R/V Roger Revelle Expedition KNOX03RR

CLIVAR/CO2 cruise I8S

Chief Scientist Weekly Report 4, 26 February 2007

1615 local; 53.2 °S, 87.4 °E; air temp 2.4 °C (37 °F), wind 15 knots from West

We have now completed 35 stations as we work our way north through the 50s (50-60 degrees South). This is a tough area to work with a large rosette, because low pressure systems circulate over these circumpolar waters, their winds building seas which can propagate great distances as long-period swell. Swell can cause large amplitude ship motions. When we are doing a rosette cast, these ship motions can move the ship's outboard block (a pulley) for the 0.322” diameter armored CTD cable down faster than the fall speed of the rosette, resulting in slack wire. The block also can move up fast enough to cause a shock load in cable tension. Slack wire followed by shock load can lead to cable kinks, and cable kinks immediately and irreparably weaken the CTD cable, potentially leading to loss of signal from the CTD or even parting of the cable and loss of the rosette.

This is all by way of saying that we spent part of the week in a battle royal with cable kinks. (The photo sent this week shows one of these.) The situation was perplexing because two years ago we carried out a similar Southern Ocean transect (in the Pacific Ocean sector) from this ship with virtually the same equipment, and under occasionally trying sea conditions, but with few cable kink problems. Expert minds considered every aspect of the problem, but a working solution was elusive.

How do we get rid of a cable kink once one occurs? When the rosette is back on deck we cut off the end of the cable (kinks almost always occur within 10-20 meters of the rosette) and re-do the electrical and mechanical connections (called a “termination”) between the CTD cable and the rosette - a several hour process. That's time lost from the in-water science programs, because the ship has a fixed distance to cover this cruise. Plus we were lowering the package extra slowly in an effort to avert kinks, which was yet more time lost.

Added to the delays caused by re-terminations and slow rosette deployments were delays caused when winds and seas were too high to permit over-the-side operations. We lost more than 24 hours earlier this week when we rode out seas blown up by sustained 40-knot winds.

Last night we switched our rosette from the brand-new 10,000-meter CTD cable installed for this trip to the older CTD cable on the second winch. (CTD winches are essential to our work, so SIO set the Revelle up ahead of time with two CTD winches each ready to go. This older cable is the one we used during the 2005 cruise.) Since then, two casts have been completed without problems, compared to kinks and reterminations with every cast the days immediately before. It thus seems plausible that the new cable had from its manufacture some intrinsic characteristic incompatible with our rosette operations in swell. We do not know this with certainty, of course, but so long as the old cable provides good performance, we are in good shape.

So it's occasionally a step backwards for every two forward, and definitely not all steaks, icebergs, and auroras. But despite the setbacks, we are still occupying every planned station, and data quality is superb.

Jim Swift and Annie Wong

chief and co-chief scientists, CLIVAR/I8S