Effects of low-frequency tropical forcing on intraseasonal tropical-extratropical interactions

A spherical nondivergent barotropic model, linearized about a 300-mb climatological January flow, is used to examine the extratropical response to low-frequency tropical forcing. A two-dimensional WKB analysis shows that the energy propagation depends on the sum of three vectors: the basic state wind vector, a vector that is parallel to the absolute vorticity contours, and the local wave vector. The latter two vectors are functions of the slowly varying background flow and forcing frequency omega . As omega decreases, the ray paths approach that of the local wave vector, so that the energy propagates in a direction perpendicular to the wave fronts. The extratropical jet streams have a stronger influence on the long period (>30 day) ray paths than on those of intermediate period ( similar to 10-30 day). Global and local energetics calculations show that the energy conversion from the zonally varying basic flow increases as omega decreases. The local energetics show that for the long period disturbances, both the energy conversion and energy redistribution due to advection and pressure work are significant along the North African-Asian jet stream. The long period disturbances are less sensitive to the location of the tropical forcing than those of intermediate period. This provides a plausible explanation for the observations showing that the long period oscillations tend to be geographically fixed at the exits of the extratropical jet streams, whereas those of intermediate period are zonally mobile wave trains. The long (intermediate) timescale disturbances dominate in the Northern (Southern) Hemisphere, where the zonal variations in the basic flow are more (less) pronounced.