POLEWARD-PROPAGATING ANGULAR-MOMENTUM PERTURBATIONS INDUCED BY ZONALLY SYMMETRICAL HEAT SOURCES IN THE TROPICS

A series of experiments has been performed using an idealized model of the global atmosphere to study the role eddies play in communicating changes in the zonal mean state between the Tropics and extratropics. When an oscillatory heating perturbation centered about the equator is imposed, the author found a poleward-propagating zonal wind anomaly e manating from the Tropics into the midlatitudes when the heat source o scillates with a period of around 25-100 days. At higher frequency, mo st of the zonal wind perturbation is confined within the Tropics, whil e at lower frequency, the main signal occurs in the midlatitudes. The angular momentum budget and Eliassen-Palm cross sections have been exa mined. The results suggest that eddies act to communicate changes in t he Tropics into the midlatitudes in at least two ways. First, changes in zonal mean zonal wind in the Tropics lead to a shift in the eddy an gular momentum divergence pattern. Second, heating in the Tropics chan ges the temperature gradients between the Tropics and midlatitudes, gi ving rise to changes in the amplitude of eddy fluxes and hence eddy mo mentum divergence. Both effects act to damp the perturbation in the Tr opics, as well as to transmit the tropical perturbation poleward into the midlatitudes. A simple three-component analytical model has been d eveloped based on these ideas, and the model reproduces the main featu res observed from the numerical model experiments. Low-frequency (peri od 200 days and longer) variability excited by tropical heating has be en examined further. When the perturbation is a single heat source cen tered on the equator, the author found that the main response appears to be a standing oscillation in the midlatitudes, with very weak polew ard-propagating signal. However, when the author added a heating sourc e at 15 degrees latitude with the opposite phase, an apparently signif icant poleward-propagating signal from the Tropics into the extratropi cs was obtained. Analyses suggest that this poleward-propagating signa l may just be an illusory superposition of two largely standing oscill ations located side by side, each with relatively weak poleward propag ating tendency of its own.