Wave-wave interaction and transport due to synoptic-scale Rossby waves are investigated by using the middle atmosphere version of the NCAR CCM3. Rossby waves penetrate the equatorial lowermost stratosphere vigorously through the "westerly duct" over the central and eastern Pacific where the mean wind at the equator is westerly because of the Walker circulation.
These waves excite Rossby-gravity (RG) waves that propagate vertically in the lower stratosphere in a very similar way to that shown by Magana and Yanai (1995). These waves are preferably excited over barotropic RG waves since their frequencies are closer to those of the Rossby waves. Although the forcing waves are barotropic, their shallow vertical scales are suitable to excite the baroclinic RG waves as well as barotropic ones. The lateral excitation is the main source of the stratospheric RG waves in the model.
The westerly duct is accompanied by a positive (negative) potential vorticity (PV) anomaly in the north (south) of the equator. The interaction of the Rossby waves and the mean flow creates an interesting longitudinal asymmetry in the PV anomalies. It opens a "window" where the gradient of PV is small at the southwest-side of an anomaly. This connects the northern hemisphere subtropics and tropics which is secluded by the subtropical jet during the northern winter. A similar week PV gradient is observed at the Atlantic westerly duct associated with the convection over American continents. These windows seem to be important for the exchange of the tropical and subtropical airmass in the lowermost stratosphere.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics