Large-scale planetary waves and circulations in the extra- tropical atmosphere are known to have important low-frequency variability, which affects weather and climate. In this project, the emphasis will be on studying the various mechanisms that can possibly couple the tropics to the extra-tropics and their role in affecting the year-to-year changes in the extra-tropical circulation and wave dynamics. The existence of the phenomenon of extra-tropical Quasi- Biennial Oscillation provides a focus for this theoretical study. Newly available observational analyses on this phenomenon also provide critical checks on the relevance of the proposed mechanisms of tropical-extra-tropical coupling. The approach will be one of hypothesis testing. Several essentially two- dimensional and some three-dimensional mechanisms have been proposed so far: through induced change in the mean meridional circulation (Hadley and Brewer-Dobson circulations), through transport of tropical perturbations by the preexisting circulation, and through modulations in the wave guide for planetary waves in the extra-tropics. The relevance of each will be tested with mechanistic model studies and by critically comparing the model results with available observation. This research is an important component of the basic theoretical studies required as an underpining for the development of prediction models for low frequency weather and climate phenomena.
