The Antarctic Zone Flux Experiment M. G. McPhee,* S. F. Ackley,+ P. Guest/ B. A. Huber @ D. G. Martinson® J. H. Morison,& R. D. Muench,** L. Padman,## and T. P. Stanton* ABSTRACT In winter the eastern Weddell Sea in the Atlantic sector of the Southern Ocean hosts some of the most dynamic air- ice-sea interactions found on earth. Sea ice in the region is kept relatively thin by heat flux from below, maintained by upper-ocean stirring associated with the passage of intense, fast-moving cyclones. Ocean stratification is so weak that the possibility of deep convection exists, and indeed, satellite imagery from the Weddell Sea in the 1970s shows a large expanse of open water (the Weddell Polynya) that persisted through several seasons and may have significantly altered global deep-water production. Understanding what environmental conditions could again trigger widespread oceanic overturn may thus be an important key in determining the role of high latitudes in deep-ocean ventilation and global atmospheric warming. During the Antarctic Zone Flux Experiment in July and August 1994, response of the upper ocean and its ice cover to a series of storms was measured at two drifting stations supported by the National Science Foundation research icebreaker Nathaniel B. Palmer. This article describes the experiment, in which fluxes of heat, mass, and momentum were measured in the upper ocean, sea ice, and lower-atmospheric boundary layer. Initial results illustrate the importance of oceanic heat flux at the ice undersurface for determining the character of the sea ice cover. They also show how the heat flux depends both on high levels of turbulent mixing during intermittent storm events and on large variability in the stratified upper ocean below the mixed layer. 1. Background seasonal ice edge and the continental shelves, that re- mained virtually ice free for several years (Zwally and The cold, abyssal waters of the World Ocean in- Gloersen 1977; Carsey 1980; Martinson et al. 1981). teract directly with the rest of the climate system in This expanse of open water, called the Weddell limited areas confined to high latitudes, where den- Polynya, was maintained by infusion of heat from be- sity structure of the upper ocean permits deep convec- low, suggesting that transient (by climatic timescales) tion. In the mid-1970s, satellite imagery showed a deep-ocean convection was occurring on a massive large region in the Weddell Sea, far from both the scale. Direct evidence of deep convection was ob- served in the region during the time of the polynya *McPhee Research Company, Naches, Washington. (Gordon 1978), and, indeed, Gordon (1982) estimates +USA Cold Regions Research and Engineering Laboratory, that deep-water formation associated with the polynya Hanover, New Hampshire. exceeded typical production in the Weddell Sea #Naval Postgraduate School, Monterey, California. (which is derived mainly from the continental shelves @ Lamont-Doherty Earth Observatory, Palisades, New York. and comprises a significant proportion of the total &Polar Science Center, University of Washington, Seattle, Washington. deep-water production) by three to six times. The **Earth and Space Research, Seattle, Washington. polynya affected a tremendous transfer of heat from ##College of Oceanic and Atmospheric Sciences, Oregon State the ocean to the atmosphere: while it was active University, Corvallis, Oregon. (1974-76), average water temperature in the upper Corresponding author address: Dr. Miles G. McPhee, 450 Clover 2000 m of the central Weddell dropped by over 0.5°C Springs Rd., Naches, WA 98937. (see Fig. 4 of Gordon 1991). The heat capacity of 2 E-mail: [email protected] In final form 11 January 1996. km of water is about three orders of magnitude greater ©1996 American Meteorological Society than that of the entire air column, so there must have J J 77 Bulletin of the American Meteorological Society Unauthenticated | Downloaded 10/08/21 10:40 PM UTC been a major impact on winter air temperature and ing deep convection and much larger heat flux asso- atmospheric circulation in the region as well. ciated with deep-water formation (Martinson 1990). Compared with most of the World Ocean, static From the standpoint of upper-ocean physics, sus- stability of the water column in the central and east- tained high heat flux through the mixed layer in the ern Weddell Sea is very weak. It appears that the air- Weddell poses interesting questions. At high latitudes, sea-ice system there exists in a precarious balance Coriolis attenuation of turbulent stress in the bound- between two quasi-stable modes that display substan- ary layer is strong [e.g., the Ekman stress spiral pre- tially different sea ice distributions and deep-water sented by McPhee and Martinson (1994)], serving to formation rates (Martinson etal. 1981;Gordon 1991). limit the depth of turbulent mixing compared with The currently active state supports a seasonal sea ice lower latitudes. Direct upwelling at the base of the cover