Observed global response of ocean stratification to climatic forcing

The global ocean plays a pivotal role in climate by taking up, storing and redistributing vast amounts of heat, carbon and other tracers. A fundamental factor shaping this role is the ocean’s stratification, which accounts for the resistance of a water column to be mixed vertically. As such, stratification modulates the transfer of climatically important properties (e.g., heat, carbon, oxygen and nutrients) between deeper oceanic layers and near-surface waters, which are in frequent contact with the atmosphere and may thus interact with the rest of the climate system.

It has traditionally been assumed that beyond the deepest extent of surface mixing in winter, ocean stratification remains approximately constant or evolves very slowly on the interannual and longer time scales of pertinence to contemporary climate variability – comprising both internal and anthropogenic changes imprinted on the historical record. As a result, most research efforts to document or understand ocean stratification and its climatic function have, to date, primarily focused on near-surface waters. Deeper in the water column, little is known about the extent to (or time scales over) which the stratification of the main pycnocline, extending to depths in excess of 1000 m, is influenced by climate variability.

Here, we test this view by performing the first global-scale, systematic investigation of the spatio-temporal variability of ocean stratification from the surface to the main pycnocline, using 20 years (2003-2022) of data from the Argo float array. We demonstrate that deep-ocean stratification varies significantly with well-defined spatio-temporal patterns. Both near-surface and main pycnocline stratifications are found to exhibit spatially-structured, vertically-coherent, global-scale variations on seasonal-to-decadal time scales, unveiling a new view of ocean stratification from surface to depth as a rapidly-evolving, readily-interactive element of the climate system. Variability in stratification is organized into well-defined patterns that replicate the spatial footprints and time scales of major climate modes such as the El Niño – Southern Oscillation, pointing to these modes of internal variability as important drivers of stratification changes. Our diagnosed patterns and forcings of stratification variability provide an important benchmark for advancing the climate models used to understand and predict the ongoing climate change.