Report of Activities on the RVIB N.B. Palmer Cruise 02-04

8 August 2002

 

Our first day on the transit to the southern sector of the Southern Ocean GLOBEC grid off Alexander and Charcot Islands was fraught with difficulty. Since we entered Crystal Sound on 5 August, there apparently had been a buildup of very dense brash ice just outside the entrance to the sound. Although we began the journey before midnight on 7 August, by mid-morning on the 8th we had made about 6 nm and our progress was a snails pace. A part of the problem was that the wind was out of the northeast at about 20 kts and this had caused the ice to pack in close to the shore. While the Palmer could make it through this pack ice with relative ease, the L.M. Gould could not. Even with the Gould very close behind, the Palmer's wake region closed up with the dense brash ice to such an extent that the Gould could not push forward through it and within a short time came to a stop. The Palmer then either had to back down to reopen the path in front of the Gould and start out again or to circle and come up alongside the Gould, cut right in front, and then move forward with the Gould again trying to follow behind. For most of the day the convoy went nowhere fast. About 1700, as we moved west beyond the vicinity of three very large icebergs, the character of the ice changed and the wake region no longer closed up so abruptly. The Gould finally could follow the Palmer and we were able to move consistently along the transit route to the southwest.

 

Several hours later while steaming off the northern end of Adelaide Island, the ice pack thinned significantly and there were pools of open water amongst the floes. An XBT survey with drops at 10 nm intervals was started in this area to see if the water column characteristics were indicative of offshore water coming into the area and contributing to an increased upward heat flux that reduced the rate of sea ice formation. Indeed, as described below, the deep water was much warmer indicative of the ACC water coming onto the shelf. The slow pace of the transit made it a good day to go computer virus hunting. The ship's PC computers were hit by the klezH@mm virus making life miserable for many of the investigators and crew using PCs and causing significant problems with the data acquisition systems. The ship's network was shut down in the morning and all of the laptop computers were brought to a main lab for virus checking and cleaning. All other PCs throughout the ship were also checked and cleaned. Virus checking had started a couple of days ago when it first turned up, but a few computers were found to still have the virus. By late afternoon, all machines had been scanned/cleaned and the network brought back up with no sign of the virus.

 

During the day, the skies were overcast and, like a couple of days ago, there was almost no contrast between sea ice and sky. Huge icebergs, which dotted the area, blended into the background so that they were barely visible. The winds were persistently out of the northeast (050) around 20 kts. The air temperature varied from -2.5C in the morning to -3.4C in the evening. The barometer continued its slow decline moving from 1006.2 mb to 1001.4 mb by evening.

 

CTD Group report (Eileen Hofmann, Baris Salihoglu, Bob Beardsley, Chris MacKay, Francisco (Chico) Viddi, Sue Beardsley)

During early evening of 8 August, we began an XBT survey along the course taken for the transit to the first process site. The XBT survey was to begin after exiting Crystal Sound and turning south along the north coast of Adelaide Island. However, the survey was started earlier because a region of thin ice with large areas of open water was encountered soon after leaving an area of extensive sea ice that covered the shelf just to the north of Adelaide Island. The thought was that warm water may be present at depth that then mixes upward, keeping surface waters above freezing and resulting in less sea ice cover.

 

The vertical temperature profiles from the first two XBTs show warm (1.44C) water at depths below 200 m to 250 m. Surface water temperatures were -1.77C to -1.78C, which is above freezing. Thus, it appears that the presence of the warmer modified Circumpolar Deep Water at depth may be related to the reduced ice cover at the surface. Understanding the dynamics of this interaction is a primary goal of many of the components of the SO GLOBEC program.

 

We are continuing to drop either T-7 (maximum depth of 760 m) or T-4 (maximum depth of 460 m) XBT probes, depending on the bottom depth, at about 10 nm intervals along the southward transit. The results from the XBT section will be described in a later report after the section is finished.

 

Nutrients (Yulia Serebrennikova and Steve Bell)

Nutrient analysis of the water samples taken from the CTD casts during the Crystal Sound survey was completed. The samples were taken from all seven CTD casts; a total of 113 seawater samples were analyzed for nitrate, nitrite, silicic acid, phosphate, and ammonium. The analyses of seawater samples have been conducted according to the JGOFS/WOCE suggested protocols.

