Effects of a freshening trend on upper-ocean stratification over the central tropical Pacific and their representation by CMIP6 models

Along with global warming, the obviously increasing trend in the freshwater flux (FWF) into the ocean in the central tropical Pacific (CP) is causing a freshening trend in seawater to reconstruct the upper oceans. In this work, the linear trend in stratification directly induced by the freshening trend is isolated by Argo data from 2005 to 2020, and a positive feedback process is proposed and verified by a 1-D model to understand how the freshening trend can modulate the temperature trend over CP. The freshening trend in the mixed layer (ML) induced by the increasing FWF trend makes the stratification in the ML more stable through thickening the barrier layer (BL) and reducing the entrainment of subsurface cold water. Therefore, there is feedback on warming from FWF and salinity effects. Furthermore, 33 CMIP6 models were used to validate the model's ability to reproduce the physical process in which the freshening trend in the ocean induces a thicker BL. 70% of the 33 models can reasonably simulate the freshening trend. Only 52% of the 33 models correctly reproduce the abovementioned physical process. Serious deviations are present in the simulation for the shoaling trend in isothermal layer depth in the remaining models.

Automated summary

This study reveals that the increasing trend in freshwater flux into the central tropical Pacific leads to seawater freshening and changes in the upper oceans. Through model verification, it is found that the increasing freshwater flux thickens the barrier layer and reduces the entrainment of subsurface cold water, resulting in a more stable stratification in the mixed layer and positive feedback on warming.