More eddying of subtropical western boundary currents boosts stratification and cools shelf seas

Abstract Ocean eddies are intensifying with climate change, especially in western boundary currents. Boundary currents separate coastal seas from the open ocean, but eddies drive cross-slope fluxes that can adjust the current and change shelf-sea conditions. Here we analyse two years of mooring observations in the Agulhas Current, diagnosing eddy dynamics and fluxes. We find that eddies converge heat and salt towards the current core over time, cooling adjacent shelf seas while broadening and stratifying the current. On the inshore edge, frequent 10-km frontal instabilities dominate, pumping cold, nutrient-rich waters up onto the shelf, while farther offshore 100-km meanders move heat onshore. The result is accelerated warming at the surface, but cooling at depth. Similar tendencies are expected in all subtropical western boundary currents as eddying increases, implying that adjacent shelf and slope seas will bear more extremes in the future, even while the strength of these currents may hold steady.