Impact of atmosphere-ocean coupling on the predictability of monsoon intraseasonal oscillations

The impact of air-sea coupling on the predictability of monsoon intraseasonal oscillations (MISO) has been investigated with an atmosphere-ocean coupled model and its atmospheric component. From a 15-yr coupled control run, 20 MISO events are selected. A series of twin perturbation experiments have been conducted for all the selected events using both the coupled model and the atmosphere-only model. Two complementary measures are used to quantify the MISO predictability: (i) the ratio of signal-to-forecast error and (ii) the spatial anomaly correlation coefficient (ACC). In the coupled model, the MISO predictability is generally higher over the Indian sector than that over the western Pacific with a maximum of 35 days in the eastern equatorial Indian Ocean. Air-sea coupling significantly improves the predictability in almost the entire Asian-western Pacific region. The mean predictability of the MISO-related rainfall over its active area (10 degrees S-30 degrees N, 60 degrees-160 degrees E) reaches about 24 days in the coupled model and is about 17 days in the atmosphere-only model. This result suggests that including an interactive ocean allows the MISO predictability of an atmosphere-only model to be extended by about a week. The extended predictability is primarily due to the coupled model capturing the two-way interactions between the MISO and underlying sea surface. The MISO forces a coherent intraseasonal SST response in underlying ocean that in return exerts an external control on the future evolutions of the MISO. The break phase of the MISO is more predictable than the active phase in both the atmosphere-only model and the coupled model as revealed in the observations. Air-sea coupling appears to extend the MISO predictability uniformly regardless of the active or break phases.