suggesting they are at least in part forced from the tropics

Tropical-Extratropical Teleconnections: PNA and PSA

• The PNA is associated with modulation of the Aleutian Low, the Asian jet, and the Pacific storm track, affecting precipitation in western North America and the frequency of Alaskan blocking events and associated cold air outbreaks over the western USA in winter.

• The PSA is associated with modulation of the westerlies over the South Pacific, effects of which include significant rainfall variations over New Zealand, changes in the nature and frequency of blocking events across the high-latitude South Pacific, and interannual variations in antarctic sea ice across the Pacific and Atlantic sectors .

Atlantic Multi-decadal Oscillation

• Instrumental observations capture only two full cycles of the AMO, so the robustness of the signal has been addressed using proxies.

• The revised AMO index indicates that North Atlantic SSTs have recently been about 0.3°C warmer than during 1970 to 1990, emphasizing the role of the AMO in suppressing tropical storm activity during that period.

Atlantic Multi-decadal Oscillation

• The AMO is likely to be a driver of multi-decadal variations in Sahel droughts, precipitation in the Caribbean, summer climate of both North America and Europe, sea ice concentration in the Greenland Sea and sea level pressure over the southern USA, the North Atlantic and southern Europe

• Walter and Graf (2002) identified a non-stationary relationship between the NAO and the AMO. – During the negative phase of the AMO, the North Atlantic SST is

strongly correlated with the NAO index. – NAO index is only weakly correlated with the North Atlantic SST

during the AMO positive phase.

Antarctic Circumpolar Wave

• The Antarctic Circumpolar Wave (ACW) is described as a pattern of variability with an approximately four-year period in the southern high-latitude ocean-atmosphere system, characterised by the eastward propagation of anomalies in antarctic sea ice extent, and coupled to anomalies in SST, sea surface height, MSLP and wind

Indian Ocean Dipole

• Indian Ocean Dipole (IOD), also referred to as the Indian Ocean Zonal Mode .This pattern manifests through a zonal gradient of tropical SST, which in one extreme phase in boreal autumn shows cooling off Sumatra and warming off Somalia in the west, combined with anomalous easterlies along the equator.

• The magnitude of the secondary rainfall maximum from October to December in East Africa is strongly correlated with positive IOD events .

• Several recent IOD events have occurred simultaneously with ENSO events and there is a significant debate on whether the IOD is an Indian Ocean pattern or whether it is triggered by ENSO in the Pacific Ocean

Indian Ocean Dipole

• Indian Ocean SSTs tend to rise about five months after the peak of ENSO in the Pacific. Monsoon variability and the SAM are also likely to play a role in triggering or intensifying IOD events.

• One argument for an independent IOD was the large episode in 1961 when no ENSO event occurred. Analysing observations from 1958 to 1997, concluded that 11 out of the 19 episodes identified as moderate to strong IOD events occurred independently of ENSO.