Ship Drift Research Articles

Variations of global mesoscale eddy energy observed from Geosat

The global distribution of eddy kinetic energy has been synoptically observed from analysis of the Geosat Exact Repeat Mission (ERM) altimeter data collected for a 2‐year period from November 1986 through November 1988. Using a technique developed by Sandwell and Zhang (1989), altimeter data from forty‐four 17‐day repeat cycles (2 years) were processed into sea surface slopes along the satellite ground track, averaged, and filtered to produce a mean sea surface slope profile having an estimated accuracy of 0.2 μrad (2 cm sea level change over 100 km distance). A series of global eddy kinetic energy maps, each averaged over 3 months, and their mean were then generated. The maximum mean eddy kinetic energy per unit mass exceeds 2000 cm2/s2 for most of the western boundary currents; however, it only reaches approximately 500 cm2/s2 for the Antarctic Circumpolar Current (ACC). More than 65% of the world ocean has relatively low variability with an eddy kinetic energy of less than 300 cm2/s2. Results obtained from this study are in general agreement with other Geosat ocean variability studies (e.g., Zlotnicki et al., 1989). However, significantly higher variability is found when compared with either Seasat or ship drift data. Significant seasonal variations were found in the Gulf Stream and Kuroshio currents. The ACC system exhibits no apparent seasonal variation.

Application of a Frictional Channel Flow Theory to Flow in the Prince of Wales Channel, Torres Strait

Abstract Turbulent flow is considered in a narrow, constant-depth channel connecting two basins having a time-dependent sea-level difference. The bosom stress is taken to be linear rather than quadratic in velocity, even when flows are quite strong. This approximation is justified theoretically by comparing appropriate linear and nonlinear solutions. Simple formulae are available for the depth-averaged current speed ū along the channel axis in terms of the sea-level gradient along that axis. Application of suitable mixing length theory shows that the stress should vary linearly with depth even for time-dependent flows. The mixing length theory predicts the current profile to be logarithmic near the bottom and slightly greater than logarithmic near the surface. The theory was applied to sea level tidal constants and some acoustic Doppler and ship drift measurements recently made in the Prince of Wales Channel, Torres Strait. Sea level gradients and tidal flows in the channel are large because the channel j...

Surface current distributions in the tropical Indian Ocean derived from compilations of surface buoy trajectories

Three different satellite‐tracked drifting buoy data sets are compiled and used to generate a monthly climatology of surface currents in the tropical Indian Ocean. Buoys were deployed between 1975 and 1987. The data density is maximum on and near the equator and decreases poleward. Drift characteristics of the different buoy configurations are compared using a structure function analysis. The differences in windage effects are consistent with the buoy designs and small compared with the signals studied. The currents in the tropical Indian Ocean during boreal winter and spring can be characterized as two counterrotating gyres. A southern clockwise rotating gyre bounded on the south by the South Equatorial Current (SEC) and on the north during winter by the Equatorial Countercurrent (ECC) and during spring by the Equatorial Jet (EJ). A northern counterclockwise rotating gyre is bounded on the south by the ECC and EJ, depending on season, and on the north by the North Equatorial Current (NEC). The two gyre systems break down during boreal summer. During this season, the SEC is located closer to the equator, and the NEC is replaced by the eastward flowing Indian Monsoon Current (IMC). The western boundary circulation becomes more complicated from late spring through early autumn with the observation of two intense smaller scale gyres. The large‐scale southern gyre reappears during boreal autumn with the reappearance of the EJ. The northern gyre begins to reappear in December, with the reversal of the IMC and the reappearance of the NEC. The monthly buoy speeds are compared with a monthly climatology generated from ship drift reports. Differences between the two climatologies are in general small except in regions of few trajectories. The annual cycles in amplitudes and phases of the major currents in the region are thus comparable.

