Impact of Planetary-scale Eddies on the Recent Trend of the Extratropical Isentropic Slope and Arctic Warming during Boreal Winter

Abstract:

Baroclinic eddies are known to arise from baroclinic instability in the region where the meridional slope of isentropic surfaces is steep. According to the baroclinic adjustment theory, these eddies in turn neutralize the atmosphere by flattening the isentropes such that the isentropic slope is maintained at its marginal state for baroclinic instability. However, contrary to this theory, according to which the isentropic slope should remain constant, the recent trend of Arctic warming raises the possibility that there could have been a systematic trend in the extratropical isentropic slope over the same time period as the warming.

Observational analyses show that tropospheric isentropes north of 50°N have been flattening significantly for the recent 25-yr period. Projecting this trend pattern to daily isentropes reveals that the trend pattern fluctuates at intraseasonal time scales. It is found that synoptic-scale (zonal wavenumber k = 4-72) eddy heat fluxes are indeed enhanced shortly after the isentropes are abnormally steep. However, following the peak of the synoptic-scale eddy heat fluxes, the expected decline in isentropic slope is rather modest. Instead, it is an enhancement of planetary scale (zonal wavenumber k = 1-3) eddy heat fluxes that lead to a significant decline in the isentropic slope. The flattening of the isentropes also coincides with Arctic warming. Deviating from baroclinic adjustment theory, the k = 1-3 wave growth is not preceded by steepened extratropical isentropes, and instead, it is preceded by an enhanced zonal asymmetry of tropical heating. These suggested causal linkages are tested by performing initial value calculations with the dynamical core of a GFDL GCM. Forced by the observed tropical and extratropical latent heating, the model calculations capture the structure of the extratropical isentropic slope trend pattern and the associated Arctic warming pattern reasonably well. From these results, it is concluded that the recent flattening trend of the extratropical isentropic slope is mostly caused by planetary scale eddy activities generated from latent heating.