RESPONSE OF THE ZONALLY ASYMMETRIC FLOW TO TIME-DEPENDENT TROPICAL HEATING

An atmospheric general circulation model is used to study the impact o f idealized zonally propagating tropical heating anomalies on the low- frequency variability in the North Pacific region. The propagating hea ting is designed to mimic the thermal forcing associated with the Madd en-Julian oscillation (MJO). Results are examined by separating the fo rced response from other variability and by comparing with runs employ ing fixed-phase (stationary) heating anomalies. For both the forced an d free circulations, the main modes of variability consist of a zonal expansion and retraction of the East Asian jet. The effective Rossby w ave forcing associated with the heating is dominated by the advection term and located in the subtropics in the regions of strong absolute v orticity gradients. Compared with cases using stationary forcing, the response to the propagating forcing is weaker and of different phase, indicating that the 40-day period used for the propagating anomalies i s too short to allow the development of the steady-state response in t he extratropics. The model's total low-frequency variability in the No rth Pacific sector is dominated by the free oscillations that are the result of local processes uncorrelated with tropical variability. The relatively small forced response appears to be partly the result of th e simplicity of the propagating heating anomaly that propagates at a c onstant phase speed and the simplifications introduced into the GCM th at do not allow transient feedback in the diabatic heating. It is sugg ested that the lack of a significant Rossby wave stretching term in th e subtropics is a distinguishing feature of the east-west dipole heati ng anomalies of the MJO and may contribute to the weakness of the resp onse compared to interannual tropical heating anomalies.