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Uwe Send

Abstract

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.

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Uwe Send

Abstract

Flow reversals during relaxation of the equatorward wind on the northern California shelf are studied with observations and a simple numerical model. Data from the CODE experiment are used to document the changes in the cross-shelf profiles of alongshelf currents and potential vorticity when the wind forcing changes from upwelling-favorable to near-zero. Upwelling vorticity dynamics are considered using a simple equation, where also the importance of stress curls is discussed. During the wind relaxation events, the usually equatorward flow reveres to poleward on the inner shelf, resulting in a region of high shear and occasionally a localized maximum in potential vorticity on the shelf. In the course of such an episode, a single eddylike structure on the shelf was observed with satellite imagery. In order to simulate the basic dynamics of the flow reversals and instability, a contour dynamics model is developed for barotropic flow on an exponentially sloping bottom, in which potential vorticity is advected and relative vorticity changes only by movement across topography or advection. Thus it is well suited for modeling the character of nonlinear shelf waves, instabilities and related processes. The model is applied to flow reversals and to local instability qualitatively similar to the observations. It is shown how an intrusion of low potential vorticity along the coast can generate a single “eddy” on the unstable flow behind it. This inshore potential vorticity minimum is likely to originate on the sheltered wide shelf north of Point Reyes. .An extra tracer has been added to some simulation and an example shows entrainment in the absence of any turbulence.

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Sunke Schmidt and Uwe Send

Abstract

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.

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SungHyun Nam and Uwe Send

Abstract

This study presents observations of the cross-sectional structure of resonant response to sea/land breezes (SLBs) off Huntington Beach (HB) in the Southern California Bight (SCB). A resonant response to local diurnal wind stress fluctuations associated with SLB forcing occurs intermittently and produces strong diurnal oscillations of flow and temperature resulting from enhanced work of the diurnal local wind on the sea surface. At nighttime (daytime), a coherent cross-sectional circulation with offshore (onshore) currents in the surface layer (upper 15 m) and onshore (offshore) currents in the intermediate layer around 20 m are generated, with a three-layered vertical structure on the outer shelf. The authors find a net cross-shore eddy heat flux (net cooling of nearshore water) during the period of strong response to SLB, that is, a rectified mean heat flux and steeper isotherms resulting from the diurnal SLB fluctuations. The steepened mean isotherms are also found to be in thermal–wind balance with intensified mean equatorward flow, which thus can also be generated by the resonant SLB dynamics. Similar rectified onshore transport of other quantities is expected, relevant for biogeochemical processes. The distribution of maximum diurnal kinetic energy in time and across the shelf supports the concept that subinertial shears create the sufficient condition for resonant response to SLB forcing.

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Uwe Send and John Marshall

Abstract

The large-scale, integral effect of convective elements (plumes) constituting an open-ocean chimney is investigated both theoretically and with a plume-resolving numerical model. The authors consider an initially homogeneous “patch” of ocean of depth H, with Coriolis parameter f, in which buoyancy is lost from the surface at a rate B. Both vorticity constraints on the convection patch and model analyses imply that, irrespective of the details of the plumes themselves, the mean vertical transport resulting from their action must be vanishingly small. Plumes are best thought of as mixing agents, which efficiently homogenize properties of the chimney.

Scaling laws are derived from dynamical arguments and tested against the model. Using an expression for the vertical mixing timescale, they relate the chimney properties, the strength of the geostrophic rim-current setup around it, and its breakup timescale by baroclinic instability to the external parameters B, f, and H. After breakup, the instability eddies may merge to form larger “cones” of convected water, which offset the buoyancy loss at the surface by laterally incorporating stratified fluid. Properties of the plumes only enter the scaling results by setting the vertical mixing timescale.

The authors argue that the plume scale may be parameterized by a mixing scheme if this implies the appropriate mixing timescale. Finally, the authors suggest that for the estimation of deep-water formation rates the volume of convectively modified fluid processed by a chimney should be computed rather than the mean vertical transport during the convection phase.

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Uwe Send, Carsten Eden, and Friedrich Schott

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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.

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Uwe Send, Lloyd Regier, and Brent Jones

Abstract

Many fixed oceanographic instruments and observing systems are deployed in the water column or on the seafloor for extended periods of time without any expression at the sea surface. To routinely communicate with such subsurface instruments in the deep ocean, here a system is presented that uses underwater gliders and commercially available acoustic modems for this task and its use is demonstrated with subsurface moorings and inverted echo sounders plus bottom pressure sensor (PIES). One recent glider mission spent 31 days in data retrieval dives, capturing 2 MB of error-free subsurface data. To acquire this volume, a total of 2.65 MB (including all retransmissions) were sent, with a success rate of 75%. A model for the energy usage of each phase of modem function was derived from laboratory measurements. While the model predicts that the glider would expend 0.21 J to acquire each data byte, the actual consumption of the glider in the field is 0.49 J byte−1. The inefficiency is due to overhead associated with establishment of the acoustic link and with the resending of data that is received with errors. Including all the time for negotiating the acoustic link and for the retransmission of erroneous data, the net data throughput are around 3 bytes s−1 in spite of the modem operating at 140 to 600 baud. Even with these limitations, the technique has shown to be useful and is being utilized routinely in a research project in the California Current to obtain data from horizontal distances up to 7 km from an instrument at depths up to 4000 m, transferring on average 6 kB of data in a day of acoustic communications.

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Arne Körtzinger, Jens Schimanski, and Uwe Send

Abstract

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.

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Xue Fan, Uwe Send, Pierre Testor, Johannes Karstensen, and Pascale Lherminier

Abstract

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 4.48°–5.34°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°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.

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Friedrich Schott, Martin Visbeck, Uwe Send, Jürgen Fischer, Lothar Stramma, and Yves Desaubies

Abstract

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.

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