Holger Klein

Currents and dispersion in the abyssal Northeast Atlantic. Results from the NOAMP field program

The NOAMP field program, carried out in the deep Northeast Atlantic from 1983 to 1986, was designed to investigate the local deep flow and the dispersion of suspended material by means of current meter, SOFAR float, turbidity and CTD data. The bottom topography of the NOAMP area (45–49°N, 17–23°W) represents a highland with hills and ridges rising up to 1000 m above the deep-sea floor (approx. 4500 m). The deep mean flow (3–5 cm s−1) runs fairly parallel to the depth contours and consists of a system of small topographically induced gyres that can trap any kind of passive admixture. During the first 50 days after release, a cluster of 14 deep-sea floats drifting at depths of around 3500 m spread steadily over an area with a diameter of about 300 km. During the following roughly 200 days a further drift of the centre of mass of the cluster—as well as a further spreading relative to the centre of mass—was hardly noticeable. Occasionally, high energetic events (benthic storms) with durations between 3 and 27 days, and maximum velocities up to 27 cm s−1 occurred directly above the deep-sea floor.

Near-bottom currents in the deep Peru Basin, DISCOL experimental area

Long-term near-bottom current meter data (September 1989 to January 1992) from the deep Peru Basin (7° 04′ S, 88° 28′ W), are analysed and discussed with respect to the dispersion of sediments mobilized due to potential deep-sea mining. The data were gathered during the DISCOL experiment and reveal a current regime that is characterized by an alternating sequence of relatively strong (>5 cm/s) quasi-unidirectional currents and weak currents (<1 to 3 cm/s) with great directional variability. The duration of these phases varies between about 2 and 5 month without a discernible periodicity or a seasonal signal. Temporarily, a bottom mixed layer of 100 to 200 m height was observed. Estimates are made of horizontal eddy diffusivities and vertical settling velocities that depend on local grain size distribution.

Some results of a diffusion experiment at a river plume front

The fluorescent tracer Rhodamine B was released at the light side of a river plume front with a strong salinity gradient. To survey greater parts of the proper front, the whole rhodamine patch could not always be completely covered with ship sections due to the fast front-parallel spreading of the rhodamine. Therefore, conventional analyzing methods are not applicable. The extraordinarily high correlation between salinity and rhodamine concentration suggested the idea to determine rhodamine concentration as a function of salinity, regarding the latter as a conservative property due to its smaller gradients and fluxes. Thereby the large scale deformation of the water mass and the advective term of the transport equation are eliminated in first approximation.

Local currents at a shoreface-connected ridge in the German Bight

Current data at a shoreface-connected ridge in the German Bight have been sampled in January 1990 and January 1991. During the 1991 experiment a violent southwest storm hit the area so that the storm-induced currents can be compared to those under moderate periods. The storm significantly changed the tide dominated spatial and temporal current regime. Estimates suggest, that all sediment grain sizes which occur in the ridge area (0.1 to 0.5 mm) can be resuspended due to the storm-induced intensification of the bottom currents.

Tidal and residual currents in the northern North Sea: Observations

One-month current and water-level time series from the northern North Sea are analyzed with regard to residual flows, volume transports, and tidal constituents. The data originate from theNORA andMOVENS field programs (along 59°N, July/August 1990 and roughly 62°N, September/October 1986 respectively). With regard to the residual flows, the results generally confirm the known circulation pattern of the northern North Sea. Differences between these and other investigations are due to the great short and long-term variability of the current systems. The main objective of this paper, however, is to describe tides and tidal streams. Firstly, we relate the tidal streams along 59°N to the spring and neap high waters at Aberdeen; secondly, up to 17 tidal constituents are determined by means of harmonic analysis.