 

The nutrients have a nearly classical winter distribution for both transects across Matha Strait and Crystal Sound. Nitrate, phosphate, and silicic acid exhibit high concentrations from near-bottom depths up to 200-m and slightly lower concentrations approaching the surface. There is a slight increase of silicic acid and decrease of nitrate and phosphate near the bottom below 700 m. The approximate deepwater concentrations (in micromolar units) are 34, 2.4, and 105 and the upper mixed layer concentrations are 31.5, 2.25, and 90 for nitrate, phosphate, and silicic acid respectively. The concentrations of these nutrients in the upper mixed layer are about 5% higher toward the inside of Crystal Sound. Elsewhere, there is no substantial horizontal gradient of these nutrients. Nitrite has essentially zero concentration throughout the water column for all 7 CTD stations. Ammonium levels are low also. The highest ammonium concentration, 0.1 micromolar, was found at station 1 in the northern part of Matha Strait.

 

ADCP/OPC/MOCNESS Studies (Ryan Dorland)

Single ping data collected by the ADCP are read continuously from the ship network and processed by software developed by Meng Zhou (University of Massachusetts Boston) and Eric Firing (University of Hawaii). The single ping amplitude data from the ADCP serve as a proxy for acoustic backscattering from particles in the water column. A near real-time display of the amplitude data, along with continuous display of Simrad EK500 data, allow for quick identification of potentially high backscattering layers and patches. A patch with a high Volume Target Strength (VTS) of -60 db was observed on 6 August (Julian Day 218) at 1900, during the second Tucker trawl of the evening. The patch extended 0.5 nautical miles along the ship track at -66 34.0′S; -67 08.0′W heading NW, and was present near the surface to a depth of 75 m. The Tucker trawl was towed through approximately 2/3 of this patch, and contained a high abundance of adult krill and other zooplankton.

 

The first 1-m MOCNESS tow of the cruise was conducted at 1345 on 7 August at -66 31.67′S; -67 19.17′W under calm conditions. The target depth of 500 m was not reached due to a sharp rise in bathymetry near the starting point of the tow, measured by the Simrad EK500 and ADCP. The MOCNESS was taken to a depth of 300 m and was fished towards the surface at a variable speed of up to 3 knots. Mounted on the MOCNESS is an Optical Plankton Counter (OPC, Focal Technologies) and flowmeter. The OPC measures the abundance and estimated spherical diameter (esd) of particles between 250 m and 2 cm passing a beam of light in a flow-through tunnel. The OPC provides rapid continuous sampling of marine organisms and allows for estimation of biomass. The count frequency and esd distribution is displayed in the dry lab while the instrument is deployed. High counts per second (>30) were observed during the first tow below 250 m, corresponding to a VTS of -85 db from ADCP measurements. Count frequency was less than 10 cps in the upper 75 m. The size distribution data from the first tow have yet to be processed.

 

On-station ADCP current measurements tended to be weak and variable in Crystal Sound, reaching maximum velocities of 10 cm/s. It was observed that noise from ice coverage eliminated ensemble velocity profiles when ship velocities were greater than 4 knots. The recent rendezvous with the Gould provided an opportunity to acquire the raw and ensemble ADCP data for the first portion of their cruise, allowing for comparison between the two ADCP systems.

 

Special thanks go out to Fred Stuart for his assistance with eliminating cross-talk problems between the MOCNESS and OPC systems, and to Andy Nunn on the Gould for retrieving the ADCP files on short notice.

 

Sea Birds (Chris Ribic and Erik Chapman)

The ship spent the day in a restricted area trying to free the L.M. Gould from the ice. We didn't cover much ground and remained on the southern side of Matha Strait, just north of Adelaide Island, during daylight. During that time, two Blue-eyed Cormorants, a Kelp Gull, and a Snow Petrel, and no penguins were observed. We hope to get in some survey time during the next few days as we head toward station #77, in the southern portion of the study grid.

 

Current Position and Conditions

The second day of our steam towards the southern sector of the SO GLOBEC grid with the L.M Gould following in our wake has gone very well. Our current position at 2305 on 9 August is -67 29.996′S; -71 29.001′W. The ice pack is 10/10 with few leads. The air temperature is -11.5C and the sea temperature is -1.835C, causing sea smoke to appear in the ship's wake. Winds are out of the east (085) at about 6 kts. The barometer is at 984.6 mb and the skies are cloudy.

 

Cheers, Peter