Endeavor Blog: Going up the river of salt one more time.
As we slowly make our way toward the salinity maximum of the north Atlantic, there is not much aboard demanding immediate attention. Ships move slowly across the face of the globe and it will take us over 8 days to get to our site. Our instruments have been tested, and we tweak gear and get used to the ship. And the pressure of proposals and reviews never leaves, so sometimes it feels like you never left the office. We are just as reachable by email out here by needy editors as we are back home.
Nonetheless, there is time to reminisce, and since I am the old guy on this cruise I have more to recall than anyone else. I can claim seniority this trip on Endeavor, I rode on her in Narragansett Bay shortly after she was delivered in 1976. Captain Rhett joined her in 1982, and we have sailed together a number of times; I had 4 cruises on Endeavor in 1982 alone. The name Endeavor comes from a famous British ship used by Captain Cook in the discovery of Australia. There is actually a piece of the stern post of Cook’s Endeavour mounted in the library and photos of the Space Shuttle Endeavor as well. A great name for a vessel of exploration.
Though small compared to the Knorr, and thus more lively, one always feels that she will come back from every roll. I have been on her in some really nasty blows in the Gulf Stream, and she will be moving around like mad, but you always felt safe. Fortunately at my age I don’t get seasick any more, and though the first few days were pretty rough, we are now under light wind conditions and Endeavor is cruising along nicely. The sun is shining and we are under the subtropical high. Though a remarkably large and intense low pressure system churns to our north, sending long swell our way, it is so much longer than the boat that we simply bob up and down and don’t roll too much. The Captain headed south of our line at first to keep us out of the nastiest weather and it has paid off. A small ship like this cannot make good speed if it’s getting pounded by the seas. He and I trade tales about the old days, from weather to prior captains, ports and cruises, when I head up to the bridge to check the forecast.
But I also reminisce about what has brought me here. For it really goes back to one of my first oceanographic cruises as a grad student in the early ‘70’s. It was on the rusty old “Trident”, URI’s first ship. The cruise was out of Barbados, and we steamed northeast toward the origin of the salinity maximum waters for a while then turned northwest and ended in Bermuda. We were testing a new free-fall profiler of Tom Rossby’s and doing CTD casts. That early profiler had lots of problems but we did see interesting stuff in the CTD casts that led to my first paper and much of my research over my whole career.
What we saw were some interesting steps in the temperature and salinity profiles. At that time Melvin Stern at URI had recently shown how such steps could appear in the laboratory when the salinity gradient was strong and “salt fingers” formed to allow the salt to fall down through the water column. Salt fingers depend on the fact that salt diffuses much more slowly than heat. The warm salty water above can lose heat to the cold fresh water below if it “fingers” downward. This makes it become cold, salty water which is heavier so it continues to sink. It passes heat to fresher water which warms, becomes buoyant and rises. Such small-scale “double diffusive” convection can have dramatic consequences for the larger-scale temperature and salinity structure of the ocean. But we did not know much about that back then. I found it fascinating and learned enough from Mel Stern to make some fundamental contributions to salt finger theory. It was great to be in on the ground floor of a relatively new field. A matter of being in the right place at the right time.
Salt fingers had brought me back to Barbados a number of times. Just east of that tropical island we find the largest vertical salt gradients in the ocean thermocline and the strongest “thermohaline staircases” ever observed. For the thermohaline staircase is the fully formed expression of salt fingers in the ocean, where their amplitudes get large and strong vertical mixing ensues. We had proved this a decade ago, with a deliberate tracer release experiment. My WHOI colleague Jim Ledwell injected a passive chemical into the center of the staircase and was able to watch it spread in the vertical by sampling nine months later. The salt finger mixing rate was ten times as high as it is in non-staircase regions, solid proof that fingers were an important ocean mixing process. In a way, you can think of the staircases as like a series of water falls in the river of salt. We are presently approaching the “headwaters” of this salt river, the place where it gets the saltiest, highest up in the water column. The ocean circulation carries this high salinity water beneath the surface off to the southwest toward Barbados in a distinct plume we call the river of salt. Each of the subtopical gyres has its own river of salt, and we study this one to learn about them all.
We were there just 6 months ago, when we started SPURS by deploying an impressive array of floats, gliders, moorings and drifters. The structure of the headwaters surprised us in several ways. The salinity maximum turned out to be a plateau, there was a highest salinity that was reached in one specific patch of surface water and it did not vary much within that patch. Outside this water the salinity would swing wildly up and down but once in the saltiest patch it tended to stay at a constant high value. Another surprise was just how salty it was. There is a global trend for salty areas to get saltier and fresh areas to get fresher, a consequence of the water cycle intensifying with global warming. These ocean salinity trends are a foreboding warning of what may be in store for mankind as the globe continues to warm. A dramatic intensification of the water cycle means more intense droughts and more extreme storms and floods. Alarmingly, we found the salinity to be saltier than had ever been observed before in that region. The waters we are entering are certainly the saltiest in the global open ocean, though marginal seas such as the Mediterranean and Red seas do have higher salt concentrations. There is strong evaporation here and little rainfall, a sort of oceanic desert where water is leaving the surface of the ocean. The salt stays behind and gives us the river of salt, the water enters the atmosphere to begin the global water cycle. It is truly the point of origin for both the atmospheric water cycle and the oceanic salt cycle; a place that has fascinated me for decades. A place where we hope to learn how to interpret what’s going on with our global water cycle. And the water cycle is truly an oceanic phenomenon, the terrestrial water cycle that we depend on for civilization is actually just a small side show compared to the water cycle over the ocean. Given the how unexplored the ocean is, we learn something new every cruise. What will we learn this time? -Ray Schmitt, 3/22/13