How the Subpolar North Atlantic Drives East Coast Sea Level Rise

Climate researchers have devoted years to identifying the processes driving East Coast sea level rise. Retreating glaciers, the expansion of warmer seawater and shifts in the Gulf Stream all contribute to the increase, and the explanation appeared largely settled.

A NASA-led research group has now identified an overlooked element. Its findings indicate that the principal cause of the coast’s long-term rise is located in a frigid, distant part of the ocean near Greenland, thousands of miles from the US shoreline.

A faint ocean signal

Sea level rise on the East Coast has two components. Global warming raises water levels worldwide by melting ice and causing the ocean to expand, while regional processes make levels rise more rapidly along certain shores than others.

What caused the additional rise along the East Coast had remained uncertain. Scientists understood that surface influences, including winds acting on the sea and heat exchanged between ocean and atmosphere, had an effect, but separating the contribution of each one was difficult.

Researchers at NASA’s Jet Propulsion Laboratory (JPL), which forms part of the California Institute of Technology, investigated the issue. The study was led by JPL oceanographer Ou Wang, who analysed the figures using an approach not previously applied in quite this way.

Two coastlines, a shared driver

The researchers selected two benchmark locations: Nantucket, Massachusetts, to represent the Northeast, and Charleston, South Carolina, for the Southeast. Each site has an extensive tidal record and stands for a distinctly different coastal region.

They used climate-model data spanning 2000 to 2100, examining how the ocean surface responded as warming continued. The model generated century-scale maps showing wind pressure and heat transfer at the sea surface.

The outcome was clear: a single influence accounted for conditions at both coastlines. Rather than originating with local winds or currents nearby, the signal was linked to a heat source far further north.

Where ocean heat remains

That influence is heat flux in the subpolar Atlantic: the cold, turbulent ocean region south of Greenland, where warm currents arriving from the south descend and circulate back. This area exchanges heat between the atmosphere and sea at exceptionally high rates.

During the model’s 100-year simulation, the balance of that exchange changed. A greater amount of heat remained in the ocean instead of being released into the atmosphere, with the additional warmth changing both the arrangement of seawater and its density.

The effect was not confined to its source. Variations in ocean pressure, together with slowly moving coastal waves, transmitted it southwards until it appeared as increased water levels at Nantucket and Charleston.

A separate study connects this same circulation system with flood risk in the Southeast.

Wind and warmth

Wind still matters considerably. The researchers concluded that wind stress, the friction created as air moves across the sea surface, was the main influence on fluctuations from year to year and from decade to decade.

A run of stormier years, followed by a quieter period, can alter the readings registered by tide gauges. Over a century, however, periods of stronger and weaker winds cancel each other out.

The gradual increase in mean sea level instead stems from the northern heat signal, rather than from conditions off the US coast. Although both factors had been considered before, nobody had measured their respective effects alongside one another across an entire century until this work.

Working back to the source

The group applied a technique known as adjoint sensitivity, which effectively runs a physics calculation backwards. Beginning at the coast, it follows the signal in reverse to determine where it originated.

Using this method, the researchers could assess the entire North Atlantic, assigning weight to every location according to its effect on sea level at Nantucket and Charleston. The strongest areas appeared far from the US shore, particularly in the subpolar Atlantic.

Previous research had suggested this sort of distant influence, but much of it depended on correlations rather than cause and effect. Adjoint sensitivity follows the causes themselves, identifying where a signal physically starts rather than simply where it changes in parallel with coastal conditions.

East Coast sea level rise

The US East Coast occupies a particularly sensitive setting. Its continental shelf is wide and shallow, allowing water accumulating offshore to build up against the shelf and move inland in ways not seen on many other coasts.

This physical layout, combined with the gradual southward movement of signals from the subpolar Atlantic, connects cities from Boston to Miami with ocean processes far to the north. All are affected by the same fundamental driver.

Other studies have documented how strongly regional sea levels can differ from the global mean. The rate of rise along US coastlines has exceeded that average, and this research identifies a specific process that may explain the difference.

What may change

The research gives climate modellers a more precise area to focus on. Improving projections of East Coast sea level rise will require better resolution in the subpolar North Atlantic.

The results also change what coastal planners in Norfolk or Wilmington, North Carolina, may need to monitor. A warmer winter in the Labrador Sea could reveal something about their local water levels 10 years later.

Before this research, East Coast sea level rise was understood as the combined result of numerous overlapping processes. The leading driver now has an identified location: the subpolar North Atlantic and the heat it retains.