Determination of the ocean circulation using Geosat altimetry

A spherical harmonic model of the sea surface topography complete to degree and order 10 and a model of the Earth's geopptential field complete to degree and order 50 have been obtained in a simultaneous solution using Geosat altimeter data and tracking data from 14 different satellites. The sea surface topography model compares well with oceanographic models computed using hydrographic data and ship drift data. Currently, errors in the estimated gravity field model limit the determination of the spherical harmonic coefficients of the general ocean circulation to degrees 10 and lower, corresponding to a minimum wavelength of 4000 km. Error analysis indicates that the correlation between the geoid and the sea surface topography model is less than 0.2, indicating good separation of the geoid and the sea surface topography at wavelengths of 4000 km or longer. Estimates of the scale factor for the significant wave height (H1/3), which is used to compute the electromagnetic bias correction and the bias for the Geosat altimeter, are obtained. The estimate of the H1/3 correction is 3.6±1.5%, and the height bias estimate is zero.

A Statistical Approach to Testing Equatorial Ocean Models with Observed Data

A model testing procedure based on multivariate statistical analysis has been developed to provide an objective measure of the fit between ocean model simulations and observations, taking into account the uncertainties in the atmospheric forcing and the inaccuracies in the oceanic data. The method is applied to the seasonal variations in an ocean model of the tropical Atlantic. A wind-driven linear multimode model with a simple mixed-layer is tested against surface currents estimated from ship drift. The uncertainties in the observations, and in the model response due to random errors in the wind stress and its interannual variability create a substantial indeterminacy in the available sample, but it is shown that they do not explain the large discrepancies that are found between observed and modeled seasonal surface currents. Nor are the uncertainties in the wind stress bulk formulation sufficient to account for the model-data differences. These can then be attributed unambiguously to the oversimplification of the model physics. The use of the method in model tuning is illustrated by determining the vertical resolution that provides an optimal fit to the observed surface currents. The linear model works best with only one vertical mode, and a mixed-layer depth of 40 m. However, the discrepancies with the observations remain too large for the improvement in model performance to be statistically significant.

The drift of the research shipMikhail somovin the pacific ocean ice massif (March‐July 1985)

Abstract Between March 25 and July 25, 1985 Mikhail Somov drifted a straight‐line distance of 328 miles (525 km). Drift velocities and directions during the different sections of this drift are related to the general patterns of atmospheric circulation, to wind velocities and directions, and to the known peculiarities of the Antarctic Coastal Current. Some preliminary analysis of the characteristics of iceberg drift in the same area is also attempted.

The drift of the research shipMikhail somovMarch‐July 1985, and climatic peculiarities of the Ross Sea

Abstract Meteorological observations carried out aboard Mikhail Somov during her drift provide a wealth of information on this little‐known area of the Antarctic. In particular they throw light on the Ross Sea Low, the deepest cell of atmospheric pressure on earth, and on the traveling cyclones associated with it. The latter result in some of the highest mean monthly wind velocities on earth along the Marie Byrd Coast, reaching 12–13 m/s in some months. Associated with these conditions are high levels of cloud cover and high humidities, making the environment of the Ross Sea one of the most severe oceanic environments in the world. Analysis of the weather data would suggest that an anomalous sequence of three cyclones in a row which approached the Russkaya Station area from the northeast (instead of the more normal direction of northwest) in March 1985 probably contributed to the ice conditions which led to Mikhail Somov's besetment. The article concludes with a detailed analysis of weather conditions recorded aboard Mikhail Somov during the various sections of her drift.

Worldwide ship drift distributions identify missing data

The geographical and temporal distributions of worldwide ship drift velocities were plotted in order to see where and when the observations were made and to identify what appear to be major gaps in the data. Curiously, large areas of the North Atlantic, North Pacific, and South Pacific were found to be devoid of observations in some months during the years 1920–1934 when the number of yearly observations is large. An estimated 700,000 observations are missing. These would significantly enhance the usefulness of the data set if they could be found and added to it, especially in the Pacific where the data density is low.

Southern Ocean Surface Characteristics from FGGE Buoys

Abstract In this analysis of satellite-tracked drifting surface buoys released in the Southern Ocean, buoy velocities are averaged along trajectories for 90 days to determine the mean circulation, and eddy kinetic energy is computed using perturbations about the 90-day mean. The 90-day mean is based upon wind-driven circulation theory, Southern Ocean wind spectra, cumulative averages of mean buoy velocity and energy, and plots of the resulting mean circulation. Excluding the Antarctic Circumpolar Current (ACC), the spatial distribution of the eddy kinetic energy is consistent with previous distributions, based upon spatially averaged ship drift and surface drifting buoy data, reported in the literature. In the ACC, values of eddy kinetic energy are generally lower than found elsewhere. The analysis indicates that the spatial structure of perturbations about the 90-day mean is zonally extended and meridionally compressed. The meridional compression may be due to β, which acts as the restoring force for pla...

