DOUGLAS G. MARTINSON, Lamont-Doherty Earth Observatory of Columbia University and Department of Earth and Environmental Sciences, Columbia University, Palisades, New York 10964
This module of the Deep Ocean Ventilation Through Antarctic Intermediate Layers (DOVETAIL) experiment (see overview by Muench, Antarctic Journal, in this issue) is designed to contribute a field component and modeling study. Initial results from our modeling activities are reported in a companion article (Holland and Martinson, Antarctic Journal , in this issue). This article addresses the field component. In particular, we have deployed six surface ice-drifters in an effort to monitor ice motion, air temperature, and pressure, providing surface forcing, or ground truthing of model-based forcing fields, for the ocean modeling component and DOVETAIL program in general.
The surface forcing will help differentiate between changes in the mixed layer due to surface fluxes and those induced by lateral displacements. These data provide the only time series of the surface forcing that drives local temporal modification of the thermohaline fields in this program.
Specifically, the ice-drifters provide hourly samples of air-temperature and pressure and an Argos transmitter to relay position, and the other data, via satellite. These measurements provide information about surface stress, geostrophic winds, and thermodynamic forcing and give estimates of surface winds and freshwater input.
The surface stress is quadratically proportional to the ice velocity; the latter extracted from ice position. The geostrophic winds will be determined from the pressure data, supplemented by the larger pressure field data available from the numerous weather stations situated throughout the Antarctic Peninsula region. The thermodynamic forcing is provided by the air-temperature observations in conjunction with standard bulk aerodynamic formulae. The near-surface air temperature will also be used to estimate the longwave back-radiation, because it is fairly well established that the ice-surface temperature is in equilibrium with average surface-air temperature (Guest personal communication). Incoming radiant heat fluxes will be computed using standard radiative transfer schemes (cloud cover will be obtained from AVHRR images when possible).
Surface winds will be estimated using the relationship of Martinson and Wamser (1990), which shows a strong correlation between ice speed and 3-day average wind speed. The relationship between wind direction and ice direction is typically 20°±20°. These surface-wind estimates will be compared to those estimated by extrapolating from the geostrophic winds to the surface (e.g., Hoeber and Gube-Lehnhardt 1987) assuming a typical stability profile from the region (e.g., Wamser and Martinson 1993; Andreas and Claffey 1995). They will also be compared to the ECMWF and NCAR gridded field values, which will be used to force the longer term model simulations, to provide some consistency checks. The winds are required for the turbulent heat-flux bulk formulae, and the derived values should be of sufficient accuracy to constrain the forcing within statistically reasonable limits (which will be tested through sensitivity studies).
The freshwater (ice melt) input is proportional to the thermal anomaly across the ice-water interface and quadratically related to the relative ice-water speed (McPhee 1994). The water temperature and surface-water velocity will be predicted by the model and diagnosed by comparison to the near-surface current meters and hydrographic survey data.
The drifters, which are low-cost, off-the-shelf devices that have 6-month battery life, are provided by Metocean Data Systems. They were deployed in August 1997 during the DOVETAIL cruise. The buoys do float but were deployed atop ice floes since they are not designed for open-ocean measurements because they are inherently unstable in a wave field. Thus, once the ice platforms drift north into warm waters and melt, the drifters will likely fail, or return only sporadic data. The ice-drifters were placed on typical ice floes at the southeastern edge of the study region (figure) after which they drifted northeasterly as seen in the figure. Originally, the drifters were to be deployed in triads (providing ice divergence); one triad deployed during each DOVETAIL cruise to provide the most continuous coverage of the study region over the longest period. Unfortunately, because of the loss of one of the cruises due to ship problems, all six drifters were deployed during the solo cruise. They were deployed in two triads. Though we lose our ability to provide long-term monitoring of the surface conditions, we gain a more robust image of the conditions during the central winter months allowing a more thorough assessment of the period monitored and its spatial variations. At the present time, the drifters are still functioning.
This work was supported by National Science Foundation grant OPP 95-27752.
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Guest, P. 1994. Personal communication.
Hoeber, H., and M. Gube-Lehnhardt. 1987. The eastern Weddell Sea drifting buoy data set of the Winter Weddell Sea Project (WWSP) 1986 . Bremerhaven, Germany: Alfred-Wegener-Institut für Polar und Meeresforschung.
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