We’re leaving Europe’s shoreline now, though it would have extended farther west back in the icy periods when the sea level was a lot lower because ice sheets locked up so much water.  Except for the ice, you could have built a forty-story hotel at the seashore about 16,000 years ago; by 12,000 years ago, the waves would have been lapping at the windows halfway up.  By 8,000 years ago, they’d have been over the top.  A number of river valleys, carved in glacial times, were drowned when the sea level rose, such as the Thames below London, San Francisco Bay, and Chesapeake Bay.

     The warm waters of the North Atlantic Current now flow up the west coast of Scotland and then continue all the way up the Norwegian coast; Oslo’s harbor is ice-free all year.  The warm current splits, some heading west toward Greenland, and the rest continuing north into the Greenland Sea, keeping a large area free of sea ice.  I mentioned that sea ice can cap the ocean surface and prevent the winds from evaporating water and leaving salt behind.  And that, along with rain and meltwaters, can interfere with sinking the surface waters.  Let me warn you that the “how” of things is going to be pretty salty.

     If you want to see cold sinking in action, pour some hot coffee into a tall transparent mug.  Wait for a few minutes for the motion to settle.  Then get some very cold cream (not half-and-half) and, using a spoon lowered to its rim in the coffee, gently pour the cream into the spoon, allowing it to overflow the rim and layer out onto the surface of the coffee.  If you do this right, as my friend Dan Hartline showed me a decade ago, you will soon be rewarded by the sight of a column of cream plunging to the bottom.  Though cream often floats because of its fat content, its density increases when cold, enough to sink through hot coffee.  Indeed, the density buildup from salt excess and evaporative cooling is what causes the North Atlantic surface waters to sink so dramatically.  (Unlike the thermohaline circulation,  Dan’s trick has a reversing feature:  Once the cream warms down on the bottom, it will come geysering up to the top again in some very pretty turbulent plumes, finally ending up in a layer on top.)

     Besides downwelling, you can create upwelling, by blowing gently from one side of the rim of the coffee cup.  You’ll push a wave of coffee across the surface, making room for deeper coffee to rise to the surface nearest your lips.  Winds cause oceans to upwell in many places, such as off the north coast of South America.

 

Surface waters are flushed regularly, even in lakes.  Twice a year they sink, carrying their load of atmospheric gases downward.  That’s because water density changes with temperature.

     Fresh water is densest at about 4°^C (a typical refrigerator setting; anything that you take out of the refrigerator, whether you place it on the kitchen counter or move it to the freezer, is going to expand a little).  A lake surface cooling down in the autumn will eventually sink into the less dense (because warmer) waters below, mixing things up.  Seawater is more complicated, not expanding much below about 5°^C but with the salt content becoming very important in determining whether water floats or sinks.  Because surface water that evaporates leaves nearly all of its salt behind, the surface becomes saltier – and if it becomes more dense than the underlying water, it sinks, sometimes in great blobs that do not mix very well with underlying waters, just like Dan’s cream.

     The fact that excess salt is flushed from surface waters has global implications, some of them recognized two centuries ago.  Salt circulates, because evaporation up north causes it to sink and be carried south by deep currents.  That the cold waters of the ocean depths came from the Arctic was posited in 1797 by the Anglo-American physicist Sir Benjamin Thompson (the Count Rumford that I mentioned back in Germany ), who also posited that, if merely to compensate, there would have to be a warmer northbound current as well.  By 1961 the oceanographer Henry Stommel was beginning to worry that these warming currents might stop flowing if too much fresh water was added to the surface of the northern seas.  By 1987 the geochemist Wallace Broecker was piecing together the paleoclimatic flip-flops with the salt-circulation story and warning that small nudges to our climate might produce “unpleasant surprises in the greenhouse.”

 

Oceans are not well mixed at any time.  Like a half-beaten cake mix, with strands of egg still visible, the ocean has a lot of blobs and streams within it.  When there has been a lot of evaporation, surface waters are saltier than usual.  Sometimes they sink to considerable depths without much mixing, as happens in the eastern Mediterranean where the surface gets salty because evaporation exceeds the input from rivers.  The salty bottom water flows west and out the bottom of the Strait of Gibraltar into the Atlantic Ocean.  This water is about 10 percent saltier than the ocean’s average, and so they sink into the depths of the Atlantic.  A nice little Amazon-sized waterfall flows over the ridge that connects Spain with Morocco, 800 feet below the surface of the Strait.

     Another underwater ridge line stretches from Greenland to Iceland and on to the Faeroe Islands and Scotland.  It, too, has a salty waterfall, which pours the hypersaline bottom waters of the Greenland Sea and the Norwegian Sea south into the lower levels of the North Atlantic Ocean.  This salty waterfall is more like thirty Amazon Rivers combined.  Why has the eastern branch of it declined more than 20 percent in the last fifty years?

     Indeed, why does it exist?  The cold dry winds blowing eastward off Canada evaporate the surface waters of the North Atlantic Current, and leave behind all their salt.  In late winter the heavy surface waters sink en masse.  These blobs, pushed down by annual repetitions of these late-winter events, flow south, down near the bottom of the Atlantic.  The same thing happens in the Labrador Sea between Canada and the southern tip of Greenland.

     Salt sinking on such a grand scale in the Nordic Seas allows warm water to flow much farther north than it might otherwise do.  It has been called the Nordic Seas heat pump.  Nothing like this happens in the Pacific Ocean (which is, in consequence, about 5°^C cooler), but the Pacific is nonetheless affected, because the sink in the Nordic Seas is part of a vast worldwide salt-conveyor belt.  Such a conveyor is needed because the Atlantic is saltier than the Pacific (water which evaporates from the Atlantic is carried by the trade winds across Central America to fall as rain in the Pacific).

     The Atlantic would be even saltier if it didn’t mix with the Pacific, in long, loopy currents.  These carry the North Atlantic’s excess salt southward from the bottom of the Atlantic, down into the southern oceans, and some continues into the Pacific Ocean.  A round trip on a grocery-checkout conveyor belt takes less than a minute.  This conveyor takes more than a thousand years to make a complete loop.

 

 

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The floor of the North Atlantic Ocean showing the major far-north downwelling sites.  The Gulf Stream can also sink at “near-north” sites near the bottom of the picture, especially when floating ice caps the far-northern sinks.

     Everything else being equal, the Coriolis effect tends to make the major currents turn towards the right in the Northern Hemisphere.  But shorelines and continental shelf walls may prevent right turns, as when the Norwegian Current continues north until attracted to the left by the sinking sites.  Gyres are counterclockwise, however; as surface waters are attracted toward sinking sites, they will turn right.