ALBERT

Description: A new approach based on statistical estimation is proposed for the analysis of tomographic traveltime data in cases of significant nonlinear dependence of the traveltimes on the sound-speed variations. Traditional tomography schemes based on linear perturbative inversions about a single, a priori fixed background state cannot properly handle such cases since the linearized model relations will lead to considerable inversion errors, depending on the extent of nonlinearity. In contrast, the background state is considered here as a variable unknown quantity to be estimated from the traveltime data, simultaneously with the peak identification function and the sound-speed perturbation. Using the maximum likelihood approach and the Gaussian assumption, the statistical estimation problem reduces to a weighted least squares problem to be solved simultaneously for the three unknown quantities. A posteriori inversion-error estimates are derived accounting also for uncertainties in the background selection and the peak identification. The proposed method is applied to nine-month-long traveltime data from the Thetis-2 experiment, conducted from January to October 1994 in the Western Mediterranean Sea, where the variability of the ocean environment gives rise to significant nonlinear dependencies between sound-speed and traveltime variations. The recovered temporal variability and stratification compare well with independent XBT observations.

Description: In the autumn of 1996 the field component of an experiment designed to observe water mass transformation began in the Labrador Sea. Intense observations of ocean convection were taken in the following two winters. The purpose of the experiment was, by a combination of meteorological and oceanographic field observations, laboratory studies, theory, and modeling, to improve understanding of the convective process in the ocean and its representation in models. The dataset that has been gathered far exceeds previous efforts to observe the convective process anywhere in the ocean, both in its scope and range of techniques deployed. Combined with a comprehensive set of meteorological and air-sea flux measurements, it is giving unprecedented insights into the dynamics and thermodynamics of a closely coupled, semienclosed system known to have direct influence on the processes that control global climate.

Description: The depth of winter convection in the central Labrador Sea is strongly influenced by the prevailing stratification in late summer. For this late summer stratification salinity is as important as temperature, and in the upper water layers salinity even dominates. To analyze the source of the spring and summer freshening in the central region, seasonal freshwater cycles have been constructed for the interior Labrador Sea, the West Greenland Current, and the Labrador Current. It is shown that none of the local freshwater sources is responsible for the spring–summer freshening in the interior, which appears to occur in two separate events in April to May and July to September. Comparing the timing and volume estimates of the seasonal freshwater cycles of the boundary currents with the central Labrador Sea helps in understanding the origin of the interior freshwater signals. The first smaller pulse cannot be attributed clearly to either of the boundary currents. The second one is about three times stronger and supplies 60% of the seasonal summer freshwater. Transport estimates and calculated mixing properties provide evidence that its source is the West Greenland Current. The finding implies a connection also on interannual time scales between Labrador Sea surface salinity and freshwater sources in the West Greenland Current and farther upstream in the East Greenland Current. The freshwater input from the West Greenland Current thus also is the likely pathway for the known modulation of Labrador Sea Water mass formation by freshwater export from the Arctic (via the East Greenland Current), which implies some predictability on longer time scales.

Description: Two state-of-the-art profiling floats were equipped with novel optode-based oceanographic oxygen sensors. Both floats were simultaneously deployed in the central Labrador Sea gyre on 7 September 2003. They drift at a depth of 800 db and perform weekly profiles of temperature, salinity, and oxygen in the upper 2000 m of the water column. The initial results from the first 6 months of operation are presented. Data are compared with a small hydrographic oxygen survey of the deployment site. They are further examined for measurement quality, including precision, accuracy, and drift aspects. The first 28 profiles obtained are of high quality and show no detectable sensor drift. A method of long-term drift control is described and a few suggestions for the operation protocol are provided.

Description: The so-called equatorial stacked jets are analyzed with ship-board observations and moored time series from the Atlantic Ocean. The features are identified and isolated by comparing vertical wavenumber spectra at the equator with those a few degrees from the equator. Mode-filtering gives clear views of the jets in meridional sections, the typical extent being ±1° in latitude. The vertical structure can be well described (explaining 82% of the variance) by N−1-stretched cosines, with a Gaussian amplitude tapering in the vertical. The stretched wavelengths are somewhat variable. Fitting jets of a fixed (stretched) wavelength to four moored sensors in the depth range 1300–1900 m, allows one to track the vertical phase of the jets with an rms error of 30°–45°. The resulting fit from a 20-month moored time series shows long periods of unchanging jet conditions and intermittent times of high variability. There is no significant vertical propagation on these timescales nor a seasonal reversal. Using a composite from many different experiments, interannual variability is visible, however. A possible mechanism for the stacked jets is inertial instability, resulting from background meridional shears at the equator. A condition is that the Ertel potential vorticity becomes zero somewhere, due to meridional asymmetries in the zonal flows. The ship-board observations show that this may be approximately fulfilled by the instantaneous zonal low-mode flows at various depths, resulting from an excess of zonal momentum south of the equator most of the time. Inertial instability should act to redistribute this zonal momentum, and our mooring data show indeed persistent northward momentum flux, but not at the depth levels expected. The momentum transport might suggest that the jets can also flux or mix other properties across the equator.

