The tropical instability waves, frequently observed in the tropical regions of the Pacific and Atlantic oceans, usually demonstrate significant air-sea interaction between the waves in the atmosphere and the ocean. The generation of these waves trapped within the atmospheric boundary layer has been considered a direct response of the atmosphere to the SST variation embedded in the wave pattern near the equator with wavelengths of about 1000 km and periods between 25 and 40 days. These instability waves not only contribute to the climate variability over the Atlantic but are also influenced by it. Our preliminary results have demonstrated these waves' remote climate impact near the west coast of Africa and the tropical Atlantic, e.g., intraseasonal precipitation variation. Moreover, previous observational and numerical studies indicate that these ocean instability waves' structure is controlled to some extent by external wind forcing, which suggests a feedback from the atmosphere to the corresponding ocean instability waves. To further explore the air-sea interaction mechanisms of these waves in the atmosphere and ocean, a regional coupled atmosphere-ocean model is developed by coupling a regional climate model to a reduced gravity ocean model.

The regional climate model (RCM) is a modified version of MM5 that has proved to be able to realistically capture similar wave response in the atmosphere to the observed daily SST prescribed in the model. The nonhydrostatic dynamics of RCM with 24 vertical sigma levels and various convection schemes allows RCM to be valid for mesoscale modeling. To cover the whole tropical Atlantic, the domain for our coupled model experiments extends from 112°W to 22°E and 31°S to 31°S. The horizontal grid spacing is 90 km. The reduced gravity ocean model (RGO) used in this study is a 2.5-layer model, consisting of two active layers: mixed layer and thermocline layer. The deep water below the thermocline layer is assumed to be infinitely deep and motionless. To resolve mesoscale eddy activities, a fine horizontal resolution of 0.25° grid spacing is used.

The daily mean net surface heat flux, net surface solar radiation flux, and surface wind stress are computed in RCM and passed down to RGO. Similarly, the computed daily mean SST from RGO is offered to RCM in exchange for the required fluxes from RCM. Preliminary result indicates that the tropical instability waves in the atmosphere may be phase locked to that in the ocean in our coupled modeling. To prevent the coupled model from drifting in a long-term climate modeling, the application of flux-correction strategy may be necessary when coupling these two regional models together.