The WOCE Hydrographic Program

The primary scientific objective of the World Ocean Circulation Experiment (WOCE) is to understand the general circulation of the global ocean well enough to be able to model its present state and predict its evolution in relation to long‐term changes in the climate. The large‐scale circulation includes ocean currents that are directly observable (for example, by current meters or ship drift) and processes of water mass formation, modification, and movement that are studied by more indirect methods. All are important to WOCE, and the determination of the “mean” The primary scientific objective of the World Ocean Circulation Experiment (WOCE) is to understand the general circulation of the global ocean well enough to be able to model its present state and predict its evolution in relation to long‐term changes in the climate. The large‐scale circulation includes ocean currents that are directly observable (for example, by current meters or ship drift) and processes of water mass formation, modification, and movement that are studied by more indirect methods. All are important to WOCE, and the determination of the “mean” circulation and also its variability on annual to decadal time and appropriate space scales is also of value.

Currents off south-eastern Australia: results from the Australian coastal experiment

The Australian Coastal Experiment was conducted off the east coast of New South Wales between September 1983 and March 1984. The experiment was conducted with arrays of current meters spanning the continental margin at three latitudes (37.5�, 34.5�, and 33.0�S.), additional shelf moorings at 29� and 42�S. coastal wind and sea-level measurements, monthly conductivity-temperature-depth probe/expendable bathythermograph (CTD/XBT) surveys, and two satellite-tracked buoys. Over the continental shelf and slope, the alongshore component of the current generally exceeded the onshore component, and the subtidal (<0.6 cpd, cycles per day) current variability greatly exceeded the mean flow. Part of the current variability was associated with two separate warm-core eddies that approached the coast, causing strong (>50 cm sec-1), persistent (>8 days), southward currents over the continental slope and outer shelf. Temperature and geostrophic velocity sections through the eddies, maps of ship's drift vectors and temperature contours at 250 m, and the satellite-tracked drifter trajectories showed that these eddies were similar in structure to those observed previously in the East Australian Current region. Both eddies migrated generally southward. Eddy currents over the shelf and slope were rare at Cape Howe (37.5�S.), more common near Sydney (34.5�S.), and frequent at Newcastle (33.0�S.), where strong northward currents were also observed. Near Sydney, the eddy currents over the slope turned clockwise with depth between 280 and 740 m, suggesting net downwelling there. Repeated CTD sections also indicated onshore transport and downwelling at shallower levels; presumably, upwelling occurred farther south where the eddy currents turned offshore. Periodic rotary currents over the continental slope near Sydney and Newcastle indicated the presence of small cyclonic eddies on the flank of a much larger anticyclonic eddy. Between early October and late January, no strong southward currents were observed over the continental margin near Sydney. Data from this 'eddy-free' period were analysed further to examine the structure and variability of the coastal currents. Much of this variability was correlated with fluctuations in coastal sea-level (at zero lag) and with the wind stress (at various lags). The coherence and phase relationships among current, wind-stress, and sea-level records at different latitudes (determined from spectral analysis and frequency-domain empirical orthogonal functions) were consistent with the equatorward propagation of coastal-trapped waves generated by winds in phase with those near Cape Howe. Time-domain empirical orthogonal functions show that the current fluctuations decayed with distance from shore and with depth, as expected of coastal-trapped waves.

Current observation system using ship drift and its applications in Kumano-nada and Enshu-nada Seas

A continuous observation system of ship drift of ferry boats was developed using Loran C, ship direction, ship speed and wind velocity measurement. A formula to estimate the wind-driven lateral drift of the boat is proposed, and a measuring system of the surface current is described, including a discussions on its usefullness to monitor day-to-day variations of the circulation patterns in the coastal boundary regions of the Kuroshio.This system was applied to observe variations of the circulation in the Kumano-nada, and Enshu-nada Seas south of Japan. Some preliminary results were obtained by using this system as well as other methods. This work focuses on the behavior of warm eddies in the Kumano-nada Sea. The surface current patterns that include these phenomena obtained by the use of the ship drift show good correspondence with currents and thermal structures observed with GEK, CTD, moored current meters and satellite thermal images.

