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1
Armitage, Thomas W. K., Sheldon Bacon, Andy L. Ridout, Alek A. Petty, Steven Wolbach, and Michel Tsamados. "Arctic Ocean surface geostrophic circulation 2003–2014." Cryosphere 11, no. 4 (July 26, 2017): 1767–80. http://dx.doi.org/10.5194/tc-11-1767-2017.
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Abstract. Monitoring the surface circulation of the ice-covered Arctic Ocean is generally limited in space, time or both. We present a new 12-year record of geostrophic currents at monthly resolution in the ice-covered and ice-free Arctic Ocean derived from satellite radar altimetry and characterise their seasonal to decadal variability from 2003 to 2014, a period of rapid environmental change in the Arctic. Geostrophic currents around the Arctic basin increased in the late 2000s, with the largest increases observed in summer. Currents in the southeastern Beaufort Gyre accelerated in late 2007 with higher current speeds sustained until 2011, after which they decreased to speeds representative of the period 2003–2006. The strength of the northwestward current in the southwest Beaufort Gyre more than doubled between 2003 and 2014. This pattern of changing currents is linked to shifting of the gyre circulation to the northwest during the time period. The Beaufort Gyre circulation and Fram Strait current are strongest in winter, modulated by the seasonal strength of the atmospheric circulation. We find high eddy kinetic energy (EKE) congruent with features of the seafloor bathymetry that are greater in winter than summer, and estimates of EKE and eddy diffusivity in the Beaufort Sea are consistent with those predicted from theoretical considerations. The variability of Arctic Ocean geostrophic circulation highlights the interplay between seasonally variable atmospheric forcing and ice conditions, on a backdrop of long-term changes to the Arctic sea ice–ocean system. Studies point to various mechanisms influencing the observed increase in Arctic Ocean surface stress, and hence geostrophic currents, in the 2000s – e.g. decreased ice concentration/thickness, changing atmospheric forcing, changing ice pack morphology; however, more work is needed to refine the representation of atmosphere–ice–ocean coupling in models before we can fully attribute causality to these increases.
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Rio, M. H., R. Santoleri, R. Bourdalle-Badie, A. Griffa, L. Piterbarg, and G. Taburet. "Improving the Altimeter-Derived Surface Currents Using High-Resolution Sea Surface Temperature Data: A Feasability Study Based on Model Outputs." Journal of Atmospheric and Oceanic Technology 33, no. 12 (December 2016): 2769–84. http://dx.doi.org/10.1175/jtech-d-16-0017.1.
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AbstractAccurate knowledge of ocean surface currents at high spatial and temporal resolutions is crucial for a gamut of applications. The altimeter observing system, by providing repeated global measurements of the sea surface height, has been by far the most exploited system to estimate ocean surface currents over the past 20 years. However, it neither permits the observation of currents moving away from the geostrophic balance nor is it capable of resolving the shortest spatial and temporal scales of the currents. Therefore, to overcome these limitations, in this study the ways in which the high-spatial-resolution and high-temporal-resolution information from sea surface temperature (SST) images can improve the altimeter current estimates are investigated. The method involves inverting the SST evolution equation for the velocity by prescribing the source and sink terms and employing the altimeter currents as the large-scale background flow. The method feasibility is tested using modeled data from the Mercator Ocean system. This study shows that the methodology may improve the altimeter velocities at spatial scales not resolved by the altimeter system (i.e., below 150 km) but also at larger scales, where the geostrophic equilibrium might not be the unique or dominant process of the ocean circulation. In particular, the major improvements (more than 30% on the meridional component) are obtained in the equatorial band, where the geostrophic assumption is not valid. Finally, the main issues anticipated when this method is applied using real datasets are investigated and discussed.
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Berta, Maristella, Lucio Bellomo, Annalisa Griffa, Marcello G. Magaldi, Anne Molcard, Carlo Mantovani, Gian Pietro Gasparini, et al. "Wind-induced variability in the Northern Current (northwestern Mediterranean Sea) as depicted by a multi-platform observing system." Ocean Science 14, no. 4 (July 25, 2018): 689–710. http://dx.doi.org/10.5194/os-14-689-2018.
