Hot Carbon: Carbon-14 and a Revolution in Science by Professor John F. Marra

Author:Professor John F. Marra [Professor John F. Marra]
Language: eng
Format: epub
ISBN: 9780231186704
Publisher: Columbia University Press
Published: 2019-06-24T21:00:00+00:00

9

OCEAN CIRCULATION

A

rnold Gordon looked with a combination of alarm and disgust at the small pen as it slowly inked its way toward deeper depths on the plotter paper (this was 1983), recording the conductivity (salinity) of

the seawater. The ship had stopped for a “test” station at the beginning of an oceanographic expedition to make sure the conductivity-temperaturedepth profiler (CTD) was working properly before heading hundreds of kilometers offshore, and beyond help.

Gordon and his crew sited the test station 1 off the coast of Cape Town, South Africa, a location called Cape Basin, in the southeast corner of the Atlantic Ocean, just north and west of the Cape of Good Hope. Looking at the trace, Arnold at first thought the pen on the plotter was stuck, and gave it a gentle rap. But the plotter pen recorded what it was told. It was responding. Well, then, maybe the sensors on the CTD were broken? The sensors consisted of a thermistor to record temperature and a conductivity cell from which to derive salinity. If they were not working, it would jeopardize the month-long cruise ahead of them. Fixing or recalibrating the sensors in those days required a company tech, not someone that is usually available in the ship’s crew or science party. Replacing the sensors would mean returning to Cape Town, squandering expensive and unrecoverable ship days.

All these options ran through Gordon’s mind in seconds. Then, his disposition went from disgust to astonishment. He recognized that the high conductivity numbers—high salinities—he was observing at depth in the Atlantic Ocean came from seawater that had originated in the neighboring Indian Ocean, far to the east, on the other side of Africa. Further stations nearby confirmed the salinities the CTD recorded. They showed an anomalous water mass: a deep ocean eddy, a slowly spinning blob of water carrying higher temperatures and salinities than its surroundings. The eddy was quietly and unobtrusively making its way into the central South Atlantic.

Waters from the western Indian Ocean become part of the Agulhas Current, hugging the east African coast along South Africa and Mozambique. The Agulhas takes its name from Cape Agulhas, the southernmost tip of Africa. Agulhas is Portuguese for “needles.” One story has it that the Portuguese, the best navigators in the early days of European exploration, noted that at this location, magnetic and geographic north were the same. The “needles” on their compasses aligned. Another story, more prosaic, has it that the Portuguese saw some needlelike rock formations along the coastline.

The Agulhas mirrors the North Atlantic’s Gulf Stream, being a current that travels toward a pole along a western boundary of an ocean. The Gulf Stream is remarkable for its velocities, but the Agulhas moves with even higher energy. When it runs out of land to help steer it, the Agulhas becomes unstable and turns back on itself—retroflects—because it interacts with the Antarctic Circumpolar Current to its south, across the subtropical front in the Southern Ocean, flowing in the opposite direction, to the east. Now

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