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Diurnal Ocean-Atmosphere Coupling and Its Climate Impacts

Diurnal Ocean-Atmosphere Coupling and Its Climate Impacts

Presenting author: D.J.Bernie

Other authors: S.J.Woolnough, E.Guilyardi, G.Madec, J.M.Slingo, J.Cole

The impact of diurnal ocean-atmosphere interaction has previously received little attention, though its role in numerous aspects of the tropical climate has previously been hypothesized and observed. This study examines the impact of the diurnal cycle of ocean-atmosphere coupling on the variability of the tropical climate.

Forced OGCM simulations are used to assess the impact of the diurnal cycle on the intraseasonal SST response to the Madden-Julian Oscillation (MJO) during TOGA-COARE. The diurnal cycle is shown in increase the magnitude if the SST response to the MJO by around 25%, implying a potential role for the diurnal cycle in the coupled nautre of the MJO. It is also demonstrated that, in the Pacific, the diurnal cycle of turbulence in the upper ocean modifies the vertical exchange of momentum between the equatorially divergent Ekman layer and geostrophic convergence at depth. This results in 10% stronger Pacific sub-tropical cells and equatorial upwelling.

The variability of the coupled system is then examined in CGCM simulations. At seasonal time scales the dynamical and thermodynamical impacts of the diurnal cycle in the upper ocean are shown to increase the strength of Bjerknes feedbacks in the tropical Pacific, leading to an increase in the seasonality of the coupled system.

Lag correlations and composites are then used to examine the impact of the diurnal cycle of ocean-atmosphere coupling on the MJO. The inclusion of the diurnal cycle is shown to lead to a more coherent MJO due to the improved representation of ocean-atmosphere coupling.