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Friedrich Schott

Abstract

A useful extension of upward-looking buoy-mounted acoustic Doppler current profilers (ADCPs), in particular for studying surface-mixed-layer dynamics from underneath, would be to combine them with a thermistor cable for obtaining simultaneous current and temperature profiles. Measurements with an ADCP, operating at 150 kHz and sampling at 2.2 m vertical bin lengths, were carried out from a barge moored in Lake San Vincente, California, over a water depth of 45 m, to investigate the effects of a thermistor string running along the axis of the instrument between the acoustic beams and held by a small auxiliary float. The biasing effect of the thermistor string itself was only of order 1%. Doppler biasing of the horizontal currents by two different kinds of Coats (one a cylindrical hard foam float with low acoustical reflectivity, the other a hollow sphere) was investigated. The floats were suspended at 15 and 30 m. For floats at 30 m, evidence was not conclusive because of apparent interference by other reflectors. The cylindrical foam float at 15 m caused no measurable Doppler biasing, while the effect of the hollow sphere was of order 40% in the depth bin around 15 m.

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Walter Düing and Friedrich Schott

Abstract

Temperature and current records were obtained from four subsurface moorings deployed in the source region of the Somali Current from mid-January to mid-July 1976.

The first part of the records from January until early April showed that the array straddled the convergence zone of the northward-flowing East African Coast Current and the southward-flowing Somali Current. During this time, except for the southernmost location off Mombasa, the mean flow at all locations was weak and variable. The predominant variability had a time-scale of 4–5 days.

Around 20 April the wind shifted to the southeast and three days later the flow in the upper 80 m turned northward and intensified. Development of strong northward flows below the thermocline took several more weeks. The observations imply that a switching mechanism took place at that time. The initially eastward-flowing Equatorial Counter Current is shifted rapidly 45° to the left to run northward along the coast. This mechanism may, in part, be responsible for the impulsive beginning of the Somali Current during the early stages of the monsoon onset south of the equator.

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Friedrich Schott and Walter Düing

Abstract

Current measurements from three stations along the east coast of Florida at about 300 m water depth with a maximum longshore separation of 180 km were used to analyze for the presence of propagating waves.

The analysis was done by solving the inverse problem of determining the most likely wave parameters from 36 independent auto spectra and cross spectra from four current meters. For the 10–13 day band a significant fit of the data by a wave cross-spectral function was found. The wavelength is 170 km and the phase propagation 17 cm s−1 toward the south. The current fluctuations are elliptically polarized with anti-cyclonic rotation and with an axis ratio of 0.30. The mean current amplitude is 14.3 cm s−1. A marginally significant fit with similar wave parameters resulted for the 7–10 day band. The results suggest that these waves are continental shelf waves, probably generated by atmospheric cold front passages.

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Martin Visbeck and Friedrich Schott

Abstract

The seasonal cycles found in moored current measurements in the equatorial Somali Current region and along the equator between 50° and 60°E are compared with the multilayer Geophysical Fluid Dynamics Laboratory model for the tropical Indian Ocean. The remote forcing of Somali Current transport variations by incident long equatorial waves from the equatorial interior subthermocline region is investigated by analyzing the model velocities of annual and semiannual period. Amplitudes and phases of linear equatorial Rossby and Kelvin waves were least-squares fitted to the model velocities between 5°S and 5°N, 55° and 86°E from 100-m to 1000-m depth. Two cases of wave fits are distinguished: the “free” Kelvin wave case, where the Kelvin waves were fitted independently, and the “reflected” Kelvin wave case, where they were coupled to the Rossby waves by the western boundary condition for a straight slanted (45° to the north) coastline. The wave field velocities explained ∼70% of the spatial variance in the equatorial model subregion and also compared reasonably well with observed current variations along the equator. At the western boundary, the short-wave alongshore transport due to reflected incident long waves was determined and found to be antisymmetric about the equator. The maximum transport variation for the semiannual period due to the short waves was about 5 × 106 m3 s−1 between 150- and 800-m depth at 3° north and south of the equator. Observational evidence for the western boundary transport variations and the sensitivity to changes in the incident wave field are discussed.

