Sensitivity of the tropical intraseasonal oscillation prediction to boundary conditions

A coupled atmosphere-ocean model is used to assess the impact of air-sea interaction on the predictability of tropical Intraseasonal Oscillations (ISO). From a 15-year coupled control run, twenty ISO events are selected. A series of ‘twin' perturbation experiments have been conducted for all the selected events with both the coupled model and the atmosphere-only model. Two complementary measures are used to quantify the ISO predictability: i) the ratio of signal-to-forecast error at a given grid point and ii) the spatial anomalous correlation coefficient (ACC) over a specific region.

When measured with the ratio of signal-to-forecast error, the predictability of the coupled model is generally higher in the Indian sector than that in the western Pacific with a maximum of 40 days in the southern tip of Indian Peninsula. Air-sea coupling improves the predictability over almost the entire Asian-western Pacific region, particularly in the northeast Indian Ocean and western North Pacific. Averaged over the Asian-western Pacific region, the mean predictability of the ISO-related rainfall in the coupled model reaches about 24 days. The same measure of predictability for the atmosphere-only model is about 17 days. Similar findings are obtained when ACC is used as the measure of predictability. These results suggest that air-sea coupling is able to extend the predictability of model's ISO-related rainfall variations by about a week in its active region.

The break phase of the ISO is more predictable than the active phase in both the atmosphere-only system and the coupled atmosphere-ocean system. This phase dependence is more obvious in the atmosphere-only model as air-sea coupling appears to reduce this phase dependence by extending the ISO predictability in the active phase.