Transition between suppressed and active phases of intraseasonal oscillations in the Indo-Pacific Warm Pool

Intraseasonal oscillations (ISOs) are important large amplitude elements of the tropical climate. Yet, ISOs are generally poorly described and are neither well simulated or predicted by numerical models. A detailed analysis of the life cycle of ISOs over the western Pacific warm pool is presented with the aim of increasing understanding of the phenomena and attempting to determine why models handle them so poorly. The focus of the paper is on the transition phase between suppressed and active convection that is hypothesized to be a defining stage for ISO development. Using data from the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean-Atmosphere Response Experiment (COARE), it was found that both atmosphere and ocean undergo important changes during the transition phases. Low-level moistening of the atmosphere appears to precondition the atmosphere for the formation of deep convection. Surface winds are strengthened and exert stress over warmer than average surface oceanic waters, leading to enhanced evaporation. Sea surface temperature is higher during the transition due to the lack of clouds and weak surface winds during the undisturbed phase. Surface evaporation appears as a necessary (but not sufficient) condition for the low-level moistening. Large-scale subsidence also appears as a condition for the large-scale moisture build-up by inhibiting formation of local deep convection. Using a cloud classification scheme based on microwave and infrared satellite data, it was observed that midtop (cloud with a top in the middle troposphere) nonprecipitating clouds are a direct response to the low-level moisture build-up. To address the issue of the generally poor simulation of ISOs by general circulation models, we evaluate an ensemble of simulations using the ECMWF seasonal forecast model during TOGA COARE transition periods. Results show that the model is not able to develop the low level moistening of the atmosphere if those conditions are not already part of the initial conditions.