Resilient northern overturning in a warming climate

Introduction

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.

Hypothesis

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.

Observations and Preliminary Results

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.

Research Approach

In ROVER, I will explore this concept through an extensive field campaign, which includes:

1. A mooring array across the boundary current.
2. 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.
• Assess its climatic importance.

Significance

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.