Momentum transport processes in the stratiform regions of mesoscale convective systems over the western Pacific warm pool

Momentum transport by the stratiform components of mesoscale convective systems (MCSs) during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment in December 1992 is investigated using a cloud-resolving model. The mesoscale momentum transport by the stratiform regions of MCSs is examined in two distinct large-scale flow regimes associated with the intraseasonal oscillation over the western Pacific warm pool. Model simulations for 14 December 1992 characterize the 'westerly onset' period, which has relatively weak low-level westerlies with easterlies above. Simulations for 23-24 December represent the 'strong westerly' regime, when westerlies extend from the upper troposphere to the surface, with a jet 2-3 km above the surface. In the westerly onset simulation, the extensive stratiform region of a MCS contained abroad region of descent that transported easterly momentum associated with the mid-level easterly jet downward. Thus, the stratiform regions acted as a negative feedback to decrease the large-scale mean westerly momentum developing at low levels. In the strong westerly regime, the mesoscale downward air motion in the stratiform regions of large MCSs transported westerly momentum downward and thus acted as a positive feedback, strengthening the already strong westerly momentum at low levels. Momentum fluxes by the mesoscale stratiform region downdraughts are shown to have a systematic and measurable impact on the large-scale momentum budget.