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Current position of the SSV Corwith Cramer. Click on the vessel to view position history. Use the tools, top right, to change the map style or view data layers. Dates and times use GMT (Greenwich Mean Time).


SEA Currents: SSV Corwith Cramer

August 12, 2015

The Strait of Gibraltar

Farley Miller, 3rd Assistant scientist

Corwith Cramer

Surface profiles from 2014 showing temperature, salinity and fluorescence through the Strait of Gibraltar

Ship's Log

Noon Position
36°11.5’N x 05°07.4’W

Description of location
the Strait of Gibraltar

Ship Heading
070M

Ship Speed
6.0 kts

Weather / Wind / Sail Plan
Westerly winds at 15 knots. All Squares and Mains’l and Stays’l set.

Critter sightings
Porpoises in our wake before dawn.

The entrance to the Mediterranean Sea is one of the most interesting and dynamic areas in the world, speaking oceanographically. The mixing of two of the largest water bodies on Earth takes place in the Strait of Gibraltar, a mere eight miles across at its narrowest and 20 miles long. At the Western edge of the Strait, a fjord like barrier lies 80-190 or so meters beneath the surface, while only a few miles on either side, depths of over 800 meters are the norm. With all the tidal flow of both oceans bottlenecked over this feature (creatively titled “The Ridge” on English charts), huge amounts of water are pushed and turned, upwelled and down-welled creating very interested (and complex) currents. We were happily motor sailing at 5 knots through the water, while our GPS tracked our course over ground (the distance per hour you plot on a chart) at 8.9 knots! We had a following current of over three knots helping us in. But it was not to last, over the course of 40 minutes, the current changed to hitting us on the port side at 2 knots, then 0.75 knots coming right back at us! It appeared strange at first, but a little deciphering revealed that we had unknowingly sailed our ship into a massive scale eddy, just as you would see forming in a river behind a rock, swirling mixing ocean water along with our ship.

Our Acoustic Doppler Current Profiler (ADCP) tracks the currents below the ship at multiple depths, allowing us to infer which mass of water is moving where. Using it, we can visualize the drastic differences between the surface currents effecting our ship (this instrument is the envy of many the navigator on non-research ships) and the deeper currents moving massive amounts of Med water west, and Atlantic water east.

The hot, salty Med mixes with the more tepid, less saline Atlantic in the strait. Surface profiles from last year’s trip show rapid changes in both temperature and salinity on either side of the strait. Chlorophyll fluorescence (a relative reading of the amount of phytoplankton present) show some extreme high chlorophyll areas in the strait, indicating high mixing, followed by much lower chlorophyll, or oligotrophic, areas in the northern Med.

- Farley

Categories: Corwith Cramer, • Topics: yard period  science • (0) Comments
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