Southern Ocean Iron Experiment (SOFeX) Cruise
January 5 - February 26, 2002

January 11, 2002: Day 7
Latitude: 55 30.00 S
Longitude: 172 00.00 W

Ken Johnson writes: Were chugging along to the north in a lumpy sea as we complete our pre-fertilization survey.So what are people doing on board?One of the busy people is Steve Pierce from Oregon State University. Steve is processing data collected from the ships Acoustic Doppler Current Profiler. The ADCP transmits pings of sound at 150 khz down into the water. If the sound wave hits a particle in the water, some of the energy is reflected back to the ship where it is detected. If the particle is moving when the sound wave hits it, then the frequency of the reflected sound is shifted slightly depending on the direction that the particle is moving - this is called a Doppler Shift. If there are enough particles in the water, the ADCP can determine current speed and velocity down to 400 m depth. One problem, if the ship is moving, then he has to subtract the ships velocity, no small problem if the ship is going much faster than the water.And if the ship is pitching, he has to correct for the fact that sound is not going straight down. So he has lots to do, but the results are great. Hes producing plots of the absolute speed of the water, which is difficult to do in real time.

His results show that there are current speeds up to 50 cm/s (about 1 knot) in the region around 53S. To our surprise, as we surveyed to the south along 172W into the SubAntarctic Front, he found that velocities dropped dramatically and the region south of 55.5 has current speeds of about 10 to 15 cm/s. There is a fairly constant temperature gradient in this region, but the chemical properties measured by Joe Jennings from Oregon State are quite uniform, about 20uM Nitrate and 1 uM Si. This is our new target area for fertilization. So instead of surveying back to the north last night we kept on to the south and we should be finished near 2200 this evening. No big surprises so far.

Well transit back to our target site at 56 S, 172 W and should be there early in the morning to begin our pre-fertilization station.That will take about 14 hours and then we begin adding iron. Dinner time now.

ADCP profile generated by Steve Pierce (Oregon State University). This is a thumbnail so you can click on it to get a larger image.

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