Lagrangian Drifters Research Articles

Topographic Upwelling off Southwest Nova Scotia

The waters off Cape Sable in southwestern Nova Scotia have anomalously low temperatures and high nutrients during summer. Using a three-dimensional tidal model as well as field observations obtained between 1978 and 1985, a new mechanism is proposed to explain the origin of these anomalous properties. The numerical model predicts several areas of strong tidally induced residual upwelling and downwelling. On the submarine ridge off Cape Sable, upwelling occurs over the eastern flank and downwelling over the western flank. This upward vertical transport is very effective in supplying cold, saline, and nutrient-rich water from deep to shallow layers. The predicted upwelling and downwelling are induced by a tidal rectification process resulting from tidal currents flowing over complex bottom topography. Hence, the process is named “topographic upwelling and downwelling.” The upwelling (downwelling) is generated by the residual currents flowing from deep to shallow (shallow to deep) waters. This process is three-dimensional so that the water parcels advected onshore from deep to shallow zones in the upwelling regime may remain in the lower layer and be carried away from the upwelling region by a longshore coastal current. Thus, the upward transfer of cold and saline water in the three-dimensional topographic upwelling may not be as efficient as the classical upwelling associated with two-dimensional (on-off shore) circulation. However, efficient vertical transfer can be accomplished in conjunction with the topographic upwelling if the cross-isobath transport associated with the upwelling carries deep water to a shallow region where there is strong vertical mixing. This combination of topographic upwelling and strong tidal mixing provides the mechanism for producing the observed cold water anomaly off Cape Sable. Topographic upwelling occurs on the eastern side of the cape, and strong tidal mixing produces a well-mixed area off the cape. These processes are verified by hydrographic, current meter, and Lagrangian drift data collected in the surrounding area. The mechanism is expected to be important in other coastal regions where strong tidal currents and large variations of bottom topography are found.

Structure of Velocity and Temperature in the Northeast Pacific as Measured with Lagrangian Drifters in Fall 1987

In October 1987, 49 Lagrangian surface drifters (TRISTAR-II) were released in a 200-km × 200-km square area southeast of Ocean Station Papa as part of the OCEAN STORMS Experiment. The drifters measured temperature at the drogue level and reported their position through ARGOS approximately 11 times per day. Thirty-one of the drifters retained drogues for longer than three months, and data from those instruments are used to describe the evolving fall 1987 pattern of current and temperature structures at 15 m in the area between 46° and 49°N, 142°W and 132°W. Time variable currents were dominated by mesoscale eddies of anticyclonic rotation with horizontal radii of 53–86 km and rotational speeds of 10–20 cm s−1. These eddies persisted for at least 90 days as evidenced by successive drifter trajectories through the eddies. Currents with periods longer than 1 day had a mean to the east of 4.4 cm s−1 and a mean to the north of 0.7 cm s−1. Background eddy kinetic energy levels were 40 cm s−2. Thus, eddy kinetic energy was four times larger than mean kinetic energy. The eastward single particle diffusivity was 1100 m2 s−1 and northward diffusivity was 1600 m2 s−1. The local change of thermal energy at 15-m depth was −2.9 W m−3, while on average, flow advected cold water to the east at a rate of 0.8 W m−3. Therefore, large-scale advective processes accounted for 28% of the thermal energy balance at 15 m. This horizontal heat convergence in the open ocean is comparable in magnitude to that produced by powerful equatorial currents in the eastern Pacific cold tongue.

Mesozooplankton standing stock during the North Atlantic spring bloom study in 1989 and its potential grazing pressure on phytoplankton: a comparison between low, medium and high latitudes

