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

2 and 3 August 2002

The Drake Passage is a dreaded stretch of water extending down below the southern tip of South America to the continental shelf region of the Western Antarctic Peninsula. High winds and high seas are frequently present and on the Southern Ocean GLOBEC cruises, the investigators have had to steel themselves for a very uncomfortable ride across the Passage to get to the study site off Marguerite Bay. On the Austral fall cruise this year, the Passage lived up to its reputation, and only a few hardy souls were up and around during the first days in the transit. During the past couple of days, however, we have been spared the fury of its storms and have made it across with moderate winds and seas during an interlude in the winter storms. That is not to say that all was well. The seas were still rough enough to make a number of individuals feel seasick.

Much of 2 August was spent steaming within the 200 mile Argentine economic zone. Argentina has severe restrictions on the scientific information that can be recorded by foreign ships such as the N.B. Palmer. Thus, most electronic sensors acquiring data by electronic means were turned off. By special arrangement, the Acoustic Doppler Current Profiler (ADCP) data were being logged as were the navigation data needed to interpret the current meter data. Also, incidental observations were made of sea birds and mammals. Once the 200-mile limit was reached (around 1630), Expendable Bathythermograph (XBT) observations commenced, as described by Eileen Hofmann below, and the along-track sensing system for recording sea surface temperature, salinity, fluorescence, PCO₂, bottom depth, and meteorological data began recording data. For a number of groups, the days steaming were used to complete the setups of their laboratory space and experimental equipment, and to rig the equipment to be deployed in the ocean.

During the morning of 2 August, the winds were in the 15 to 25 kt range out of the west. Around 0800 at -57º 18.83′S; -64º 14.73′W, we crossed a front (possibly related to the Subantarctic Front) and in a matter of minutes the sea surface temperature went from 2.2ºC to -0.63ºC. There was a similar shift in the air temperature, although not as abrupt. The skies were partly cloudy throughout the day and the sea was ice free, although the first iceberg of the cruise was spotted during the day. In the evening, there were light rain showers. Around midnight as we were approaching 60 S, we encountered the northern edge of the pack ice in the form of isolated patches of sea ice. This position is much further north than where we encountered it last year at this same time of year. In the morning of 3 August, overcast skies in the Passage again gave way to mixture of sun and clouds. Bright sun highlighted the white pack ice and the numerous moderate sized icebergs that we steamed past on our way to the Boyd Strait - our entrance between Snow and Smith Islands into the inner passageway leading to the south along the Gerlache and Bismark straits. The winds stayed out of the west below 25 kts, but during the day it was cold on deck with the air temperature around -2ºC. In the late afternoon, snow squalls enveloped the Palmer and the ship's speed had to be reduced for a time because of the poor visibility.

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

The CTD group for the fourth U.S. Southern Ocean GLOBEC (SO GLOBEC) cruise consists of Bob Beardsley and Sue Beardsley, from the Woods Hole Oceanographic Institution; Eileen Hofmann and Baris Salihoglu, from Old Dominion University; and Chris MacKay, from RGL Consulting Limited. Francisco (Chico) Viddi from Universidad Austral de Chile provides the sixth member of the CTD group during times when he is not busy making cetacean observations, his primary focus for the cruise.

The CTD group began hydrographic observations for NBP02-04 after passing the 200-mile limit of Argentine coastal waters late in the afternoon of 2 August. During the transit across Drake Passage, XBTs were dropped at 10-nm intervals, with the first at the 200-mile limit and the last at the 500-m isobath on the continental shelf to the north of Smith and Snow Islands, which are part of the South Shetland Islands.

Temperature measurements were made with T-7 XBT probes, which provide data to 760 m, using a hand-held launcher. Rough seas during the first half of the transect necessitated launching the XBT probes from the 01 deck. After calmer conditions prevailed, the XBTs were launched over the ship's stern from the main deck.

Overall the XBT probes worked well. The only problems encountered were during deployments in heavy sea ice (8/10 to 10/10 coverage) when the wire broke prior to the end of the cast due to interference from ice floes. However, deploying the XBTs directly over the stern into the ship's wake minimized this problem.

The Drake Passage temperature section shows the basic structure of the Antarctic Circumpolar Current (ACC). The northern-most ACC front, the Subantarctic Front, was crossed just after passing the 200-mile limit at -58º 56.55′S; -63º 42.39′W. The structure of this front below 200 m is similar to that observed during the previous U.S. SO GLOBEC cruise (NBP02-02). South of the Subantarctic Front is the Subantarctic Zone, which is characterized by relatively flat isotherms at depths below 200 m.

The Polar Front was crossed in the morning of 3 August at -61º 15.715′S; -62º 54.578′W. This front was further south than expected from its position determined from historical data. However, it is consistent with the location observed during NBP02-02. Why the Polar Front is further south is not known at this time. It is one of the observations that will require further study, especially in terms of what it implies for Circumpolar Deep Water intrusions along the west Antarctic Peninsula continental shelf. The Polar Frontal Zone to the south of the Polar Front was reduced in extent, being only about 30 km to 40 km wide.

