The impact of El Nino-Southern Oscillation on the temperature field over Canada

The impact of the two phases of El Nino-Southern Oscillation (ENSO), namely EI Nino and La Nina, on the surface and lower tropospheric temperature fields over Canada is documented. Gridded surface temperature data for 91 years (1900-1990) and 500-1000 hPa thickness data for 49 years (1946-1994) have been analyzed statistically in the context of EI Nino, La Nina and normal years. Using a composite analysis, the present study conclusively demonstrates that significant positive surface temperature anomalies spread eastward from the west coast of Canada to the Labrador coast from the late fall to early spring (November through May) following the onset of El Nino episodes. The accompanying temperatures in the lower troposphere show a transition from the Pacific/North American (PNA) pattern to the Tropical/Northern Hemisphere (TNH) pattern over the North American sector during the same period. Conversely, significant negative surface temperature anomalies spread southeastward from the Yukon and extend into the upper Great Lakes region by the winter season following the onset of La Nina episodes. Furthermore, the lower tropospheric temperatures show a negatively-phased PNA-like pattern in early winter which weakens considerably by May of the following year. Thus, while western Canadian surface temperatures are influenced during both phases of ENSO, eastern Canadian surface temperature effects are found during the El Nino phase only. The impact of ENSO on the Canadian surface temperatures is the strongest during the winter season and nearly disappears by spring (April and May). The largest positive (negative) anomalies are found to be centred over two separate regions, one over the Yukon and the other just west of Hudson Bay in the EI Nino (La Nina) years. Over western Canada, mean wintertime temperature distribution of the EI Nino (La Nina) years is found to be shifted towards warmer (colder) values relative to the distribution of the normal years. This study suggests the possibility of developing a long-range forecasting technique for Canada using ENSO related indices.