Advective Influences on Forced Tropical Motions

Abstract

The nonlinear equilibrium response to idealized low-latitude heating is examined and compared with the linear response using a spectral, equatorial ?-plane shallow-water model. Three categories of mass forcing are considered: wave-only forcing, wave plus latitudinally symmetric ?Hadley? zonal forcing centered on the equator, and wave plus latitudinally antisymmetric ?monsoonal? zonal forcing. The forced zonal wavenumber index is either m =2 or m = 8; nonlinear effects ?scatter? energy into harmonies m = 4 or 16. Sixteen distinct solutions are analyzed to identify the mechanistic relations between forcing patterns and flow responses. Significant nonlinear modifications occur for the latitudinally symmetric and antisymmetric zonal forcing runs; these are mainly the result of changes in the zonal and forced wave modes. In the four wave-only experiments, most nonlinear differences are small and due to the presence of higher harmonics, but a preference for easterly anomalous winds is noted. When zonally symmetric forcing is added, there result stronger wave-mean flow interactions and greater changes due to nonlinearity. Significant regions where the balance equation would be nonelliptic are found near the equator. For latitudinally symmetric zonal forcing centered on the equator, the overall flow is generally weakened when there is a zonal mass source and strengthened when there is a zonal mass sink. In the latter four cases, the magnitude of the primary divergence centers is enhanced by approximately 100%. Anomalous mean easterlies near the equator occur for all of the symmetric zonal runs. With latitudinally antisymmetric zonal forcing, the nonlinear modification of divergence amplitude is only 10%, but the strength of the equatorial easterly anomalies is much greater than in the preceding experiments. Implications for selected observed phenomena (e.g., the ITCZ and ENSO) are discussed.