The equatorial response to higher-latitude forcing

The classic view, following Charney and Webster and Holton, is that signifi cant midlatitude forcing of the Tropics can be expected only in regions wit h westerly winds in the upper troposphere because it is only in these regio ns that stationary Rossby waves will be able to propagate toward the equato r. Here it is shown that higher-latitude forcing can project directly onto equatorial waves and give a significant tropical response in both easterly and westerly tropical flow. The equatorial response to higher-latitude forcing is considered in the con text of a dry atmosphere and a localized higher-latitude forcing with eastw ard or westward phase speed. Previous ideas of the Doppler shifting of equa torial waves by zonal Rows are extended to include consideration of a forci ng involving a range of zonal wavenumbers. A Gill-type model suggests that there can be significant forcing of equatorial waves by either vorticity fo rcing or heating in higher latitudes. In agreement with the theory, the Kel vin wave response to eastward forcing is peaked at high frequencies/short p eriods but reduces only slowly with decreasing frequency. Primitive-equatio n experiments confirm the strong equatorial response associated with a deep Kelvin wave for forcing in midlatitudes. The response is strongest in the Eastern Hemisphere with its equatorial, upper-tropospheric easterlies. The possible importance of this equatorial response in the organization of larg e-scale, deep tropical convection and the initiation of the Madden-Julian o scillation is discussed. The ability of westward forcing in higher latitude s to trigger Rossby-gravity and Rossby waves is found in the primitive-equa tion model to be significant but rather less robust. These wave signatures are dearest in the lower troposphere. For shorter periods the Rossby-gravit y wave dominates, and for upper-tropospheric forcing, downward and eastward wave activity propagation is seen. Upper-tropospheric westerlies are found to enhance the response.