I'm aboard the RSS James Clark Ross, having left Punta Arenas in the far south of Chile. We're "steaming", to use the nautical term, for Bird Island in the South Georgia island chain. We will be dropping off some lucky scientists who will be spending some time there taking lake sediment samples, before making a turn to the south. It's after this turn where my work really begins. I'm charged with measuring the temperature and salinity "profiles" (that is, measuring the temperature and salinity as they change with depth) along a "transect" (A23 to be precise - I'll explain what that means in my next post). Why do oceanographers bother measuring T & S? And how, in a deep, constantly moving ocean, do we do it? Today, I'll briefly explain the former question. A little later, I'll describe the later.
Why are T & S important? Well, for a start, animals that live in the ocean are generally adapted to certain temperatures and salinities. Coral is a great example, and one that gets a lot of press. Tropical corals can only survive in a narrow temperature range: when the waters get too warm (as is already occurring in some parts of the world as global warming starts to leave its mark on the planet), the corals get stressed and, eventually, die. Almost every animal in the ocean exploits some ecological niche, and both T & S play a key role in defining them.
However, for a physical oceanographer such as myself, temperature and salinity are so much more than just two ingredients essential for marine life on Earth. They are also primary drivers of the ocean circulation - a fundamental component of the climate system.
Both temperature and salinity influence ocean circulation through their influence on the density (that is the mass of the water in a particular volume, say one liter - sometimes called the buoyancy) of ocean water. Warmer water, all things being equal, is less dense (we could also say "lighter" or "more buoyant") while colder water is more dense ("heavier" or "less buoyant"). Salt has a similar effect: after all, if you 10g/oz/whatever quantity to a bottle of water, it will weigh 10g/oz/whatever more than before and hence be more dense (as the volume of liquid hasn't really changed). If you drop denser water in lighter water, it will sink (yes, yes, you the oceanographer in the crowd, I know it will entrain fluid and mix as it does. Shut up for a second and let me go on) until it reaches a level where its density is the same as the density of the surrounding fluid. This effect is why the densest ocean waters - the coldest and the saltiest - are the deepest: they've had time to sink the bottom and "fill" the ocean from the bottom up. The change in the density of seawater with its depth is what oceanographers call "stratification" and it's a fundamental property of the ocean that affects all sorts of things, such as ocean turbulence and how similar the currents at the surface are to the currents at depth.
The temperature and salinity of the oceans change depending on where you are, and in what season. Obviously, water is colder - hence denser - at the poles than in the tropics, simply because the tropic get more and stronger sun than the Arctic or Antarctic. However, some regions of the tropics get a lot of rain, which dilutes the salt in the water, while some sub-tropical regions have very little cloud cover, so the sun beams in a bakes the ocean surface. In these "horse latitudes" the salinity is very high as much of the surface water evaporates, leaving more concentrated salt. Hence water in the sub-tropics can often be quite dense, even though the water can be quite warm.
By collecting thousands upon thousands (upon thousands) of water samples we can start to see patterns between the temperature and salinity of the water, and where it has come from. For example, as I was describing before, warm, salty water was probably formed in the sub-tropical regions, while warm, yet non-salty water (so called "fresh water", even though it still may have plenty of salt in it) might have come from the wet-tropics. We call these unique(ish) combinations of temperature and salinity "water masses", and any attendee of an oceanographic conference will hear a barrage of odd names and acronyms - NADW, AAIW, Mode Water, etc... that are the names for different water masses. Each water mass comes with baggage: a story about how it managed to find itself with the T & S it possesses. And finding a particular water mass a long way from home can help give us an idea of the pathway that it took to get from its point of formation to wherever it has ended up, which can in turn help us understand how the ocean currents move.
On the WAPITI cruise we'll be chasing so-called "Antarctic Bottom Water" (abbreviated AABW). This water mass is the densest you can find: it's cold because it comes from the coasts of Antarctica, one of the coldest regions on the planet. It's salty, because near Antarctica lots of sea-ice is formed. The water that makes up sea-ice is quite fresh - sea-ice rejects salt from the water as it forms - and this salt ends up in the water below the sea-ice. Thus, AABW has a pretty clear signature: colder than -0.2 degrees C (it won't freeze because it's both too salty and under pressure) and saltier than about 34.6 psu (that's Practical Salinity Units for those not up on their oceanographic metrology). AABW is so dense that it sinks pretty-much straight to the bottom upon being formed. But it doesn't just lay around on the ocean floor like Brad Pitt in the underrated Tarantino film True Romance (you have no idea what I'm talking about, do you?). We find AABW all through the Atlantic Ocean, indicating that it has spread to the north after being formed. What's more, AABW is rich in dissolved CO2 that it has absorbed from the atmosphere. Since AABW stays on the ocean floor for centuries or even millennia, its CO2 stays there with it, where it can't contribute to global warming.
But wait, there's more! Through the magic of a simplified description of ocean currents, called the geostrophic balance approximation (which says that the Corriolis force and the pressure gradient force are the dominant forces in the ocean, and that the two act to balance each other), we can directly relate the ocean's density structure to the ocean currents themselves. So if we know the ocean's density structure (that is, how the density changes in latitude, longitude and depth) we get an approximation of the ocean current speed for free (although we miss a very, very important constant factor called the "barotropic current" that can be thought of as the average of the current speed in the water column). On the WAPITI cruise, we'll use our "transects" or "sections" of T & S to estimate the geostrophic current speeds in the Weddell Sea, which can give us an idea about how rapidly the AABW is moved from its formation regions to the open ocean.
This (longish) post explains a little about why T & S are important to measure. It's a simplified picture, and I've skipped a bunch of details like the effect of mixing, internal waves and the importance and difficulty of estimating the barotropic current. But it should give you the gist. In my next post, I'll show you how we measure temperature and salinity from ships, with some real world examples from our first CTD casts.
I'm a physical oceanographer at LOCEAN Paris. My research concentrates on the Southern Ocean and its role in the climate system.