Currents and near-bottom suspended matter dynamics in the central north sea during stormy weather — Results of the PIPE’98 field experiment —

In order to investigate near-bottom suspended participate matter (SPM) dynamics and near-bottom currents, between January 18 and 25,1998, a small array of optical backscatter meters and current meters was deployed in the central North Sea close to the pipelines NORFRA and ZEEPIPE I. The data were supplemented by local wave measurements (WAVEC buoy) and shipborne measurements as CTD profiles, water samples for the determination of the SPM concentration, and meteorological data. At the position of the PIPE site previous Side Scan surveys indicated scouring processes at the ZEEPIPE I. Therefore, possible interactions between the pipe and SPM dynamics and currents should be investigated. A special challenge was the positioning of the instruments at a water depth of 60 metres. In the North Sea, January is known for rough weather and as intended two heavy storms hit the area. Due to rough sea during the first days of the experiment, we did not succeed to place the instruments directly beside the pipes. Unfortunately, two near-bottom current meters failed. However, the interaction between wind, waves, currents, and SPM concentration during periods with wind speeds up to 43 kn and peak waves of 11 m was observed and will be discussed. For the interpretation of the field data we used also the results of the European Shelf Wave Model which proved to be very reliable. High swell, generated by remote wind fields, proved to be the key mechanism for sediment mobilization. The vertical range which is influenced by wind, wind waves, and swell and its temporal variation was estimated.

Near-bottom currents and bottom boundary layer variability over manganese nodule fields in the peru basin, se-pacific

Near-bottom current meter data recorded between 0.6 and 500 metres above bottom (mab) at two manganese nodule sites in the deep Peru Basin are discussed with respect to the vertical current structure and the formation and variability of the benthic boundary layer (BBL) and its sublayers. The data reveal a weak current regime with great spatial and temporal variability and a bottom intensification at about 30–50 mab. The BBL height varies according to the daily mean values of flow above the BBL, reaching maximally 150 to 200 metres.

On the current profile in the German Bight: A two-month BroadBand ADCP time series of high vertical resolution

A bottom mounted BroadBand ADCP was deployed for about two months in the German Bight (54° 15.3′, N, 7° 35.5′ E) at 40 m depth. The ADCP recorded current data and echo intensity between 3 and 38m depth with a vertical resolution of 1 m. The data, which provide continuous vertical current profiles, are investigated with regard to the vertical variations of residual and tidal currents and kinetic energies. To investigate the temporal variability of the current profiles, the time series are subdivided into 8-day intervals. Furthermore, the data are compared with historical data. According to the ADCP profiles, the water column can be subdivided into three layers: A primarily wind-driven near-surface layer (∼10m), a middepth layer with great directional variability, and a near-bottom layer of about 10 m thickness with a strictly logarithmic current profile.

Two counterrotating eddies in the Eastern North Atlantic: Lagrangian observations

The formation of a cyclonic cold-core eddy, separating from a branch of the North Atlantic current system at about 49° N and 21° W, could be observed during a hydrographic survey in April 1985. One month later the eddy was revisited, now 40 nautical miles northeastwards of its previous position and detached from its parent current. During a second hydrographic survey a neutrally buoyant SOFAR float was launched into the core of the eddy. The 9-months trajectory shows the signature of the cyclonic eddy for about 2 months. Later—after passing regions of strong horizontal shear—the float follows the movements of a counterrotating, i. e. anticyclonic mesoscale eddy. A discussion of the Lagrangian data will be given in this paper.

Currents at the light-vessel “Deutsche Bucht”: A comparison between ADCP measurements and the BSH forecast model

Between June 15 and August 10,1999, an Acoustic Doppler Current Profiler (ADCP) and a water level recorder (WLR) were deployed in the close vicinity of the unmanned light-vessel “Deutsche Bucht” in the German Bight. Together with locally recorded meteorological and hydrographical data (salinity and temperature), this in-situ data set is compared to the forecasts of the BSH circulation model “BSHcmod”. The meteorological measurements are used to assess the meteorological model input generated by an atmospheric model of the German Weather Service. Generally, we observed a good agreement between in-situ and model data. Differences between measured and predicted currents were observed at the surface and at mid-depth. They were caused mainly by missing ADCP data within the first 3 metres under the surface and by the coarse vertical grid spacing of the model which prevents the representation of sharp vertical gradients.