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- Author or Editor: Luca Centurioni x
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Fulfilling Observing System Implementation Requirements with the Global Drifter ArrayAbstract
The Global Ocean Observing System (GOOS) requirements for in situ surface temperature and velocity measurements call for observations at 5° × 5° resolution. A key component of the GOOS that measures these essential climate variables is the global array of surface drifters. In this study, statistical observing system sampling experiments are performed to evaluate how many drifters are required to achieve the GOOS requirements, both with and without the presence of a completed global tropical moored buoy array at 5°S–5°N. The statistics for these simulations are derived from the evolution of the actual global drifter array. It is concluded that drifters should be deployed within the near-equatorial band even though that band is also in principle covered by the tropical moored array, as the benefits of not doing so are marginal. It is also concluded that an optimal design half-life for the drifters is ~450 days, neglecting external sources of death, such as running aground or being picked up. Finally, it is concluded that comparing the drifter array size to the number of static 5° × 5° open-ocean bins is not an ideal performance indicator for system evaluation; a better performance indicator is the fraction of 5° × 5° open-ocean bins sampled, neglecting bins with high drifter death rates.
Abstract
The Global Ocean Observing System (GOOS) requirements for in situ surface temperature and velocity measurements call for observations at 5° × 5° resolution. A key component of the GOOS that measures these essential climate variables is the global array of surface drifters. In this study, statistical observing system sampling experiments are performed to evaluate how many drifters are required to achieve the GOOS requirements, both with and without the presence of a completed global tropical moored buoy array at 5°S–5°N. The statistics for these simulations are derived from the evolution of the actual global drifter array. It is concluded that drifters should be deployed within the near-equatorial band even though that band is also in principle covered by the tropical moored array, as the benefits of not doing so are marginal. It is also concluded that an optimal design half-life for the drifters is ~450 days, neglecting external sources of death, such as running aground or being picked up. Finally, it is concluded that comparing the drifter array size to the number of static 5° × 5° open-ocean bins is not an ideal performance indicator for system evaluation; a better performance indicator is the fraction of 5° × 5° open-ocean bins sampled, neglecting bins with high drifter death rates.
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.
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.
On Characterizing Ocean Kinematics from Surface DriftersAbstract
Horizontal kinematic properties, such as vorticity, divergence, and lateral strain rate, are estimated from drifter clusters using three approaches. At submesoscale horizontal length scales
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.
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
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.
Abstract
Satellite-tracked drifting buoys of the Global Drifter Program have drogues, centered at 15-m depth, to minimize direct wind forcing and Stokes drift. Drogue presence has historically been determined from submergence or tether strain records. However, recent studies have revealed that a significant fraction of drifters believed to be drogued have actually lost their drogues, a problem that peaked in the mid-2000s before the majority of drifters in the global array switched from submergence to tether strain sensors. In this study, a methodology is applied to the data to automatically reanalyze drogue presence based on anomalous downwind ageostrophic motion. Results indicate that the downwind slip of undrogued drifters is approximately 50% higher than previously believed. The reanalyzed results no longer exhibit the dramatic and spurious interannual variations seen in the original data. These results, along with information from submergence/tether strain and transmission frequency variations, are now being used to conduct a systematic manual reevaluation of drogue presence for each drifter in the post-1992 dataset.
Abstract
Satellite-tracked drifting buoys of the Global Drifter Program have drogues, centered at 15-m depth, to minimize direct wind forcing and Stokes drift. Drogue presence has historically been determined from submergence or tether strain records. However, recent studies have revealed that a significant fraction of drifters believed to be drogued have actually lost their drogues, a problem that peaked in the mid-2000s before the majority of drifters in the global array switched from submergence to tether strain sensors. In this study, a methodology is applied to the data to automatically reanalyze drogue presence based on anomalous downwind ageostrophic motion. Results indicate that the downwind slip of undrogued drifters is approximately 50% higher than previously believed. The reanalyzed results no longer exhibit the dramatic and spurious interannual variations seen in the original data. These results, along with information from submergence/tether strain and transmission frequency variations, are now being used to conduct a systematic manual reevaluation of drogue presence for each drifter in the post-1992 dataset.
Conditions for Reliable Divergence Estimates from Drifter TripletsAbstract
Horizontal velocity gradients of a flow field and the related kinematic properties (KPs) of divergence, vorticity, and strain rate can be estimated from dense drifter deployments, e.g., the spatiotemporal average divergence (and other KPs) over a triangular area defined by three drifters and over a given time interval can be computed from the initial and final areas of said triangle. Unfortunately, this computation can be subject to large errors, especially when the triangle shape is far from equilateral. Therefore, samples with small aspect ratios are generally discarded. Here we derive the thresholds on two shape metrics that optimize the balance between retention of good and removal of bad divergence estimates. The primary tool is a high-resolution regional ocean model simulation, where a baseline for the average divergence can be established, so that actual errors are available. A value of 0.2 for the scaled aspect ratio Λ and a value of 0.86π for the largest interior angle θ are found to be equally effective thresholds, especially at scales of 5 km and below. While discarding samples with low Λ or high θ values necessarily biases the distribution of divergence estimates slightly toward positive values, this bias is small compared to (and in the opposite direction of) the Lagrangian sampling bias due to drifters preferably sampling convergence regions. Errors due to position uncertainty are suppressed by the shape-based subsampling. The subsampling also improves the identification of the areas of extreme divergence or convergence. An application to an observational dataset demonstrates that these model-derived thresholds can be effectively used on actual drifter data.
