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Re-setting the CTD.
1. Re-setting the CTD.

Watching CTD deployment
2. Watching CTD deployment

The CTD brought back on deck.
3. The CTD brought back on deck.

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From: John Prytherch
Date: Mon, 15 Dec 2008 23:12
Subject: About half-way

We are now steaming south and are approaching the half way point of Drake' Passage. The Southern Ocean and Drake's in particular are famous for extreme weather but we have been lucky and the wind and waves have been mostly light.

Despite the calm conditions, the rolling of the JCR has had several of us inexperienced sailors feeling a bit green, but after a day or two everyone has just about got their sea legs.

Margaret and I have been assigned the night shift - 8pm to 8am. After a difficult first shift, 24 hours awake and contending with seasickness, we have settled into a good routine. The main work revolves around CTD casts: sending a package of instruments on the end of a cable to the sea floor and back again, making measurements and collecting water samples on the way. At the start of the cruise we were sending the instruments down every 2 miles or so as the high variability of the sea floor in this part of the ocean produces effects we want to capture. This made for a very busy work schedule! Now we are approaching the middle of Drake's passage the CTD casts are deeper (taking longer) and more spaced out, giving us time to analyse the measurements we are recording (and to write this blog!).

The most important measurements made by the CTD are of temperature and salinity (saltiness). These are made continuously as the instrument sinks and then rises through the water column. The temperature and salinity allow us to calculate the density of the seawater. By making these measurements in a line (referred to as a section) across Drake's Passage, we can make an estimate of the mean flow of water across the whole section.

The work itself is a combination of computer-based instrument monitoring and physically collecting samples and resetting instruments when the CTD is back on deck. This can be ‘entertaining’ when the weather is bad and the ship is rolling.

John