Why is seasonal density stratification in shelf seas expected to increase under future climate change?

Coastal and shelf seas provide a diverse range of ecosystem services, which are often mediated by seasonal density stratification through its control on biogeochemical cycles. These seas are highly vulnerable to climate change and downscaling studies consistently project an increase in seasonal stratification over the next century, but without a clear explanation. Here we revisit a well-established theory of coastal ocean mixing and demonstrate with a new ensemble of downscaled simulations for the Northwest European continental shelf seas to 2100 that the increase of expansivity with temperature is sufficient to consistently increase the seasonal stratification. Where there is a closed balance between buoyancy input and mixing, small changes in expansivity are amplified to a large relative change in stratification. This simple link between global heating and stratification substantially reduces uncertainty in projections of this key parameter in seas around the world.