Abstract
Abstract. The wintertime influence of tropical Pacific sea surface temperature (SST) variability on subseasonal variability is revisited by identifying the dominant mode of covariability between 10–60 d band-pass-filtered surface air temperature (SAT) variability over the North American continent and winter-mean SST over the tropical Pacific. We find that the El Niño–Southern Oscillation (ENSO) explains a dominant fraction of the year-to-year changes in subseasonal SAT variability that are covarying with SST and thus likely more predictable. In agreement with previous studies, we find a tendency for La Niña conditions to enhance the subseasonal SAT variability over western North America. This modulation of subseasonal variability is achieved through interactions between subseasonal eddies and La Niña-related changes in the winter-mean circulation. Specifically, eastward-propagating quasi-stationary eddies over the North Pacific are more efficient in extracting energy from the mean flow through the baroclinic conversion during La Niña. Structural changes of these eddies are crucial to enhance the efficiency of the energy conversion via amplified downgradient heat fluxes that energize subseasonal eddy thermal anomalies. The enhanced likelihood of cold extremes over western North America is associated with both an increased subseasonal SAT variability and the cold winter-mean response to La Niña.
Highlights
The El Niño–Southern Oscillation (ENSO), the leading mode of sea surface temperature (SST) variability over the tropical Pacific, has far-reaching impacts on the atmospheric circulation in the Northern Hemisphere through the generation and propagation of a stationary Rossby wave train to the extratropics (Trenberth et al, 1998)
By identifying the dominant mode of interannual covariability between winter-mean tropical SST and subseasonal surface air temperature (SAT) variability, this study confirms the prominent role of ENSO in modulating the SAT variability over North America
Among the classical ENSO indices, the Niño 3 and Niño 3.4 indices are most closely correlated with the mode of variability identified in this work
Summary
The El Niño–Southern Oscillation (ENSO), the leading mode of sea surface temperature (SST) variability over the tropical Pacific, has far-reaching impacts on the atmospheric circulation in the Northern Hemisphere through the generation and propagation of a stationary Rossby wave train to the extratropics (Trenberth et al, 1998). Nakamura (1996) identified a mode of year-toyear covariability between the winter-mean tropospheric circulation and subseasonal variability over the North Pacific sector This mode is characterized by extratropical wintermean circulation anomalies that strongly resemble the atmospheric response to ENSO and the PNA pattern. Renwick and Wallace (1996) noted an increase in subseasonal variability over the North Pacific in La Niña winters, and Lin and Derome (1997) documented an enhancement of subseasonal variability in negative PNA years
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