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Friedrich Schott and Detlef R. Quadfasel

Abstract

An array of six current-meter moorings and several coastal temperature recorders was deployed on the shelf and continental slope off northern Somalia from March to July 1979; a seventh mooring was placed near 2°S. In addition, four deep-sea moorings were deployed for a period of one month in May–June farther offshore.

Already during the late northeast monsoon in March the Somali Current north of 5°N was flowing northeastward in the top 150 m. Underneath, in the depth range 150–400 m, a narrow southward undercurrent was observed from March to June. After the first onset of the southwest monsoon, which occurred around 5 May when winds shifted from easterly to southwesterly parallel to the coast, the near-surface temperatures on the shelf decreased immediately with no detectable phase difference between 6 and 10°N, but no change was observed in the offshore circulation pattern. The final monsoon onset around 10 June was characterized by a drastic increase in wind speeds and the establishment of a strong anticyclonic wind-stress curl over the northern Somali Basin. The current measurements showed that within a few days after this onset the northern Somali gyre spun up over the deep sea and then propagated northwestward toward the coast with a speed of 12 cm s−1. These findings are in good agreement with results of satellite infrared imagery. The observed gyre kinematics can be explained by locally generated non-equatorial Rossby waves.

When the onset reaches the coast the shallow coastal undercurrent is extinguished.

Superimposed on the gyre-scale variability were fluctuations in the period ranges of weeks to months and of 3–5 days. There is evidence that the energies of the latter were related to the development of the Somali Current.

Significant differences were found in a comparison of the 1979 current measurements south of the equator with observations obtained there during the monsoon onset of 1976.

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Jürgen Fischer and Friedrich A. Schott

Abstract

Fifteen profiling floats were injected into the deep boundary current off Labrador. They were ballasted to drift in the core depth of Labrador Sea Water (LSW) at 1500-m depth and were deployed in two groups during March and July/August 1997. Initially, for about three months, the floats were drifting within the boundary current, and the flow vectors were used to determine the mean horizontal structure of the Deep Labrador Current, which was found to be about 100 km wide with an average core speed of 18 cm s−1. North of Flemish Cap the boundary current encounters complicated topography around “Orphan Knoll,” and there the LSW outflow splits up into different routes. One obvious LSW path is eastward through the Charlie Gibbs Fracture Zone and another route is a narrow recirculation toward the central Labrador Sea. A surprising result was that none of the floats were able to follow the boundary current southward to the Grand Banks area and exit into the subtropics. Trajectories and temperature profiles of the eastward drifting floats indicate the importance of the North Atlantic Current for dispersing the floats, even at the level of LSW.

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Detlef R. Quadfasel and Friedrich Schott

Abstract

Hydrographic observations at intermediate depth in the Somali Basin have been made during and after the transition from the northeast monsoon to the southwest monsoon, 1979. The data are discussed in relation to measurements of absolute currents during the same time period. Earlier water-mass identification by Warren et al. (1966) is confirmed and extended by introducing a well-defined Equatorial Water Mass. The distribution of water masses and its temporal evolution with the changing monsoonal wind field is analyzed on three different spatial scales. Equatorial and near-coastal undercurrents are an important factor in the large-scale redistribution of water masses in the intermediate layers. Cross-equatorial exchange of water, in particular highly saline Red Sea water, is largely confined to a narrow region off the East African Coast. No obvious response of these currents to the onset of the southwest monsoon is detected. In the mesoscale range anticyclonic subsurface eddies containing Equatorial Water are observed in the northern and southern Somali Basin. A possible formation mechanism of these features through equatorial westward subsurface jets in discussed. Double-diffusive intrusions with vertical scales of over 100 m are observed near strong fronts. Their characteristics compare well with theoretical predictions (Ruddick and Turner, 1979). A stability analysis for the upper and lower boundaries of these intrusions shows significantly that double-diffusive processes are acting also on these intrusions themselves.

