RRS Discovery Cruise 232, 04 Apr-21 Apr 1998. Gibraltar exchange processes

Abstract

The three principal objectives for RRS Discovery Cruise 232 were to carry out mooring operations associated with long-term monitoring of the exchange between the Atlantic and the Mediterranean through the Strait of Gibraltar, to study nonlinear processes resulting from the strong currents in the Strait with new instrumentation uniquely available on RRS Discovery, and to measure biogeochemical fluxes associated with the upper layer inflow of Atlantic water and the lower layer outflow of Mediterranean water through the Strait.

Eleven moorings deployed by scientists from Southampton Oceanography Centre (SOC), University of Malaga (UM), and Institut fur Meereskunde (IFM) in Kiel, who are cooperating in a two-year monitoring of the exchange through the Strait of Gibraltar using moored current meters under a multi-disciplinary CEC targeted programme called CANIGO, were scheduled for recovery during the cruise. Eight moorings were successfully recovered: two moorings were pre-released due to an error by an American collaborator and one mooring remains entangled with its anchor at the sill.

The principal nonlinear process studied was the development of a bore on the outgoing tide near the sill, its release as the tide turns, and its conversion into a nonlinear wave train as it propagates eastward into the Mediterranean. Dramatic signatures of 100m amplitude internal waves were measured by acoustic backscatter using EK500 underway profiling. Robust evidence for the waves was simultaneously derived from current profiles measured by the shipboard acoustic Doppler current profiler (ADCP), not only from the horizontal velocity but also from the directly measured vertical velocity, and from tow-yo CTD profiles up and down through the interfacial region between Atlantic and Mediterranean waters.

From hydrographic sections across the eastern and western entrances to the Strait of Gibraltar, we aimed to measure the biogeochemical fluxes through the Strait. Water samples analysed for oxygen, nutrients, chlorofluorocarbons, trace metals and dissolved organic carbon are to be combined with CTD and lowered ADCP velocity profiles to determine the fluxes directly. Such flux calculations represent a challenging sampling and analysis problem due to the tidal variations in the inflow and outflow currents as well as in the depth of the interface between the Mediterranean and Atlantic waters.

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Identifiers

ORCID for H.L. Bryden: orcid.org/0000-0002-8216-6359

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