Resilient northern overturning in a warming climate

A vigorous Atlantic Meridional Overturning Circulation (AMOC) is crucial for the mild northern European climate; it brings warm water northward near the surface and returns cold, dense water at depth. In a warming climate the AMOC is projected to weaken or even approach a tipping point. Contrary to this established view, I hypothesize that an overlooked, climate-change induced mechanism may impart resilience to the overturning: As the sea ice recedes, increasing stretches of the boundary current system around the Nordic Seas and Arctic Ocean become exposed to the atmosphere. The resulting increased ocean heat loss in winter further densifies the water in the boundary current, which is a direct pathway supplying the lower limb of the AMOC. Enhanced dense-water formation is counter-intuitive in a warming climate and not represented by current climate models, but has the potential to safeguard the northern overturning and maintain a steady supply of dense water to the AMOC. Sparse observations and preliminary results from a 1D model indicate that water mass transformation occurs in the increasingly ice-free boundary current, but its extent, importance, and future development are unknown. In ROVER, I will explore this concept through an extensive field campaign, which includes a mooring array across the boundary current and an unprecedented wintertime survey of this severely under-sampled area. Combined with targeted high-resolution modeling, I will use the comprehensive data set to document the occurrence of this process, understand its dynamics, quantify its extent, and assess its climatic importance. Dense-water formation in the boundary current system that may safeguard the northern overturning would represent a paradigm shift for water mass transformation at high latitudes and the stability of the overturning circulation. As such, ROVER is timely and will have a substantial and significant impact on the science of climate change and climate impact assessment.