Zonal Momentum Balance in the Tropical Atmospheric Circulation during the Global Monsoon Mature Months

In this paper, zonal momentum balances of the tropical atmospheric circulation during the global monsoon mature months (January and July) are analyzed in three dimensions based on the ECMWF Interim ReAnalysis (ERA-Interim). It is found that the dominant terms in the balance of the atmospheric boundary layer (ABL) in both months are the pressure gradient force, the Coriolis force, and friction. The nonlinear advection term plays a significant role only in the Asian summer monsoon regions within the ABL. In the upper troposphere, the pressure gradient force, the Coriolis force, and the nonlinear advection are the dominant terms. The transient eddy force and the residual force (which can be explained as convective momentum transfer over open oceans) are secondary, yet cannot be neglected near the equator. Zonal-mean equatorial upper-troposphere easterlies are maintained by the absolute angular momentum advection associated with the cross-equatorial Hadley circulation. Equatorial upper-troposphere easterlies over theAsian monsoon regions are also controlled by the absolute angularmomentum advection but aremainlymaintained by the pressure gradient force in January. The equivalent linear Rayleigh friction, which is widely applied in simple tropical models, is calculated and the corresponding spatial distribution of the local coefficient and damping time scale are estimated from the linear regression. It is found that the linear momentummodel is in general capable of crudely describing the tropical atmospheric circulation dynamics, yet the caveat should be kept in mind that the friction coefficient is not uniformly distributed and is even negative in some regions.