1,089 research outputs found
RRS Charles Darwin Cruise 139, 01 Mar-15 Apr 2002. Trans-Indian Hydrographic Section across 32°S
No full textA modern hydrographic section was made across the Indian Ocean at a latitude of about 32°S during a 46-day voyage from Durban to Fremantle aboard RRS Charles Darwin in March-April 2002. The principal goal of this work was to measure the flows of mass, heat, freshwater, inorganic and organic nutrients, and carbon dioxide across the southern boundary of the Indian Ocean in order to determine the meridional overturning circulation for the Indian Ocean, to define the heat, freshwater, nutrientand carbon transports across 32°S, and to produce overall physical and biogeochemical budgets for the Indian Ocean. A second goal was to examine the climate variability in ocean circulation from comparisons of these new measurements with previous surveys in 1936, 1965, 1987 and 1995. A total of 146 hydrographic stations were made along this transoceanic section. At each station an instrument package consisting principally of a CTD, 3 Lowered ADCP's and 24 10-litre sampling bottles was lowered from the surface down to the ocean bottom to measure temperature, salinity, oxygen and eastward and northward current profiles throughout the water column. On the way back to the surface, 24 water samples were collected at various depths and these samples were analysed on board ship for salinity and oxygen (to calibrate the continuous electronic profiles), for inorganicnutrients, constituents of the carbon system, and chlorofluorocarbons. Samples were also collected and stored for later, shore-based analyses of helium, tritium, and organic nutrients. Throughout the cruise velocity data in the upper few hundred meters of the water column were provided by an ADCP mounted in the ship's hull, meteorological variables were monitored and samples of air and rainfall were periodically collected. In addition, 25 Argo floats were launched along the section to provide continuing profiles over the next 5 years. This report describes the methods used to acquire and process the measurements on board ship during the cruise
RRS Discovery Cruise 232, 04 Apr-21 Apr 1998. Gibraltar exchange processes
No full textThe 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
Discovery and quantification of the Atlantic Meridional Overturning Circulation: the importance of 25N
No full textHere we present a review of the history of modern understanding of the strength of the Atlantic Meridional Overturning Circulation (MOC), which arguably originates in 1957. This was the year that the Discovery cruises not only observed the Atlantic deep western boundary current for the first time, but also completed a transatlantic section along 24ºN, from which reliable estimates of the size and structure of the MOC were later obtained. It was also the year Stommel began to publish his estimates of the size of the Atlantic overturning. These key developments are put into the context of early qualitative pictures of the Atlantic MOC which can be traced back to 1798. The early proposals differed significantly from Wüst’s qualitativepicture of layered interhemispheric exchange, published in 1935 but still broadly accepted today, and on which subsequent quantification relied. Early estimates of the Atlantic MOC strength, as by-products of regional circulation schemes, were by today’s standard weak at 6-8 Sv. Stommel’s work from 1957 and later developments in the 1980’s produced much stronger overturning. Recognition of the importance of the MOC’s role in meridional heat transport, necessitating studies dedicated to its quantification, led to a consensus regarding its strength in the early 1980’s. The accepted 16-18 Sv MOC resulting from the 1957 Discovery section analysis supported Stommel’s 1957 work and has since been verified by independent observations. We examine only the steady state MOC here, understanding and quantification of its variability are still very much evolving
Estimation of the transports through the Strait of Gibraltar
No full textAn acoustic doppler current profiler (ADCP) is used to measure the currents and estimate the transports over the Camarinal Sill at the Strait of Gibraltar. The deepest measurements of the ADCP compare well with an underlying conventional current meter. The exchange interface between the Atlantic and the Mediterranean water is defined as the depth of the maximum vertical shear. The mean depth of the shear interface is 147 m. The time series of the depth of the interface and the currents are used to estimate the transports across the Strait. The resulting values are 0.78 Sv for the Atlantic inflow and ?0.67 Sv for the Mediterranean outflow. The time series of the shear interface include fortnightly oscillations of 19 m. The time series of the transports are compared with the pressure and sea level difference records across the Strait. Linear multiple regression is used to