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William E. Johns and Friedrich Schott

Abstract

Current meter observations were collected from a three-dimensional array moored in the Florida Straits between December 1983 and June 1984 as part of the Subtropical Atlantic Climate Studies (STACS) program. Approximately one-fourth of the total subinertial velocity and temperature variance contained in these records is associated with meandering of the Florida Current on time scales ranging from several days to a few weeks. There approach to be no strong correlation between the occurrence of meanders and variations in Florida Current volume transport or local wind forcing.

Utilizing frequency-domain empirical mode analysis we find the most coherent, energetic meandering signals within two limited frequency bands centered near periods of 12 days and 5 days. These meanders propagate downstream (northward) with phase speeds and wavelengths of approximately (28 km d−1, 340 km) and (36 km d−1, 170 km) respectively. Periodic waveforms composed from these modes indicate an asymmetric meander pattern with wave crests and troughs leading on the eastern side of the Florida Straits. These meanders appear to be giving up significant energy to the mean flow through up-gradient eddy momentum and buoyancy fluxes in the cyclonic shear zone of the Florida Current, with the net energy transfer being generally dominated by barotropic (eddy momentum flux) processes.

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Friedrich Schott and Kevin D. Leaman

Abstract

In the Golfe du Lion, south of France, favorable conditions for deep winter convection exist and were documented by the MEDOC experiments during 1969–75. A renewed investigation of that regime with upward-looking moored acoustic Doppler current profilers (ADCPs) was carried out during 24 January–5 March 1987, to record the three-dimensional currents associated with the deep mixing. While in the earlier studies initial deep convection did not begin until fairly late in the winter season, a very strong Mistral around 10 January 1987 had already generated a 1arge deep-mixed patch, homogeneous down to around 2000 m at deployment time. Three ADCPs, two working at 150 kHz and one at 75 kHz, were moored in a triangle of 15 km sidelength at 550–780 m depth. Full records at 1-hour ensemble time intervals, 400 pings per ensemble, 8 m bin lengths were obtained by the 75 kHz and one of the 150 kHz ADCPs.

In mid-February, a second Mistral hit the region. With the onset of strong winds and surface cooling the occurrence of short-period current fluctuations, in the period range of hours, was observed which lasted for the duration of the negative heat flux.

The vertical currents recorded by the ADCPs during this period included downward events of 5–10 cm s−1 velocity with weaker upward motion in between. These events appeared to occur simultaneously over the depth range of several 100 m covered by the ADCPs. An interpretation of these events as frozen structures, advected by with the mean current, yielded a horizontal scale estimate of only order 1 km. The mean vertical velocity during the Mistral week due to the integrated effect of these events was of order 1 cm s−1 downward.

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Kevin D. Leaman and Friedrich A. Schott

Abstract

Deep winter convection in the northwestern Mediterranean (Gulf of Lions) and the subsequent formation of Mediterranean Deep Water were observed using advanced oceanographic instrumentation during a six-week long experiment in 1987. The severe 1987 European winter forced an intense outbreak of the Mistral, a cold, dry wind blowing down the Rhone valley, in mid-January. Surface cooling and evaporation were of sufficient intensity to cause an initial episode of deep convection shortly before the experiment described here began. However, several more Mistral events took place during the experiment.

During several cruises into the area, CTD and absolute horizontal velocity profiles were measured in the mixed area over the Rhone fan as well as across the southern front of this region; in addition, continuous records of shipboard meteorological and oceanographic parameters (air temperature, surface salinity, etc.) were made.

An early-February Mistral apparently did not produce enough surface cooling to reinitiate convection. In contrast, further convection during an intense, mid-February Mistral was observed in both hydrographic and current-meter data. The 0.02°C cooling observed in CTD data at 1950 m, as well as in data from moored temperature sensors at other depths by the end of this storm, is consistent with what would be expected if the estimated surface heat loss were mixed over the 2200 m depth of the water column.

Analysis of CTD data shows that the presence of unstably stratified surface layers was correlated with periods of strong surface cooling. An inverse relation appears to exist between the thickness and density excess of these layers. Comparison with surface cooling rates suggests that these layers could be formed in periods as short as one hour.

Finally, comparison of our results to historical data suggests that deep water formed in the northwestern Mediterranean has become progressively warmer and saltier over the past several decades.

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