Accuracy of current profile measurements: Effect of tropical and midlatitude internal waves

The effect of midlatitude and tropical internal wave variability on current profile measurements is investigated and quantified to yield practical error estimates. First, a data set of Pegasus current profiles from the tropical Atlantic (6°S to 6°N) is analyzed for their rms down/up differences, which are compared with predictions from Garrett‐Munk type internal wave theory and with statistics derived from current meter moorings in the same region. The agreement in terms of amplitudes and vertical distribution proves that most of those differences are due to internal waves and not instrumental errors. Nonetheless, this is the noise of the measurements, if low‐frequency motions are sought, and the errors can thus be quantified using the same internal wave theories. At midlatitudes the error variance is the usual 44(N/3 cph) cm2/s2 with some latitude dependence, and the effect of averaging in the vertical or summing several profiles (e.g., up and down) is estimated. The same is done for equatorial situations, where construction of a crude equatorial frequency spectrum for internal waves yields 77(N/3 cph)cm2/s2 for the error variance. Again, error reduction due to averaging is estimated

Intercomparing drifts from eddy-resolving and cycling floats in the deep western boundary current along the Mid-Atlanic Ridge

The Lagrangian nature of cycling floats is evaluated in the framework defined by the Deep Western Boundary Current of the Mid-Atlantic Ridge. In a statistical approach, speeds and drifts are estimated for an APEX cycling float and compared with the velocities inferred from a park ensemble of four eddy-resolving RAFOS floats. They were deployed at the same location and ballasted for drifting at the same mission depth. Displacement errors induced by geostrophic shear and wind forced currents are analyzed. We observe that the velocity estimated from the RAFOS floats is not statistically different from the velocity estimated from the APEX float. Likewise, the initial separation between the cycling float and a simultaneously deployed RAFOS float has been studied in terms of the turbulent diffusivity. Though the performance of this study in comparable cases without a mean current field may be limited, these oceanic observations support exploiting the Lagrangian nature of the cycling floats

Comparación de flotadores RAFOS y derivadores-perfiladores autónomos APEX en la Corriente Profunda de Frontera Oeste de la Dorsal CentroAtlántica

The Lagrangian nature of cycling floats is evaluated in the framework defined by the Deep Western Boundary Current of the Mid-Atlantic Ridge. In a statistical approach, speeds and drifts are estimated for an APEX cycling float and compared with the velocities inferred from a park ensemble of four eddy-resolving RAFOS floats. They were deployed at the same location and ballasted for drifting at the same mission depth. Displacement errors induced by geostrophic shear and wind forced currents are analyzed. We observe that the velocity estimated from the RAFOS floats is not statistically different from the velocity estimated from the APEX float. Likewise, the initial separation between the cycling float and a simultaneously deployed RAFOS float has been studied in terms of the turbulent diffusivity. Though the performance of this study in comparable cases without a mean current field may be limited, these oceanic observations support exploiting the Lagrangian nature of the cycling floats.En este trabajo se examina la naturaleza lagrangiana de los derivadores-perfiladores autónomos en el contexto definido por la Corriente Profunda de Frontera Oeste de la Dorsal Centroatlántica. Desde un punto de vista estadístico, se comparan la velocidad y trayectoria subsuperficiales de una boya APEX con las estimadas a partir de un conjunto de cuatro boyas subsuperficiales RAFOS. Todas fueron lanzadas en la misma posición y lastradas para derivar a la misma profundidad. Asimismo, se ha analizado el error inducido por el cizallamiento vertical de la velocidad debida tanto a la geostrofía como a Ekman. En este sentido, hemos comprobado que la velocidad estimada por medio de los derivadores RAFOS no es estadísticamente diferente de la estimada por el perfilador APEX. Por otro lado, la separación inicial de las boyas se ha estudiado en términos de la difusividad turbulenta del medio. Finalmente, podemos afirmar que estas observaciones apoyan el aprovechamiento de la naturaleza lagrangiana de los derivadores- perfiladores

Relative-time inversions in the Labrador Sea acoustic tomography experiment.

One-year long travel-time data from the second deployment period of the Labrador Sea acoustic tomography experiment are analyzed, using a relative-time matched-peak approach, in order to invert for the sound-speed field and simultaneously solve for an unknown travel-time offset. From the relative-time inversions oceanographic information in terms of vertically averaged temperatures are deduced, yielding satisfactory matching with respect to available independent observations. The estimated offsets can be attributed to differential clock drifts, showing a clear parabolic behaviour over the course of the experiment, reaching maximum deviations from linear clock drift corrections (end-point calibrations) of the order of 50 ms. By applying the estimated second-order corrections to the travel-time data, absolute-time matched-peak inversions can then be performed. The used matched-peak approach accounts for the non-linear behaviour of travel times, which is due to the seasonally variable acoustic propagation conditions in the probed region, and turns out to be an appropriate tool in dealing with unknown travel-time offsets

Transport estimates in the Strait of Gibraltar with a tidal inverse model

To estimate the volume transport through the Strait of Gibraltar and to study the spatial structure of the time-variable flow, a varying number of current meter moorings were maintained at the eastern entrance of the strait between October 1994 and April 1998, and was complemented with intensive shipboard measurements during the European Union project Canary Island Azores Gibraltar Experiment (CANIGO). A tidal inverse model is used to merge these data sets in order to investigate the flow at the eastern entrance of the strait. The two-dimensional structure of the tidal flow was described by simple analytical functions. Harmonics with the seven most important tidal frequencies were used as temporal functions. With this model, the tidal currents can be predicted for any time and location at the eastern entrance of the strait, and more than 92% of the variance of the lower layer flow is explained. It was used to remove the tidal currents from the individual measurements and to calculate the mean flow through the strait from the residuals. Combined with a similar inverse model for determining the depth of the interface between Mediterranean and Atlantic water, the volume transport was estimated to be 0.81 ± 0.07 Sv for the upper layer and −0.76 ± 0.07 Sv for the lower layer. The correlation of the tidal currents and the fluctuations of the interface accounts for ∼7% of the transport at the eastern entrance