Returning a week later, we headed for station 67, planning to repeat four stations to be sure of picking up the whole of the Polar Front again. We conducted an XBT survey on our approach to the station, which indicated that we were close to the front. However the sea surface temperatures stayed frustratingly high. The repeat CTD at the position of the old 67 confirmed that the Polar Front was still to the south of us. Despite much complaint from everyone who had expected to be turning north towards home, we set off south in search of the cold waters marking the front. In the end, we had to return to the position of the old station 65 to encounter the waters which had previously occured at 71, over 100 km further north. We imagine that a meander in the Antarctic Circumpolar Current had advected in from the west, moving the Polar Front to the south. Despite the blow of having to go further south again, we were pleased to have picked up the section within 12 hours of arriving back at the work area after a week away. Also, we now have two northbound CTD sections, plus zonal and meridional XBT and ADCP sections, so we now feel we have a flavour of the variability of this very dynamic region. It was frustrating that we have not been able to get cloudfree sea surface temperature imagery of this area to assist our station planning. In the event, we were repeating station 71 exactly 7 days from its first occupation, to within hours. Clearly had we not had the full 7 days extension to the cruise, we would have been in great difficulty to complete the section.
Since then, we have continued northwards varying the station spacing according to the positions of fronts and steep topography. The Polar Front was well resolved, as was the wider and much faster (> 50 cm/s) Subantarctic Front. In the last few hours we have crossed the Subtropical Front and now have sea surface temperatures exceeding 17C ! We are at station 94 at this moment, in glorious sunshine, and visibility has increased markedly recently. Our first bbq is planned for Saturday.
Data processing continues apace. Sections of all the nutrient and oxygen data have been produced and nicely complement the CTD temperatures, salinities and geostrophic currents. It has been satisfying to map the increasing signal of the North Atlantic Deep Water, and the descent of the Antarctic Intermediate Water. Yesterday there were celebrations over the contoured plots of the CFC data (freons 11, 12, 113 and carbon tetrachloride) produced from the now calibrated and clean stations up to 71. We are all very pleased with the results so far.
The race to do the most stations has now been won decisively by the CTDs, but it is still neck and neck between XBTs and met balloons. We will run out of XBTs at 84 and currently have about 15 left; however the met team are running perilously short of helium gas after they opened another new cylinder the other day only to find it virtually empty, presumably having leaked.
The main tribulation of the last week has been the winch system. We are using the big 17 mm cable so that we can do stations in depths approaching 6000 m. Although the cable is designed to be used with this winch with more than 8000 m of wire out, it was found that deeper than 5000 m there was not enough friction on the traction winch to hold the CTD ! Imagine the scene - the CTD is 150 m off the bottom when suddenly it begins to plummett downwards, the winch is unable to hold it. A sharp cry from Paul Woodroffe, "stop the winch !!!!". Simon to the rescue, and the CTD is halted 50 m from the seabed. Phew!
The following few stations were plagued with winch problems - two were aborted in the top few hundred metres, and the deeper ones progressed slowly with a large number of stops for winch tests, and extreme caution below 5000 m, paying out at less than 15 metres per minute. Touch wood, these difficulties have now been solved by Simon, who can't have had much sleep for several days.
The last few days have confirmed the 12-4 "we do it more often" watch as a Star Team par excellence. They would turn in at the end of their watch having just completed a station, and then get up again 8 hours later only to find us about to do the following station. They wouldn't believe our stories of aborted casts and sitting around waiting for winches to work. They are convinced by the Conspiracy Theory. Now it is necessary to bribe Steve, Elaine, Andrew and Paul Woodroffe with Mars bars. Paul is of course our Hero having successfully navigated the CTD to the deepest stations under adverse conditions, and, most importantly, got it back ! There were moments when the PSO could hardly bear to watch, but Paul is made of sterner stuff.
An excellent talk was given by Jane and Russell on Tuesday evening, on isotopes. The subtitle was taken from a quote by our ever witty Sparky Mike (radio officer) "gosh isn't science exciting" ! The doubles table tennis is into the third round, and a singles tournament has started. Brian and Andy are proving to be the hidden talent on the scientists' side. Other social events include the nightly videos in the saloon, and persistent attempts to teach the PSO to play bridge.
JB, the Mate, has entered into the spirit of the science with enthusiasm. He has launched a met balloon, fired the bottles on the CTD, and launched an ALACE float. We now have only a few more ALACEs to deploy. We were pleased to get news (thanks to Ray Peterson of Scripps) of the latest position fixes of the first four floats we deployed. Three went east with the Antarctic Circumpolar Current as expected, and one went the other way !
Now that we are north of the subtropical front, we are making up for lost time with some widely spaced stations (60 miles) before the final onslaught of close stations up and down the Rio Grande Rise, across the Vema Channel, and across the Brazil Current. There has been little in the way of exciting wildlife this week, and the weather has been reasonable. It seems very strange for the outside air temperatures to be warm.