ANT-XXV/4 Weekly Report No. 1
28.03.2009
Punta Arenas – Punta Arenas
Departure and successful mooring recovery
• Departure
We started to work on Polarstern 2 days before departure in order to install equipment. As hydrological stations were planned to begin soon after departure across the entrance of the Magellan Strait and we had to be ready in time. Moreover, one of us, Christian, could not participate in the cruise for medical reasons and his expertise was needed to install equipment in the chemical laboratories. Thanks to the efficiency of AWI logistics department and the crew, all the equipment was on board and available, so everybody was grateful and happy and prepared the instruments and laboratories. Christian disembarked and Polarstern left Cabo Negro, a methane terminal just outside Punta Arenas, on this sunny Saturday evening at 18:00. On board are 45 crew members and 45 scientists from nine nations: Germany, France, Argentina, China, Korea, Netherlands, Austria, Spain, Chile, and Italy.
Punta Arenas slowly disappeared in the distance while black and white commerson dolphins were surfing the small waves of Magellan Strait around Polarstern.
A meeting in the evening was useful to recall the scientific and technical objectives of the cruise as well as for introducing ourselves to each other.
• Scientific objectives and work plan
The Antarctic Circumpolar Current (ACC), the world largest current, is a key element of the global climate system. This 2,000 km broad ring of cold water which encircles the Antarctic continent is pushed eastward by the strong westerly wind belt. At Drake Passage the ACC is constricted to its narrowest extent (700 km), thus a convenient place for observations.
Monitoring the ACC transport and water mass characteristics is essential for understanding the coupling of this major current with climate change. Observing the ACC is not an easy matter since the current is concentrated in highly variable narrow bands of swift currents, meanders and energetic eddies of all sizes are numerous. Our experimental set up is designed to combine satellite and in situ observations. Satellite altimetry measures the sea level of the ocean along tracks revisited every 10 days, with an horizontal resolution of 7 km. These sea level observations can be used to estimate ocean currents. In-situ measurements will provide information on the vertical structure of the ACC, information that cannot be obtained by satellites.
925hPa depression coming in
In-situ measurements consist of current-meter moorings which gather time series of the currents and CTD/LADCP/rosette stations. Mooring work can only be performed during daylight, while CTD/LADCP/rosette stations can be carried out around the clock. Thus, those responsible for hydrography work in shifts. The CTD/LADCP records vertical profiles of temperature, salinity, oxygen, horizontal velocity, chlorophyll fluorescence and turbidity. The system is equipped with 22 bottles of 12 litre capacity that can be closed at desired depths. The water samples are divided up amongst the chemists on board who measure various properties of the water. On Polarstern after each cast the instrument does not stay on deck outside, rather is entered into a wet lab and the people taking samples are comfortably protected from foul weather.
Nice wave in Drake Passage
This is our third cruise in Drake Passage on board Polarstern. The first one (ANT-XXII/3) took place in January February 2006. During that cruise we deployed ten current-meter moorings along a ground track of Jason altimeter satellite and carried out a refined array of hydrographic stations with numerous chemical tracers to properly examine the water masses characteristics, source, age, mixing, modifications since the WOCE A21 cruise carried out in 1990. The current-meter moorings were recovered during ANT-XXIV/3 and 5 were redeployed in the northern half of Drake Passage where the ACC is concentrated. During ANT-XXV/4, we want to recover the 5 moorings and occupy hydrographic stations along two satellite ground tracks: track 104, as we did in 2006, and track 29, which straddles over the Shackleton Fracture zone which acts as a barrier for deep waters in the southern Drake Passage.
For some of us, this is our third cruise on Polarstern and we feel like at home. We watch the new comers that are fascinated with Polarstern facilities and comfort, and enjoy the excellent cuisine.
Recovering M4 by night
• Day to day development
We performed the security fire drills and started work in the mouth of Magellan Strait, performing successfully 9 CTD stations on March 22.
Weather soon took command on the ship operations and work development. A very important person on board is the meteorologist, who analyses the weather evolution and makes forecast of wind and waves. The forecast was most unfavorable with a low of 925 hPa in the Drake Passage. It was sunny and windy as we entered LeMaire Strait. The spectacle was gorgeous between Tierra de Fuego and Isla de Los Estados. We attempted to perform a CTD station and soon stopped as the wind and waves were too strong and would damage the equipment.
The forecast being very unfavorable we had to keep sheltered behind State Island hoping for better times. The doctor distributed a number of patches and medication against sea-sickness that were very efficient and soon everybody was fine and enjoying the fascinating scenery at 11 Beaufort. We felt safe, very comfortable watching the waves from the bridge.
We finally performed three CTD in Le Maire Strait and headed towards Drake Passage on the 24th. We went back for shelter again in LeMaire Strait navigating in circles at low speed. 11 Beaufort again. Finally, we managed to recover mooring M1 the northernmost mooring on the 25th, in the presence of a fair amount of high swell leftover from the gale. We attempted to reach M2 on the same day, but the wind again picked up in the evening. Thus, we went back to perform the CTD stations. We managed to recover M2 early 26 March with a nice weather. We took advantage of the calm weather to perform hydrographic stations, readily arrived at M3 early on the 27 swiftly recovered the mooring and continued onto M4. M4 was recovered at dusk as the wind was picking up and rain pouring. We spent the 28 March performing CTD stations and recovered the last mooring M5 on the 29 in a calm and foggy morning.
All the moorings were recovered!
The wind picked up again in the afternoon with Beaufort 8, preventing deploying the CTD.
Thus, as usual in Drake Passage, weather governed our work this week and the rhythm of the two main activities: the CTD/LADCP/ rosette stations and the mooring recoveries.
During this first week (Saturday March 21- Sunday night 29) between depressions we managed to recover the 5 current meter moorings and carry out 31 hydrological stations. Everyone now is adjusted to the ship motions. The members of the crew look after us so well. They are doing their best to make our stay profitable and comfortable. With much success.
On top of it, the food is delicious, varied, generous.
With best wishes from a highly performing and highly spirited ship.
Christine Provost