The diagnosis of synoptic-scale vertical motion in the seasonal thermocline

The form of the vertical motion field on the ocean synoptic scale in the seasonal thermocline can be diagnosed by applying the ω-equation developed in meteorology. The results show a pattern typical of an unstable baroclinic wave. This was achieved by combining hydrographic data from the seasonal thermocline, collected by a CTD in a towed fish, with surface currents measured from ship's drift using an electromagnetic log.

Tropical Atlantic geostrophic currents and ship drifts

Historical ship drifts and geostrophic surface currents obtained from hydrographic data are compared in the tropical Atlantic Ocean. The major components of the current system (North Equatorial Current, North Equatorial Countercurrent, South Equatorial Current) are clearly depicted by the two data sets. The main difference between the two fields is the weakness of the geostrophic currents except in the near‐equatorial band, where on the contrary, they are much larger than the ship drifts. The amplitudes of the annual signals differ, but the phases are in rather good agreement. The uncertainty concerning the ship drift data, and the limitation of the geostrophic assumption, can explain a part of the differences encountered in this study. A much better comparison is obtained if we add to the geostrophic current ug an Ekman drift current ue computed from the climatological wind stress and a mean constant vertical viscosity value. Then the total velocity u = ug + ue corresponds very closely in phase and amplitude to the ship drift velocity, except at the equator.

Seasonal cycle of velocity in the Atlantic North Equatorial Countercurrent as measured by surface drifters, current meters, and ship drifts

This report describes the general circulation and seasonal variation of currents in the equatorial Atlantic, concentrating on the North Equatorial Countercurrent (NECC), using data collected as part of the Seasonal Response of the Equatorial Atlantic and Programme Français Océan et Climat dans l'Atlantique Equatorial experiments plus historical ship drifts. During 1983–1985 the Lagrangian circulation was studied by launching and tracking 30 freely drifting drogued buoys in the North Equatorial Countercurrent and 23 in the South Equatorial Current (SEC). In addition, continuously recording current meters were moored for 20 months at depths from 20 to 300 m near the center of the NECC, at 6°N, 28°W. These measurements are the longest series ever obtained in this current and provide information about its seasonal and interannual variations and zonal, meridional, and vertical structure. The seasonal cycle of the NECC is very regular from year to year. Each year the NECC starts up in May–June. It flows eastward across the Atlantic with surface speeds of up to 143 cm/s in the west, extending down to 350 m at 28°W, and flowing into both the Guinea Current and North Equatorial Current. It disappears or reverses from about January–June west of 18°W. Some near‐surface water of the NECC is inferred to downwell and flow equatorward toward the Equatorial Undercurrent. The SEC flows into the North Brazil Current, which during spring continues up the coast into the Caribbean. During fall, however, the whole North Brazil Current retroflects, or turns back on itself, between 45° and 50°W, forming the western NECC. The retroflection establishes a meander pattern in the NECC that slowly propagates westward during fall at a speed of 4 cm/s. The meanders have a displacement of about 300 km in latitude, a wavelength of 900 km, and meridional velocity fluctuations of up to 100 cm/s. The swift currents and time‐dependent meanders in the western NECC cause a high eddy kinetic energy, ∼2400 cm2/s2, equivalent to that of the energetic part of the Gulf Stream.

The seasonal variations of surface currents in the tropical Atlantic Ocean: A comparison of ship drift data with results from a general circulation model

Historical ship drifts from the tropical Atlantic Ocean are compared with surface currents from a general circulation model forced with monthly mean climatological winds. The model accurately reproduces the spatial structure of the currents and its time dependence, which varies considerably from the eastern side of the basin, where a semiannual harmonic is prominent, to the western side, where an annual harmonic is dominant. However, the amplitude of the simulated surface currents in the western side of the basin is too large. Mixing processes in the model appear to be too weak, especially when the winds are weak in regions where the thermocline is deep. High‐frequency fluctuations of the winds need to be taken into account, and parameterization of the mixing needs to be improved, especially when the Richardson number is small.

