If you follow the latitude lines from much of Europe westward across the Atlantic, you tend to run into Canada. Even if you go to the southern tip of Spain, you're not much further south than the Virginias. Canada, of course, has a reputation for being rather frozen and inhospitable, while Europe goes to pieces if it snows for more than an hour or two. The difference is mainly due to ocean currents.
At the north edge of the Atlantic Ocean, warm surface water cools off and sinks, drawing in more warm surface water from the south, generating a warm surface current along Europe's Atlantic coast. Portions of this current comes in from the tropics near Africa and South America, and more is drawn in from the Indian and Pacific Oceans. In addition to making Europe a pleasant place to live, this current also provides the main source of ocean mixing—that is, these currents act to cool the ocean surface temperatures and heat the deep ocean.
This thermohaline circulation is more complicated than I have outlined because, as its name implies, ocean salinity also plays a role: surface rain and melt water from glaciers and permanent ice, which change the local salinity of the ocean, also act as drivers for convective flow. In addition, because the warm water sinks in the North Atlantic, there must be an upwelling somewhere. Major sites of upwelling include the northern part of the Indian Ocean and the North Pacific. But, the North Pacific is kind of cold as well... so why does it upwell there?
Since the oceans are a critical part of climate modeling, the more confidence we have in our understanding of the dynamics of these currents, the better. This is precisely what Anna von der Heydt from Utrecht University is trying to do.
Her studies focus on finding stable solutions to the flow dynamics and examining how the stability of these solutions are influenced by various factors, including the northern extent of the two major ocean basins (Atlantic and Pacific) and their connectivity.
First, she modeled the Atlantic and Pacific oceans as having both the same size with a single connection lying to the far south. She found that there were four stable solutions: the one we see today—warm surface currents sweeping northward in the Atlantic Ocean from the Pacific; a reversed solution—warm currents flowing south out of the Atlantic and into the Pacific; and two solutions that involved no global circulation at all, just local convective cells sited at the northern ends of the Atlantic and Pacific oceans.
Geography appears to play a major role in dictating what happens in the real world. The Pacific Ocean closes off in a different location than the Atlantic does, and adding this to the model effectively eliminated the reverse global circulation solution.
We know that the thermohaline circulation is not just relatively stable, but also the one that actually exists—the geologic record shows that it has been in existence for a while. So, why that instead of the other solutions? To answer this question, we need to look back a bit further, when the two Americas were still separate. The ocean basins were still largely the same—the Atlantic was a bit smaller, the Pacific a bit larger, but overall not that different. However, their connectivity was very different, with a large channel near the equator joining the two oceans.
Adding this connectivity into the mix essentially eliminated the global circulation solution. It did still occur for some conditions, but it was a much weaker current, while the two locally circulating solutions were still found to be stable. Qualitatively, this is what has been observed—sediment data suggests that there was no global circulation when North and South America were separated—indicating that nature does indeed like the alternative solutions as well, even if the details may be slightly different.
The final part of the talk looked at how we transition from one stable solution to another, a situation called bi-stability. Imagine you are where we are now, in one of the circulating solutions. Now, let's increase surface temperatures: as we do so, we move along a line where the flow characteristics of the current only change a little bit. Then, with a final small temperature change, the solution becomes unstable, and the system begins the transition to one of the non-circulating solutions. Other research indicates that this transition takes place on the order of about 100 years.
"No problem" says our local village idiot, we just drop the temperature a fraction and everything will start back up again. But, sadly, no, this is not what occurs in a bi-stable system. Instead, the system is in a new stable solution whose parameters overlap with that of the previous solution. We need to keep reducing the temperature until we exit the basin of stability for the non-circulating solution. Only then will the system jump to a new solution (quite possibly the old solution) of global circulation.
For Europe, which may experience dramatic changes to its regional climates if the thermohaline circulation changes, this represents a very scary scenario, because we don't know where the point of instability is. We also don't know how we would get back to a circulating solution should the worst occur. And the measurements we presently have of the current are so noisy that we aren't likely to see it coming until it has already happened.