Abstract Within the framework of the JGOFS Pilot Study in 1989 mesozooplankton (0.2–20 mm) was sampled by means of a Hydro-Bios multinet in five depth strata (0–25, 25–50, 50–100, 100–200, 200–500 m) during four Lagrangian drift experiments of 8–14 days' duration at 18, 33, 46 and 58°N, to follow the seasonal progress of the phytoplankton spring bloom development in the northeast Atlantic. Mesozooplankton standing stock, measured as dry weight and ash-free dry weight, increased by a factor of about 6 from 18 to 58°N. Day/night differences amounted to 10–20% of the average and were—with one exception at 18°N—not statistically significant. Using the data on weight-specific respiration rates measured by colleagues on the same cruise, the ingestion rates and potential community grazing of mesozooplankton on phytoplankton within the upper 100 m of the water column were calculated. During all four drift experiments, quasi-steady-state conditions were observed in phyto- and zooplankton standing stock, primary production and daily sedimentation at 100 m depth. The maximum potential grazing rate by mesozooplankton accounted for about half of the daily primary production. Since sedimentation of fresh phytoplankton was negligible, it is concluded that the grazing pressure exercised by mesozooplankton together with micro- and nanozooplankton was responsible for keeping the phytoplankton standing stock at a more or less constant level during the investigated spring bloom in the four areas. Particle flux was thus dominated by zooplankton faecal material.

Obtaining Smooth Hydrographic Profiles from a Buoy Deployed in Sea Ice

Abstract SALARGOS buoys that measure upper-ocean temperature and salinity in ice-covered seas have been collecting data in the Arctic basin for several years. The buoys consist of a 300-m-long string of six temperature-conductivity sensors at fixed depths, with a pressure sensor at the bottom. The electronics housing, including an Argos transmitter, is frozen into the sea ice. The buoy drifts with the sea ice, sampling a region described by the Lagrangian drift of the surrounding ice pack. When the ice is moving slowly, the relative water velocity in the upper ocean is low and the buoy simply produces hydrographic time series at six depths. In periods of rapid drift the buoy cable is displaced upward and the sensors sample other depths. Collecting such data over time can produce plots with relatively high vertical resolution. But to use this information, one needs a reasonable curve fit to these data similar to a smoothed version of a CTD (conductivity-temperature-depth) cast. Two methods have been invest...

A simple method for calculating the velocity field beneath irregular waves

The need for estimating water velocities beneath ocean waves for engineering calculations has produced ad hoc methods that are based on the relation between sea surface elevation and the velocity potential given by linear theory. These methods depend on some assumptions, which are, in general, clearly inappropriate for ocean waves. In this paper, we present a new, theoretically consistent method of superposition for a spectrum of linear waves and examine its characteristics with respect to several existing methods. Numerical simulations and laboratory data, collected using a surface-following device and an acoustic current meter, are used to test the relative accuracy of the new method and four existing methods of predicting wave-induced velocities. The results show that linear methods can provide reasonably accurate predictions of wave-induced velocities both at depth and near the free surface. In particular, predictions obtained using coordinate stretching and modified stretching methods as well as the new superposition method are in close agreement with measurements, while those given by the methods of extrapolation and direct linear theory grossly overpredict the near surface crest velocities. Rms errors indicate that, overall, the proposed method of superposition provides the most accurate predictions of wave-induced velocities. In addition, the new method satisfies Laplace's equation and also predicts correctly both the Eulerian and Lagrangian drift velocities under irregular waves, whereas the stretching methods fail in both these regards.

Evolution and kinematics of a loop current eddy in the Gulf of Mexico during 1985

The large eddy that broke off from the Loop Current in July 1985 was the most extensively studied eddy ever to appear in the Gulf of Mexico. Other investigators have described its early evolution based on Lagrangian drifters and its later evolution using moored current meters in the western gulf. This paper provides additional insight on the early evolution of the eddy using results from air dropped expendable bathythermographs and air dropped expendable current profilers in early May, a hydrographic ship survey in mid July, and a detailed ship survey in August using expendable bathythermographs and a current profiler. The May survey established a center of circulation at about 26°N but showed that the eddy had not separated from the Loop Current. A maximum velocity of 171 cm/s was observed near the northern edge of the feature. The evidence suggests that a large elongated eddy then separated from the Loop Current and later split into two smaller eddies. The July hydrographic cruise showed a clear separation of the large eddy from the Loop Current to the southeast. Two weeks later, the August survey showed an asymmetric eddy, with the maximum surface current 178 cm/s south of the center of circulation and 132 cm/s to the north. A western eddy named Ghost Eddy then separated from an eastern eddy named Fast Eddy. Using the current profiles and data from the drifters, we constructed a simple kinematic feature model for eddies in the Gulf of Mexico.

