Exploring ocean stratification in CMIP6 models: biases and evolution in a warming world

In the context of global warming, understanding the effects of rising ocean surface temperatures on ocean stratification and mixing is essential. Recent research challenges the traditional notion of a linear relationship between surface warming and increased stratification, raising critical questions about the ocean's interior response to climate forcing and future climate change. 

This study addresses these questions by analyzing upper-ocean stratification using historical simulations and two different warming scenarios from a representative set of CMIP6 models. We characterize the ocean's vertical structure by applying the sharp homogenization/diffusive retreat (SHDR) algorithm to fit vertical density profiles with an analytical one-dimensional model. This approach integrates various stratification metrics, including mixed layer depth, pycnocline amplitude, and slope, providing a comprehensive representation of the upper ocean. 

Our analysis reveals critical gaps in the representation of ocean stratification in historical simulations, including an asymmetric bias in the main pycnocline slope between hemispheres. Building on these findings, we examine how stratification responds and evolves under SSP2-4.5 and SSP5-8.5 scenarios, addressing these discrepancies within the context of a warming climate. These insights contribute to our understanding of ocean stratification dynamics and their implications for future climate projections.