Critical Salinity as an early warning of Tipping Point in the North Atlantic Subpolar Gyre

The subpolar gyre (SPG) of the North Atlantic plays a pivotal role in the Atlantic Meridional Overturning Circulation (AMOC) and climate through various teleconnections. This study examines the tipping point in thisregion within CMIP6 projections using three models: CESM2-WACCM, MRI-ESM2-0, and NorESM2-LM. These models, selected for exhibiting tipping patterns in at least one emission scenario, reveal distinct yet converging patterns of change, suggesting a destabilization of the subpolar region driven by shifts in salinity, temperature, and density profiles. A consistent feature across the models is pronounced freshening in the upper 150 meters of the water column. This results in a strong stratification, accompanied by cooling in the top 250 meters and warming between 150 and 1500 meters. The resulting enhancement in water column stability leads to a marked reduction in mixed layer depth (MLD). These changes disrupt vertical mixing, weaken nutrient transport, and alter regional circulation dynamics, with cascading effects on marine ecosystems and climate feedback mechanisms. We employed a density-based approach that accounts for the combined effects of temperature and salinity on water density to identify the critical surface salinity leading to the tipping of the SPG. This critical salinity represents a threshold for the salinity level beyond which density-driven stratification results in a stable water column. For stability to break, surface salinity must exceed this critical salinity. All three models consistently identify a critical salinity threshold of approximately 33.8 g·kg⁻¹. When surface salinity drops below this threshold, the subpolar region experiences rapid cooling, reduced convection, and potentially irreversible transitions. The tipping point of the SPG is preceded by an expansion of areas in the SPG where surface salinity falls below this critical threshold, accompanied by a decrease in MLD. To complement our analyses, we used the ISAS dataset to assess how far the system is from an SPG tipping point. Our next step is to establish an observable spatial pattern of early warning. Our findings underscore the vulnerability of the North Atlantic subpolar region to salinity-driven tipping points, which may lead to potentially irreversible transitions. This highlights the critical need for precise monitoring and advanced modeling of salinity dynamics to enhance predictability in future climate scenarios.