Impacts of low frequency variability modes on Canadian winter temperature

The impacts of El Nino-Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the North Atlantic Oscillation (NAO) on winter (January-February-March) temperature variability over Canada are analysed for the period 1900-1995. Through linear regression, regional characteristics in the interannual temperature variability explained by these oscillations are firstly identified. Modulation of El Nino and La Nina winter temperature responses by various combinations of the PDO and NAO is then investigated via composite analysis. Results show the NAO as the dominant low frequency variability mode affecting winter temperature, however, the effects are mainly confined to north-eastern regions of the country. The ENSO and PDO influences are somewhat weaker and occur over western and central Canada. It is determined that the associated winter PDO pattern has a significant modulating effect on ENSO related temperature responses. Impacts are stronger and more spatially coherent during El Nino episodes when positive PDO winters are associated with strong positive temperature anomalies over most of Canada, neutral PDO with weaker anomalies (positive in the west and negative in the east), and negative PDO with strong negative anomalies over western Canada. Analysis also suggests that La Nina and negative PDO combinations are associated with negative temperature anomalies, especially in the far west. Over eastern regions of Canada, El Nino (La Nina) events modulate the typical positive (negative) NAO temperature responses by generally making them warmer (colder). All observed relationships are explained by variations in associated mid-tropospheric circulation over the north Pacific (i.e. Aleutian low region), and western and central North America. Results from this investigation aid in the understanding of relationships between low frequency oscillations and winter temperature variability over Canada. They also suggest the possibility of improved winter temperature forecasts based on conditions in the tropical and north Pacific, and the north Atlantic. Copyright (C) 2001 Royal Meteorological Society.