UNDERSTANDING OCEAN CURRENTS

Much recent conjecture in both popular culture and scientific circles about how melting polar ice could cause major ocean currents to slow, or even possibly stop.  Such an event, of course, would have catastrophic consequences including severe storms, major climatic shifts and dramatic impact on marine life around the world. To appreciate such a phenomenon, it is important to first understand the forces that create currents and how basic oceanographic measuring devices, including the thermosalinograph “TSG” used in SeaKeeper 1000 monitoring systems, provides data critical to scientists observing for such oceanographic changes.

Ocean currents are like rivers, perhaps thousands of miles in length; they may be hot or cold.  Some are found closer to the surface; others run deep.  Depending on location and season, surface currents are defined as the top thousand feet or so. (Between the surface and deep layers there is also a density discontinuity, or boundary layer, called the “pycnocline.”)

Looking at ocean currents on the largest scale, the major forces might be put in order as:  solar heating, winds, gravity, and Coriolis (caused by the earth's rotation). Whereas surface currents are driven mainly by the wind, the other forces as well as land masses have great effect on the deeper current. Huge circular patterns called current gyres, which spin clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere, are also significant ocean phenomena. These gyres actually form mounds, or elevated areas, in the ocean.

Particularly in the equatorial regions the heating of water is a dominant factor. The movement of currents and gyres away from the equator helps spread the sun’s heat to far northern and southern regions. Warmer water holds more salt in solution, increasing its density. As these rivers of warmer, more saline water approach the colder northern and southern latitudes, they cool, becoming even denser, causing the water to sink. These same rivers, now at the bottom of the sea, are drawn back toward the equatorial waters to fill the void left by rising, sun-warmed seas. The sinking at the poles and the rising at the equators is the basic engine that drives ocean currents.

Deep ocean currents make up about 90% of the oceans. This thermohaline circulation, also known as the ocean's conveyor belt, flows far under the surface in the deep ocean basins and thus mostly hidden from immediate detection, except in areas of significant upwelling where ocean water is forced upward due to strong winds that blow warmer surface water away, or topographical features such as undersea ridges or continental shelves.

Knowledge of surface ocean currents is essential for many things, including reducing costs of shipping by lowering fuel costs.  Ocean currents are also very important in dispersing many marine life forms. For example, spiny lobsters of Florida and the Bahamas, are actually spawned off Venezuela and Cuba and ride the Gulfstream to their home reefs while in their larval stage.

Perhaps the best known current is the Gulf Stream, which is partly responsible for Western Europe’s relatively balmy climate.  Starting in the Gulf of Mexico and Caribbean, it moves along the eastern shore of North America, then crosses to Europe, finally going deep in the region off Greenland, where it sinks to the ocean floor and returns south.

 In 2005, a study of the North Atlantic provided dramatic findings of a 30% reduction in the warm currents that carry water north from the Gulf Stream.  Quoted in New Scientist magazine, Dr. Harry Bryden from the highly respected National Oceanography Centre in Southampton, England, says he is not yet sure if the change is temporary or signals a long-term trend. "We don’t want to say the circulation will shut down; but we are nervous about our findings. They have come as quite a surprise." Speculation about the finding raised fears that failing warm water currents might plunge Europe into a mini ice age.

This study also revealed that currents on the Canadian side of Greenland still seems to be functioning as normal, though on the European side seawater is not sinking as rapidly as usual, sending only half as much deep water south as before.

Possible explanations include large additions of fresh water (less saline and less dense) perhaps from melting sea ice, from the melting Greenland ice cap, or increased flow from Siberian rivers.   A decrease in density of surface water would prevent it from sinking, which in turn would slow the flow of tropical water from the south. Any of these scenarios could be triggered by effects of climate change. Some climate models predict that global warming could lead to such an ocean current shutdown later this century.

The last shutdown, which prompted a temperature drop of 5°C to 10°C in Western Europe, was probably at the end of the last ice age, 12,000 years ago. There may also have been a slowing of Atlantic circulation during the Little Ice Age, which lasted sporadically from 1300 to about 1850 and created temperatures low enough to freeze the River Thames in London.

When previously unanalyzed data was factored into his research, Dr. Bryden found a similar pattern. This suggests that his measurements are not unique, and that most of the slow-down happened between 1992 and 1998.  Despite the alarming implications of Dr. Bryden’s findings, most scientists believe more study is needed to understand and identify ocean current patterns and cycles.

A number of ways exist to measure currents and estimate the volume of water they are transporting.  In addition to direct measurement of the currents, scientists look for the correlation with the driving forces described above.  To understand the forces behind the currents, temperature and salinity has to be accurately measured to determine density.  To measure temperature and salinity, scientists use the thermosalinograph or “TSG.”

The TSG is a core component in the SeaKeeper 1000, the Society’s innovative, automated, atmospheric and oceanographic monitoring system.  Now installed on approximately 60 private yachts, cruise ships, piers, NOAA ocean buoys, a Mediterranean car ferry, and a US Coast Guard ice breaker, these systems yield extremely precise data for scientists at an exceptionally economical cost.  Data is sampled down to three decimal places and logged every minute; the data is then transmitted by satellite every three hours. In addition to its existing deployment of monitoring systems, SeaKeepers is excited to currently have another 20 systems in various stages of installation.

The highly accurate data from SeaKeeper 1000 systems is combined with data from arrays of deep ocean sensors and satellite data to form an accurate picture of the vast oceans.  The oceans are changing significantly in many ways, not just in temperature and salinity. (In previous issues of SeaKeepers Report, (see www.seakeepers.org) we have described the increasing acidification of the sea, diminishing plankton populations, etc. 

Accurate data is more important than ever to monitor the seas’ vital signs to understand these changes and the challenges they represent.  With the increasing importance of such things as changing ocean current patterns, SeaKeepers is proud that the system the Society pioneered 10 years ago is proving to be even more relevant today. 

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