A.F. AMOS, A.R. WICKHAM, and C.C. ROWE, University of Texas at Austin, Marine Science Institute, Port Aransas, Texas 78373
The AMLR program has completed eight austral summer seasons (mid-January to mid-March 1990-1997) surveying the upper waters around Elephant Island. These surveys are designed to describe the relationships between krill, their predators, and environmental variables. During each austral summer from 1990 to 1996, a research cruise with two 30-day legs was completed. A large-area survey of 91 conductivity-temperature-depth (CTD)/rosette stations on a grid with 15-nautical-mile spacing between stations was completed once per leg. This season, the large-area survey grid was expanded to 106 stations; new stations were added west of Livingston Island and south of King George Island in Bransfield Strait. The new grid was successfully completed on Leg I. Because of a delay for ship repair, however, Leg II was significantly shortened (see Martin, Hewitt, and Holt, Antarctic Journal , in this issue).
At each of the 106 stations on Leg I (Survey A), the physical oceanography group obtained vertical CTD profiles to 750 meters. In addition to temperature and salinity, profiles of dissolved oxygen, optical parameters, and in situ chlorophyll- a fluorescence were obtained along with discrete water samples for nutrient and phytoplankton analyses. This year, a new Sea-Bird SBE-9/11 PLUS CTD/carousel water sampler was used for the CTD/carousel casts.
No similar CTD/carousel casts were made on the shortened Leg II; however, a Sea-Bird Seacat (model SBE16) was used for CTD casts (limited to 250 meters depth) along the 55°W meridian, a section of interest to the international community (CCAMLR, 1995). As on all previous AMLR cruises, weather, sea temperature, salinity, water clarity, chlorophyll, and solar radiation data were continuously acquired. This season, new equipment (Coastal Environmental WeatherPak and a Fluke Data Bucket) consolidated the acquisition of these data.
As in the past, we classified and grouped stations with similar vertical temperature/salinity (T/S) characteristics. We have identified five water zones, designated I through V. It should be noted that these zones are based on the T/S curves from the surface to 750 meters (or to the bottom in water shallower than 750 meters). For example, Water Zone I is based on these multiple characteristics:
Water Zone I is the oceanic water of the Drake Passage. In the Bransfield Strait and south of Elephant Island, Water Zone IV dominates. Here, bottom waters are around -1°C, and the subsurface extrema are far less prominent, although a slight "crook" in the curve is characteristic. In between, there are transition zones where adjacent water zones mix. In this article, we report only on the Leg I survey, which took place late January to mid-February.
Figure 1 shows the composite T/S scatter diagram for all stations of Survey A. The stations shown in the inset map are shaded ac-cording to water zone. The two major zones can clearly be seen. Separation of the transition zones along the main frontal boundary by their T/S characteristics is also reflected in the chlorophyll- a profiles shown in Holm-Hansen et al. ( Antarctic Journal , in this issue). This year on Leg I, surface waters exceeded 4°C for the first time in the 8 years of AMLR surveys in which surface conditions were monitored continuously (figure 2). Data are taken from the underway environmental data-collection system rather than from the CTD to increase the resolution. During "warm years" such as 1993 (Amos 1993), temperatures above 3°C were occasionally found in the same region in January and February and, more frequently, in February and March. This year the condition was reversed with the record-setting temperatures occurring on Leg I and much cooler surface temperatures on Leg II. Of course, Leg II was considerably reduced in scope and encompassed only the last several days of March, when the atmosphere began to cool down approaching the austral autumn. Consequently, 1997 looks as though it might be an anomalous year for the AMLR program.
We have noted that in more recent AMLR cruises, especially in January and February, the perception is that calmer atmospheric conditions prevailed compared to the earliest AMLR cruises. During Leg I, the mean wind speed below 60°S was 6.3 meters per second; this was even calmer than last year, which had been the record-holder for "serene" conditions. The few days of Leg II below 60°S were much windier: the mean wind was 10.1 meters per second. Although the maximum wind of 21.9 meters per second on Leg I was higher than last year, these high winds occurred only once at the beginning of the leg. For 21 days, the maximum wind speed did not exceed 15.5 meters per second. Air temperatures were below freezing for only a few hours during Leg I, but there were a few days of freezing temperatures on the abbreviated Leg II with a low of -2.3°C.
This work was supported by National Oceanic and Atmospheric Administration (NOAA) contract number 50ABNF600015. We thank the crew and captain of the R/V Yuzhmorgeologiya , especially the scientific support staff under Oleg Pivovarchuck.
Amos, A.F. 1993. AMLR program: Interannual variability in the Elephant Island surface waters in the austral summer. Antarctic Journal of the U.S., 28(5), 201-204.
Convention on the Conservation of Antarctic Marine Living Resources (CCAMLR). 1995. Report of the Working Group on Ecosystem Monitoring and Management. Appendix I. SC-CAMLR-XIV/3, Siena, Italy, 24 July to 3 August 1995.
Holm-Hansen, O., C.D. Hewes, J. Maturana, L. Rios, and G. Gonzalez-Rodas. 1997. AMLR program: Phytoplankton distribution and its relationship to different water zones characterized by physical oceanographic parameters, January and February 1997. Antarctic Journal of the U.S., 32(5).
Martin, J.E., R.P. Hewitt, and R.S. Holt. 1997. The U.S. Antarctic Marine Living Resources (AMLR) program: 1996-1997 field season activities. Antarctic Journal of the U.S., 32(5).