· 8 min read
There was no ambiguity to the title of the juried science journal article: “Warning of a forthcoming collapse of the Atlantic Meridional Overturning Circulation.” Any year within this century, ocean currents will completely stop if we continue to emit the same level of greenhouse gases as we do today. When the movement of the Atlantic Ocean ceases, it will become too cold north of London for trees to grow, and equatorial latitudes, including Florida, will sizzle.
A close read of this article reveals that “indications” that “suggest” ocean currents shift a bit in intensity have been found. Weaker or stronger is not clear in the data. This is not surprising because we know that an arm of the Labrador Current flowing through the narrows between Georges Bank and Browns Bank into the Gulf of Maine varies from year to year. Some years, it does not. We have no idea when Labrador Current Water will enter and when more Continental Slope Water will flow into this sea beside the Atlantic. The oceanographic terms are barn door open, barn door closed, and barn door ajar.
The Atlantic Meridional Overturning Circulation (AMOC) is part of the world’s ocean current. It begins in the Greenland Sea north of Iceland, travels around the Atlantic into the Pacific onto the Indian Ocean around the Antarctic continent, and back counterclockwise around the Atlantic Ocean, past Norway into the Arctic Ocean. It bears right, once around the Arctic Ocean before the circulation is completed southbound in the Greenland Sea. A bucket of seawater of the right density poured into the Greenland Sea will take about 1,600 years to circulate.
Wallace S. Broecker first described the Great Ocean Conveyor Belt that wraps around the world. His discovery came after several decades of tracing the routes and using radioisotopes to date the age of deep waters. Ocean circulation determines local climates. The world turns to the East, causing equatorial ocean currents to flow West and to bend to the right, driving the Gulf Stream north to warm Britain and Scandinavia. In the Denmark Strait between Iceland and Greenland, cold nutrient-rich water from the Arctic Ocean meets and dives 11,000 feet below warm nutrient-Atlantic water. The East Greenland Current bears right around Cape Farewell to flow north by West Greenland into Baffin Bay and south back into the Labrador Sea between Greenland and Labrador. The Labrador Current, strengthened by water from Hudson Bay, cools Newfoundland and New England.
Broecker’s interests in connections between the ocean and atmosphere were heightened when ice cores from Greenland indicated 25 climate changes during the ice age of the last 150 million years. The abrupt switching between ice age conditions and warming conditions, a difference of 11 to 15 degrees Celsius, happened over decades. Research of foraminifera in deep ocean sediments found concurrent stop-and-go action to the Great Ocean Conveyor Belt. The quest for what caused climatic switches is on. All eyes now turn to the Greenland Sea as we seek to understand the phenomena that turn ocean circulation and global climates on or off.
Scientists warn that ocean circulation may cease within the century and base that conclusion on some sparse measurements in the Greenland Sea collected over seven years. They said the sea surface temperatures are “fingerprints testifying to the strength of the AMOC.” Ocean circulation ensures that surface waters are connected to deep ocean currents.
With a pint glass of tap water, we can look for surface water fingerprints and what drives the Great Ocean Conveyor Belt around the world. A tablespoon of salt mixed into a pint of water is 40 parts per thousand of salt. This is the saltiness of the Mediterranean Sea, which is higher than the Atlantic Ocean, which is 36 ppt. Wallace Broecker observed that the Atlantic Ocean should have a salinity of 35 ppt, except for evaporation, which raised it to 36 ppt. He postulated that a warming sea might have more evaporation. The Gulf of Maine, a sea beside the Atlantic Ocean, is less salty at 34 ppt due to the freshwater warmed by the land that enters from rivers.
Place an ice cube into the glass of briny water that is no longer circulating from the stirred salt. Drip a drop or two of food coloring onto the ice cube. The color will pool on the cube and then slide off with the meltwater to spread out on top of the salt water. Density is both temperature and salinity. Although cold water is more dense than hot water, the freshwater will form a band on top of the salt water.
There are no fingerprints here because water bodies, defined by temperature and salinity, stay independent and can travel in their own directions. The southbound Labrador Current flows easily beneath the Gulf Stream. The Labrador Current delivered an iceberg to the Titanic.
