“Modes of
climate variability” are distinct and robust features of variability in the
climate system with identifiable characteristics, affecting particular regions
over certain time periods. Generally, these features alternate or “oscillate”
between one set of patterns and an alternate set. A familiar example is the El
Niño–Southern Oscillation (ENSO), but there are other modes of variability also
discussed in this report.
El Niño–Southern Oscillation and
Indian Ocean Dipole
ENSO is a
quasi-periodic variation in sea surface temperature and other related
variables, such as surface pressure and surface wind, lasting about three to
five years and situated mainly over the tropical eastern Pacific Ocean. ENSO
affects much of the tropics and subtropics, but also influences the
mid-latitudes of both hemispheres, including Canada. The warm phase of ENSO is
known as El Niño (warm waters in the tropical eastern Pacific Ocean) and the
cool phase as La Niña (cool waters in the tropical eastern Pacific Ocean). The
warm phase tends to be associated with warmer winter air temperatures and drier
conditions over much of Canada. The opposite is true during La Niña. Related to
ENSO is the Indian Ocean Dipole (IOD), a variation in sea surface temperature
centred in the Indian Ocean, with a typical timescale of about two years.
Pacific Decadal Oscillation and
Interdecadal Pacific Oscillation
The Pacific
Decadal Oscillation (PDO) is a recurring pattern of sea surface temperature
variability centred over the northern mid-latitude Pacific Ocean. The PDO has
varied irregularly, with a characteristic timescale ranging from as short as a
few years to as long as several decades. As with ENSO, the warm (positive)
phase of the PDO tends to be associated with warmer winter air temperatures
over much of Canada (Shabbar and Yu, 2012). At times over the past century,
this mode of variability has exerted a strong influence on continental surface
air temperature and precipitation, from California to Alaska. The Interdecadal
Pacific Oscillation (IPO) is related to the PDO, but with a much wider
geographic range of influence (Salinger et al., 2001).
Arctic Oscillation and North Atlantic
Oscillation
The Arctic
Oscillation (AO), sometimes referred to as the Northern Annular Mode, is the
dominant pattern of variability of sea level pressure and atmospheric pressure
north of about 20º north latitude. If the pressures are high over the Arctic,
they are low over the mid-latitudes, and vice versa. The AO varies over time,
with no particular periodicity. The positive phase of the AO tends to be associated
in winter with warmer air temperatures over western Canada, and colder
temperatures in the north and east. The North Atlantic Oscillation (NAO) is
related to the AO but is centred over the North Atlantic Ocean rather than the
whole of the Northern Hemisphere. The NAO has a strong influence on the
strength and direction of westerly winds and the location of the storm track
over the North Atlantic Ocean. The positive phase of the NAO is also associated
with warm winter temperatures over much of western Canada, and cold winter
temperatures over eastern Canada.
Atlantic Multi-decadal Oscillation
The Atlantic
Multi-decadal Oscillation (AMO) is a recurring pattern of sea surface
temperature of the North Atlantic Ocean (north of the equator and south of
about 80º north latitude), with a characteristic timescale of 60 to 80 years.
The AMO has been known to influence hurricane activity, as well as rainfall
patterns and intensity, across the North Atlantic Ocean.
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