A recent decline in North Atlantic subtropical mode water formation

Over 60 years of hydrographic data from southeast of Bermuda show the temporal variability of North Atlantic subtropical mode water. Between 2010 and 2018, there was an 86-93% loss of thickness, suggesting weaker production of mode water that is expected to continue with warming. As a manifestation of mixing dynamics in the upper ocean, interannual and decadal variability of subtropical mode water (STMW) properties in the North Atlantic Ocean provides a valuable insight into ocean-atmosphere interaction in a changing climate. Here, we use hydrographic data from the Bermuda Atlantic Time-Series Study and Hydrostation S sites near Bermuda, as well as various ocean reanalysis products, to evaluate the modern variability of STMW properties. Our study finds an 86-93% loss of STMW thickness at these sites between 2010 and 2018 and a comparable loss throughout the western subtropical gyre, culminating in the weakest STMW pentad on record. We correlate this decline with a reduction in the annual outcropping volume and northward excursions of the formation region, suggesting a gyre-wide signal of weakening STMW generation. The outcropping volume of STMW is anti-correlated with surface ocean heat content, foreshadowing future STMW loss in the face of continued warming.