Due to a shrinking ice cap, the Arctic Ocean has twice the open seawater; therefore, greater volumes of water freeze in the fall. When ice forms, salts concentrate in the adjacent liquid. The water around the floating ice becomes saltier. Very cold and denser, it sinks. This pumping action of more deep water puts more energy into the thermohaline circulation to strengthen it.
The Gulf Stream transports warm water northwards at a velocity ranging between 85 to 105 sverdrup (1 sverdrup = 106 cubic meters per second). The Amazon River, the largest river in the world, carries 0.2 sverdrup. The Gulf Stream is five times stronger than the AMOC!
More seawater was observed coursing the Gulf Stream in October 2011 when the intensified Stream meandered in a deeper curve, swinging further northwest up onto the continental shelf. For a couple of months, the Gulf Stream sped closer to Rhode Island than ever before. The Gulf Stream is like a river that gains much energy cascading down a mountain. When the water’s descent is stopped by hitting a plain, the energy must go somewhere rapidly. Energy must dissipate, and so the river meanders. With more volume and energy, it curves back and forth instead of flowing in a straight line. The greater the river’s departure from the straight and narrow, the greater its “sinuosity,” as it gives up potential energy. Viewed from a distance on high, the river's course can remind one of a freight train crash, dissipating energy by sending cars every which way.
The Gulf Stream is forced to accelerate through the Florida Straits. More water volume through the Straits results in more energy being released. This water energy, the potential-vorticity budget, is spent on the structural distortion of flow in a meander compared to the straight portion. Meanders bunch up and circle back to cut off, forming “ox-bows,” warm-core eddies that spin westerly, and cold-core eddies that drift east into the Sargasso Sea. The unprecedented large meander onto the Continental Shelf off Rhode Island indicates more warm water in the Gulf Stream barreling north to the Arctic Ocean.
This brings us back to the narwhals mentioned at the beginning of this tale. They live and carry out their sophisticated monitoring processes on the threshold between the Arctic and Atlantic Oceans.
To find evidence of seawater warming in Baffin Bay, researchers found historical data sets collected between 1950 and 2003. Bottom water temperatures exhibited statistically significant warming at depths between 1,300 and 7,000 feet. The maximum warming was found in a water mass between 2,000 and 2,600 feet. This warming of Intermediate Water was as large as 0.2°C (0.36°F) per decade.
Given the challenges of deploying CTD Rosette water trapping arrays over the side of a ship and the amount of sea ice in Baffin Bay that could wreak havoc to a ship tethered for the better part of a day by hydro-winch cable to gear over the side, researchers turned to the local narwhal population for assistance.
Narwhals dive to the ocean floors of Baffin Bay ten to twenty-four times a day for Greenland halibut. Fourteen narwhals were captured and fitted with satellite‐linked time‐depth‐temperature recorders. When they surfaced, data was transmitted to the researchers. CTD-Rosettes were used from ships and helicopters to verify the accuracy of the whales' findings.
Join us to finish up the story at the bottom of Baffin Bay in Part 5 of our Narwhals' Tale series.
See you over there,
Rob