of~8x 106 km2 (Zwally et al. 1983) and coastal mixed layer, or strong diffusion in the underlying deep/bottom-water formation (e.g., Gill 1973; Foster pycnocline (sharp density gradient), also tends to keep and Carmack 1976). The alternative mode (probably the mixed layer shallow. The ice apparently reaches exemplified regionally by the Weddell Polynya) is its mean thickness during a short period of rapid characterized by winterlong deep convection driving growth early in the winter and, on average, grows very strong vertical fluxes of heat, salt, and atmospherically little thereafter, eliminating brine-driven convection active gases. The convection feeds significant open- as a major factor in mixed-layer dynamics. At tem- ocean deep-water formation (Gordon 1982; Martinson peratures near freezing, buoyancy flux is determined 1991), and the associated vertical heat flux prevents almost entirely by salinity flux. Thus, several factors the formation of an insulating sea ice cover (Killworth seem to work against high heat flux and deep mixed 1979; Martinson et al. 1981). If the convective mode layers in the central and eastern Weddell, yet previ- were to encompass the whole Weddell Gyre, a lack ous winter expeditions found mixed layers averaging of winter sea ice would almost certainly affect the glo- nearly 100 m thick and inferred average upward heat bal climate, contributing significantly to global atmo- flux during the winter as high as 40 W m~2 (Gordon spheric warming (Schlesinger and Mitchell 1985; and Huber 1990). Results of numerical modeling stud- Rind et al. 1995), with intense cooling and ventila- ies of the magnitude of oceanic heat flux in the re- tion of the abyssal ocean. gion have been ambiguous; for example, Martinson It has been nearly two decades since an extensive (1990) suggested that heat flux from below the mixed 2 area of open water persisted through the entire win- layer of 20-40 W m~ was necessary to maintain the ter in the central Weddell, yet several recent winter upper ocean-ice regime, while Lemke et al. (1990) oceanographic expeditions through the region have found oceanic heat flux to the ice to be an order of demonstrated that the ice cover remains relatively magnitude less in their coupled ocean-sea ice model. thin, the mixed layer is deep by polar standards, and McPhee (1994) showed that high heat flux and deep that there is strong inferential evidence (e.g., thin ice, mixed layers could coexist in his numerical bound- elevation of mixed-layer temperature above freezing, ary layer model, but only if the surface momentum entrainment of geochemical tracers from below) for flux was concentrated in storm events with episodes significant heat flux out of the deep ocean (Gordon of very intense turbulence. If, in fact, oceanic heat flux and Huber 1990; Schlosser et al. 1990). Away from plays a pivotal role in maintaining the tenuous sea- a relatively narrow region of multiyear ice east of the sonal ice cover of the Weddell, understanding the Antarctic Peninsula, mean ice thickness over much small-scale mechanisms governing vertical flux in the of the Weddell reaches only 0.6-0.7 m or less, even mixed layer and pycnocline is an important first step in late winter (Wadhams et al. 1987). At latitudes typi- in developing capability for predicting what environ- cal of the Weddell, the thermodynamic balance is mental changes could again trigger widespread deep such that the ice would be expected to grow to at least convection. twice this thickness in the absence of oceanic heat flux Against this backdrop, a group of scientists inter- or persistent ice divergence. In many parts of the ested in air-sea-ice interactions recommended a Weddell, salt rejected by a few tens of centimeters winter expedition into the Weddell Sea as one com- of additional ice growth could initiate deep convec- ponent of the international Antarctic Zone Program tion. So in an almost paradoxical way, it appears that (ANZONE). ANZONE is a relatively informal but anomalously large heat flux into the mixed layer from active association of scientists interested in oceanic below prevents additional ice growth, thus forestall- processes south of the Polar Front in the Southern 1222 Vol. 77, No. 6, June 7 996 Unauthenticated | Downloaded 10/08/21 10:40 PM UTC Ocean. It has targeted the Weddell Sea for intensive balance studies, plus automated stations for measur- study and provided, for example, the forum for orga- ing ice temperature at closely spaced intervals in the nizing the highly successful U.S.-Russian Ice Station ice (J. Wettlaufer 1994, personal communication). Weddell (ISW; Gordon et al. 1993). The suggested Shipboard oceanographic systems included a hull- project, called the ANZONE Flux Experiment mounted, 150-kHz acoustic Doppler current profiler (ANZFLUX), was designed primarily to measure (ADCP); a conductivity-temperature-depth (CTD) the response of the upper ocean and sea ice to storm recorder with rosette water sampler; plus a second events.