Role of Surface Forcing for Stratosphere-Troposphere Coupling in an Idealized Model

Forcing from the lower boundary has received little attention so far in simulating stratospheric and tropospheric circulations with idealized models. For example, models typically employ a wave-number-two topography and uniform surface drag of some magnitude. But the way how the drag is specified may be important: it can shift the tropospheric westerlies, which may impact the wave driving and the circulation of the stratosphere. Here, we address this issue using GFDL’s dry dynamical core with realistic topography and zonally asymmetric equilibrium temperatures. We systematically vary the model’s prescribed surface drag in terms of magnitude and land-sea contrast. We demonstrate that the surface drag is a key factor for the vertical coupling between the lower and middle atmosphere. By controlling jet location and eddy activity, surface drag modulates the amount of wave energy entering into the stratosphere, the polar vortex strength, the number of SSWs, and the characteristics of the downward propagating pattern. Based on these outcomes, we present an idealized model with optimized surface drag, which simulates a more Earth-like circulation than previous studies.