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Our Changing Gulf: The Atlantic Meridional Overturning Circulation

Take a look at your coffee cup when you add a teaspoon of milk. The cold milk sinks to the bottom of the cup. Blow across the top of the coffee cup. The top layer of the coffee will move and the milk below slowly rises to the top, turning your black coffee a smooth shade of brown. That, in its simplest form, is a convection cell. Now think about the Atlantic Ocean. At the equatorial latitudes, the water is hot. The air is hot. Everything is hot. At the northern latitudes, the water and air are cold. A northward flow of warm surface water, powered by the wind, moves from the south along the eastern coast until it ends up in the chilly North Atlantic. The warmer water rapidly loses its heat to the cold atmosphere and becomes more saline due to evaporation. Colder, saltier water is dense; the water sinks in the ocean off Greenland and Iceland and begins a slow process of moving at great depth back to the equator, where the process begins again. This convective motion is called the Atlantic Meridional Overturning Circulation (AMOC).

The AMOC is a major circulation cell in the Atlantic, impacting deep water in the Gulf of Maine. Image courtesy of Nature.

The Gulf of Maine is located at the border of cold northern water (the Labrador Current) and the warm surface current (the Gulf Stream). Because the Gulf is largely an enclosed sea, deep water enters it primarily through the Northeast Channel. The source and temperature of that water is largely controlled by the AMOC. The warming global climate has affected the AMOC’s strength. As more ice melts in the Arctic and Labrador Seas, more freshwater enters the North Atlantic. The warm southerly surface water that enters the region is diluted by the freshwater, reducing its salinity. The water then does not sink but rather remains in a cold layer at the ocean surface, effectively turning off the convective current. In addition, the oceans are growing steadily warmer, particularly at the northern latitudes, reducing the temperature contrast between southern and northern regions. This too has diminished the AMOC. Scientists estimate that the AMOC has weakened by about 15% since the mid-twentieth century.

The weakened AMOC, in turn, has had an effect on the Gulf of Maine. The Gulf Stream has shifted northward; the cold Labrador current flowing into the Gulf of Maine is weaker, allowing the warmer Gulf Stream water to enter into the Gulf of Maine. As a 2019 article in Oceanography, co-authored by Nick Record, Bigelow Laboratory for Ocean Sciences, Jeffrey Runge, Gulf of Maine Research Institute, and others noted, changes in the AMOC have resulted in deeper waters of the Gulf of Maine warming at twice the fastest surface rate.

Very small and vitally important to the Gulf of Maine food web, C. finmarchicus is affected by changes in water temperature at depth. NOAA image.

Warmer water at depth has had a marked effect on a multitude of species. But it is the behavior of the tiny copepod Calanus finmarchicus that has drawn researchers’ attention. C. finmarchicus is the preferred food of the North Atlantic right whale. C. finmarchicus goes dormant in deep water, below 100 meters, through late summer into winter. Record and Runge’s article reported that in the deep water of the Gulf of Maine, “The fastest warming rates occurred at depth in the late summer, autumn, and winter months. Warming was fastest at depths of 20–150 m, from August to February, reaching as high as 0.5°C per year, twice the extreme warming rates of 0.23°C per year reported by Pershing et al. for surface waters.” Too much warm water at depth may be one reason that C. finmarchicus abundance has changed in the Gulf of Maine. Right whales once traveled to the mouth of the Bay of Fundy each summer, where they stocked up on C. finmarchicus in large groups. Today the whales have become more rare in that area. Record and Runge suggest that in the eastern Gulf of Maine, the warming at depth has been so great as to make the area inhospitable to dormant C. finmarchicus.

As the authors note, “Warming has been most rapid in deep water during these seasons , likely reflecting changes in flow through the Northeast Channel. The decline in C. finmarchicus is likely a combination of reduced supply and a more direct effect of deepwater temperatures.”

Meanwhile, C. finmarchicus has been abundant in the western Gulf of Maine. The abundance is largely due to the transport of the copepods to the region by the Maine Coastal Current and is not due to changes in the AMOC. In Cape Cod Bay and south of Nantucket, right whales have shown up in large numbers in recent years, presumably to forage on the copepods. It’s a long way from the Gulf of Maine to the Arctic Ocean. Yet the steadily warming temperatures in that region and resulting ice melt are causing fundamental changes to the Gulf’s currents and ecology and, inevitably, its commercial fisheries.

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