2. Heat content across the Mediterranean ("THETIS-2").
During much of 1994, a tomography array of 7 systems (250/400Hz) was deployed across the western Mediterranean (scales up to 600km) in a joint effort with our previous THETIS partners. The purpose was to observe the seasonal basin-scale heat content cycle, to provide a data set for assimilation studies, and to demonstrate the feasibility of a shore-based acoustic monitoring system. Full 9-month long time series were obtained successfully with at least one-way transmissions along all transmission paths. The analysis is still in progress, but preliminary inversions for total water-column heat content appear very accurate and comparison with ECMWF surface heat fluxes agree currently to within 10% or better. Also, an assimilation study demonstrated the complementary nature of these data with satellite altimetry. Various other assimilation studies with this data set are in progress, and more accurate surface heat flux calibrations are expected.
3. Transport observations in the Strait of Gibraltar ("CANIGO").
In 1996, a month-long pilot experiment was conducted in the Strait of Gibraltar in a joint project with SIO/San Diego. High-frequency (2kHz) acoustic transmissions across the strait in a perpendicular and diagonal direction (16-20km) were used to test various methods of observing the horizontally integrated transport through the strait. Such a capability is important, since the cross-strait structure of the flow is rather complicated and cannot be integrated well with a few moorings to yield the transport. Therefore particularly intensive moored and ship-board measurements of the actual currents during the acoustic measurements provided the comparison data.
The preliminary analyses show that both the reciprocal traveltime differences along the diagonal path and the horizontal phase differences between two receivers for the perpendicular path, are in qualitative agreement with the flow observations.
4. Monitoring of thermohaline overturning processes.
In 1996, the first phase of a German special research initiative was started, which has the topic of variability of thermohaline systems. This is a CLIVAR-related topic and the emphasis is on the (variability of) processes taking place in the subpolar North Atlantic, which is the source region for the global thermohaline "conveyor belt" circulation. These processes and regions include overflows from the polar seas, entrainment and mixing of overflow waters, convection and deep water mass formation in the Labrador and Irminger Seas, source and driving processes for the deep water originating in this region. As part of this first 3-year phase, intensive observations of convection (driving, spatial extent, variability) and spreading of water in/from the Labrador Sea are taking place, and a tomography array has been deployed in summer 1996. This will be maintained with between 3 and 5 instruments for the next 3 years, and complements the large number of other techniques deployed (in cooperation with US and Canadian groups). The hope is to extend the tomography array in future phases and in the framework of CLIVAR and GOOS, to monitor the water masses on the scale of the entire supolar gyre. This cannot be maintained by one group alone and cooperation with other laboratories capable of applying tomography is solicited.
Acknowldegements :
The THETIS and THETIS-2 projects were supported by the EC MAST and MAST-2 programs under contracts MAST-CT90-0008 and MAS2-CT91-0006.
The Gibraltar project is embedded in the EC MAST-3 program CANIGO (contract MAS3-CT96-0060) and the SIO participation is ONR funded. The Labrador Sea project is part of the "Sonderforschungsbereich" 460 funded by the German Science Foundation (DFG).