The Southern ACC Front was crossed at -61º 54.118′S; -62º 39.920′W just before moving up the continental slope of the shelf to the north of the South Shetland Islands. This front was identified by deepening and closing of the 2.0ºC isotherm. Just south of this feature, a “V-shaped” feature was observed in the isotherms, which was followed by the southern ACC boundary just off of the continental shelf. The “V-shaped” feature is characteristic of Antarctic shelf/slope regions. The water mass structure along the Drake Passage temperature section consists of a thick layer of Winter Water in the upper 75 m to 100 m. Below this is Circumpolar Deep Water which extends from about 200 m to the final depth of 760 m that is reached by the XBT probe. Water over the continental shelf is well mixed to about 200 m. Below this the water warms to almost 1.0ºC at 400 m, the bottom depth.

The XBT section shows that the upper water column has cooled considerably in the 8 weeks since the NBP02-02 cruise. In the northern part of Drake Passage, north of -59º 26.313′S; -63º 32.230′W, water above 200 m is between -1.0ºC and -1.4ºC. South of this point, the upper water column is between -1.5ºC and -1.85ºC. The appearance of water of -1.8 C and colder coincided with the first sighting of sea ice, which was near -59º 56.117′S; -63º 21.950′W, about half-way across Drake Passage. The regions where upper water column temperatures were less than -1.8ºC were areas where sea ice was extensive and appeared to be forming. The coldest water of -1.85ºC was found at -61º 35.103′S; -62º 43.732′W at about 25 m.

The XBT section across Drake Passage ended in the early evening of 3 August. The CTD group is now starting a second section that extends into Bransfield Strait, through Boyd Strait between Smith and Snow Islands. This section will continue until the Palmer turns south and west to transit Gerlache Strait. At this point a third XBT section will be done along the axis of Gerlache Strait. The results of these sections will be discussed in tomorrow's report.

The CTD group would like to thank Erik Chapman, Tom Bailey, and Jose Torres for providing assistance with some of the XBT casts.

Sea Birds (Chris Ribic and Erik Chapman)

Today (3 August), the ship approached the continental shelf off Livingston Island after first encountering sea ice at 60° latitude at about midnight last night. After sunrise, ice coverage steadily increased from about 5/10ths to closer to 9/10ths coverage toward the end of the day. The ice was mainly pancake interspersed with slush and as the ship moved closer to shore, it moved through older, thicker ice about 1 m thick. Judging from the root-beer colored undersurface of some of the larger pancakes overturned by the ship, some of these pancakes may be still carrying remnant algae from the late fall bloom that we observed during the April-May cruise this year.

Bird observations were conducted periodically throughout the day, though observations were made to the horizon rather than within the 300 m transect that we will survey while in the study area near Marguerite Bay. Mainly, the goal was to look for a concentration of Adélie penguins along the ice edge and if possible, to collect diet samples from them. This area was of particular interest in light of the movement of Adélie penguins fixed with satellite tags by Dr. William R. Fraser's group at Avian Island during the last cruise. Two of these birds have moved more than 300 miles north, near our location today, in recent weeks. Judging from recent satellite imagery, it appears as if the Adélies were tracking along the ice edge as it extends north from Marguerite Bay, and these birds may be indicating some interesting biology in this area. However, during 3 hours of observations, just a single, first year Adélie penguin was seen. First year Adélies, or chicks that fledged last austral summer, are easily distinguished from older birds by white feathers on their neck that are replaced by black feathers after they molt during their second year. Otherwise, 28 Snow Petrels were observed along with 3 Southern Fulmars, 3 Blue Petrels, and a Cape Petrel. Snow Petrels were observed dip feeding at the surface and picking food out from within the slush between pancakes. While the Blue Petrels were observed flying over larger leads, it is very unusual to see these Sub-Antarctic breeders foraging near ice. During the SO GLOBEC cruises, we have only seen Blue Petrels in open water, far from any sea ice. We also observed two Elephant Seals and an Antarctic Fur Seal hauled out onto the ice.

Marine Mammal report (Francisco Chico' Viddi)

Early on 2 August, the Drake Passage woke us up just to give us a “welcome” to its territory. For many of us it was quite difficult to begin the day, but fortunately we survived this first day. Beaufort was 5 and 6 for most of the day, so only two effort hours were achieved from the swaying Ice Tower. The observation effort hours were interrupted during the day in order to have better conditions (and give time to the whale observer to recover from being "sea-sick"). Even though the sea conditions could have been much worse, it was still quite rough. In spite of this there was a whale sighting at 1022 (-57º 41.54′S; -64º 07.28′W), 32º to port at 800 meters. It was a single whale, but since it was so rough, it could not identified. Before this observation, we had a wonderful sighting of an iceberg of about 15 meters long. This, the first iceberg, was our first “welcome” to Antarctica.

We are currently steaming at 11 kts on a 158 course in the Boyd Strait in relatively ice free water. Our position at 2341 on 3 August is -63º 08.55′S; -61º 44.42′W. The air temperature is -2.2ºC and the sea temperature is -1.585ºC. Winds are out of SW (236) at 20 to 30 kts and the barometer is at 989.2 mb.

Cheers, Peter