OC103 Lesson #17: Deep Ocean Circulation In the last lesson we covered the wind-driven currents of the surface ocean. While the surface circulation is the more obvious to us, it turns out it affects only 10% of ocean water volume. The remaining 90% of ocean water is too deep to be affected by wind. This lesson covers where that deep water comes from, and how it circulates through the deep ocean in slow currents driven by density differences between water masses with different temperatures and salinities. This slow movement of deep water helps regulate our climate, and delivers nutrients to organisms living in the deep. Density-Driven Water Flow Deep-water circulation is driven by density differences (as in heavier water sinks, and lighter water rises), and is called “Thermohaline Circulation” because temperature and salinity (T&S) together are what determine the density of seawater. Processes that cause variations in temperature and salinity occur mainly at the surface (warming and cooling by the atmosphere, evaporation, freezing). So once a parcel of water becomes dense and sinks away from the surface it tends to keep its T&S characteristics for a long time (1000 years or longer), until it eventually circulates back up to the surface. Density Variation in Sea Water Most ocean water is within a narrow range of T&S. The figure at right shows the natural range of T&S of ocean waters. 75% of ocean water has the very narrow range of T&S characteristics covered by the small, dark blue square near the bottom of the figure at 0–5°C and 34–35‰, while 99% of ocean water has T&S characteristics that fit within the light blue area. But even these small variations in T&S are enough to create the density differences that drive deep- water circulation.Water Masses The figure below is a north-south cross section through the Atlantic Ocean showing the locations and directions of water flowing beneath the surface. The different packages or “tongues” of water with distinctive combinations of temperature and salinity are called water masses. Each water mass gained its unique characteristics when it formed at the surface in a particular area. For example, the water mass that forms in the Mediterranean Sea is warm and salty (due to the warm, dry climate causing a lot of evaporation there), and when it flows out into the Atlantic it can still be distinguished from the surrounding water by its distinctive combination of temperature and salinity. It is the small water mass labeled MIW in the figure below. Water masses are usually named for where they formed and/or where they flow in the ocean. For example, the Mediterranean water mass is called Mediterranean Intermediate Water (MIW) because it formed in the Med, and flows through the ocean at an intermediate depth (about 2 km deep, below the less salty, less dense surface waters and above the denser, colder deep waters). We are not interested in the gory details of all the codes for the different water masses, but an important thing to notice in this figure is that the two cold, salty water masses way down near the bottom (AABW - Antarctic Bottom Water; and NABW - North Atlantic Bottom Water) form near the poles and flow down into the deepest part of the ocean. kmWhere does the Deep Water Come From? The densest water masses are cold and salty, and are made that way by freezing winds at high latitudes. For example, the dense water masses that form in the North Atlantic are North Atlantic Bottom Water (NABW) and North Atlantic Deep Water (NADW); and the one that forms near Antarctica is Antarctic Bottom Water (AABW). The cold atmosphere in these regions causes the surface of the ocean to freeze, and since the structure of ice pretty much excludes salts, the freezing of sea ice removes fresh water from the fluid ocean, leaving behind salty, very cold (and thus dense) seawater. That dense water sinks and flows away from the poles (see figure at right). The map at left shows where the main deep water masses form in the far north and south, and the colored arrows show where the water masses flow to once they sink into the deep ocean.Importance of Deep Flow Deep water contains O2 and CO2 from when it was in contact with the atmosphere. CO2 content is important because it determines the CCD (remember the Carbonate Compensation Depth is the depth below which carbonate shells dissolve faster than they can accumulate in the sediment), which determines how much CaCO3 is preserved on the ocean floor as sediments and how much dissolves back into the ocean to be used by other organisms. CO2 is also used for plant photosynthesis, but this is not an issue in the deep ocean because it is too dark for plants to survive anyway. There are, however, animals in the deep ocean, and O2 content is important for animal respiration. It also helps determine how much organic carbon accumulates in sediments without being oxidized. If there is not much O2, the organic carbon in the sediments can convert to CH4 (methane gas), which, if it makes it back into the atmosphere will act as a greenhouse gas just like CO2. If the methane gas stays in the sediments, it could one day be a valuable source of energy. Several countries (including the U.S.) are investigating the possibility of tapping these gas sources to burn in place of petroleum. A key factor in determining whether the methane gas stays locked in the sediments versus escaping into the ocean and atmosphere appears to be the temperature of the deepest water next to the sediments. If the deep water is cold enough, the methane remains frozen in the sediments. If the deep water warms up, the methane is released. It is therefore a matter of great concern that as our climate gets warmer it could cause the deep water to warm up enough to allow the methane to be released, and since methane is a potent greenhouse gas it would exaggerate the warming affect and cause even faster and more extreme climate change. The deep ocean, being 90% of the ocean water, also has an enormous capacity to absorb and release heat and greenhouse gasses, and can act something like a buffer to absorb excess heat and gasses (although some of the dangers of that were mentioned above). So the rate, temperature, and composition of seawater circulating through the deep ocean is an important factor in regulating climate change. It takes a water mass about 1000–1500 years to make the roundtrip from the surface to the deep ocean and