On the Role of Planetary-Scale Waves in the Abrupt Seasonal Transition of the Northern Hemisphere General Circulation

The role of planetary-scale waves in the abrupt seasonal transition of the Northern Hemisphere (NH) general circulation is studied. In reanalysis data, the winter-to-summer transition involves the growth of planetary-scale wave latent heat and momentum transports in the region of monsoons and anticyclones that dominate over the zonal-mean transport beginning in midspring. The wave-dominated regime coincides with an abrupt northward expansion of the cross-equatorial circulation and reversal of the trade winds. In the upper troposphere, the transition coincides with the growth of cross-equatorial planetary-scale wave momentum transport and a poleward shift of subplanetary-scale wave transport and jet stream. The dynamics of the seasonal transition are captured by idealized aquaplanet model simulations with a prescribed subtropical planetary-scale wave sea surface temperature (SST) perturbation. The SST perturbation generates subtropical planetary-scale wave streamfunction variance and transport in the lower and upper troposphere consistent with quasigeostrophic theory. Beyond a threshold SST, a transition of the zonal-mean circulation occurs, which coincides with a localized reversal of absolute vorticity in the NH tropical upper troposphere. The transition is abrupt in the lower troposphere because of the quadratic dependence of the wave transport on the SST perturbation and involves seasonal-time-scale feedbacks between the wave and zonal-mean flow in the upper troposphere, including cross-equatorial wave propagation. The zonal-mean vertical and meridional flows associated with the circulation response are in balance with the planetary-scale wave momentum and latent heat meridional flux divergences. The results highlight the leading-order role of monsoon-anticyclone transport in the seasonal transition, including its impact on the meridional extent of the Hadley and Ferrel cells. They can also be used to explain why the transition is less abrupt in the Southern Hemisphere.