On the diabatic heating of the North Atlantic storm-track

A new decomposition of the time mean sea level pressure, precipitation and pressure vertical velocity is applied to ERA-Interim reanalysis data over the North Atlantic ocean for the December-February 1979-2011 time period. The decomposition suggests that on the majority of wintertime days (90%), the atmospheric state over the Gulf Stream is dominated by a continuous series of synoptic systems, or baroclinic waves, propagating across the region. The time mean value of precipitation and vertical motion is accordingly set by the propagating waves, with a major contribution from extreme synoptic events. The result is particularly striking for vertical velocity considering that ascent and descent could reasonably be expected to cancel out in such a series of waves.

These results shed a new light on analyses of the storm track heat budget in which the residual between diabatic heating and “transient” eddy heat fluxes (singled out through band pass time filtering or spatial Fourier analysis) is interpreted as a Rossby wave source. This interpretation is questioned because, as a consequence of the filtering used, these studies prevent any direct contribution of the “transients” to the time mean vertical motion, attributing the latter entirely to the circulation associated with the thermally forced Rossby wave. The fact that “transients” directly contribute to the observed time mean upward motion over the Gulf Stream might also explain the discrepancy between observed and predicted response of the vertical motion field to heating in midlatitudes.