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Global Hydrography |
| Physical Oceanography |
A global section was assembled from 674 full-depth hydrographic profiles beginning northwest of the Ross Sea, north through the Pacific and Bering Sea, crossing the Arctic Ocean and Nordic Seas, south through the Atlantic Ocean, finally crossing the Weddell Sea. Most stations are at 50 km spacing. The results show clearly the primary thermohaline regimes of the World Ocean in relation to source regions and bathymetry.
The plot of potential temperature (potential temperature is temperature slightly corrected for pressure effects) shows what we expect for the surface waters: warmest in the tropics and coldest in the polar regions with an extension into the Nordic Seas of warm water from the Atlantic. Along with the plot of salinity this shows quite dramatically that the North Atlantic Ocean is overall the saltiest and warmest of the oceans. The reasons for this are to some extent still debated but focus on geography: The Mediterranean Sea (not on this plot) takes relatively fresh surface waters in and returns very salty water to the Atlantic. And the Nordic Seas take in relatively salty surface waters, cool them, and return them at great density, filling the North Atlantic abyss with salty waters. Other factors include precipitation and circulation patterns.
The most obvious global deformations are the huge upward adjustments in the Antarctic region where the Antarctic Circumpolar Current runs from west to east around Antarctica. The deep isopleths are compelled by the field of motion and the earth's rotation to rise close to the surface around Antarctica, where they are cooled and exchange gases. A huge tongue of relatively fresh Antarctic Intermediate water intrudes from the Antarctic into the South Atlantic at ca. 1000 meters. A similar but less dramatic tongue in the South Pacific does not show in salinity with this color scheme. A slightly more subtle deformation is seen in the bowl shaped regions in the upper layers in the subtropics of each of the oceans.
At any given level the densities of the Arctic Ocean and Nordic Seas are the greatest in the World Ocean. This occurs primarily because already-salty surface waters enter the Nordic Seas and are cooled there, so the unique combination of cold and salt makes them very dense. They can sink to the bottom there, but the Greenland to Scotland Ridge retains the densest waters north of Iceland. Some very dense, cold, salty water does spill out. Where the overflows abut the remnants of Antarctic-source bottom waters in the North Atlantic these mix to form the lower North Atlantic Deep Water.
Where the salt carried by North Atlantic Deep Water influences the Antarctic circumpolar waters, and in turn where these relatively salty waters are brought near the surface by the patterns of circulation (for example in the Weddell Sea), the Antarctic forms its densest waters, aided by shelf processes there that provide extremely cold waters. The present vigor of the thermohaline circulation seems to owe much to the North Atlantic.
Note the near-bottom tongues of cold water extending into the Atlantic and Pacific from the Antarctic. Although the Antarctic upper layer waters are as a whole relatively fresh, some are saltier than others and when they get very cold they can become quite dense. These are less severely restricted by submarine ridges in spreading north than the Arctic deep waters are in spreading south, so much of the abyssal World Ocean is filled from the densest available Antarctic waters.
Scripps faculty researching the hydrographic structure of the global ocean include:
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Physical Oceanography at Scripps