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Abstract. The variability and evolution of the Northern Current (NC) in the area off Toulon is studied for 2 weeks in December 2011 using data from a glider, a high-frequency (HF) radar network, vessel surveys, a weather station, and an atmospheric model. The NC variability is dominated by a synoptic response to wind events, even though the dataset also evidences early stages of transition from late summer to fall–winter conditions. With weak winds, the current is mostly zonal and in geostrophic balance even at the surface, with a zonal transport associated with the NC of ≈1 Sv. Strong westerly wind events (longer than 2–3 days) induce an interplay between the direct-wind-induced ageostrophic response and the geostrophic component: upwelling is observed, with offshore surface transport, surface cooling, flattening of the isopycnals, and reduced zonal geostrophic transport (0.5–0.7 Sv). The sea surface response to wind events, as observed by the HF radar, shows total currents rotated at ≈-55 to -90∘ to the right of the wind. Performing a decomposition between geostrophic and ageostrophic components of the surface currents, the wind-driven ageostrophic component is found to rotate by ≈-25 to -30∘ to the right of the wind. The ageostrophic component magnitude corresponds to ≈2 % of the wind speed.
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Centurioni, L. R., J. C. Ohlmann, and P. P. Niiler. "Permanent Meanders in the California Current System." Journal of Physical Oceanography 38, no. 8 (August 1, 2008): 1690–710. http://dx.doi.org/10.1175/2008jpo3746.1.
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Abstract 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 near-surface vorticity field.
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Chaudhary, A., N. Agarwal, and R. Sharma. "Estimation of currents using SARAL/AltiKa in the coastal regions of India." ISPRS - International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences XL-8 (December 23, 2014): 1365–67. http://dx.doi.org/10.5194/isprsarchives-xl-8-1365-2014.
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The present study explores the possibility of deriving the across track currents along the Indian coastal region from SARAL/AltiKa mission. The across track surface geostrophic currents obtained from along track SARAL altimeter data are directly compared (qualitatively) with high frequency (HF) radar observations of surface currents in the Bay of Bengal. The velocity component from HF radar which is perpendicular to the altimeter tracks is considered. Since the ageostrophic velocity contribution is ignored for the moment, the surface geostrophic currents SARAL compare well only under low wind conditions. Due to high along track resolution of SARAL there are large variations in velocity which are not captured by the HF radar observations. In general, the magnitude and variations in surface currents derived from SARAL altimeter are comparable with HF radar observations.
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Ollitrault, Michel, and Alain Colin de Verdière. "The Ocean General Circulation near 1000-m Depth." Journal of Physical Oceanography 44, no. 1 (January 1, 2014): 384–409. http://dx.doi.org/10.1175/jpo-d-13-030.1.
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Abstract The mean ocean circulation near 1000-m depth is estimated with 100-km resolution from the Argo float displacements collected before 1 January 2010. After a thorough validation, the 400 000 or so displacements found in the 950–1150 dbar layer and with parking times between 4 and 17 days allow the currents to be mapped at intermediate depths with unprecedented details. The Antarctic Circumpolar Current (ACC) is the most prominent feature, but western boundary currents (and their recirculations) and alternating zonal jets in the tropical Atlantic and Pacific are also well defined. Eddy kinetic energy (EKE) gives the mesoscale variability (on the order of 10 cm2 s−2 in the interior), which is compared to the surface geostrophic altimetric EKE showing e-folding depths greater than 700 m in the ACC and northern subpolar regions. Assuming planetary geostrophy, the geopotential height of the 1000-dbar isobar is estimated to obtain an absolute and deep reference level worldwide. This is done by solving numerically the Poisson equation that results from taking the divergence of the geostrophic equations on the sphere, assuming Neumann boundary conditions.
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Cadden, Dara D. H., Richard Styles, and Bulusu Subrahmanyam. "Estimates of Geostrophic Surface Currents in the South Atlantic Bight." Marine Geodesy 32, no. 3 (August 11, 2009): 334–41. http://dx.doi.org/10.1080/01490410903094908.
