Benjamin Franklin made the first widely publicized contribution to knowledge of ocean circulation. He was inspired in this by his stay in Paris during the summer of 1783---a summer of busy negotiations, but also one in which he observed the 'volcanic fog' from the eruption of the Icelandic volcano Laki--and experienced the exceptionally severe winter of 1783-1784 back in the USA. Could large volcanic eruptions cause dust clouds big enough to cool the Earth temporarily? Franklin thought so---and expanded on his ideas about weather and climate in a speech he gave in 1784 titled 'Meteorological Imaginations and Conjectures' which he later submitted as a paper to the Memoirs of the Literary and Philosophical Society of Manchester where it was published in pages 373-377 in their 1789 folio.
Of more relevance to the topic of ocean circulation, Franklin wrote the first serious scientific paper about ocean circulation in 1786 which he submitted to the American Philosophical Society . Google has a link to it, but none of the pages are up. I'm linking anyway hoping that his article is added later.
Maritime Observations, the title of Franklin's monograph, is mostly about the Gulf Stream. Mariners had known about ocean currents for centuries, but Franklin was the first to write about them in a science publication that got wide circulation--therefore making the topic 'stick' in the scientific community. Franklin was the first to give the Gulf Stream credit for Europe being so mild when it was so far north--Paris being further north than Montreal, and Great Britain and Ireland at the latitude of Labrador. He got the broad outline of the North Atlantic surface ocean circulation right--the trade winds pushing the tropical waters west, where they entered the Caribbean and Gulf of Mexico, turned north off the coast of the eastern United States, and then northeast towards Europe, carrying warmth.
It is interesting to read accounts from the first settlers in the 1600s about the weather. New York and Boston are at the latitude of central and southern Italy, and Jamestown is at the latitude of Gibraltar. Colonists were shocked to discover that it "freezes and snows severely in winter".
This was noticed and discussed by scientists in the 17th and 18th centuries, and before Franklin it was believed that forests kept places colder---that snow lingered in the shade of trees and kept the land colder as a feedback. Thomas Jefferson had written that as the land was cleared and the forests replaced by fields and towns, that American would get warmer---the widespread belief of educated people in the 1700s. We know now that was wrong.
For the next hundred years---what else was added to knowledge of the ocean circulation system? Not much (and truth be told, the oceanographic expeditions of the late 19th century and early 20th century didn't add much knowledge either) But there was one expedition that generated some interesting ideas.
The Challenger Expedition was perhaps the greatest scientific expedition of the 19th century. It went world-wide travelling more than 70,000 nautical miles (80,000 statute miles), the longest scientific journey undertaken to that point. Not until astronauts went to the moon and retrieved moon rocks did a scientific expedition travel further. Before Challenger, people had only observed the top few fathoms of the ocean.
To enable her to probe the depths, the Challenger's guns were removed and her spars reduced to make more space available. Laboratories, extra cabins and a special dredging platform were installed. She was loaded with specimen jars, filled with alcohol for preservation of samples, microscopes and chemical apparatus, trawls and dredges, thermometers and water sampling bottles, sounding leads and devices to collect sediment from the sea bed and great lengths of rope with which to suspend the equipment into the ocean depths. Because of the novelty of the expedition, some of the equipment was invented or specially modified for the occasion. In all she was supplied with 181 miles (291 km) of Italian hemp for sounding, trawling and dredging.
Challenger returned to Spithead, Hampshire, on 24 May 1876, having spent 713 days at sea out of the intervening 1,606. On her 68,890-nautical-mile (127,580 km) journey, she conducted 492 deep sea soundings, 133 bottom dredges, 151 open water trawls, 263 serial water temperature observations, and discovered about 4,700 new species of marine life. Copies of the written records of the Challenger Expedition are now stored in several marine institutions around the UK including the National Oceanography Centre, Southampton and the Dove Marine Laboratory in Cullercoats, Tyne and Wear. The complete set of reports of the Challenger Expedition, written between 1877 and 1895, are available online at http://19thcenturyscience.org.
Given the achievements of the Challenger Expedition, it is perhaps a bit harsh to say that knowledge of the deep ocean was not advanced very much. It did expand knowledge many times over, but from virtually nothing to very very small. It did make many interesting observations.
The most important was that globally, the deep oceans more than a few hundred yards down are very very cold. Close to freezing. Everywhere. And that most of the cold water in the deep oceans originated from the margins of Antarctica. Cold, oxygen-rich water sank near Antarctica, and to a much lesser extent near Greenland and in the Arctic Ocean. After analyzing the observations, it was believed that water rose near the equator where solar heat warmed the waters and currents flowed poleward to balance the cold water sinking.