Significance Statement
Divergence in the ocean indicates how fast floating objects in the ocean spread apart, while convergence (negative divergence) captures how fast they accumulate. Measuring divergence in the ocean, however, remains challenging. One method is to estimate divergence from the trajectories of drifting buoys. This study provides guidance under what circumstances these estimates should be discarded because they are too likely to have large errors. The criteria proposed here are less stringent than some of the ad hoc criteria previously used. This will allow users to retain more of their estimates. We consider how position uncertainty affects the reliability of the divergence estimates. An observational dataset collected in the Mediterranean is used to illustrate an application of these reliability criteria.
Abstract
Horizontal velocity gradients of a flow field and the related kinematic properties (KPs) of divergence, vorticity, and strain rate can be estimated from dense drifter deployments, e.g., the spatiotemporal average divergence (and other KPs) over a triangular area defined by three drifters and over a given time interval can be computed from the initial and final areas of said triangle. Unfortunately, this computation can be subject to large errors, especially when the triangle shape is far from equilateral. Therefore, samples with small aspect ratios are generally discarded. Here we derive the thresholds on two shape metrics that optimize the balance between retention of good and removal of bad divergence estimates. The primary tool is a high-resolution regional ocean model simulation, where a baseline for the average divergence can be established, so that actual errors are available. A value of 0.2 for the scaled aspect ratio Λ and a value of 0.86π for the largest interior angle θ are found to be equally effective thresholds, especially at scales of 5 km and below. While discarding samples with low Λ or high θ values necessarily biases the distribution of divergence estimates slightly toward positive values, this bias is small compared to (and in the opposite direction of) the Lagrangian sampling bias due to drifters preferably sampling convergence regions. Errors due to position uncertainty are suppressed by the shape-based subsampling. The subsampling also improves the identification of the areas of extreme divergence or convergence. An application to an observational dataset demonstrates that these model-derived thresholds can be effectively used on actual drifter data.
Significance Statement
Divergence in the ocean indicates how fast floating objects in the ocean spread apart, while convergence (negative divergence) captures how fast they accumulate. Measuring divergence in the ocean, however, remains challenging. One method is to estimate divergence from the trajectories of drifting buoys. This study provides guidance under what circumstances these estimates should be discarded because they are too likely to have large errors. The criteria proposed here are less stringent than some of the ad hoc criteria previously used. This will allow users to retain more of their estimates. We consider how position uncertainty affects the reliability of the divergence estimates. An observational dataset collected in the Mediterranean is used to illustrate an application of these reliability criteria.
Abstract
Presented here are three mean dynamic topography maps derived with different methodologies. The first method combines sea level observed by the high-accuracy satellite radar altimetry with the geoid model of the Gravity Recovery and Climate Experiment (GRACE), which has recently measured the earth’s gravity with unprecedented spatial resolution and accuracy. The second one synthesizes near-surface velocities from a network of ocean drifters, hydrographic profiles, and ocean winds sorted according to the horizontal scales. In the third method, these global datasets are used in the context of the ocean surface momentum balance. The second and third methods are used to improve accuracy of the dynamic topography on fine space scales poorly resolved in the first method. When they are used to compute a multiyear time-mean global ocean surface circulation on a 0.5° horizontal resolution, both contain very similar, new small-scale midocean current patterns. In particular, extensions of western boundary currents appear narrow and strong despite temporal variability and exhibit persistent meanders and multiple branching. Also, the locations of the velocity concentrations in the Antarctic Circumpolar Current become well defined. Ageostrophic velocities reveal convergent zones in each subtropical basin. These maps present a new context in which to view the continued ocean monitoring with in situ instruments and satellites.
Abstract
Presented here are three mean dynamic topography maps derived with different methodologies. The first method combines sea level observed by the high-accuracy satellite radar altimetry with the geoid model of the Gravity Recovery and Climate Experiment (GRACE), which has recently measured the earth’s gravity with unprecedented spatial resolution and accuracy. The second one synthesizes near-surface velocities from a network of ocean drifters, hydrographic profiles, and ocean winds sorted according to the horizontal scales. In the third method, these global datasets are used in the context of the ocean surface momentum balance. The second and third methods are used to improve accuracy of the dynamic topography on fine space scales poorly resolved in the first method. When they are used to compute a multiyear time-mean global ocean surface circulation on a 0.5° horizontal resolution, both contain very similar, new small-scale midocean current patterns. In particular, extensions of western boundary currents appear narrow and strong despite temporal variability and exhibit persistent meanders and multiple branching. Also, the locations of the velocity concentrations in the Antarctic Circumpolar Current become well defined. Ageostrophic velocities reveal convergent zones in each subtropical basin. These maps present a new context in which to view the continued ocean monitoring with in situ instruments and satellites.
