Day 5 - Charting a course
September 10, 2011

Course of the Kilo Moana shown as a black line. Lengths of arrows indicate speeds of the currents.

I was lucky and got to sleep in this morning, while my roommate, Peter Alpert, ran the 6:00 a.m. CTD cast. Peter has also taken on the sometimes thankless task of negotiating which scientists get water samples from each of the 24 Niskin bottles that are brought up during each CTD cast. Apparently this challenging allocation process has been made even trickier because some bottles are leaking or closing at the wrong depths. But one way or another, the science goes on.

Around 10:00 this morning, we had a meeting of the science crew. It was somewhat sparsely attended because some researchers could not leave their labs due to ongoing experiments. Others had literally been up all night, and were trying to get a little sleep before the next CTD cast came on deck. The chief scientist, Julie Robidart, led the meeting, despite having gotten only a few hours of sleep each night since the cruise started.

At the science meeting, we first discussed the challenges we’d been having with the CTD casts. Then Julie described the reasons for the byzantine path that the Kilo Moana has been traveling over the last three days. It’s all about following the Environmental Sample Processor (ESP).

As I described in my blog entry for September 7, the Kilo Moana first headed north, then turned around and back-tracked a little bit. Our goal was to find an area where the currents were strong enough so that the ESP would not just stay in one spot.

The map above shows the course of the Kilo Moana as a black line heading north from O’ahu. The lengths of the arrows pointing away from the black line indicate the speeds of the currents that were measured by a current meter on board the ship. You can see we ended up at a spot where the currents were relatively strong, as indicated by the long blue arrows.

In the three days since the ESP was deployed, it has drifted about 58 kilometers to the northeast of its original position. The Kilo Moana has dutifully followed along, staying about a mile away—close enough so that we are more or less sampling the same water mass as is the ESP.

Computer analyses projects ESP will follow the currents around an open-ocean eddy that is 100 kilometers across.

At the meeting this morning, Julie showed us several computer analyses performed by the folks at the University of Hawaii, which explained why the ESP is moving to the northwest. It turns out that we and the ESP are being carried slowly around the edge of an open-ocean eddy that is about 100 kilometers across (see illustration at left).

We expect the ESP to curve around to the east as it follows the currents around this eddy. Craig Taylor pointed out that the position beacon on the SID and IPS arrays suggests that they are already starting to move in a more easterly direction, just as the University of Hawaii current prediction suggests.

The ESP will remain the center of our universe as long as it is in the water. However, at the moment there is not a lot going on at this particular spot in the ocean. Early data from the ESP and from ship-board DNA analyses suggest that Trichodesmium and several other nitrogen-fixing bacteria were present in large numbers yesterday, but that there are very few left today.

Kim standing next to CTD
Kim Fulton-Bennett standing next to CTD.

In a possibly related observation, several researchers on board the ship noted large amounts of what they called “crud” clogging their filters and visible under the microscope today. We don’t know for sure, but this crud could be the decomposed remains of bacteria that recently bloomed and then died off.

Faced with a lack of exciting biological activity at our current location, we had several options: 1) explore the center of the eddy and see if anything is happening there; 2) chase filaments of water spinning off the eddy, which could be carrying bacteria away from the eddy; or 3) return to where we deployed the ESP, since we know there was more biological activity there. In the end, the team decided that we will take the ship back to where we deployed the ESP to see if anything is happening there. The most recent satellite images show higher chlorophyll concentrations in this area, and as Julie put it, “Everybody likes chlorophyll.”

As I write this, we’re gliding southwest across an ocean that is as smooth and black as obsidian. Behind us, the waxing gibbous moon rises. We leave behind a wake like a trail of glittering diamonds. Tomorrow morning, before sunrise, we’ll take a CTD cast or two at the location where we deployed the ESP. Unless we see something really exciting, we’ll then head back to the northeast and continue shepherding our menagerie of instruments as they drift around the Pacific Ocean.

—Kim Fulton-Bennett

Watching a spectacular sunset.
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