Effects of mixed-layer depth and an isolated coral reef on the strong complexing capacity of oligotropic waters

The strong copper complexing capacity of seawater was determined along three transects enclosing Wreck Reef, an isolated reef in the Coral Sea. Measurements of ship's drift and geostrophic calculations relative to 300 m indicated that little of the surface waters flowed through the reef system. The values of copper complexing capacity and dissolved organic carbon close to Wreck Reef were similar to those found within the Great Barrier Reef system. Within the top 100 m, values of complexing capacity ranged from 2.5 to 26 nM, with the elevated values being associated with a deep mixed layer on the leeward side of the reef and possibly with processes occurring within the reef itself. There was no correlation between copper complexing capacity and concentrations of nutrients or dissolved organic carbon. The copper complexing capacity within the euphotic zone was generally higher than that observed previously for surface waters of south-eastern Australia and the North West Shelf of Australia.

Mapping climatological seasonal variations of surface currents in the tropical Atlantic using ship drifts

The seasonal variability of current velocities in the tropical Atlantic was studied by grouping ship drift velocity observations into 2° × 5° boxes and calculating monthly mean velocity values. These values were used to calculate and map the annual mean velocity, the seasonal variation about the mean, the annual and semiannual harmonics, and the first two empirical orthogonal functions (EOFs). The seasonal variation is primarily zonal in the equatorial band and in the North Equatorial Countercurrent (NECC) and primarily alongshore near the coast of South America. Maxima of seasonal variation of 23 cm/s are centered in the NECC near 6°N, 42.5°W and in the Gulf of Guinea near 2°N, 7.5°W. Most (∼80%) of the variance in the NECC and along the western boundary of the area studied has an annual period; most of the variance along the equator in the mid‐Atlantic has a semiannual period. Over the whole region, 49% of the seasonal variance is explained by the annual harmonic, and 69% is explained by a combination of the annual and semiannual harmonics. The second EOF contains 29% of the variance of the data set and shows a simultaneous speeding (slowing) of the major equatorial currents (the South Equatorial Current (SEC), the North Brazil Current, the NECC, and the Guinea Current) along their principal axes of variation with a concurrent slowing (speeding) of the Guyana Current and the Brazil Current. The pattern of variation resembles a large equatorial gyre centered along 4°N that flows clockwise from May to September and counterclockwise most of the rest of the year. The first EOF accounts for 35% of the variance and shows a simultaneous speeding (slowing) of eastward velocity from 5°S to 10°N except in the western SEC, the western end of the North Brazil Current, and the Guinea Current. The out‐of‐phase fluctuations apparent in the western SEC near the equator are explained by the slowing and reversal of the South Equatorial Current near 2°N, 40°W between April and June. This slowing of the SEC is associated with the start up of the NECC, which accelerates eastward in May, June, and July and flows eastward across the Atlantic from July through December. The SEC divides seasonally near the eastern tip of Brazil, where residual alongshore velocities are northward for half the year (peaking during May and June) and southward for the other half of the year. A second division can be seen north of the Amazon, where the North Brazil Current continues up the coast into the Guyana Current during the first half of the year but partially retroflects into the NECC during the last half of the year. A fast eastward flow in the NECC coincides with below average velocities in the Guyana Current and above average velocities in the western sections of the North Brazil Current and the SEC (north of the equator).

The analysis of currents measured from a moving ship in the region of the North Atlantic Polar Front

A streamfunction has been obtained from sea surface currents measured by ship's drift without using CTD data. The method used involves first calculating the relative vorticity and then solving a Poisson equation to obtain the streamfunction.

WOCE/TOGA Historical Oceanographic Data Subgroup

The Oceanographic Historical Data Subgroup of the World Ocean Circulation Experiment (WOCE)/Tropical Ocean and Global Atmosphere program (TOGA) Data Management Working Group was organized to assess both the quantity and quality of historical standard oceanographic data files (e.g., temperature and salinity profiles and current meter, drifter, and ship drift data) for WOCE/TOGA studies and to estimate the amount of existing data that is not to be found in National Oceanogrphic Data Center (NODC) files and formulate a strategy for adding these data to the historical files.