On the role of stationary waves in the abrupt seasonal transition of the Northern Hemisphere general circulation

The role of stationary wave transport in the abrupt seasonal transition of the Northern Hemisphere (NH) general circulation is investigated. In reanalysis data the transition from winter to summer involves the dramatic growth of eddy moisture and momentum transports in the lower troposphere, which occur in the region of subtropical anticyclones and Monsoons, and dominate over the zonal-mean beginning in mid spring. The eddy-dominated regime is associated with an abrupt expansion of the region of zonal-mean upwelling into the NH subtropics. In the upper troposphere, stationary eddy transport is important throughout the seasonal cycle and the transition coincides with the growth of cross-equatorial and subtropical eddy momentum transport, the expansion of the region of equatorward motion into the NH, an upward shift in the tropical tropopause, and a dramatic poleward shift of the NH jet stream.

The dynamics of the seasonal transition are shown to be captured by idealized aqua planet model simulations with a prescribed subtropical zonally-asymmetric planetary-scale sea surface temperature (SST) perturbation. For a sufficiently large SST perturbation there is an abrupt transition to an eddy-dominated circulation in the NH, which captures all of the features seen in reanalysis data. Eddy latent heat transport and variance in the lower troposphere and eddy momentum transport in the upper troposphere control the zonal-mean vertical and meridional flow, respectively. The coupling of the subtropical lower and upper troposphere occurs via a baroclinic stationary wave. The adjustment in the tropical upper troposphere involves seasonal timescale feedbacks between the zonal-mean flow and eddies, including cross-equatorial wave propagation. The results highlight the leading-order role of transport by subtropical anticyclones and Monsoons in the abrupt seasonal transition of the NH general circulation and their impact on the meridional extent of the Hadley and Ferrel cells. The results also suggest that stationary eddy transport should be included in theories of the general circulation and when interpreting the response to climate change.