What are the sources of mechanical damping in Matsuno–Gill type models?

The Matsuno-Gill model has been widely used to study the tropical large-scale circulations and atmosphere-ocean interactions. However, a common critique of this model is that it requires a very strong damping to get realistic response and it is unclear what could provide such damping in the upper troposphere. This study analyzed the momentum budget in the tropics using 15 years of daily NCEP/NCAR and ECMWF reanalyses. The results show that in the Walker circulation, the pressure gradient force is strong at both the lower and upper levels, and requires a strong (~ 5 day) damping to balance it. This supports the use of strong damping in the Matsuno-Gill type models. At the upper levels, the damping comes from convective momentum transport (CMT; also called "cumulus friction") in the deep convection region, and from nonlinear advection in the shallow convection region. At the lower levels above the boundary layer, the damping comes from CMT in both regions. Damping is also strong in the Asian summer monsoon, and in oscillations such as the ENSO and the MJO.