Description: The upper ocean, including the biologically productive euphotic zone and the mixed layer, has great relevance for studies of physical, biogeochemical, and ecosystem processes and their interaction. Observing this layer with a continuous presence, sampling many of the relevant variables, and with sufficient vertical resolution, has remained a challenge. Here a system is presented which can be deployed on the top of deep-ocean moorings, with a drive mechanism at depths of 150-200m, which mechanically winches a large sensor float and smaller communications float tethered above it to the surface and back down again, typically twice per day for periods up to 1 year. The sensor float can carry several sizeable sensors, and it has enough buoyancy to reach the near surface and for the communications float to pierce the surface even in the presence of strong currents. The system can survive mooring blow-over to 1000m depth. The battery-powered design is made possible by using a balanced energy-conserving principle. Reliability is enhanced with a drive assembly that employs a single rotating part that has no slip rings or rotating seals. The profiling bodies can break the surface to sample the near-surface layer and to establish satellite communication for data relay or reception of new commands. An inductive pass-through mode allows communication with other mooring components throughout the water column beneath the system. A number of successful demonstration deployments have been completed.

Description: During December 1991 to April 1992 measurements with moored acoustic Doppler current profiler (ADCP) stations and shipboard surveys were carried out in the convection regime of the Gulf of Lions, northwestern Mediterranean. First significant mixed layer deepening and generation of internal waves in the stratified intermediate layer occurred during a mistral cooling phase in late December. Mixed layer deepening to about 400 m, eroding the salinity maximum layer of saltier and warmer Levantine Intermediate Water and causing temporary surface-layer warming, followed during a second cooling period of late January. During a mistral cooling period from 18 to 23 February 1992, convection to 1500-m depth was observed, where the size of the convection regime was 50–100 km extent. Vertical velocities 40–640 m deep, recorded by four ADCPs of a triangular moored array of 2 km sidelength in the center of the convection regime, exceeded 5 cm s−1 and were not correlated over the separation of the moorings. Horizontal scales estimated from event duration and advection velocity were only around 500 m, in agreement with scaling arguments for convective plumes. Plume activity during nighttime cooling was larger than daytime daytime. Significant evidence for rotation of the plumes could not be found. Overall, plume energy, and the degree of mixing accomplished by them, was much lower than observed during a stronger mistral in February 1987. The mean vertical velocity over the mistral period, determined from the four ADCPs, was near zero, confirming the role of plumes as mixing agents rather than as part of a mean downdraft in a convection regime. The cyclonic rim current around the convection regime was confined to a strip of 〈20 km width with an average velocity of about 10 cm s−1, which is in agreement with near-zero vertical mean velocity in the interior based on potential vorticity conservation. A relation between variations of the larger-scale cyclonic North Mediterranean Current along the boundary and the deep convection could not be identified. An unexplained feature still is the cover of the convection regime by a shallow layer of light water that moves in rather quickly from the sides after the cooling ends.

Description: A number of geophysical observing techniques, including ocean acoustic tomography, obtain sequences of records of which the observed relative maxima (“peaks”) are used to infer properties of the system via inversions. Traditionally, these peaks first are tracked (followed from one record to another) and identified separately, before they can be used in an inversion scheme. In this paper, a method is presented for identifying and tracking ensembles of such peaks in one step and simultaneously with the inversion. A priori information, in our case knowledge about the ocean, can thus be used to constrain the allowed peak identifications, enabling the usage of irregularly appearing or more closely spaced peaks. The best identification is defined to be the one that upon inversion minimizes a cost function that involves data residual and smoothness in time, subject to two constraints bounding the solution and residual size. For the presented cases, the minimum can be found by simply trying inversions with all possible peak identifications. Sample applications of the method from an acoustic tomography experiment are shown in order to illustrate the approach and results.

Description: Mesoscale anticyclonic eddies in the Irminger Sea are observed using a mooring and a glider. Between 2002 and 2009, the mooring observed 53 anticyclones. Using a kinematic model, objective estimates of eddy length scales and velocity structure are made for 16 eddies. Anticyclones had a mean core diameter of 12 km, and their mean peak observed azimuthal speed was 0.1 m s(-1). They had core salinities and potential temperatures of 34.91-34.98 and 4.488-5.34 degrees C, respectively, making them warm and salty features. These properties represent a typical salinity anomaly of 0.03 and a temperature anomaly of 0.28 degrees C from noneddy values. All eddies had small (〈〈 1) Rossby numbers. In 2006, the glider observed two anticyclones having diameters of about 20 km and peak azimuthal speeds of about 0.3 m s(-1). Similar salinity anomalies were detected throughout the Irminger Sea by floats profiling in anticyclones. Two formation regions for the eddies are identified: one to the west of the Reykjanes Ridge and the other off the East Greenland Irminger Current near Cape Farewell close to the mooring. Observations indicate that eddies formed in the former region are larger than eddies observed at the mooring. A clear increase in eddy salinity is observed between 2002 and 2009. The observed breakup of these eddies in winter implies that they are a source of salt for the central gyre. The anticyclones are similar to those found in both the Labrador Sea and Norwegian Sea, making them a ubiquitous feature of the subpolar North Atlantic basins.