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  • Author or Editor: Verena Hormann x
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Verena Hormann
,
Rick Lumpkin
, and
Renellys C. Perez

Abstract

A generalized method is developed to determine the position of the Atlantic northern cold tongue front across its zonal extent from satellite sea surface temperature (SST) data. Previous approaches estimated the frontal position subjectively or individually, calling for a more objective technique that is suitable for large datasets. The developed methodology is based on a median frontal SST, and associated positional uncertainties are on the order of 0.3° latitude for the period 1998–2011. Frontal characteristics are generally consistent with tropical instability waves (TIWs) and interannual variations are large. Application to drifter observations shows how the new methodology can be used to better understand circulation features near the northern cold tongue front. A drifter pair deployed on the eastern side of a passing TIW crest north of the front revealed that the trajectories of the drifters were clearly influenced by the shape of the front and they did not cross the front, but rather stayed close together about 2.5° north of the front. In a more complete analysis using all available drifters near the Atlantic northern cold tongue front, only about 12% of the trajectories crossed the front. Analyses in an along- and cross-frontal frame of reference complement isopycnal coordinate mapping, and tropical Atlantic drifter velocities averaged in frontal coordinates indicate a broadened shear zone between the northern branch of the South Equatorial Current and North Equatorial Countercurrent as well as meridional convergence near the front.

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Verena Hormann
,
Luca R. Centurioni
, and
Gilles Reverdin

Abstract

Salinity measurements from drifters constitute an important in situ dataset for the calibration and validation of the sea surface salinity satellite missions. A total of 114 satellite-tracked salinity drifters were deployed within the framework of the first Salinity Processes in the Upper Ocean Regional Study (SPURS) experiment in the subtropical North Atlantic focusing on the period August 2012–April 2014. In this study, a subset of 83 drifters, which provided useful salinity measurements in the central SPURS region from a few weeks to more than one year, is evaluated and an ad hoc quality-control procedure based on previously published work and the new observations is described. It was found that the sampling algorithm of the drifters introduces a predominantly fresh bias in the noise level of the salinity data, probably caused by the presence of air bubbles within the measuring cell. Since such noise is difficult to eliminate using statistical methods, extensive editing was done manually instead. Such quality-control procedures cannot be routinely applied to the real-time data stream from the drifters. Therefore, a revision of the sampling algorithm of the drifter’s salinity sensor is needed. Comparisons of the drifter’s salinity measurements with independent datasets further indicate that the sensor can provide reliable observations for up to one year. Finally, little evidence was found that the quality of the drifter’s salinity measurements depends on the presence of the drogue.

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Sebastian Essink
,
Verena Hormann
,
Luca R. Centurioni
, and
Amala Mahadevan

Abstract

Horizontal kinematic properties, such as vorticity, divergence, and lateral strain rate, are estimated from drifter clusters using three approaches. At submesoscale horizontal length scales O ( 1 10 ) km , kinematic properties become as large as planetary vorticity f, but challenging to observe because they evolve on short time scales O ( hours to days ) . By simulating surface drifters in a model flow field, we quantify the sources of uncertainty in the kinematic property calculations due to the deformation of cluster shape. Uncertainties arise primarily due to (i) violation of the linear estimation methods and (ii) aliasing of unresolved scales. Systematic uncertainties (iii) due to GPS errors, are secondary but can become as large as (i) and (ii) when aspect ratios are small. Ideal cluster parameters (number of drifters, length scale, and aspect ratio) are determined and error functions estimated empirically and theoretically. The most robust method—a two-dimensional, linear least squares fit—is applied to the first few days of a drifter dataset from the Bay of Bengal. Application of the length scale and aspect-ratio criteria minimizes errors (i) and (ii), and reduces the total number of clusters and so computational cost. The drifter-estimated kinematic properties map out a cyclonic mesoscale eddy with a surface, submesoscale fronts at its perimeter. Our analyses suggest methodological guidance for computing the two-dimensional kinematic properties in submesoscale flows, given the recently increasing quantity and quality of drifter observations, while also highlighting challenges and limitations.

Significance Statement

The purpose of this study is to provide insights and guidance for computing horizontal velocity gradients from clusters (i.e., three or more) of Lagrangian surface ocean drifters. The uncertainty in velocity gradient estimates depends strongly on the shape deformation of drifter clusters by the ocean currents. We propose criteria for drifter cluster length scales and aspect ratios to reduce uncertainties and develop ways of estimating the magnitude of the resulting errors. The findings are applied to a real ocean dataset from the Bay of Bengal.

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