Peter Niiler

Pathways of marine debris derived from trajectories of Lagrangian drifters.

Global set of trajectories of satellite-tracked Lagrangian drifters is used to study the dynamics of marine debris. A probabilistic model is developed to eliminate the bias in spatial distribution of drifter data due to heterogeneous deployments. Model experiments, simulating long-term evolution of initially homogeneous drifter array, reveal five main sites of drifter aggregation, located in the subtropics and maintained by converging Ekman currents. The paper characterizes the geography and structure of the collection regions and discusses factors that determine their dynamics. A new scale R(c)=(4k/|D|)(½) is introduced to characterize tracer distribution under competing effects of horizontal divergence D and diffusion k. Existence and locations of all five accumulation zones have been recently confirmed by direct measurements of microplastic at the sea surface.

Mean Dynamic Topography of the Ocean Derived from Satellite and Drifting Buoy Data Using Three Different Techniques

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 c...

North Atlantic Ocean surface currents

[1]xa0Close to 1800 surface drifters are used to investigate the 15 m circulation of the North Atlantic Ocean. The data are used to describe structures of the average Eulerian circulation and of the associated eddy variability. The data resolve scales on the order of 50 km, which have hitherto not been systematically described, in particular, near shelf breaks and near the most intense currents, the Gulf Stream, the North Atlantic Current (NAC), and the frontal currents of the subpolar gyre. This reveals a complex series of quasi-permanent eddies, meanders, and recirculation patterns. Gulf Stream intensity is portrayed as changing abruptly near 54°W, east of which, it is identified as two current branches centered near 39° and 41.5°N, the northern one connecting more directly with the NAC, and the southern one with the recirculation gyre and the Azores Current (AC). Many features of the currents are controlled by topography, in particular, currents are often intensified near shelf breaks or parallel to ridges in the subpolar gyre. However, the largest northward branch of the NAC in the Icelandic basin is located near the deepest bathymetry, not near steep bathymetry. Other currents, in particular, in the subtropical gyre, are less clearly related to topography: for instance, the AC is featured as a zonal eastward current extending far west of the Mid-Atlantic Ridge (MAR) to at least 55°W and possibly 63°W. In the interior and away from topographic features the eddy kinetic energy (EKE) is the largest where the mean currents are the largest. In the subpolar gyre, there are striking differences in EKE between southward flowing currents (the Labrador and east Greenland Current) and the northward flowing currents (west Greenland Current and branches of the NAC), which have higher EKE. The areas of weakest variability are located in the southwest part of the subpolar gyre, northeast of Iceland, and in the eastern Atlantic south of 45°N. The AC eddies and the mesoscales south of the Canary Islands transect this eastern eddy desert. Drifter trajectories are used as realizations of Lagrangian particles in the vicinity of current cores. These illustrate the variety of paths or connections between different current systems and demonstrate cross-stream dispersions.

Separation of the Kuroshio water and its penetration onto the continental shelf west of Kyushu

The separation of Kuroshio water west of Kyushu and its penetration onto the continental shelf of the East China Sea were investigated by analyzing trajectories of satellite-tracked surface drifters deployed during 1989–1996. Conductivity, temperature, and depth (CTD) data collected in the eastern East China Sea in December 1993 and late April/early May 1995 were also analyzed. Composite trajectories of 172 drogued drifters provided direct evidence of the separation of the northeastward flowing Kuroshio into two parts at the western mouth of the deep trough southwest of Kyushu. The Kuroshio main stream turns to the east toward the Tokara Strait, and a northward flowing branch current penetrates onto the shelf across the continental slope west of the trough. Analysis of the CTD data and drifter trajectories, concurrently observed, shows that the branch current was part of the inshore Kuroshio, just upstream before its separation. During the periods of CTD observations, the separated, inshore Kuroshio water intruded northward in a tongue shape along the shelf break of the trough and penetrated onto the shelf after crossing obliquely the western continental slope of the trough. The water that penetrated onto the shelf continued to flow northward toward the Korea Strait. The main path of this water was easily traced by its high salinity, which is characteristic of the inshore Kuroshio water. Application of an inverse method to the observed CTD data also supports the separation and penetration of the Kuroshio and calculates the volume transport of the northward branch to be about 4.0×106 m3/s for the two different surveys. The tongue-shaped intrusion of the inshore Kuroshio water onto the outer shelf may take place in close association with the eastward turning of the Kuroshio main stream and it might be caused by the vorticity adjustment created by the eastward turning of the Kuroshio.

Permanent Meanders in the California Current System

Surface Velocity Program (SVP) drifter data from 1987 through 2005; Archiving, Validation, and Interpretation of Satellite Oceanographic data (AVISO) sea level anomalies; and NCEP reanalysis winds are used to assemble a time-averaged map of the 15-m-deep geostrophic velocity field in the California Current System seaward of about 50 km from the coast. The wind data are used to compute the Ekman currents, which are then subtracted from the drifter velocity measurements. The resulting proxy for geostrophic velocity anomalies computed from drifters and from satellite sea level measurements are combined to form an unbiased mean geostrophic circulation map. The result shows a California Current System that flows southward with four permanent meanders that can extend seaward for more than 800 km. Bands of alternating eastward and westward zonal currents are connected to the meanders and extend several thousand kilometers into the Pacific Ocean. This observed time-mean circulation and its associated eddy energy are compared to those produced by various high-resolution OGCM solutions: Regional Ocean Modeling System (ROMS; 5 km), Parallel Ocean Program model (POP; 1/10°), Hybrid Coordinate Ocean Model (HYCOM; 1/12°), and Naval Research Laboratory (NRL) Layered Ocean Model (NLOM; 1/32°). Simulations in closest agreement with observations come from ROMS, which also produces four meanders, geostrophic time-mean currents, and geostrophic eddy energy consistent with the observed values. The time-mean ageostrophic velocity in ROMS is strongest within the cyclonic part of the meanders and is similar to the ageostrophic velocity produced by nonlinear interaction of Ekman currents with the nearsurface vorticity field.

