Atmosphere, ocean, and coupled ocean–atmosphere models in a single code

Abstract A new coupled ocean–atmosphere model is presented. It permits the study of turbulent interactions between the atmosphere and the ocean at very fine scales within a single coherent model code (the Meso–NH model). First, an ocean model is developed from the Meso–NH atmospheric model with only a few modifications. By exploiting the isomorphism between the equations governing the two fluids, the same hydrodynamical algorithm is used to simulate both the atmosphere and the ocean. The main difference in the numerical code is related to the distinct state equations describing the ocean and the atmosphere. The ocean model is then applied to idealised cases of shallow and deep convection, as well as to a diurnal warm‐layer case observed during Cooperative Indian Ocean Experiment on Intraseasonal Variability/Dynamics of the Madden–Julian Oscillation. Because atmospheric and oceanic models share a single code base, they automatically share many key features, such as numerical schemes and subgrid turbulent representations. The coupling between the ocean and the atmosphere is therefore straightforward, internally consistent, and does not require an external coupler. The ocean model, as well as the coupled one, benefits directly from all existing and future developments of the atmospheric model, such as grid nesting, turbulence statistics, budget diagnostics, maintenance, and ongoing computational improvements. Finally, a first test of the coupled model at fine scales (large‐eddy simulations) is presented.