The El Niño-Southern Oscillation (ENSO) is a complex phenomenon involving interactions between the atmosphere and the ocean in the tropical Pacific Ocean. It has global impact, variously causing drought and floods, mainly in the tropics and to a lesser extent in the mid-latitudes. The ENSO has been shown to significantly influence the rainfall in the tropics from Africa to Australia as well as the subtropics of North American. Temperature is also influenced in many parts of the globe.

The atmospheric component known as the Southern Oscillation, has been described as a seesaw of atmospheric pressure between the eastern equatorial Pacific and the Indo-Australian areas (Glantz et al, 1991). The Southern Oscillation Index (SOI) is used to characterize the strength of this atmospheric phenomenon. It is based on the monthly average of the surface pressure differences between Tahiti and Darwin.

The Oceanic component consists of a periodic displacement of the normally cold water that upwells along the coast of South America and across the Pacific in a cold tongue due to the tropical easterlies. When the easterlies relax, upwelling ceases and warm water moves in from the west, extending across the equatorial Pacific. It is known as an El Niño. When Sea Surface Temperatures (SST) conditions in the tropical central Pacific are anomalously cold, it is know as a La Niña.

A key link to this periodic oscillation of surface pressure and sea surface temperature patterns lies in the normally present tropical easterly winds. When they weaken in the eastern tropical Pacific Ocean, the upwelling of cold water ceases and warm surface waters spread across the Pacific Ocean. The underlying reason for these interannual variations in the strength of the Easterlies is not yet understood; however, progress is being made in modelling the atmospheric circulation and reproducing the phenomenon.

Significant ENSO events occur periodically every two to seven years and vary in strength. The relative strength is measured by the two key indices: the SOI and SST anomalies. Other indicators of the state of the ENSO phase are the strength of tropical easterly winds and the location of widespread tropical convection. Observations of these parameters can yield useful insights into upcoming seasonal climate conditions in Canada and around the globe.

The ENSO event is one part of the predictable component of seasonal variability. It is believed that about 30-40% of the total variations in the seasonal climate can be predicted. The balance of the seasonal climatic variations are due to other not-well-researched forcing phenomenon and a remaining large part is due to the chaotic nature of the atmosphere.

While a full understanding of the physical processes linking ENSO events and regional climates is not yet established, there are fairly robust time-dependent statistical correlations among the tropical SST anomalies, the state of the ENSO and the anomalies of temperature and precipitation in Western Canada.

Shabbar (1996) has investigated correlations involving temperature. Linkages to other regional climates have been described by numerous research (Ropelewski and Halpert; Shea and Madden). Their collective works underlie the development of statistical climate prediction models and the Analogue Neural Network models developed by CICS. The Neural Network models simulate seasonal temperature and precipitation anomalies in Western Canada by reproducing the observed time lag (of up to several months) between tropical phenomenon and the response of the Western Canadian seasonal climate.

Climate Response to ENSO

It is well documented that the cold season climate of Western Canada tends to be warmer and drier during El Nino's and colder and wetter during La Nina's.

Figure 1 on the following page shows the departures from normal, computed from selected El Nino analogues of the current ENSO phase for the Southern Prairies. The chart begins with month 8 (August 1997) and ends with month 20 (September 1998) Note that the solid "middle" line is the median of all analogues selected for three month averages (e.g. Winter is defined as (December-January-February). The upper and lower solid lines represent the envelope of extreme observed temperatures during "El Nino"s which occur monthly. This serves to illustrate that during "El Nino"s, cold departures from normal months are not infrequently.

Figure 1: Southern Prairie Provinces temperature departures from normal due to El Nino.

Figure 2: BC Peace River and Central Alberta precipitation departures from normal during El Nino.

It can be seen that there is a discernible climatological response to "El Nino"s. Clearly, each El Nino creates a different response and in applying the Analogue Neural Network approach described below, CICS endeavours to differentiate between them and provide added value for business decision makers.

The dashed line on each of the two Figures at the left represent the seasonal predictions of temperature and precipitation. The difference from the median line indicates how this El Nino will cause a different climatic response from "average El Nino" events.

Click on the following link for more on how CICS uses the ENSO signal to produce seasonal climate predictions.

This information is reproduced from an address given by Rick Lee, Manager of Climate Applications to the Canadian Dehydrators Association, October, 1997.