13.3 Observations of the Deep Circulation
The abyssal circulation is less well known than the upper-ocean circulation. Direct observations from moored current meters or deep-drifting floats were difficult to make until recently, and there are few long-term direct measurements of current. In addition, the measurements do not produce a stable mean value for the deep currents. For example, if the deep circulation takes roughly 1,000 years to transport water from the north Atlantic to the Antarctic Circumpolar Current and then to the north Pacific, the mean flow is about 1mm/s. Observing this small mean flow in the presence of typical deep currents having variable velocities of up to 10 cm/s or greater, is very difficult.
Most of our knowledge of the deep circulation is inferred from measured distribution of temperature, salinity, oxygen, silicate, tritium, fluorocarbons and other tracers. These measurements are much more stable than direct current measurements, and observations made decades apart can be used to trace the circulation. Tomczak (1999) carefully describes how the techniques can be made quantitative and how they can be applied in practice.
Tomczak (1999) defines a water mass as a
Plots of salinity as a function of temperature, called T-S plots, are used to delineate water masses and their geographical distribution, to describe mixing among water masses, and to infer motion of water in the deep ocean. Here's why the plots are so useful: water properties, such as temperature and salinity, are formed only when the water is at the surface or in the mixed layer. Heating, cooling, rain, and evaporation all contribute. Once the water sinks below the mixed layer, temperature and salinity can change only by mixing with adjacent water masses. Thus water from a particular region has a particular temperature associated with a particular salinity, and the relationship changes little as the water moves through the deep ocean.
Thus temperature and salinity are not independent variables. For example, the temperature and salinity of the water at different depths below the Gulf Stream are uniquely related (Figure 13.6, right), indicating they came from the same source region, even though they do not appear related if temperature and salinity are plotted independently as a function of depth (Figure 13.6, left).
Temperature and salinity are conservative properties because there are no sources or sinks of heat and salt in the interior of the ocean. Other properties, such as oxygen are non-conservative. For example, oxygen content may change slowly due to oxidation of organic material and respiration by animals.
Each point in the T-S plot is a water type. This is a mathematical ideal. Some water masses may be very homogeneous and they are almost points on the plot. Other water masses are less homogeneous, and they occupy regions on the plot.
Mixing two water types leads to a straight line on a T-S diagram (Figure 13.7). Because the lines of constant density on a T-S plot are curved, mixing increases the density of the water. This is called densification (Figure 13.8).
Water Masses and the Deep Circulation
The plot indicates that the same water masses can be found throughout the western basins in the south Atlantic. Now let's use a cross section of salinity to trace the movement of the water masses using the core method.
A core is a layer of water with extreme value (in the mathematical sense) of salinity or other property as a function of depth. An extreme value is a local maximum or minimum of the quantity as a function of depth. The method assumes that the flow is along the core. Water in the core mixes with the water masses above and below the core and it gradually loses its identity. Furthermore, the flow tends to be along surfaces of constant potential density.
Let's apply the method to the data from the south Atlantic to nd the source of the water masses. As you might expect, this will explain their names.
We start with a north-south cross section of salinity in the western basins of the Atlantic (Figure 13.10). It we locate the maxima and minima of salinity as a function of depth at different latitudes, we can see two clearly defined cores. The upper low-salinity core starts near 55°S and it extends northward at depths near 1000m. This water originates at the Antarctic Polar Front zone. This is the Antarctic Intermediate Water. Below this water mass is a core of salty water originating in the north Atlantic. This is the North Atlantic Deep Water. Below this is the most dense water, the Antarctic Bottom Water. It originates in winter when cold, dense, saline water forms in the Weddell Sea and other shallow seas around Antarctica. The water sinks along the continental slope and mixes with Circumpolar Deep Water. It then fills the deep basins of the south Pacific, Atlantic, and Indian Oceans.
The Circumpolar Deep Water is mostly North Atlantic Deep Water that has been carried around Antarctica. As it is carried along, it mixes with deep waters of the Indian and Pacific Oceans to form the circumpolar water.
The flow is probably not along the arrows shown in Figure 13.10. The distribution of properties in the abyss can be explained by a combination of slow flow in the direction of the arrows plus horizontal mixing along surfaces of constant potential density with some weak vertical mixing. The vertical mixing probably occurs at the places where the density surface reaches the sea bottom at a lateral boundary such as seamounts, mid-ocean ridges, and along the western boundary. Flow in a plane perpendicular to that of the figure may be at least as strong as the flow in the plane of the Figure shown by the arrows.
The core method can be applied only to a tracer that does not influence density. Hence temperature is usually a poor choice. If the tracer controls density, then flow will be around the core according to ideas of geostrophy, not along core as assumed by the core method.
The core method works especially well in the south Atlantic with its clearly defined water masses. In other ocean basins, the T-S relationship is more complicated. The abyssal waters in the other basins are a complex mixture of waters coming from different areas in the ocean (Figure 13.11). For example, warm, salty water from the Mediterranean Sea enters the north Atlantic and spreads out at intermediate depths displacing intermediate water from Antarctica in the north Atlantic, adding additional complexity to the flow as seen in the lower right part of the figure.
Various tracers meet these criteria to a greater or lesser extent, and they are used to follow the deep and intermediate water in the ocean. Here are some of the most widely used tracers.
Each tracer has its usefulness, and each provides additional information about the flow.
|Department of Oceanography, Texas A&M University
Robert H. Stewart, firstname.lastname@example.org
All contents copyright © 2005 Robert H. Stewart,
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Updated on November 2, 2007