Use of simulated drifter tracks to investigate general transport patterns and residence times in the Coastal Transition Zone

General transport patterns and residence times in the upper 100 m of the coastal transition zone (CTZ) were studied using simulated Lagrangian drifter experiments. The circulation fields used for the drifter experiments were obtained from a regional primitive equation model that incorporates coastal geometry and bottom topography that is representative of the CTZ region. The simulated circulation fields show velocity patterns that are consistent with those associated with the filaments and jets observed in the CTZ region. Drifters were released at 880 points in the model domain in a pattern that bracketed, both horizontally and vertically, the region in which offshore‐flowing filaments were observed to form. At each location, drifters were released at depths of 30, 60, 90 m and tracked for 30 days. The depth of these drifters varied along their trajectories in response to the vertical velocities experienced by the drifters. An additional set of drifters was released at 30 m and was constrained to remain at this depth. The composite drifter trajectories show that transport patterns and residence times in the upper 100 m are determined by the proximity of the drifter release point to the offshore‐flowing filament. Drifters released inshore of the filament are transported southward with the mean flow of the model California Current. Drifters released near the region in which the filament originates are transported rapidly offshore. The point at which this difference in drifter fate occurs is approximately 100 km offshore. The time required for a drifter to be transported from the nearshore region to the outer boundary of the area covered in the CTZ model domain is approximately 20 days. Comparison of the free‐floating drifters with the 30‐m fixed‐depth drifters shows that the deeper drifters are displaced southward as they are transported offshore in the jet. The magnitude of the southward displacement for the 90‐m drifters is about 15 km, implying substantial cross‐jet exchange. Comparison of the simulated drifter trajectories with the trajectory followed by a drifter released during the 1988 CTZ field sampling program support the hypothesis that the deepening of the abundance maxima observed in the distributions of Dolioletta gegenbauri, juvenile forms of Euphausia pacifica and Eucalanus californicus resulted from these populations being downwelled as they were transported offshore by the flow associated with the filament observed in 1988.

Observations of the Mindanao Current during the western equatorial Pacific Ocean circulation study

The Western Equatorial Pacific Ocean Circulation Study (WEPOCS) III expedition was conducted from June 18 through July 31, 1988, in the far western equatorial Pacific Ocean to observe the low‐latitude western boundary circulation there, with emphasis on the Mindanao Current. This survey provides the first quasi‐synoptic set of current measurements which resolve all of the important upper‐ocean currents in the western tropical Pacific. Observations were made of the temperature, salinity, dissolved oxygen, and current profiles with depth; of water mass properties including transient tracers; and of evolving surface flows with a dense array of Lagrangian drifters. This paper provides a summary of the measurements and a preliminary description of the results. The Mindanao Current was found to be a narrow, southward‐flowing current along the eastward side of the southern Philippine Islands, extending from 14°N to the south end of Mindanao near 6°N, where it then separates from the coast and penetrates into the Celebes Sea. The current strengthens to the south and is narrowest at 10°N. Direct current measurements reveal transports in the upper 300 m increasing from 13 Sv to 33 Sv (1 Sverdrup = 1 × 106 m3 s−1) between 10°N and 5.5°N. A portion of the Mindanao Current appears to recurve cyclonically in the Celebes Sea to feed the North Equatorial Countercurrent, merging with waters from the South Equatorial Current and the New Guinea Coastal Undercurrent. Another portion of the Mindanao Current appears to flow directly into the NECC without entering the Celebes Sea. The turning of the currents into the NECC is associated with the Mindanao and Halmahera eddies.

