The inability of winter to forget the memory of tropical cyclones

The existence of wind on Earth largely results from the gradient of temperature from the poles to the equator caused by varying insolation with latitude. It has long been established and recently quantified, that this redistribution of temperature is accomplished through oceanic currents, atmospheric currents, and latent heat release. Implied within all three is the role of tropical cyclones (TCs) that move out of the tropics and poleward of the Hadley cell. It remains a curious climate wildcard that although the pole to equator temperature gradient does not directly produce TCs, TCs weaken this gradient when escaping the Hadley cell. Yet, the magnitude of this TC role has remained nebulous and qualitative.

By examining the winter midlatitude climate following varying recurving TC seasons over 50 years, the role of recurving TCs in the redistribution of energy is laterally approximated. The results suggest that a recurving TC, on average, may weaken the subsequent role of winter baroclinic eddies (and, implicitly, the mean pole-to-equator temperature gradient) by approximately one percent. This reduction of meridional eddy temperature flux implies a decrease in the available potential energy (APE) of the atmosphere, either as a direct or indirect result of the anomalous recurving TCs. Simultaneous with the decrease in APE, the static stability (N) of the midlatitudes is decreased as a consequence of the enhanced recurving TCs. These two competing changes result in a shift in winter extratropical cyclone (XC) existence: Following an active recurving TC season, intense XCs are slightly more frequent (owing to the decrease in N), but weak to moderate XCs are far less frequent (owing to the decrease in APE). It remains to be determined conclusively how the accumulated impacts of the TCs, and/or the resulting large-scale circulation changes, retain memory for months following the TC season.

Speculation is provided based on preliminary analyses of hemispheric snowcover leading up to the start of the winter.

Improved seasonal forecasting of higher latitude winter climate anomalies may be possible when the hemispheric frequency of recurving TCs is anomalous. Unfortunately, with this frequency minimally predictable, some component of winter midlatitude climate may be necessarily unpredictable until the TC season has nearly ended. This study suggests a view wherein TCs may be integral to the variability and forcing of climate, rather than incidental and purely responsive to other forcings, such as ENSO.