Uwe Send

Open ocean deep convection, Mediterranean and Greenland Seas

Recent observations within deep convection regimes of the Gulf of Lions and Greenland Sea all confirm the existence of small-scale plumes of only a few 100 m horizontal scale during cooling periods, in agreement with scaling arguments and non-hydrostatic modelling results. The integral effect of the plumes is that of a mixing agent rather than carrying water downward in a mean motion. It depends on the intensity and duration of the cooling how complete the mixing within the depth range of the plumes is. In the Greenland Sea, the role of the ice through brine rejection was found to be important in the preconditioning period (November – February) rather than for the deep convection itself (March) which occurred when the water was ice-free.

The Deep Water Regime in the Equatorial Atlantic

The importance of the Deep Western Boundary Current (DWBC) in the Atlantic for the interhemispheric exchange of water masses and of heat is well known, but data to estimate transports and to follow its pathways are sparse, especially in the equatorial Atlantic. New insight into the distribution of water masses in the DWBC, their transports and their variability off Brazil were gained in a contribution to the WOCE (World Ocean Circulation Experiment) program. In these studies, moored current meter measurements were combined with shipboard data from three cruises in the years 1990, 1991 and 1992. Besides tracer (Chlorofluoromethanes CFMs, components F11 and F12) and hydrographic data, direct velocity measurements were carried out using a lowered ADCP attached to the CTD, and the Pegasus profiling system.

Aspects of acoustic transponder surveys and acoustic navigation

New off-the-shelf hardware has allowed improved techniques for acoustic transponder deployment and surveying, saving time and improving the accuracy of results. In particular, affordable acoustic deck units which allow continuous computer sampling of acoustic ranges while the ship is under way (merged with simultaneous GPS position data), allow application of a variety of survey techniques, to determine the exact transponder positions or separations. Apart from a particular hardware setup used, various possible analysis techniques are summarized and results compared from a number of applications. In addition, the positioning problem using 2 or 3 acoustic transponders is discussed. The solutions are presented together with their respective errors, allowing simple rule-of-thumb estimates for positioning (or velocity) accuracy, as a function of the uncertainty in input parameters (e.g. transponder positions, acoustic travel time, depth measurement).