Evidence of thermal equilibration in large-scale atmospheric blocking episodes over Europe and the North Atlantic during winter

Abstract

Atmospheric thermal equilibration occurs when all thermal forcings diminish. Characteristics of equilibration include low diabatic heating and calm conditions throughout the atmospheric column. Large-scale planetary waves can remain in these equilibrated states for long time periods, as identified in previous work. Relationships between thermal equilibration and large-scale atmospheric blocking episodes however have not been explored in much detail previously. In this paper we evaluate whether or not thermal equilibration becomes more evident in blocking episodes. We identify wintertime, large-scale blocking episodes over the North Atlantic and Europe using a dynamical blocking index on the ECMWF ERA40 reanalysis dataset. After identifying episodes, evidence of thermal equilibration is presented and analyzed through global composites of atmospheric variables in blocked and nonblocked situations. Relationships between potential vorticity and streamlines are also described and presented through scatter plots. Results from composites, and primarily scatter plots, provide significant evidence that thermal equilibration is more probable in blocking episodes than in non-blocked states.