estimate the (statistical) contribution of each parameter on the variation of transports. The cross strait sea level difference is well correlated with the Atlantic inflow and accounts for 57% of the variability of the transport records which improves to 78% when the fortnightly and monthly cycles are included in the linear regression. The Mediterranean outflow is best correlated with the along strait sea level difference which accounts for only 10% of the variability of the transport record. Again the addition of the Msf and the MM cycles improves the percentage of the variance accounted for to 37%. The local, along strait wind component is significantly correlated with, both the Atlantic inflow and the across strait sea level difference
Observing the Atlantic Meridional Overturning Circulation yields a decade of inevitable surprises
No full textThe importance of the Atlantic Meridional Overturning Circulation (AMOC) heat transport for climate is well acknowledged. Climate models predict that the AMOC will slow down under global warming, with substantial impacts, but measurements of ocean circulation have been inadequate to evaluate these predictions. Observations over the past decade have changed that situation, providing a detailed picture of variations in the AMOC. These observations reveal a surprising degree of AMOC variability in terms of the intraannual range, the amplitude and phase of the seasonal cycle, the interannual changes in strength affecting the ocean heat content, and the decline of the AMOC over the decade, both of the latter two exceeding the variations seen in climate models
Investigating the transport of Mediterranean water in the eastern north Atlantic using hydrographic and model data (abstract of paper presented at ASLO/AGU Ocean Sciences Meeting, Honolulu, HI, 11-15 Feb 2002)
No full textRole of the Agulhas Current in Indian Ocean circulation and associated heat and freshwater fluxes
No full textA reduced estimate of Agulhas Current transport provides the motivation to examine the sensitivity of Indian Ocean circulation and meridional heat transport to the strength of the western boundary current. The new transport estimate is 70 Sv, much smaller than the previous value of 85 Sv. Consideration of three case studies for a large, medium and small Agulhas Current transport demonstrate that the divergence of heat transport over the Indian Ocean north of 32°S has a sensitivity of 0.08 PW per 10 Sv of Agulhas transport, and freshwater convergence has a sensitivity of 0.03×109 kg s?1 per 10 Sv of transport. Moreover, a smaller Agulhas Current leads to a better silica balance and a smaller meridional overturning circulation for the Indian Ocean. The mean Agulhas Current transport estimated from time-series current meter measurements is used to constrain the geostrophic transport in the western boundary region in order to re-evaluate the circulation, heat and freshwater transports across 32°S. The Indonesian Throughflow is taken to be 12 Sv at an average temperature of 18°C. The constrained circulation exhibits a vertical–meridional circulation with a net northward flow below 2000 dbar of 10.1 Sv. The heat transport divergence is estimated to be 0.66 PW, the freshwater convergence to be 0.54×109 kg s?1, and the silica convergence to be 335 kmol s?1. Meridional transports are separated into barotropic, baroclinic and horizontal components, with each component conserving mass. The barotropic component is strongly dependent on the estimated size of the Indonesian Throughflow. Surprisingly, the baroclinic component depends principally on the large-scale density distribution and is nearly invariant to the size of the overturning circulation. The horizontal heat and freshwater flux components are strongly influenced by the size of the Agulhas Current because it is warmer and saltier than the mid-ocean. The horizontal fluxes of heat and salt penetrate down to 1500 m depth, suggesting that warm and salty Red Sea Water may be involved in converting the intermediate and upper deep waters which enter the Indian Ocean from the Southern Ocean into warmer and saltier waters before they exit in the Agulhas Current
Changes in ocean water mass properties: oscillations or trends?
No full textA new transindian hydrographic section across 32°S reveals that thermocline mode waters have become saltier and colder since 1987. This change almost entirely reverses the observed freshening of mode waters from the 1960s to 1987 that has been interpreted to be the result of anthropogenic climate change on the basis of coupled climate models. Here, we compare five hydrographic sections from 1936, 1965, 1987, 1995, and 2002 to show that upper thermocline waters (10°C to 17°C) changed little from 1936to 1965, freshened from 1965 to 1987, and since 1987 have become saltier. These results demonstrate substantial oscillations in mode-water properties
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