A freshwater influx drains strength from the Gulf Stream
This story expands on the ArcheanWeb article “A chilly mystery in the North Atlantic” published 3-Jul-2020
Daily news stories constantly remind us the planet is warming. Temperatures are rising across most of the globe, but not everywhere. Far from land, in the stormy open seas of the North Atlantic Ocean, something strange is happening. A vast ocean area to the south of Greenland is cooling almost as fast as the rest of the planet warms, creating a cold spot on the global warming maps.
The Arctic and Antarctica are warming several times faster than the rest of the planet. Heatwaves bake areas in Siberia above the Arctic circle, melting permafrost, setting off massive wildfires, and providing some highly visible reminders of climate change. But other less obvious changes are taking place in the earth’s seas. More than 90 percent of the excess heat absorbed by the earth ends up in the oceans, making them the primary heat sink for our warming planet.
The general effect of this marine heating is an increase in average ocean surface temperatures and a corresponding rise in atmospheric temperatures. This process has been in swing for the past 120 years. However, the North Atlantic did not receive the message and bucked the global trend, cooling by about 1.6 degrees F over that time, and presenting a chilly mystery.
Making sense of this climate change anomaly requires an understanding of the North Atlantic circulation system—the Atlantic Meridional Overturning Circulation (AMOC). This system drives the famous Gulf Stream, which carries warm water from the tropics to the North Atlantic. Thereby providing Europe with its mild winters from heat supplied by the Gulf Stream.
Ocean circuitry
Oceans form a critical part of the earth’s heat redistribution system. Solar radiation is the primary source of heat for the planet, providing more heat to the tropics than the poles. Therefore, ocean currents help redistribute heat from the tropics to higher latitudes. These currents create the planetary circuitry of heat dissipation. But global warming is causing the oceans to rewire themselves in ways that interrupt existing circulation patterns.
One critical enabler of both shallow and deep marine currents is overturning (or thermohaline) circulation. It’s a process connecting surface currents with deep ocean systems. The only two places where this connection happens continuously, are the North Atlantic and the Southern Ocean around Antarctica. Seawater density provides the physical driver for these systems.
The process starts with cold, salty surface water. This water is denser than the surrounding ocean water, and it sinks. Its descent into the abyss is uninterrupted until the water lands on the seafloor. Flowing from there, it forms a river of dense, cold water hugging the sea bottom and traveling around the globe from ocean to ocean.
Eventually, when the deep currents warm, they rise and transform into surface currents. The Gulf Stream is part of this surface circulation system, forming the last leg of the AMOC before the water descends again to the abyss. Remember, when the dense North Atlantic water sinks, it creates a void of sorts, and warm water from the tropics and Caribbean flows northward to fill that gap. So, the real driver of the Gulf Stream is dense Arctic water. When less North Atlantic water sinks, then less tropical water flows in from the south. The Gulf Stream has waxed and waned over the millennia, changing in tandem with the strength of the Atlantic Meridional Overturning Circulation (AMOC).
History of the Gulf Stream as told by foraminifera
When dense surface water sinks to the ocean floor, it carries sediments and the shells of tiny single-cell organisms called foraminifera. The sediments accumulate at the seafloor, layer upon layer. Carbon isotope ratios, detected over time in the shells, reveal changes in the AMOC circulation strength. Eirik Vinje Galaasen used an 820-foot core from the seafloor off the southern tip of Greenland to investigate circulation history. He and his colleagues at the University of Bergen analyzed 500,000 years of overturning circulation in the North Atlantic.
Their analysis pointed to several periods of fluctuation where the AMOC weakened, thus weakening the Gulf Stream. Slowdowns were detected at 423,000, 335,000 and 245,000 years.
More recently, between the years 1300 and 1850, Europe experienced a general cooling of its climate. The average temperatures in Europe dropped by about 3.6°F. The name given to this period is the ‘Little Ice Age.’
