Differences of boreal summer intraseasonal oscillations simulated in an atmosphere-ocean coupled model and an atmosphere-only model

A series of small-perturbation experiments has been conducted to demonstrate that an atmosphere-ocean coupled model and an atmosphere-only model produce significantly different intensities of boreal summer intraseasonal oscillation (BSISO) and phase relationships between convection and underlying SST associated with BSISO. The coupled model not only simulates a stronger BSISO than the atmosphere-only model, but also generates a realistic phase relationship between intraseasonal convection and underlying SST. In the coupled model, positive (negative) SST fluctuations are highly correlated with more (less) precipitation with a time lead of 10 days as in the observations, suggesting that intraseasonal SST is a result of atmospheric convection, but at the same time, positively feeds back to increase the intensity of the convection. In the atmosphere-only model, however, SST is only a boundary forcing for the atmosphere. The intraseasonal convection in the atmosphere-only model is actually less correlated with underlying SST. The maximum correlation between convection and SST occurs when they are in phase with each other, which is in contrast to the observations. These results indicate that an atmosphere-ocean coupled model produces a more realistic ISO compared to an atmosphere-only model.