Recovery of Near-Surface Velocity from Undrogued Drifters

Abstract The authors have quality controlled six global datasets of drifting buoy data, made comparisons of 15-m drogued and undrogued buoy observations, and developed a 2D linear regression model of the difference between drogued and undrogued drifter velocity as a function of wind. The data were acquired from 2334 Surface Velocity Program (SVP) drifters, including 1845 SVP drifters after they lost their drogues; 704 AN/WSQ-6 Navy drifter buoys; and 503 First Global GARP Experiment (FGGE) drifter buoys. Meridional and zonal surface wind velocity components from the global synoptic FNMOC model, the global synoptic ECMWF model, and the global synoptic NCEP model were interpolated to naval AN/WSQ-6, WOCE–TOGA buoy, or FGGE buoy positions and date/times in the datasets. Two-day mean buoy drift velocities and positions were computed: 122u2009101 SVP drifter mean velocities before they lost their drogues and 58u2009201 SVP drifter mean velocities after they lost their drogues, 21u2009799 Navy drifter mean velocities, and ...

Eddy energy and shelf interactions in the Gulf of Mexico

Sea surface height anomaly data from satellite are continuously available for the entire Gulf of Mexico. Surface current velocities derived from these remotely sensed data are compared with surface velocities from drifting buoys. The comparison shows that satellite altimetry does an excellent job resolving gulf eddies over the shelf rise (depths between ---200 and 2000 m) if the proper length scale is used. Correlations between altimeter- and drifter-derived velocities are statistically significant (r > 0.5) when the surface slope is computed over 125 km, indicating that remotely sensed sea surface height anomaly data can be used to aid the understanding of circulation over the shelf rise. Velocity variance over the shelf rise from the altimetry data shows regions of pronounced eddy energy south of the Mississippi outflow, south of the Texas-Louisiana shelf, and in the northwest and northeast corners of the gulf. These are the same locations where surface drifters are most likely to cross the shelf rise, suggesting gulf eddies promote cross-shore flows. This is clearly exemplified with both warm and cold eddies. Finally, the contribution of gulf eddies and wind stress to changes in the mean circulation are compared. Results indicate that the eddy-generated vorticity flux to the mean flow is greater than the contribution from the surface wind stress curl, especially in the region of the Loop current and along the shelf rise base in the western gulf. Future modeling efforts must not neglect the role of eddies in driving gulf circulation over the shelf rise.

GPS-Cellular Drifter Technology for Coastal Ocean Observing Systems

Abstract A drifter for observing small spatial and temporal scales of motion in the coastal zone is presented. The drifter uses GPS to determine its position, and the Mobitex terrestrial cellular communications system to transmit the position data in near–real time. This configuration allows position data with order meter accuracy to be sampled every few minutes and transmitted inexpensively. Near-real-time transmission of highly accurate position data enables the drifters to be retrieved and redeployed, further increasing economy. Drifter slip measurements indicate that the drifter follows water to within ∼1–2 cm s−1 during light wind periods. Slip values >1 cm s−1 are aligned with the direction of surface wave propagation and are 180° out of phase, so that the drifter “walks” down waves. Nearly 200 drifter tracks collected off the Santa Barbara, California, coast show comparisons with high-frequency (HF) radar observations of near-surface currents that improve by roughly 50% when the average drifter val...

Surface Salinity Measurements—COSMOS 2005 Experiment in the Bay of Biscay

Abstract Sea surface salinity (SSS) data were collected in the Bay of Biscay between April and November 2005. The major source of data is 15 surface drifters deployed during the COSMOS experiment in early April and early May 2005 [12 from the Scripps Instution of Oceanography (SIO) and 3 from METOCEAN]. This is complemented by thermosalinograph (TSG) data from four French research vessels and four merchant vessels, from salinity profiles collected by Argo profiling floats and CTD casts, and from surface samples during two cruises. Time during the two cruises was dedicated to direct inspection of the drifters, recovering some, and providing validation data. This dataset provides a unique opportunity to estimate the accuracy of the SSS data and to evaluate the long-term performance of the drifter salinities. Some of the TSG SSS data were noisy, presumably from bubbles. The TSG data from the research vessels needed to be corrected from biases, which are very commonly larger than 0.1 pss-78 (practical salinit...

A two-dimensional response to a tropical storm on the Gulf of Mexico shelf

Abstract Surface current data from drifting buoys and remotely sensed wind data recorded over the continental shelf in the northeastern Gulf of Mexico during the passage of tropical storm Josephine in October 1996 are examined. Drifter data show the existence of a strong surface jet (velocities reaching 1 m s −1 ) that moves up the west Florida shelf and westward along the Louisiana–Texas shelf, and lasts for nearly 1 week. The coastal jet occurs during an intense synoptic scale wind event where wind speeds reach 15 m s −1 . A simple force balance and statistical analysis are performed to assess the role of strong wind forcing. The primary balance shows an Ekman-type current. The role of local acceleration is greatest when winds are directed along bathymetry. A simple two-dimensional strongly forced shelf response model developed from the linear steady-state momentum equations also indicates larger along-shore currents due to both Ekman-type forcing by cross-shore winds and a cross-shore pressure gradient arising from conservation of mass. Model parameters fit empirically are within 15% of theoretical values. The simple model explains 30% and 46% of the variance in the observed along-shore and cross-shore surface currents, respectively.