Off Watch

Cruising World - - Contents - Herb Mccormick is CW’S ex­ec­u­tive edi­tor.

Cross­ing the Gulf Stream is a rite of pas­sage for many East Coast off­shore sailors. I’ll never for­get my first time. The Stream seems to gen­er­ate its own weather, and the pre­cip­i­tous clouds on the hori­zon ahead were the ini­tial clue that we were ap­proach­ing. Then there was the sud­den spike in the tem­per­a­ture of the ocean, up into the high 70s, as con­firmed by the boat’s sea­wa­ter ther­mome­ter. The wa­ter it­self shifted to a darker shade of blue, flecked with yel­low patches of sar­gas­sum. And with a slight northerly breeze (thank God it was slight) lean­ing into the north­ward flow­ing cur­rent, the waves stood up into a pro­gres­sion of steep but rea­son­able, ne­go­tiable hills, the boat ris­ing and falling with their flow. It was all very mem­o­rable. The Gulf Stream, the so-called “river in the sea” that trucks along in places at a good 4 to 5 knots, is truly a force of na­ture.

And, re­mark­ably, it is also slow­ing down.

Such was the con­clu­sion of a pair of re­cent sci­en­tific stud­ies fo­cused on the At­lantic merid­ional over­turn­ing cir­cu­la­tion (AMOC) — an ocean cir­cu­la­tion sys­tem that in­cludes the Gulf Stream — re­cently pub­lished in the jour­nal Na­ture. Sim­ply put, a sec­tion of the warm, salty sur­face wa­ters of the Stream even­tu­ally peel off to form the North At­lantic Cur­rent which as­cends into the high, chilled Nordic lat­i­tudes, heat­ing the at­mos­phere on the way. There, the wa­ter cools and the weight of its salin­ity causes it to plunge into the depths, where it be­gins a re­turn jour­ney down the coasts of North and South Amer­ica. The en­tire cy­cle has been likened to an “ocean con­veyer belt” that plays a key role in Earth’s cli­mate by ex­chang­ing warm wa­ter from the equa­tor with cold wa­ter from the Arctic.

And the con­veyer belt, so cru­cial to dis­tribut­ing heat across the planet, is not as quick as it used to be.

Ac­cord­ing to the Na­ture re­port, the two stud­ies that is­sued this find­ing used vastly dif­fer­ent method­olo­gies: “clas­sic ex­am­ples of ‘top-down’ and ‘bot­tom-up’ ap­proaches.” The for­mer based its re­port on di­rect mea­sure­ments of sea­sur­face tem­per­a­tures; the lat­ter re­lied on mea­sur­ing deep-sea sed­i­ment cores that re­flect the speeds of the bot­tom wa­ters on their re­turn flow of the AMOC (stronger currents move thicker grains of sand). But both de­ter­mined that the weak­ened AMOC has de­clined in strength by 15 per­cent, which the Wash­ing­ton Post re­ported as “a de­crease of 3 mil­lion cu­bic meters of wa­ter per sec­ond, the equiv­a­lent of 15 Ama­zon Rivers.” Mercy. In sci­ence, as in life, one thing leads to another. In this case, per­haps iron­i­cally, the rapidly melt­ing glaciers of the Green­land ice cap and the van­ish­ing sea ice above the Arctic Cir­cle are play­ing a sig­nif­i­cant role in this equa­tion. Yes, those wa­ters are cold, but they’re also fresh, and float atop the sur­face; their mass in­fu­sion into the briny sea dis­rupts the for­ma­tion of the dense wa­ter that’s a key com­po­nent in the spin­ning AMOC.

So, what does that mean to sailors and, you know, hu­man be­ings? Well, should the brakes on the At­lantic cir­cu­la­tion con­tinue to be pumped, among other things, it could re­sult in dras­tic changes in Euro­pean weather, dra­matic fluc­tu­a­tions in hur­ri­cane fre­quency and an abrupt rise in sea lev­els on the East Coast, with a backed-up Gulf Stream hav­ing no place else to go. There are lots of other sce­nar­ios, and none of them are great.

One of the cen­tral ques­tions of our time, of course, is the de­gree to which man-made sources have con­trib­uted to cli­mate change, and in this case, what role they played in the AMOC slow­down. In­ter­est­ingly, the au­thors of one study at­tribute the cause mainly to hu­man-in­duced fac­tors (i.e., those that have played a part in the rel­a­tively new phe­nom­e­non of swiftly melt­ing ice), while those of the other sug­gest it prob­a­bly be­gan for nat­u­ral rea­sons around 150 years ago but has since been aided and abet­ted by our own col­lec­tive im­pact on the chang­ing cli­mate.

As with con­tem­po­rary pol­i­tics, there is no clear con­sen­sus.

So, for now at least, we’ll leave it to the noted oceanog­ra­pher/poet Bob Dy­lan, who once sang, “Come gather round peo­ple; Wher­ever you roam; And ad­mit that the wa­ters; Around you have grown; And ac­cept it that soon; You’ll be drenched to the bone.” For, yes, the times they are a-changin’.

Com­pos­ite im­agery from sev­eral high-seas weather mod­els shows the path of the Gulf Stream up the U.S. East Coast.

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