Analysis of Tropical–Extratropical Interactions with Influence Functions of a Barotropic Model

Abstract

Influence functions (IFs) of a linear barotropic vorticity equation model 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 rather 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 identification of comma influence regions in the Tropics and subtropics for their main action centers. The subtropical west and central-east Pacific, north of the equator, is an important source region for the Pacific North America (PNA) pattern, and the South Atlantic convergence zone (SACZ) for the Eurasian and North Atlantic patterns. The IFs and the barotropic model results, as well as the evolution of the anomalous OLR fields 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 control of the SACZ over the Atlantic and Eurasian pattern, which possibly connects back to the western Pacific. This connection can also occur at international timescales during ENSO events. An explanation of the relative insensitivity of the PNA pattern to the longitudinal position of the Pacific equatorial convection anomaly, reported by Geisler et al., is proposed.