Small-Scale Process Field Study

The first set of objectives entails a winter field study planned for August-September, 2005. The experiment is designed to assess the impact of NES processes as the density of the surface mixed layer approaches that of the underlying Weddell Deep Water. By carefully monitoring the important terms in the turbulent kinetic energy (TKE) budget (including shear production, buoyancy production, vertical transport of TKE, and TKE dissipation), our aim is gauge the rate at which turbulence derives energy from thermobaricity deep in the water column.

Based on experience from previous ice stations and from buoy records, we do not underestimate the difficulty of operating in the extreme environment of the Weddell Sea during winter, and we recognize that the chances of encountering a large polynya in a particular year are not great. On the other hand, we believe there is strong evidence that we will see conditions of instability or marginal stability as discussed above, and that we are capable of making upper-ocean measurements in such conditions that are uniquely valuable. In nearly every year, there are areas of abnormally low ice concentration around Maud Rise in late winter. In addition, analysis of winter T/S

profiles from the Weddell (McPhee 2003) indicatedthat the least thermobarically stable profiles clustered along the flanks of Maud Rise, typically between the 2500 and 3000 m isobaths. Many of these profiles would have reached thermobaric instability (assuming horizontal homogeneity) by the end of September with an average heat loss of ~25 W m-2 or less. Estimates of average heat loss in the Weddell range from 25-40 W m-2 (Gordon and Huber 1990; McPhee et al. 1999).

We envision three possible successive phases for the late winter core process experiment. Assuming that active deep convection and polynya formation are not detected immediately, we will first establish a drift station in a region selected from our survey, with emphasis on measuring near surface fluxes and corresponding fluxes near the pycnocline (typically 100 m), as well as in sometimes active layers below the cold SML. We will maintain this station for long enough to get good baseline estimates of the response of the upper ocean to surface forcing when NES effects are relatively minor. Given the intense storms of the Weddell, drift stations may move tens of kilometers in the span of a day or two, away from the original underlying water column. At some point as the season progresses and the density contrast between the SML and underlying WDW becomes small, we will shift our emphasis to a second mode of operation, in which we track a specific water mass by homing acoustically on Lagrangian drifters, stationed to cycle below the SML. During this phase we anticipate moving the ship a few kilometers at a time in between passive drifts with shorter periods of intense measurements. We will concentrate on quantifying the energy changes and fluxes associated with converting potential energy of the water column to kinetic, particularly near the base of the SML and in active IMLs, should they develop. In the first and second phases, we assume we will be able to operate in ice-covered water with little vertical surface motion (during ANZFLUX we were able to measure SML turbulent covariance statistics with surface winds approaching hurricane force).

The final phase will depend on finding conditions in which the ice cover has been removed by heat from deep convection (or by other means) so that the upper ocean is in a state of purely thermal convection (Gordon’s [1991] thermal mode). Here we will adjust our sampling strategy toward that of an open ocean environment, concentrating on atmospheric fluxes and changes in upper ocean characteristics. While the progression to this final phase represents an ideal experiment, we emphasize that the appearance of a large polynya is not critical to the overall success of the project— most if not all of the objectives regarding the NES questions can be accomplished by competent execution of the first two phases.

Study participantes: Miles McPhee Eric D'Asaro Peter Guest James Morison R.D Muench L. Padman Tim Stanton