Multidecadal Trends of the Mixed Layer Depth and Their Relation to the Wind in Global Ocean Models Forced by an Atmospheric Reanalysis

The surface mixed layer of the ocean plays a key role in ocean-atmosphere interactions. Despite the ocean surface warming in the past four decades, which increased the stratification, the mixed layer depth (MLD) has been found to increase, most notably in the Southern Ocean in summer. We use 12 models from the Ocean Model Intercomparison Project (OMIP) at different resolutions, forced by the atmospheric reanalysis JRA55-do, to assess their capability to represent the MLD trends over the period 1970–2018 and to investigate their origin. The MLD evolution in the OMIP models is extremely well correlated across models at interannual time scales, especially in summer. Correlations are lower in high resolution models because of the chaotic nature of the mesoscale variability. OMIP models reproduce consistently the deepening trend of the mixed layer in summer in the Southern Ocean and confirm its relation to the wind speed. The MLD deepening is weaker in the models than in observations, probably due to the fact that the wind speed trend is underestimated in the atmospheric reanalysis. We find however that the MLD deepening is not a simple one-dimensional response to the increase of the wind speed at a given location, but that the three-dimensional processes that control the stratification also play a part. This study gives confidence in the capacity of ocean models to project the response of the mixed layer to future changes in wind speed.