ANALYSIS OF TROPICAL EXTRATROPICAL INTERACTIONS WITH INFLUENCE FUNCTIONS OF A BAROTROPIC MODEL

Influence functions (IFs) of a linear barotropic vorticity equation mo del are computed in order to determine the regions in which anomalous divergence at upper levels, related to tropical heating anomalies, has the largest impact on certain prominent low-frequency anomalies. The present computation differs from that of Branstator in two aspects: (a ) the model includes the effects of the basic-flow divergence and the advection by anomalous divergent wind and (b) the influence functions directly assess the influence of upper-level divergence anomalies rath er than sources of vorticity. The IFs are applied to the study of low- frequency tropical-extratropical interactions at the interannual (ENSO ) and intraseasonal (30-60-day oscillation) timescales. The origin of well-known teleconnection patterns is explored through the identificat ion of common influence regions in the Tropics and subtropics for thei r main action centers. The subtropical west and central-east Pacific, north of the equator, is an important source region for the Pacific No rth America (PNA) pattern, and the South Atlantic convergence zone (SA CZ) for the Eurasian and North Atlantic patterns. The IFs and the baro tropic model results, as well as the evolution of the anomalous OLR fi elds associated with the 30-60-day oscillation, indicate the existence of a chain of connections. This chain constitutes a link between the South Pacific convergence zone (SPCZ) and the SACZ, as well as the con trol of the SACZ over the Atlantic and Eurasian pattern, which possibl y connects back to the western Pacific. This connection can also occur at interannual timescales during ENSO events. An explanation of the r elative insensitivity of the PNA pattern to the longitudinal position of the Pacific equatorial convection anomaly, reported by Geisler et a l., is proposed.