Tropical and Extratropical Forcing of the Large-Scale Circulation: A Diagnostic Study

Abstract

Seasonal anomalies in the large-scale circulation are studied in relation to seasonal mean tropical forcing and extratropical transient eddy forcing using nine years of tropical wind and outgoing longwave radiation (OLR) data. The OLR is used as a proxy for tropical conviction, and the transient eddy forcing is estimated from the extended Eliassen-Palm flux or E-vector formulation (Hoskins et al.). Results show that in the interannual time scale, extreme swings in anomalous convection over the maritime continent of Indonesia/Borneo and the equatorial central Pacific an linked, respectively, by an east-west seesaw or a dipole pattern. identified with major fluctuations of the Pacific Walker circulation. It is found that extratropical circulation anomalies are related to the fluctuation of this dipole beat source. The overall extratropical circulation seems to be more responsive to the dipole heating with enhanced convection over the maritime continent. While the characteristics of the transient eddies, especially near the exit region of the East Asia jet stream (EAJ), change substantially between maximum or minimum conviction episodes, the transient eddies (defined as transients from the seasonal mean) appear to be unimportant in forcing the seasonal mean circulation anomalies. However, the possible separate roles of low- and high-frequency eddies are not addressed in this study. It is also found that large anomalies in the streamfunction field, including the Pacific-North American teleconnection pattern are related to the interannual variation of the zero flow in the jet-core region of the EAJ. The increase and decrease in the strength of the EAJ is accompanied by an eastward expansion into or contraction of the jet core away from the central Pacific region. This variation is also coupled to convection over the maritime continent via the East Asia Hadley circulation.