Modes of climate variability

 

“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.