We will undertake the first ever comprehensive hydrographic transect along the North Scotia Ridge (Figure 1), additionally measuring dissolved oxygen, nutrients (silicate, nitrate and phosphate) and current profiles. We will deploy current meter moorings at the three sills of Shag Rocks Passage. The mean and time dependent transfer of deep waters across the ridge is an important, but as yet unmeasured, component of the present-day global thermohaline circulation. The measured near-bottom currents will aid interpretation of the variability in palaeoclimate observed in nearby sediment cores.
Figure 1 Proposed cruise track along the North Scotia Ridge (thick solid line). The bathymetric contour interval is 500 m. Depths greater than 4500 m are white. Thick dashed lines denote the mean position of the Subantarctic Front (SAF)4 and Polar Front (PF)3.
The North Scotia Ridge (NSR), between the Falkland Islands and South Georgia, is traversed by much of the Antarctic Circumpolar Current (ACC). The ACC plays an important role in the global climate system, constituting the major conduit for oceanic mass, heat, and freshwater between the Atlantic, Indian, and Pacific Oceans. The ACC comprises a number of narrow jets, namely the SAF, the PF and the Southern ACC Front4. The SAF and PF both cross the NSR (Figure 1) and must be responsible for transporting Antarctic waters towards the South Atlantic basins. It is thought that the SAF passes through a 1700 m gap at 55°W, and that the PF crosses the NSR near Shag Rocks Passage (53°S 48°W). The net heat and mass transports via deep flows across the NSR need quantifying to constrain concepts and models of global climate. The World Data Center for Oceanography has virtually no deep data in the vicinity of the NSR.
The ACC and the NSR constitute hydrodynamic and topographic barriers which the deep Antarctic waters must negotiate in their northward transit. Recently available satellite-derived bathymetry5 suggests four main locations for these overflows (55°W, 48°W, 45°W and 40°W). Shag Rocks Passage, at approximately 3000 m, is by far the deepest. A near-bottom current meter record of 1 year duration6 contained an episodic northwestward flow of cold Antarctic water through Shag Rocks Passage. Peak velocities were higher than for any other deep flows ever measured in the Southern Ocean. At that time, it was thought that Shag Rocks Passage was a single sill, but it is now known5 that there are three separate 3000 m deep sills, each providing a possible pathway for deep water to leave the Scotia Sea. It is not known whether the extreme variations in observed current speed observed6 are due to changes in the net flow through Shag Rocks Passage, or due to diversion of the flow to a different sill. The partitioning of flows between these passages, and the variability of each, are unknown, as are the dynamical processes controlling the variability.
The cruise track (Figure 1) commences at the Falkland Islands, progresses southwards to Burdwood Bank and then eastwards along the crest of the NSR to South Georgia. This enables us to determine where water masses are able to overflow the ridge. Stations will spaced at most 50km apart, and closer when crossing fronts. Over steep bathymetry stations will be undertaken at 500 m isobath intervals.
At each station full depth CTDO2 casts will be carried out and samples collected using a 24 bottle rosette for measurement of nutrients (silicate, nitrate and phosphate) and dissolved oxygen. The nutrient and oxygen measurements are essential in distinguishing the pathways for each of the water masses which flow through the Scotia Sea. In addition we shall deploy a Lowered Acoustic Doppler Current Profiler (LADCP) attached to the CTDO2 rosette package to obtain profiles of absolute current. The LADCP enables us to measure flows of deep water in gaps where it is only possible to locate one deep CTD station. The shipborne ADCP will be used throughout the cruise in conjunction with the 3 dimensional GPS system installed on the RRS James Clark Ross. This will also enable referencing of the geostrophic shear and calculation of bottom currents. This is vital in the ACC which has a significant barotropic component.
Three moorings (3 near bottom current meters, 1 ADCP) will be deployed at the sills of the deep passages of the Shag Rocks Passage as the hydrographic transect is conducted. They will remain in place for at least 1 year. Instrumentation should be available from the National Marine Equipment Pool following forthcoming purchases funded by JIF. Recovery can be undertaken by either of the BAS vessels with minimal disruption to their normal operation. CTDs should be conducted at the mooring sites upon recovery to enable quantification of sensor drift.
Objectives (1) and (2) will be addressed by analysing temperature, salinity and tracer properties, and geostrophic transports referenced to shipborne ADCP, LADCP and moored current meter velocities. Objectives (3) and (4) will be addressed by analysing current meter data, which will also show if the flow of deep water is invariant across the passage as a whole but intermittent across individual sills. Objective (5) will be addressed by undertaking a joint study incorporating these data with those collected as part of the ALBATROSS project1. A box inverse analysis will provide measurements of exchanges and mixing between water masses. Objective (vi) will be addressed by combining current meter data with results from existing BAS cores2.
1Heywood, K.J. and D.P. Stevens, UEA Cruise Report No. 6, 2000.
2Howe, J.A. and C.J. Pudsey, Journal of Sedimentary Research , 69, 847-861, 1999.
3Moore, J.K., M.R. Abbott and J.G. Richman, Journal of Geophysical Research, 104, 3059-3073, 1999.
4Orsi, A.H., T. Whitworth III, and W.D. Nowlin, Jr., Deep Sea Research, 42, 641-673, 1995.
5Smith, W.H.F and D.T. Sandwell, Science, 277, 1956--1962, 1997.
6Zenk, W., Science, 213, 1113--1114, 1981.