Lagrangian Motions in Simple Kinematic Oscillatory Flow Field

A kinematic approach is taken to examine analytically the Euler‐Lagrange transformation and the basic behavior of fluid parcel trajectories associated with a monochromatic oscillatory current field having progressive wave characteristics. This study examines (1) The dependency of the Lagrangian motions on the initial release conditions; and (2) the distinction between the Eulerian and Lagrangian time scales and its implications on the residual Lagrangian flow field. While the instantaneous trajectory of a labeled parcel depends strongly on the initial conditions, the appropriate period in the Lagrangian framework TLag and the residual Lagrangian drift over such a period are independent of the initial conditions. Because Tlag and the Eulerian time scale Teu, defined as the period of the oscillatory Eulerian current, may be quite different, the residual Lagrangian drift over a complete Teu may show strong dependency with the initial release conditions. It was also found that while the Eulerian current is monochromatic, the Lagrangian trajectory exhibits multiple harmonics, with components at both higher and lower frequencies relative to the fundamental frequency.

A Lagrangian description of motion in Northern California coastal transition filaments

Lagrangian drifters deployed during May 1987 as part of the Coastal Transition Zone experiment were used to examine the motion in cold‐water features seen in satellite AVHRR imagery. The drifters were drogued at 15 m depth and had temperature sensors at 0 m, 12 m, and 18 m. Drogue positions were obtained via service ARGOS at an average of 8 times per day. A cluster of nine drifters was deployed on May 18 near the base of a cold‐water feature off Pt. Reyes. Drifter trajectories confirm the presence of strong (> 50 cm s−1) currents along the axis of the feature. Six of the drifters moved northward following a cyclonic circulation pattern between the Pt. Reyes jet and another feature originating near Pt. Arena. The remaining three drifters, together with three more deployed on May 20, moved offshore in the positive vorticity portion of the Reyes jet. Cluster analysis of the northern tracks indicates large convergence (∼0.5f;), but because relative vorticity during the same few‐day period is found to be constant (∼−0.2f), a simple vorticity balance does not emerge. This is attributed to insufficient resolution of divergence of the water parcel by the small number in the cluster. Drifters reside on the negative vorticity side of the jet while the flow is upwind but on the positive vorticity side while the flow is downwind. Such behavior is consistent with the convergence or divergence patterns expected when along‐jet winds blow over such strong and narrow ocean currents producing significant advection of relative vorticity. Temperature‐salinity data from CTD surveys during the experiment show how the jets that were revealed in both the imagery and the drifter trajectories were advecting different water masses. In the nearshore area where the drifters were deployed a column of cold and salty water had upwelled about 80 m since leaving the source region far offshore. Within the offshore extension of the jets as traced by the drifters, this same water is found about 20 m deeper than it is in the nearshore area. We thus observe that cold filaments seen in AVHRR transport upwelled water offshore through the coastal transition zone. That water subducts in the offshore extension of the filaments. Analysis of the high‐frequency motion from a cluster of five drifters in the Reyes jet shows a multitude of mixing scales. For periods shorter than a day, the cluster shows coherent oscillations of tidal/inertial period whose motions lead to excursions on the order of 2 to 4 km. This suggests that motions on these scales are organized and not random or turbulent. Conversely, motions at scales of 1 km and less appear turbulent. Over longer time periods (several days), the particles exchange places over the cross‐jet scale of the feature (10 to 20 km).

The Weddell‐Scotia confluence in midwinter

The southern central Scotia Sea, site of the Weddell‐Scotia Confluence where outflowing Weddell Sea waters converge with the eastward flowing waters of the Scotia Sea, was sampled during June‐August (austral winter) 1988 with respect to temperature and salinity. Both drogued and ice‐mounted drifters, tracked by Argos, were deployed in the region and yielded Lagrangian drift tracks of ice and water motion. The data substantiate past accounts of the region, based upon summer field research, as dominated by eastward flow upon which a complex array of mesoscale features is superimposed. Weddell‐Scotia Confluence Water, documented by past summer work in the region and characterized by decreased static stability, was not detected, and the Scotia Front was not well defined. The region was one of intense mixing activity and primarily anticyclonic mesoscale features. Two such features, one an eddy and the other either an eddy or a meander in the Scotia Front, dominated the mesoscale field. With warm cores and containing Polar Front Water, they may have been advected eastward from Drake Passage or may have formed as detached eddies from a sharp northward bend in the Polar Front which typically lies just west of the study region. Several smaller eddies, primarily anticyclonic and some having warm cores, were also detected. There was no evidence of the deep convective mixing which has been hypothesized, on the basis of past summer data, to occur in winter, and vigorous vertical mixing was limited to a 100‐m‐thick upper mixed layer. Vertical stability in the upper layers was enhanced by low‐salinity water derived from melting ice. Temperature‐salinity analyses show that winter water in the study region can be derived through isopycnal mixing between waters from the Scotia Sea and waters from the northwestern Weddell Sea. This is in apparent contrast with summer conditions, wherein conditioning of water either through vertical mixing or via lateral mixing on continental margins has been invoked to arrive at the water mass characteristics which typify the Weddell‐Scotia Confluence.

