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 emanating from the Tropics into the midlatitudes when the heat source oscillates with a period of around 25–100 days. At higher frequency, most of the zonal wind perturbation is confined within the Tropics, while at lower frequency, the main signal occurs in the midlatitudes. The angular momentum budget and Eliassen–Palm cross sections have been examined. The results suggest that eddies act to communicate changes in the 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 angular momentum divergence pattern. Second, heating in the Tropics changes the temperature gradients between the Tropics and midlatitudes, giving rise to changes in the amplitude of eddy fluxes and hence eddy momentum divergence. Both effects act to damp the perturbation in the Tropics, as well as to transmit the tropical perturbation poleward into the midlatitudes. A simple three-component analytical model has been developed based on these ideas, and the model reproduces the main features observed from the numerical model experiments. Low-frequency (period 200 days and longer) variability excited by tropical heating has been examined further. When the perturbation is a single heat source centered on the equator, the author found that the main response appears to be a standing oscillation in the midlatitudes, with very weak poleward-propagating signal. However, when the author added a heating source at 15° latitude with the opposite phase, an apparently significant poleward-propagating signal from the Tropics into the extratropics was obtained. Analyses suggest that this poleward-propagating signal may just be an illusory superposition of two largely standing oscillations located side by side, each with relatively weak poleward propagating tendency of its own.
Journal of the Atmospheric Sciences – American Meteorological Society
Published: Nov 12, 1996