Magazine article Oceanus

A Primer on Ocean Currents: Measurements and Lingo of Physical Oceanographers

Magazine article Oceanus

A Primer on Ocean Currents: Measurements and Lingo of Physical Oceanographers

Article excerpt

Volumes of Water Moving in Currents

Oceanographers express current flow in "millions of cubic meters per second," a term difficult for most people to comprehend. A large current, such as the Gulf Stream south of Nova Scotia, transports more than 150 million cubic meters per second, and typical transports for the smaller deep western boundary currents are 10 to 20 million cubic meters per second. Various dense overflows from marginal seas such as the Mediterranean are even smaller, 1 to 3 million cubic meters per second. For comparison, the sum of all the rivers flowing into the Atlantic is about 0.6 million cubic meters per second. The Amazon contributes about a third of that total, while the Mississippi River, whose rampages plagued the midwest last summer, accounts for only about 0.02 million cubic meters per second, roughly one ten-thousandth of the Gulf Stream's transport!

Properties of Seawater

Salinity: Though seawater's salinity is expressed in grams of dissolved solids per kilogram of seawater, today's methods measure seawater's electrical conductivity and then employ a well-known conversion algorithm to determine salinity. While coastal waters can exhibit a wide range of salinity as a result of freshwater runoff, most of the world ocean lies in the narrow salinity range 33.8 to 36.8.

Temperature: Oceanographers commonly refer to the "potential" temperature of a parcel of water. This recognizes that a parcel of water sinking from a surface source will, if it does not mix or exchange heat with surrounding waters, become slightly warmer as the pressure on it increases with depth. For example, if a parcel of surface seawater starts with a temperature of 0 [degree] C and salinity 35 and descends to 3,000 meters as part of an overflow, it may warm as much as 0.3 [degree] C. An instrument lowered into this overflow would sense this warmer temperature, which is called the "in situ" temperature. However, for most calculations and analyses oceanographers use the potential temperature, which corrects for this effect of pressure and thus remains 0.0 [degrees] C.

The North Atlantic is the warmest and most saline of the world's oceans, having a mean potential temperature of 5.08 [degrees] C and mean salinity of 35.09, compared to the global average of 3.51 [degrees] C and salinity of 34.72. Most of the warmer and more saline waters of the world are concentrated in the upper kilometer of the subtropical and tropical circulation regimes in what is called the main thermocline (a region of rapid decrease in temperature with depth, in the North Atlantic typically the upper kilometer). About 77 percent of world-ocean volume is colder than 4 [degrees] C, with salinities in the relatively narrow range 34.1 to 35.1. At the sea surface, only about 26 percent of the surface area is colder than 4 [degrees] C, and it is within this area that the large volume of cold water acquires its characteristics before sinking and traveling along paths like those discussed in this publication.

Water Masses

The large volume of cold water described above comprises the "deep" and "bottom" waters. In the Atlantic there are several sources for these waters in both hemispheres. Each hemisphere's sources are blended by circulation and mixing. The net effect of northern sources dominates the deep water, while the net effect of southern sources dominates the bottom water. In regions where the two water masses coexist, the bottom water lies beneath the deep water, although because of their mixing there is no sharp demarcation between the two water masses away from their sources. …

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