Impact of Subgrid-Scale Orography on Equatorial Angular Momentum Budget and the Cold Surges in a General Circulation Model

The dynamical relations between equatorial atmospheric angular momentum (EAAM), equatorial mountain torques, and cold surges are analyzed in a general circulation model (GCM). First, the authors show that the global EAAM budget is well closed in the GCM, much better than in the NCEP–NCAR reanalysis. They then confirm that the equatorial torques due to the Tibetan Plateau, the Rockies, and the Andes are well related to the cold surges developing over East Asia, North America, and South America, respectively. For all these mountains, a peak in the equatorial mountain torque components precedes by few days the development of a cold surge, confirming that the cold surge’s “preconditioning” is dynamically driven by large-scale mountains. The authors also analyze the contribution of the subgrid-scale orography (SSO) parameterizations and find that they contribute substantially to the torques. In experiments where these parameterizations are almost entirely reduced over a given massif, the authors find that the explicit pressure torques produced by that massif largely compensate the reduction in the parameterized torques. On the one hand, this proves that the explicitly resolved equatorial mountain torques are effective dynamical drivers of the flow dynamics, since they are enhanced when a parameterized torque is reduced. On the other hand, this shows that the cold surges can be captured in GCMs, provided that the synoptic conditions prior to their onset are realistic. The compensation between torques is nevertheless not complete and some weakening of the cold surges is found when the parameterized mountain torques are reduced.