Observations of phytoplankton and nutrients from a Lagrangian drifter off northern California

A Lagrangian drifter was deployed in a cold filament off northern California as part of the Coastal Transition Zone program. The drifter was equipped with an optical package (consisting of a spectroradiometer, a fluorometer, and a beam transmissometer) suspended at 8.5‐m depth and a water sampler suspended at 16.3‐m depth. The drifter was recovered after 8 days. Optical, chemical, and biological properties changed considerably as the drifter moved offshore in the cold filament. Concentrations of phytoplankton chlorophyll increased rapidly in the first 2 days, in parallel with the disappearance of nitrate and nitrite. After this initial period, chlorophyll decreased gradually over the next 6 days with prominent diurnal fluctuations present in the last 3 days. Water transparency also showed similar long‐term as well as diurnal fluctuations. The phytoplankton community became increasingly dominated by large centric diatoms throughout the deployment. Although total cell volume was higher towards the middle of the deployment, this increase occurred without a parallel increase in chlorophyll. In addition, total particulate concentrations were highest nearshore. Although the drifter slippage was approximately 1 cm/s, the biological, chemical, and physical characteristics of the water were affected by both in situ changes and vertical motions of the water. These results are generally consistent with results from other up welling studies.

Observations of phytoplankton and nutrients from a Lagrangian drifter off northern California: Abbott, M.R. et al., 1990. J. geophys. Res., 95(C6): 9393–9409

Observations of phytoplankton and nutrients from a Lagrangian drifter off northern California: Abbott, M.R. et al., 1990. J. geophys. Res., 95(C6): 9393–9409

Flow Distribution in the St. Lawrence River System at Wolfe Island, Kingston Basin, Lake Ontario

Determination of the mass output through the St. Lawrence River outflow system is an important component in computing a mass balance of chemical loadings in Lake Ontario. Toward this goal, during 1986 and 1987, a network of self recording current meter moorings were deployed in the main channels of the St. Lawrence River system in Lake Ontario. On two occasions in 1987, detailed Lagrangian experiments were carried out using a cluster of satellite drifters. The time series data from the current meter network and Lagrangian drifter experiments provided the horizontal and vertical distribution of currents that are adequate to compute the distribution of water transport through the St. Lawrence outflow. The flow distribution in the St. Lawrence River at Wolfe Island is approximately 55 percent in the south channel and 45 percent in the north channel. The total flow rate through the St. Lawrence River system from the present study is in good agreement with the direct measurements at the power plant control facilities at Cornwall.

A second approximation to the time-mean Lagrangian drift beneath a series of progressive gravity waves: Swan, C. and J.F.A. Sleath, 1990. Ocean Engng, 17(1/2):65–79

A second approximation to the time-mean Lagrangian drift beneath a series of progressive gravity waves: Swan, C. and J.F.A. Sleath, 1990. Ocean Engng, 17(1/2):65–79

A second approximation to the time-mean lagrangian drift beneath a series of progressive gravity waves

A fourth-order solution is derived for the mean drift induced by a steady train of waves in water of constant depth. New measurements are carried out of the drift in the body of the fluid and the drift velocity gradient at the free surface. Comparison of theory and experiment shows significantly better agreement with the present fourth-order solution than with the previous second-order solution of Longuet-Higgins, M.S., 1953 [ Phil. Trans. R. Soc. 245, 535–581]. In particular, the present solution reproduces the observed tendency of the surface drift velocity to rise in shallow water and to level-off in very deep water.

