In the South Atlantic Ocean
Veteran travelers are well aware that most of the surface of the Earth (71% of it) is covered by oceans, and the oceans are largest in the Southern Hemisphere. (I should say "in its present configuration," because the distribution of land and oceans has changed with tectonic history.) We have learned about the great Southern Ocean, where water circulates around the Antarctic continent unimpeded by land, and the great basins of the Pacific, Atlantic, and Indian Oceans, delineated by the intervening southern continents. The geometry and spin of the Earth, and the position of its landmasses, influence the movement of water in currents that, in turn, affect climate, water temperature, nutrient status, productivity, and the distribution of life. This has been one of the themes of our voyage across the South Atlantic, as expostulated by oceanographer Jim Kelley. Today, instead of a photograph I offer a chart of ocean temperatures, as recorded by the National Geographic Explorer at four-hour intervals throughout our voyage.
We began our trip at Ushuaia, near the tip of South America where the Atlantic, Pacific, and Southern Oceans meet below one of the world's great geographic landmarks: Cape Horn. Southern Ocean water, originating around Antarctica and coming from the west as the West Wind Drift, must squeeze through the narrow waist of the Drake Passage. Once past Cape Horn, some of the water spreads to the north, into the South Atlantic Ocean, as the Falklands Current. We entered this water on our way to the Falkland Islands. It is an area of high productivity and abundant wildlife, fueled by dissolved nutrients of Antarctic Origin.
The great Antarctic Oasis of South Georgia lies somewhat south but mostly east of the Falklands. To reach South Georgia we passed through another great oceanographic boundary: the Antarctic Convergence or South Polar Front, where cold Antarctic water abuts warmer Subantractic water. The rich nutrient load of the cold water gives rise to the tremendous richness of wildlife of South Georgia—the birds and seals that we saw breeding on land and soaring over or swimming through the water. Nearly all life on South Georgia depends, directly or indirectly, on the bounty of the Southern Ocean.
Leaving South Georgia, we quickly passed through the Antarctic Convergence again. Water temperature rose sharply and continued to climb. The abundance of seabirds seen during our passage declined both with distance from breeding sites and diminished ocean productivity. Ah, but note how water temperature dropped again around the set of islands of Tristan da Cunha. Perhaps these oceanic islands, rising from the seafloor below, impede the movement of the currents and cause local upwelling of colder, nutrient-rich water. The islands support numerous breeding sea and land birds; several are endemic species that breed only here.
A striking feature of the trip was the drop-off in birds seen after we left Tristan. As warm-blooded and warm-loving mammals, we humans tend to associate warm conditions with verdant productivity, but this is a feature of land, not of the oceans. The deep cobalt-blue color of the water attests to a lack of nutrients to fuel the productivity of phytoplankton. Without plankton to reflect light, the water swallows the medium and longer wavelengths of sunlight, giving back only the blue. This feature is used as a proxy measure of phytoplankton abundance as recorded from Earth-orbiting satellites, but seabird abundance might equally well serve as a measure.
At this writing, water temperature is 20° C or 68° F, and bird abundance remains low even as we are nearing the Coast of Africa. Near the African coast, there is another great upwelling of nutrient-rich water that forms the Benguela current, but that is a matter for another day and another trip.