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Sudre, Joël, Christophe Maes, and Véronique Garçon. "On the global estimates of geostrophic and Ekman surface currents." Limnology and Oceanography: Fluids and Environments 3, no. 1 (February 2013): 1–20. http://dx.doi.org/10.1215/21573689-2071927.
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Zhang, ZiZhan, Yang Lu, and HouTse Hsu. "Detecting surface geostrophic currents using wavelet filter from satellite geodesy." Science in China Series D: Earth Sciences 50, no. 6 (June 2007): 918–26. http://dx.doi.org/10.1007/s11430-007-0038-4.
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Poulain, Pierre-Marie, Milena Menna, and Elena Mauri. "Surface Geostrophic Circulation of the Mediterranean Sea Derived from Drifter and Satellite Altimeter Data." Journal of Physical Oceanography 42, no. 6 (June 1, 2012): 973–90. http://dx.doi.org/10.1175/jpo-d-11-0159.1.
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Abstract Drifter observations and satellite-derived sea surface height data are used to quantitatively study the surface geostrophic circulation of the entire Mediterranean Sea for the period spanning 1992–2010. After removal of the wind-driven components from the drifter velocities and low-pass filtering in bins of 1° × 1° × 1 week, maps of surface geostrophic circulation (mean flow and kinetic energy levels) are produced using the drifter and/or satellite data. The mean currents and kinetic energy levels derived from the drifter data appear stronger/higher with respect to those obtained from satellite altimeter data. The maps of mean circulation estimated from the drifter data and from a combination of drifter and altimeter data are, however, qualitatively similar. In the western basin they show the main pathways of the surface waters flowing eastward from the Strait of Gibraltar to the Sicily Channel and the current transporting waters back westward along the Italian, French, and Spanish coasts. Intermittent and long-lived subbasin-scale eddies and gyres abound in the Tyrrhenian and Algerian Seas. In the eastern basin, the surface waters are transported eastward by several currents but recirculate in numerous eddies and gyres before reaching the northward coastal current off Israel, Lebanon, and Syria and veering westward off Turkey. In the Ionian Sea, the mean geostrophic velocity maps were also produced separately for the two extended seasons and for multiyear periods. Significant variations are confirmed, with seasonal reversals of the currents in the south and changes of the circulation from anticyclonic (prior to 1 July 2007) to cyclonic and back to anticyclonic after 31 December 2005.
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Jin, Shuanggen, Guiping Feng, and Ole Andersen. "Errors of Mean Dynamic Topography and Geostrophic Current Estimates in China’s Marginal Seas from GOCE and Satellite Altimetry." Journal of Atmospheric and Oceanic Technology 31, no. 11 (November 2014): 2544–55. http://dx.doi.org/10.1175/jtech-d-13-00243.1.
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AbstractThe Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) and satellite altimetry can provide very detailed and accurate estimates of the mean dynamic topography (MDT) and geostrophic currents in China’s marginal seas, such as, the newest high-resolution GOCE gravity field model GO-CONS-GCF-2-TIM-R4 and the new Centre National d’Etudes Spatiales mean sea surface model MSS_CNES_CLS_11 from satellite altimetry. However, errors and uncertainties of MDT and geostrophic current estimates from satellite observations are not generally quantified. In this paper, errors and uncertainties of MDT and geostrophic current estimates from satellite gravimetry and altimetry are investigated and evaluated in China’s marginal seas. The cumulative error in MDT from GOCE is reduced from 22.75 to 9.89 cm when compared to the Gravity Recovery and Climate Experiment (GRACE) gravity field model ITG-Grace2010 results in the region. The errors of the geostrophic currents from GRACE are smaller than from GOCE with the truncation degrees 90 and 120. However, when the truncation degree is higher than 150, the GRACE mean errors increase rapidly and become significantly larger than the GOCE results. The geostrophic velocities based on GOCE-TIM4 have higher accuracy and spatial resolution, and the mean error is about 12.6 cm s−1, which is more consistent with the in situ drifter’s results than using GRACE data.