Loadings of Selected Chemicals into the St. Lawrence River System from Lake Ontario, 1986/87

Abstract Determination of the mass output through the St. Lawrence River outflow system is an important component in computing mass balance of chemical loadings to Lake Ontario. The total flow rate in the St. Lawrence River System at the Wolfe Island area was calculated from detailed time series current meter measurements from a network of current meters and Lagrangian drifter experiments. This flow is roughly distributed in the ratio of 55% to 45% in the South and North channel, respectively. Loading estimates of selected chemicals have been made by combining the above transport calculations with the ongoing chemical monitoring data at the St. Lawrence outflow. A vertical gradient in the concentration of some organic and inorganic chemicals was observed. The measured concentration for some of the chemicals was higher during the summer months and also is higher in the South Channel than in the North Channel of the St. Lawrence River. These loading estimates are useful not only for modelling the mass balance of chemicals in Lake Ontario but also for serving as input loadings to the St. Lawrence River system from Lake Ontario.

A performance comparison for two lagrangian drifter designs

Abstract We compare the water‐parcel‐following and position‐reporting performance of two recent drifter designs. One (Tristar‐II ) uses the ARGOS satellite navigation system, has a very large drogue: float frontal area ratio and a small velocity error resulting from wind drag and surface wave forces. The second design uses an internal Loran receiver and transmits its position every half hour by radio. Its drogue: float frontal area ratio is much smaller than the TRISTAR's; we wished to know if this difference caused a significant change in drifter trajectory. Over a 78‐h joint deployment, the two designs had nearly identical trajectories. Measured drift speeds ranged from 10 to 30 cms−1. Wind speeds (an important potential biasing factor) ranged up to 15 ms−l. Net velocity difference over the full deployment was only 0.1 cms−1. Frontal trapping of both drifters during the latter part of the comparison period may have helped to minimize this difference. But we also observed little or no downwind slippage (...

Synoptic three-dimensional circulation in an onshore-flowing filament of the California Current

Hydrographic (CTD and XBT casts) and Lagrangian particle drift (satellite-tracked mixed layer drifters) measurements taken off Southern California and northern Baja California in July 1985 are in the region where, historically, a stable, broad onshore flow is indicated in the 0/500 db relative dynamic topography. A 50 km wide onshore-flowing filament of cold and low-salinity water is resolved by the new measurements. Water mass properties (temperature, salinity and spiciness) are used to trace the different water masses and describe the circulation in the surveyed region. A comparison of the 15/300 db relative geostrophic currents to the drifter-inferred velocities shows a significant absolute geostrophic flow at the 300 db level, with coherent patterns similar to the surface mesoscale circulation. Estimates of vorticity calculated from the geostrophic and Lagrangian data are in qualitative agreement. Surface divergence and the implied vertical velocities of the order of 0.5 m day −1 are resolved at scales of 50–100 km. The spatial distribution of these estimates shows significant upwelling within the core of the filament.

Mixing Characteristics of the Niagara River Plume in Lake Ontario

Abstract Data collected between 1982 and 1985 from Lagrangian drifter experiments in which about ten drifters were tracked for ten to twelve hours from their release across the Niagara River mouth, and from concurrently taken ship-based temperature soundings at fixed grid stations off the river mouth, are used to develop a conceptual model of the mixing characteristics of the Niagara River in Lake Ontario. The data obtained suggest a three-stage mixing process. In the initial stage, the river inflow momentum dominates and the plume is well mixed vertically. In the intermediate stage, the interaction of the well mixed, buoyant river plume with colder water from deeper depths of the lake generates a sharp thermal front. In the final stage, the river plume responds to the prevailing winds and the general circulation of the lake. The correlation between these observed plume characteristics and the distribution of toxic contaminants such as mercury and mirex in Lake Ontario sediments attributed to Niagara River outflow is illustrated.