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Dispatch 4: Overturning

2nd August 2012

I like to visit the bridge a couple of times a day to watch the pros expertly con the ship and to exchange sea stories with Piers, the Second Mate, but it’s an unequal exchange; he’s been at sea professionally since he was fifteen.  This afternoon he gestured me to the portside bridge wing.  “Have you ever seen this?”

It was a fogbow, like a rainbow but of different origin.  I had seen them in the Arctic before, but this one was unusual in my experience.  It arced horizontally, instead of vertically, perhaps because the fog bank was low, patchy blue sky visible above it.  To witness strange atmospheric phenomena is one of the abiding pleasures of Arctic sailing.  Farther north we may be treated to the Northern Lights, but this is not the optimum time of year for Aurora watching.

Our present position is 67°38’ by 24°02’, about sixty nautical miles NNW of Iceland (one degree of latitude equals 60 nautical miles, and one nautical mile equals 1.15 statute miles), in the Denmark Strait.  We’ve been talking about the circulatory significance of the strait, so now let’s try to place it in a broader hemispheric context by taking an imaginary—but quite naturalistic—drift from the subtropical Atlantic back up here to the subarctic aboard a small boat with no power, letting the current do the work (we’ll assume fair weather).  The Straits of Florida might be a good place to start for the sake of perspective and a sense of the interconnectedness of the oceanic system.

The bruise-blue Gulf Stream water is blowing through the close quarters between Florida and the Bahamas at a steady four knots, transporting some 25 million cubic meters of water every second past Miami, Ft. Lauderdale, and Palm Beach.  North of the straits, we slow a bit, but the Gulf Stream, hugging the continental shelf, still sprints northward toward the bulge of Cape Hatteras, North Carolina at high speed.  If the weather’s clear we’ll get a glimpse of Hatteras before the Steam bends our course northeastward out into the open North Atlantic.  It’s still not definitively clear just why the Gulf Stream departs the coast and heads toward Europe.  Though it’s called a surface current, the Gulf Stream’s influence reaches down thousands of meters, so it’s probably being steered northeastward by bottom topography, bathymetry in technical parlance.  Now, unconstrained by land, in very deep water our current begins to meander freely, sinuously.  If we don’t get deflected into one of the big eddies that wheel in opposite directions on either side of the main flow, we’ll ride on toward Europe.
However, contrary to popular belief and usage, the Gulf Stream proper never reaches Europe.  The fast, clearly defined current changes radically in character in the vicinity of the Grand Banks seaward of Newfoundland, dissolving into filaments of flow that bend southward in a broad, slow drift bound back toward the tropics on the east side of the Atlantic basin.  But we’re not going that way.  
We’ve been entrained in an extension of the warm Gulf Stream called the North Atlantic Current (NAC) now veering off to the north from near Flemish Cap on the Grand Banks—toward the British Isles.  Most people have heard of those palm trees growing in the south of England and even in Scotland.  By geographical rights, there should be no palm trees in the UK.  London’s latitude is 50° North.  Fifty degrees North on the other side of the Atlantic cuts through frozen Labrador.  The incongruous palms are nurtured by the NAC and the prevailing westerly winds that carry its warmth onto the land.  And in this we have a perfect example of the relationship between ocean circulation and climate.
But as our boat passes north of Scotland, things are changing deep beneath the surface.  Though our surface current hardly notices, we’re passing over a submarine mountain range running from northern Scotland west, through Iceland, and on across the Denmark Strait to Greenland.  In fact, it’s called the Greenland-Scotland Ridge.  Crossing the ridge, we enter the Nordic Seas.  Here the NAC gains a name change, to the Norwegian Atlantic Current, but that’s just nomenclature.  It’s the same warm current that graced Britain and now does the same for Norway, engendering ice-free conditions year around well north of the Arctic Circle.

'And in this we have a perfect example of the relationship between ocean circulation and climate'

Now autumn is coming on, and the current is quickly relinquishing its warmth to the cold air.  Cold water, like cold air, is heavier—denser—than warm, and the tropical-origin salinity increases its density still more.  Soon, with the onset of winter, the cold, salty water begins to sink.  The trip’s getting uncomfortable and dangerous now; it’s time to go ashore and let the ocean circulate without us.  Besides, the surface current, stream-like a few hundred miles back to the southwest, is growing complex and confusing.  There’s no telling where our boat would end up if we’d stayed aboard.  It might drift off over the top of Norway into the Barents Sea.  It might travel with an arm of the Norwegian Current over Spitsbergen into the Arctic Ocean.  Or perhaps it would flow around the edge of the basin—currents like to hug the edges of ocean basins—and down the East Greenland coast.  In any case, an “ocean problem” has arisen:

Those millions upon millions of cubic meters of Atlantic-origin water flowing into the Nordic Seas basin must now find a way out.  Nature insists that mass must be conserved.  Therefore, if a quantity of water flows north, then an equal quantity must flow back south.  If it didn’t, then Western Europe would have been submerged eons ago.  But where can the water go?  It has already gotten a start by sinking behind the Greenland-Scotland Ridge.  Now it can flow—or as they say, “leak”—back south through gaps in the ridge.  The majority of the southbound flow spills over the relatively shallow sill in the Denmark Strait and plunges into the deep Atlantic.  Though an extremely complex, only partially understood process, this violent avalanche of water reforms itself into a narrow stream, the so-called Deep Western Boundary Current, one of the great wonders of ocean dynamics, and proceeds southward at depth beneath the Gulf Stream toward the equator.   Thus mass is conserved, the circle complete, and a steady state achieved. 
The elegant, magnificent system I’ve only cursorily described—the northward surface flow of warm, salty water from the Gulf Stream System and the return of cold, dense water at depth—is called the Meridional Overturning Circulation (MOC).  The stability of global climate literally depends on the integrity of the MOC.  Should any segment of the circle be severed—say, by an influx of fresh water from the melting Greenland Ice Sheet or some other feedback from global warming—then climatic disaster will result.  No serious scientist is going so far as to say that will happen, only that it could happen; the potential is inherent to the system.  Ocean circulation is one of the great geophysical forces that shape the climatic conditions of our world—that’s reason enough for scientists to study its ways and means, and, since we live in the world, reason enough for us to look over their shoulders as they do so.  Besides, no one knows where the endeavor may lead.  In the ocean may lie answers to questions humankind have not yet thought to ask.