I've briefly mentioned thermohaline circulation as a mechanism by which oceanic temperatures can be regulated through mixing. The most important example of this is the Global Thermohaline Circulation, which is responsible for warm surface currents like the Gulf Stream that keep Western Europe so pleasantly toasty.
The main driver for this global cycle is a region of forcefully sinking water in the north Atlantic and around Greenland. The water arriving from equitorial regions already has a salinity surplus due to evaporation further south. Upon reaching the Arctic, two new processes kick in. First, the cold air temperature leads to rapid cooling of this water. Second, the seasonal formation of ice shelves expels additional salt into the surface waters. The resulting chilled brine sinks like a stone, and thus keeps the whole "conveyor" moving.
That's why I was rather alarmed when I ran across this.
CLIMATE change researchers have detected the first signs of a slowdown in the Gulf Stream — the mighty ocean current that keeps Britain and Europe from freezing.
They have found that one of the “engines” driving the Gulf Stream — the sinking of supercooled water in the Greenland Sea — has weakened to less than a quarter of its former strength.
Besides a couple of UK papers, I've hardly seen this reported anywhere.
Thermohaline circulation first came onto most people's radar five or six years ago, when computer models of global warming first began to meaningfully include ocean currents. Mostly reported as an oh-isn't-that-interesting story, it was noticed that in extreme cases it was possible for global warming to shut down the "great conveyor," as the press dubbed it at the time, leading to all sorts of climatic disruption potentially including a European mini-ice age. However, since those models were outlyers and nobody really believed the ocean could change all that fast, this wasn't considered a scenario likely enough to call for much concern.
Now it appears that this may have been a naïve attitude. For the past several years, Professor Wadhams's Polar Ocean Physics Group out of Cambridge has been hitching rides on Royal Navy submarines under the Arctic ice to study the actual sinking process. It turns out, this ocean process appears able to turn on a dime:
Peter Wadhams, Ph.D.: "The problem has been that in the last few years in fact, since 1997 this area of ice has not formed in the Greenland Sea. Part of the reason is global warming and partly because the wind system has changed. So, that part of the Greenland Sea remains ice free and there is not the driving force for the sinking of the cold water.
This means that the volume and depth of this sinking process, or convection, has been reduced. That's weakening the circulation of water in the Atlantic and there is a reduction in the amount of warm water that once headed north...
We've gone there every winter and summer for the past four years. We found that the sinking happens in the form of current 'chimneys' that's a kind of narrow 10-kilometer-wide rotating column of water where the surface water is sinking through that column down into the deep ocean. This seems to be the mechanism by which the circulation happens.
We found that these 'chimneys' are very long-lived. But there are only two of them in existence at the moment, while seven to eight years ago when they were first studied, there were at least a dozen.
If you're interested (and you should be) I'd encourage you to read this entire interview with Prof. Wadhams. While I get the impression that the interviewer is trying pretty hard to pump some kind of crazy-alarmist soundbite out of him, he does an admirable job of giving even-keeled answers that stick to what he actually knows, but manage to convey the potential gravity of the situation.
One thing he points out, which the breathless news piece I led with misses completely, is that all we know so far is that the density-driven sinking has significantly decreased recently. The wind-driven component of the surface currents seems to be chugging along just fine, for now. Moreover, since these observations span only a few years, nobody yet has a good handle on how much variability we'd expect, anyway.
But now we do know that an apparently very stable hydrodynamic feature of the north Atlantic isn't anymore. And we also know that the Odden ice shelf has stopped forming for the first time in, perhaps, centuries. Don't Panic probably remains sound advice, but I might double-check the location of my towel.