On the intraseasonal oscillation in a coupled GCM

Tropical Intraseasonal Oscillation (IO, hereafter) simulated by an atmosphere - ocean coupled GCM is investigated in this study. The model used for the study is a flux-adjusted version of PNU (Pusan National University) CGCM. The CGCM successfully simulates eastward moving IO with period of 30 - 60 days over Indian Ocean and western Pacific, such is not simulated in the atmospheric component of the model stand-alone experiment. The simulated IO shows seasonality in its occurrence with maximum activities in December - May as observation. The zonal wave number and the baroclinic structure of the simulated IO are also similar to those of the observation, although the amplitudes of the simulated IO are still weaker. The simulation of IO with the CGCM indicates that the oscillation can be also understood in the framework of an atmosphere - ocean coupled system.

In this study, the characteristics and propagating mechanisms of the oscillation are also suggested from the coupled model result. The lagged correlation patterns are used to investigate relationships between the simulated atmospheric and oceanic variables associated with IO. The correlation patterns reveal that there are systematic relationships between convection, low-level wind, evaporation, downward shortwave radiation flux, low-level moisture convergence, upper-level zonal current and SST associated with the oscillation. Evaporation is increased (decreased) to the west (east) of the convective region, which is consistent with the observations. The result indicates that evaporation-wind feedback is not the dominant mechanism for promoting the eastward propagation of the IO. The relationship between convection and SST shows systematic changes in SST with warming to the east and cooling to the west of the convection. The increase of the SST in the east of the convection is related to downwelling, reduced evaporation and increased downward shortwave radiation flux, and the decrease of the SST in the west is due to upwelling, increased evaporation and decreased downward shortwave flux. The positive SST anomaly seems to provide favorable condition for convection to develop in the east of the convection region.

The results suggest that the role of interactive ocean is vital in the CGCM experiment for the simulation of IO and that the variation of SST as well as the low-level moisture convergence seems to be important to the eastward migration of the convection associated with IO.