The Challenger expedition discovered another interesting feature. The Mediterranean Sea is considerably saltier than that world ocean (part of the reason why the Atlantic is saltier than the Pacific) and this warm salty water sinks about 5,000 feet to form a distinctive and identifiable layer throughout most of the Atlantic!
That the Atlantic is saltier than the Pacific and Indian Oceans was something of a surprise, given that such huge rivers drain into it. The Amazon river of course, and the Congo river. The Parana river system. The Mississippi river is only 4th!
On the west coast of the Americas, the Columbia River is the only sizable river. In East Asia, the Amur, Yangtze, and Mekong are major rivers, but the whole flow from East Asia is smaller than the Amazon alone. Australia contributes almost nothing to the Pacific.
Even stranger, the Arctic Ocean is really an arm of the Atlantic, and receives several large Rivers in the Russian Arctic--the Ob, Yenisey and Lena rivers--and the Arctic is the least salty of all oceans and mixes freely with the Atlantic. The Mediterranean inflow explains a little of the difference, but only a few percent. The salty Atlantic was not explained until the 1960s.
The deep warm water layer from the Mediterranean interested an American geologist, Thomas Crowder Chamberlin. Chamberlin (1843-1928) was an interesting person---together with astronomer Forest Ray Moulton he crafted the Chamberlin-Moulton planetesimal hypothesis to how the solar system formed--two suns nearly colliding and ripping off solar material which condensed into planets. He believed solar systems were very rare, since such stellar near collisions would be very rare--and calculated that our sun and the other sun in the near collision might have the only solar systems in the Milky Way's galactic arms! That hypothesis was wrong, but Chamberlin did make a lot of valid contributions to geology.
In 1904, he was going over the Challenger Expedition results, leafing through them. The Mediterranean water layer struck him. The Atlantic was already saltier than the rest of the oceans--what if it got saltier? If a local climatic change decreased the flows of the Amazon and Congo rivers, what would happen then?
Chamberlin thought that the tropical oceans would get so salty that the whole ocean circulation system would flip. Instead of cold water sinking near the poles, warm, but very salty water would sink in the tropical Atlantic, and instead of warm surface currents flowing north, cold currents like the Labrador current would flow much further south, cooling the climate and triggering an ice age!
Chamberlin thought that it would work like this. The sun heats the equatorial regions most strongly during the equinox when it passes overhead at noon. The equator gets less sun during the solstices, even though the day length stays the same. Chamberlin thought that when the precession of the equinoxes caused the sun to be closest to the Earth near the solstice, rainfall would decline with less convection triggered, causing the Amazon and Congo river flows to decline, which would cause salinity to increase--the Gulf stream to collapse, causing more cooling, and the cooling would trigger ice sheet formation, cooling the whole world, and cooling the tropics, making the tropical water less prone to evaporate, making the tropics drier, and the tropical water even denser--in a feedback.
The feedback would be broken when the sun was closest to the earth during the equinox again, and trigger more convection, triggering more rainfall, triggering more river flows, lowering the density of the tropical oceans, stopping the sinking of the tropical oceans, and letting the Gulf stream flow again, and the whole feedback operating in reverse.
Chamberlin's ideas triggered a lot of discussion when they were published in 1906. He was known for coming up with wild 'out there' ideas like his solar near collision hypothesis, and tossing them out into the scientific community to be discussed and picked apart. But triggering questions and research.
Having ice ages triggered by changes in tropical rainfall and ocean currents was an original idea, and was a good reminder that ice ages are global phenomena, and that their causes and triggers might be found outside the polar regions.
Chamberlin's ocean circulation reversal idea is wrong on several points, but it did generate for a time some interest in researching how ocean circulation works and how it changes---and how that might impact the climate.
Unfortunately, technology for detailed mapping of the ocean circulation system was not available 100 years ago. There were some proposals for new expeditions, using submarines this time to help map the deep ocean circulation system (which the Challenger Expedition did not have the technology to map---it generated hypotheses and some information on the cold state of the deep ocean, but no answers). But World War I brought an end to those ideas, and after World War I the idea was no longer in fashion, for whatever reason. The Chamberlin hypothesis faded away.
Chamberlin's hypothesis had one glaring fault. Evaporation and rainfall is a circular process. Water evaporating from the tropical Atlantic falls back into the sea, or flows back in rivers. Increasing the evaporation rate increases river flow back into the Atlantic. Decreasing the evaporation decreases the river flow back. It doesn't change the salinity appreciably.
But Chamberlin was the first to hypothesize that the ocean circulation system can have a big impact on climate. And it was an original idea--unlike ice ages and greenhouse warming, which have many fathers, going from person to person and being strengthened---he thought of this all by himself. No one had thought of it before.