ENSO’s influence on North American subseasonal surface air temperature variability

Abstract:

The influence of the El Niño-Southern Oscillation on subseasonal variability in winter is revisited by identifying the dominant mode of covariability between winter-mean sea surface temperature over the tropical Pacific sector and 10-60 day band-pass-filtered surface air temperature variability over North America. It is found, in agreement with previous studies, that La Niña conditions enhance subseasonal surface air temperature variability over western North America. This modulation of subseasonal variability is achieved via interactions between subseasonal eddies and La Niña-related changes in the winter-mean circulation. We find that eastward-propagating quasi-stationary eddies over the North Pacific extract energy more efficiently from the mean flow through baroclinic energy conversion during La Niña. Modulations of the vertical structure of these eddies are crucial to enhance the downgradient heat fluxes energizing subseasonal thermal anomalies. The increase of subseasonal surface air temperature variability and the cold winter-mean response to La Niña over western North America contribute together to enhance the likelihood of cold extremes.