Comprehensive Coupling Approach for the Ocean and the Atmosphere – COCOA

In the current context of global change, numerical models are key tools to explore the characteristics of the Earth climate and anticipate its evolution. Despite the increasing complexity of climate models (CMs), their representation of air-sea interactions remains a fundamental issue. This includes a wide range of aspects: parameterizations of atmospheric and oceanic boundary layers (BLs), estimation of air-sea fluxes, time-space numerical schemes, matching of different grids at the interface, coupling algorithms...
The coupling effort in CMs, which started in the 90’s, primarily addressed modularity of model interfaces and conservation of energy and water. More recent studies led to additional improvements, but these were generally performed independently from each other. In this context, our project aims at revisiting the overall representation of air-sea interactions in CMs, by coherently considering physical, mathematical, numerical and algorithmic aspects. To achieve this goal, this project brings together expert scientists in those disciplines.
Turbulent air-sea fluxes ensure a large part of the coupling between the ocean and the atmosphere. They are estimated in numerical models from values of physical quantities (wind and current, temperature, humidity) near the air-sea interface, and are heavily dependent on the parameterizations chosen for the oceanic and atmospheric BLs and for the flux computation (bulk formulae). In this project, we intend to improve the coherence of these parameterizations (generally chosen independently) by designing objective characterization criteria, and to study its influence on model results. We will also enrich these parameterizations by adding representations of high impact phenomena (gustiness, oceanic warm layers, SST front effects on boundary layer), while assessing the relevant level of complexity to be considered for global coupled ocean-atmosphere (OA) applications. The interest of these new capabilities will be assessed both from a deterministic (e.g. representation of specific events) and statistical (e.g. intra-seasonal and inter-annual variability) viewpoint. We will focus on the tropical regions, where latent heat transfers largely condition the atmospheric water content and where the most important biases are observed.
In order to actually improve climate simulations with a better physical representation of coupling processes, it is also necessary to improve the mathematical and algorithmic aspects. Current CMs suffer from inconsistencies linked to the discretization in space and in time. In time, current asynchronous coupling algorithms do not allow for a correct phasing between the ocean and the atmosphere, and hence between their diurnal cycles, potentially inducing biases in the estimation of daily maxima. We will correct this defect by introducing an iterative coupling algorithm based on Schwarz methods, which additional cost will be limited by locally making use of reduced models. Considering the problem in space, physical inconsistencies result today from the difference in the numerical grids used by atmospheric and oceanic models. These inconsistencies will be mitigated by implementing intermediate exchange grids to compute the surface fluxes.
The improvements described above will first be implemented and tested in a series of 1D test cases, involving a coupled 1D OA model and reference solutions from coupled large eddy simulations that will be specifically developed. Transfer into real CMs and evaluation of the impact will then be performed through 3D simulations in the two French CMs (IPSL-CM and CNRM-CM).
A number of outcomes of this project are not tied to a specific CM but virtually apply to any coupled OA model. Moreover particular attention will be paid to document and distribute our new tools (1D coupled OA model, coupled LES simulations, specific test cases and associated metrics) to the scientific community.