With its powerful eastward-flowing Antarctic Circumpolar Current and, further south, its westward-flowing East Wind Drift, and its vigorous sea-ice formation, the Southern Ocean makes its own water masses. It is also continuously receiving and entraining migrating water masses from all other world oceans. It has even been dubbed ‘the southern mixing ring” by some investigators, as it mixes water masses from everywhere.
The array of water masses to be observed here is exceptional. Their differing properties will yield valuable data to be compared and added to the existing scientific knowledge bank. We’ll highlight some representative examples of such water masses in this blog.
Photo caption: This photo shows several generations of Antarctic sea ice. Background left: tabular berg is an iceberg calved from shelf ice, permanent ice on the water near the edges of polar landmasses. Middleground: year-old to-several-year-old sea ice. Foreground: newly forming sea ice appears as a “slick” in lower center with catwalk on deck of ship RV Nathaniel B. Palmer, on right. Photo courtesy of Professor Sherwood Willing Wise, Jr., FSU/NSF Antarctic Research Facility, Tallahassee, Florida, U.S.A., from SHALDRIL II cruise, 2006.
Revving up the engine...
While some of the ocean’s surface currents are familiar to many of us and constitute part of the ocean’s discernibly separate water masses, surface ocean currents are moving only about 10 per cent of the ocean water at any given time. They ultimately skirt the western boundaries of ocean basins moving warm water from tropical latitudes toward each pole.
The Gulf Stream in the Northern Hemisphere is an example already discussed, and we’ll discuss it further. And of course we rode the Agulhas Current in the Southern Hemisphere during our first week cruising. Each one of these and other major surface currents move 100s to 1000s of times the flow of the Mississippi River; some of them move it at speeds approaching or reaching 6 miles per hour.
By far the greater component of the ocean’s water transport, at about 90 per cent of the ocean water in motion at a given time, is its deeper circulation. As opposed to the surface currents, swift and driven by prevailing winds, the deeper circulation is very slow, driven by high-density water under gravity’s influence. It takes about a whole year to move the deeper water the same distance a surface current can carry it in just one hour!
So why is all this water moving? How do we get from the surface to the deep circulation?
Cold temperature and increased saltiness drive the deeper circulation. Both, especially together, increase the ocean water’s density enough to make it sink, giving this type of circulation its name. “Thermo” derives from temperature; “haline” derives from salt, and the deep circulation is called the thermohaline circulation or THC.
Sufficient densities for sinking result from just the right combination of surface-water conditions. Critical density thresholds are reached in some high-latitude locations in both the North Atlantic and the Southern Oceans, The Norwegian Sea is a well-known North Atlantic sinking location, as is the Weddell Sea in the Southern Ocean.
Due to its salts, seawater has a colder freezing point than freshwater, so it’s still liquid below what most people think of as water’s freezing point. In the process of freezing, extremely cold seawater excludes its salts, simultaneously forming essentially fresh-water ice and extra-high-salinity water beneath the forming ice with its ejected salts. Saltier than the water around it. the water beneath the forming ice reaches a sufficient density to sink to the bottom.
The sinking North Atlantic water is called the North Atlantic Deep Water (NADW). The Southern Ocean equivalent of the NADW is called the Antarctic Bottom Water (AABW). It sinks to the deepest depths of the Southern Ocean.
Extremely cold freshwater left behind by melting sea ice can also cool enough to sink. This process forms a less dense water mass than the bottom water, and it occupies intermediate depths in the Southern Ocean. It’s called the Antarctic Intermediate Water (AAIW).
All three of these sinking water masses reach their respective ocean depths at high latitudes and begin their extremely sluggish return flow toward the opposite pole. The AABW is the densest of the three and in moving north, it ultimately underflows its less dense northern counterpart, the NADW. These water masses will remain isolated from surface water and atmospheric influences for hundreds to perhaps a thousand or more years before resurfacing via upwelling in other parts of the ocean.
Getting it in perspective...
The volume of this very dense ocean water sinking in, e.g., the North Atlantic, is enormous — nearly at the limits of comprehension — a volume equal to about 100 Amazon Rivers. (Just for perspective, the Amazon alone disgorges about 20% of all the river water on earth and four times as much as the 2nd highest-volume-discharge river, West Africa’s Congo).
