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
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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 oceanatmosphere 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.