Synoptic‐scale Rossby waves and the geographic distribution of lateral transport routes between the tropics and the extratropics in the lower stratosphere

Abstract

Atmospheric transport between the tropics and the extratropics, in the lowest part of the stratosphere during Northern Hemisphere winter, is investigated. The role of synoptic‐scale disturbances that propagate laterally into the tropics is examined using the middle atmosphere version of the National Center for Atmospheric Research Community Climate Model Version 3 general circulation model. In the lower stratosphere, synoptic‐scale Rossby waves propagate vigorously from the northern (i.e., winter) extratropics through two “westerly ducts,” where the westerly zonal mean winds near the equator are favorable to Rossby wave propagation. The waves break in the westerly ducts and modify the mean potential vorticity (PV) structure to connect subtropical and equatorial regions of sharp PV gradients. Frequent wave breaking and the wave‐induced PV structure create distinct routes where transport occurs vigorously between the tropics and the northern extratropics. Interhemispheric transport also occurs through regions associated with the westerly ducts. In the Southern (summer) Hemisphere lower stratosphere, synoptic‐scale disturbances propagate mainly as “tongues” of PV elongated from extratropical disturbances. The transport between the tropics and the southern extratropics has a strong geographic preference but is dominated by the monsoon circulation, as was shown for the upper troposphere by Chen [1995]. PV tongues and other transient anomalies are of secondary importance.