Water from the Gulf Stream, swifter than any other surface current, surges in to replenish this enormous volume of sinking North Atlantic water, much as the water in the non-drain end of your bathtub rushes toward the drain after you’ve pulled the plug. The freezing and consequent sinking of all this dense water are literally drawing the Gulf Stream’s waters along on their powerful poleward rush.
Ventilating the deep ocean...
This downward transit of huge volumes of water occurs at polar latitudes near both poles and ventilates the deep ocean with oxygen. We now know this source of oxygen sustains life at extreme depths where, not so many decades ago, no life was believed to exist.
Due to gases’ retrograde solubility, dissolving more readily in cold water than in warm water, the sinking low-temperature water can hold more dissolved oxygen than warm water would be able to hold. On its sluggish return journey, the deep water also picks up nutrients from detritus decaying on the ocean bottom.
The marine smorgasbord . . .
Cold and nutrient rich, this water spreads into all the world’s ocean basins, eventually surfacing at the equator and at a variety of coastal upwelling zones, many along eastern boundaries of ocean basins, especially at capes. Some of the water is thought to surface also above bottom features that direct the flowing water sharply upward.
Some of the deep circulation can even travel almost as far as the polar area opposite the pole near which it formed and into other parts of the world ocean, only some of which have been identified. The eventual rising of all this nutrient-rich water elsewhere underpins productive ecosystems and fisheries in all the world’s ocean basins.
The ocean conveyor belt...
The overall circulation scheme, with surface water going down at extreme limbs of the circulation, and bottom- and intermediate water coming back
up at various locations, resembles a conveyor belt. So the entire circulation is often called the ocean conveyor belt. Total surface circulation time at the air/sea interface is very short, while time cut off from atmospheric contact and surface-ocean influences is, by comparison, extremely long.
The conveyor belt carryies solar-heated water poleward on the surface and sinking polar-chilled water back toward the tropics on the bottom. It moderates world climate and ventilates the deep ocean with atmospheric input. Some investigators have likened it to a giant heat pump, a great moderator of earth’s temperature.
Without the benefit of this exchange system, the poles would be even colder and tropical heat could build up to a greater extent than it now does with more profound polar/equatorial temperature differences than we currently experience.
Why does it really matter?
Picture now the warm Gulf Stream moving away from the U.S. East Coast as it passes Cape Hatteras, N.C., and continues northeast. The atmosphere over the current is warmed by the heat it carries. That moderated air, carried on to the east by prevailing westerly winds, gives an unexpectedly mild climate to some coastal areas of Northern Europe. Southwestern parts of the British Isles, at the same latitude as Newfoundland but much warmer, can and do grow palm trees and lemon trees.
Scientists have studied the ocean conveyor belt phenomenon now for several decades. They have found evidence in climate history that during times when the conveyor belt’s vigor diminished, a pronounced cooling trend set in over Northern Europe.
A well-known example is the prehistoric period called the Younger Dryas. About 12,000 to 11,000 years ago, several millennia after the waning of the latest glacial period, northern Europe lapsed abruptly back into ice-age conditions for this thousand-year anomaly. Less extensive and more recent agriculturally disruptive cooling periods may be connected to a slackening of the conveyor belt’s strength, as well.
That could be triggered by warming from greenhouse gas increase and melting of enough Greenland ice cover to freshen North Atlantic water beyond the density threshold needed to maintain seawater sinking in the North Atlantic.
Coupled with the possibility of an unventilated deep ocean if ocean water no longer delivers its life-giving dose of oxygen, we would face the possibility of a stratified ocean and cessation of rising nutrient-rich water in some of our most productive marine areas. Mass collapses of ocean ecosystems and plummeting marine diversity might be expected to follow.
When we consider all the things we are just beginning to recognize about the ocean’s many life-sustaining functions, we are forced to realize we cannot really comprehend more than a few of the potential risks we might face if our ocean circulation stops. Earth systems are simply too complex. We need to concentrate on learning as much as we can to avoid taking chances that our activities might disrupt our own life support systems.
We feel privileged to be here, learning all we can about the complexities of our natural systems’ functions and what they need to keep them healthy. This cruise, with the funding support of the National Science Foundation, including ship time, is a valued opportunity to make our own small but vital contribution to that effort. Thanks, NSF!