Inness, P. M. and D. Gregory, 1995: Analysis of tropical intraseasonal variability in the U.K. Met Office Unified Model. Abstracts of the First International AMIP Scientific Conference, Monterey, California, 61.

There is a wide spectrum of time and space scales for the variability of tropical convection, ranging from small cloud clusters embedded in easterly waves, with a period of a few days, up to ENSO events with a period of several years. An important mode of variability is that which occurs on an intraseasonal (25 to 60 day) period, with an eastward propagating disturbance which enhances convective activity being observed in the atmosphere. The Indian Monsoon and convective activity in the West Pacific are strongly modulated on this period, and the variability of organized tropical convection on an intraseasonal period is thought to be important in the excitation of planetary waves with teleconnections to extra-tropical latitudes. Thus an accurate representation of this mode is important in numerical models which are being used for forecasting on a range of timescales, and in models being used for long term climate simulation and climate change prediction. There is also observational evidence that the strength of this intraseasonal mode is modulated on an interannual timescale which seems to be linked to ENSO.

The character of the intraseasonal (Madden-Julian) oscillation in the Hadley Centre Unified Model has been analyzed in the UKMO AMIP integration. The model is shown to produce a reasonably realistic oscillation signal, although in common with other GCMs, the dominant period of the oscillation is slightly too fast and the signal is somewhat noisy. The oscillation in the model also shows a degree of interannual variability, the oscillation being enhanced during the onset phase of ENSO and suppressed during the mature and decay phases, in agreement with observational studies. This result may provide an important insight into the mechanisms involved in the generation and maintenance of the Madden-Julian oscillation. The strength of the oscillation is also seen to be sensitive to changes in the convective parameterization used in the model. The implementation of a convective momentum transport scheme, whilst giving improvements in several aspects of the mean circulation of the model, is seen to destroy the simulation of the Madden-Julian oscillation.