Visible effects of this cooling were noted all across Europe. The Baltic Sea froze over in winters, along with rivers and lakes across Europe. Also, winters were bitterly cold, and the summers were cool. Because of the colder climate, crop failure was prevalent, leading to famine, population decline, and social unrest. Mountain snowlines dropped to lower elevations, and glaciers expanded, wiping out villages and farms. Additionally, as the North Atlantic turned into a cold spot, Arctic Ocean ice packs extended southward, making winter shipping impossible in some areas. Multiple theories try to explain the cause of the Little Ice Age; one of them is a weakening of the Gulf Stream. But the answer to how the North Atlantic circulation weakens lies in understanding variations in ocean salinity.
Saltwater and saltier water
Making saltwater even saltier is easy, just remove some of the fresh water. Mother nature provides two easy ways to do this by either adding or removing heat. Adding heat to warm Gulf Stream water increases its salinity. Hot sun and a lack of rain in the lower latitudes cause evaporation at the ocean’s surface. When water evaporates, the process only removes fresh water, thus leaving the remaining seawater saltier.
The second mechanism works in the frigid north as winter ice forms over the Arctic Ocean. Sea ice is not salty, so when sea ice forms, salt is forced out of the freezing water in a process called “brine exclusion.” The net effect is the remaining seawater becomes saltier, hence denser.
Saltwater has a lower freezing point than freshwater — 32 degrees F for fresh water versus 28.4 degrees F for normal seawater. Therefore, water leaving the Arctic ocean and flowing into the North Atlantic is extremely dense from being super-chilled and extra salty.
Dense, highly saline water continually floods into the North Atlantic from both these sources.
Greenland, a freshwater showstopper
Researchers from the University of Bergen suggest the reason for ancient slowdowns in the AMOC, and hence the Gulf Stream, was a freshwater influx from the melting of Greenland’s ice sheet. As this fresh water flooded into the North Atlantic and mingled with the seawater, the net effect was to decrease the density of ocean surface water. When the North Atlantic water was not dense enough to sink, the tap for deep-ocean circulation turned off.
The Greenland Ice Sheet has lost almost four trillion tons of ice since 1992, according to a study from NASA and the European Space Agency (ESA). The massive volumes of fresh water flowing into the North Atlantic lower the salinity, and hence the density, of surface waters. So, with less water sinking to the ocean bottom, the Gulf stream slows down and thus carries less heat to the North Atlantic. With less heat, the ocean cools, solving the puzzle of why this area of the North Atlantic is chilling and creating a cold spot as the rest of the planet heats up.
More from ArcheanWeb:
Riding the global conveyor belt (Source: ArcheanWeb) – https://archeanweb.com/2020/04/06/riding-the-global-conveyor-belt/ Also:
Thermohaline circulation (Source: ArcheanWeb) – https://archeanweb.com/2020/04/03/thermohaline-circulation/ Also:
Sources:
Ancient warming threw this crucial Atlantic current into chaos. It could happen again (By Paul Voosen; Science – AAAS) – https://www.sciencemag.org/news/2020/03/ancient-warming-threw-crucial-atlantic-current-chaos-it-could-happen-again Also:
If you doubt that the AMOC has weakened, read this (Source: RealClimate) – http://www.realclimate.org/index.php/archives/2018/05/if-you-doubt-that-the-amoc-has-weakened-read-this/ Also:
Why Earth has a stubborn spot that’s cooling (By MARK KAUFMAN; Mashable) – https://mashable.com/article/cold-blob-atlantic-ocean-climate-change/ Also:
Greenland’s Rapid Melt Will Mean More Flooding (Source: NASA) – https://www.jpl.nasa.gov/news/news.php?feature=7556 Also:
Feature Image: Ocean Storm (Modified) – Credit (Original) – Fer Nando; Unsplash – Public Domain – https://images.unsplash.com/photo-1532943126512-e2bcf68a488c?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=500&q=60
