Multiscale Dynamical Processes Underlying The Wintertime Atlantic Blockings

The atmospheric blocking over the North Atlantic in winter is investigated using a newly developed methodology, namely, localized multiscale energy and vorticity analysis (MS-EVA), and the theory of canonical energy transfer. Through a multiscale window transform (MWT), the atmospheric fields from the ERA-40 reanalysis data are reconstructed on three scale ranges or scale windows: basic flow window, blocking window, and synoptic window. The blocking event is obtained by compositing the blocking episodes, and a clear blocking high cell is identified on the blocking window. Likewise, the local multiscale energetics following the westward retrograding signal are composited. It is found that a lifecycle of the blocking high cell may be divided into three phases: onset phase, amplification phase, and decay phase. Different phases have different mechanisms in play. In general, pressure work and the canonical transfer from the synoptic eddies initiate the generation of the blocking, while the latter contributes to its amplification. The blocking decays as the system transports the KE away and as it converts KE into available potential energy(APE) through buoyancy conversion. For the APE on the blocking window, its evolution experiences two maxima and, correspondingly, two phases can be distinguished. In the first maximum phase, the dominating mechanism is baroclinic instability; in the second, buoyancy conversion takes place. These are also the mechanisms that make the warm core of the blocking in the troposphere.