Dynamical Heating of the Arctic Atmosphere during the Springtime Transition.

The Arctic undergoes an abrupt transition from the quasi-steady climate of winter to a period of rapid warming in spring. To explore the atmospheric dynamics of this transition, an extended simulation using a global atmospheric model driven by a fixed repeating annual cycle of sea surface temperatures and sea ice cover is analyzed. The model reproduces the timing, structure, and interannual variability of the observed spring onset, thus providing a platform for addressing its dynamics. It is found that atmospheric eddy heat fluxes across the Arctic boundary, highly variable in winter but much less so in spring, shape the transition and determine its timing. Together with the rapid springtime increase of solar heating, the decreased variability in dynamical heating creates the abrupt appearance of the spring transition. Perpetual season simulations for winter, early spring, and late spring further reveal the dynamics of seasonally varying dynamical heating. The eddy heat flux is less variable in spring than winter because the variance of the eddy meridional wind and the stationary wave in temperature, resulting from land-sea contrast, both weaken. Further analysis shows that the strong wintertime variance in meridional wind is associated with traveling planetary wavenumber 1, which amplifies when its phase corresponds to an east-west dipole spanning the Greenland Sea. In this configuration the transient wind-stationary thermal interaction releases zonal available potential energy into wavenumber 1. Thus the highly variable wintertime dynamical heating of the Arctic arises from a baroclinic mechanism, but one distinct from baroclinic instability or cyclogenesis. [ABSTRACT FROM AUTHOR]