A patch of fresh water on top of saltwater is known to dinghy sailors racing around a course as a “slippery sea.” Tidal currents move one way, while the lens of less density pushed by the wind may slide in a different direction and speed. Sea-savvy sailors win more races.
Let’s take another glass for a comparison of the two water bodies in a pint glass. Add an ice cube to a pint of fresh water. Drip the food coloring on the berg, watch the colored icy water cascade to the bottom, and feather out laterally. Cold water is denser than warm water and sinks. When the fronds of colored water warm to the temperature of the surrounding water, the water will mix thoroughly to be one color.
Touch the freshwater pint glass and feel the temperature. Compare this to the saltwater body in the other glass. Which one is warmer? Touch the colored water on top of the salt water. This water body may be so much colder that condensation will likely form outside the glass. The ice cube in freshwater has entirely melted away, while in the water body above saltwater, the ice cube survives (much to the distress of the Titanic). The temperature of surface water has no relationship to the temperature of the water below.
The Great Ocean Conveyor Belt may remind you of a cafeteria where trays of dirty dishes are placed on a belt to “ensure” they are “connected” to the dishwasher. In the ocean, the sinking of cold, dense water powers ocean circulation. The Earth’s rotation has set up the direction of flow that is strengthened by thermohaline circulation.
The Arctic Ocean is often overlooked as the source of increased ocean circulation. Our maps have blinkered many not to see the Arctic Ocean as part of the belt and instead assume the start is at the top of the projection, to the right of Greenland.
Once, the Arctic Ocean was covered by sea ice for nearly two-thirds of its surface at the end of summer. In recent years, summer sea ice has diminished to cover less than a third of the Arctic Ocean. When sea ice forms in October, the freezing action involves more than twice the area of surface waters.
Pure water will lock into the ice matrix at 32 degrees F. The sea ice is composed of fresh water. In a process known as brine exclusion, the salt accumulates in the adjacent frigid water. This becomes the densest water in the world and sinks to drive the thermohaline circulation of the Great Ocean Conveyor Belt. (It also happens in the Southern Ocean around the Antarctic.)
The Svalbard archipelago on the threshold between the Atlantic and the Arctic Oceans is the other end of the transport of the Atlantic Meridional Overturning Circulation. In 2007, warm Atlantic water surfaced in Svalbard to cause glaciers to melt on the islands. With the Gulf Stream flowing stronger, more warm water is going into the Arctic Ocean beneath the surface water body. This is why the sea ice melted back, exposing more open water than atmospheric scientists had expected. They could not see beneath the ocean’s surface (or read reports from Svalbard).
An indication of a stronger flow for a more robust AMOC was observed in 2011 when the Gulf Stream meandered up onto the continental shelf closer to Rhode Island than ever recorded before. Rivers meander to dissipate energy. For the Gulf Stream, it is necessary to dissipate energy by meandering after squeezing through the Florida Straits between Florida and the Bahamas.
The Gulf Stream jets through the Florida Straits at a clip of 28-32 Sverdrups with a seasonal variation of 3.5 Sverdrups (slower in winter). A Sverdrup equals 1 million cubic meters per second, or the flow of five Amazon Rivers—all the rivers in the world discharge about 1.2 Sverdrups into the ocean. Flooding northwards, the Gulf Stream picks up water. When the Gulf Stream is East of New England, the mighty current moves at 150 Sverdrups per second or about 700 Amazon Rivers. Greenland meltwater and greenhouse gases will not be slowing down this freight train.
Raise a glass to the health of the ocean. The immensity and power are beyond comprehension by all but the most experienced seamen who are humbled by it. However, we know a pint glass and can see how a slippery puddle of fresh water can do nothing to the briny deep. With cold melt water sitting on salty warm water, we see the ocean is not a bathtub where one can check the water temperature with a toe. The ocean is deep, with layers of water bodies defined by thermohaline density moving in different directions at different speeds. Don’t judge an ocean by its surface and underestimate it at your peril.
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