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Usoltsev, Igor I., Talgat R. Kilmatov, and Aleksander N. Vrazhkin. "Analysis and forecast of the surface currents in the Okhotsk Sea on the base of observations on drift of buoys." Izvestiya TINRO 189, no. 2 (June 30, 2017): 131–38. http://dx.doi.org/10.26428/1606-9919-2017-189-131-138.
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Data of observations on drifting buoys in the western Okhotsk Sea are presented. Quasi-stochastic mode of the buoys drift under forcing of atmospheric cyclone is noted. The drift is analyzed jointly with analysis of the wind field and the sea surface satellite altimetry. The buoy drift trajectories are modeled under separate influence of the wind-driven and geostrophic flows. There is concluded that both wind-driven and geostrophic currents at the sea surface should be accounted for forecasting of drift for buoys or any floating objects.
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Roach, Christopher J., Helen E. Phillips, Nathaniel L. Bindoff, and Stephen R. Rintoul. "Detecting and Characterizing Ekman Currents in the Southern Ocean." Journal of Physical Oceanography 45, no. 5 (May 2015): 1205–23. http://dx.doi.org/10.1175/jpo-d-14-0115.1.
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AbstractThis study presents a unique array of velocity profiles from Electromagnetic Autonomous Profiling Explorer (EM-APEX) profiling floats in the Antarctic Circumpolar Current (ACC) north of Kerguelen. The authors use these profiles to examine the nature of Ekman spirals, formed by the action of the wind on the ocean’s surface, in light of Ekman’s classical linear theory and more recent enhancements. Vertical decay scales of the Ekman spirals were estimated independently from current amplitude and rotation. Assuming a vertically uniform geostrophic current, decay scales from the Ekman current heading were twice as large as those from the current speed decay, indicating a compressed spiral, consistent with prior observations and violating the classical theory. However, if geostrophic shear is accurately removed, the observed Ekman spiral is as predicted by classical theory and decay scales estimated from amplitude decay and rotation converge toward a common value. No statistically robust relationship is found between stratification and Ekman decay scales. The results indicate that compressed spirals observed in the Southern Ocean arise from aliasing of depth-varying geostrophic currents into the Ekman spiral, as opposed to surface trapping of Ekman currents associated with stratification, and extends the geographical area of similar results from Drake Passage (Polton et al. 2013). Accounting for this effect, the authors find that constant viscosity Ekman models offer a reasonable description of momentum mixing into the upper ocean in the ACC north of Kerguelen. These results demonstrate the effectiveness of a new method and provide additional evidence that the same processes are active for the entire Southern Ocean.
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Umbert, Marta, Eva De-Andrés, Rafael Gonçalves-Araujo, Marina Gutiérrez, Roshin Raj, Laurent Bertino, Carolina Gabarró, and Jordi Isern-Fontanet. "Surface and Interior Dynamics of Arctic Seas Using Surface Quasi-Geostrophic Approach." Remote Sensing 15, no. 7 (March 23, 2023): 1722. http://dx.doi.org/10.3390/rs15071722.
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This study assesses the capability of Surface Quasi-Geostrophy (SQG) to reconstruct the three-dimensional (3D) dynamics in four critical areas of the Arctic Ocean: the Nordic, Barents, East Siberian, and Beaufort Seas. We first reconstruct the upper ocean dynamics from TOPAZ4 reanalysis of sea surface height (SSH), surface buoyancy (SSB), and surface velocities (SSV) and validate the results with the geostrophic and total TOPAZ4 velocities. The reconstruction of upper ocean dynamics using SSH fields is in high agreement with the geostrophic velocities, with correlation coefficients greater than 0.8 for the upper 400 m. SSH reconstructions outperform surface buoyancy reconstructions, even in places near freshwater inputs from river discharges, melting sea ice, and glaciers. Surface buoyancy fails due to the uncorrelation of SSB and subsurface potential vorticity (PV). Reconstruction from surface currents correlates to the total TOPAZ4 velocities with correlation coefficients greater than 0.6 up to 200 m. In the second part, we apply the SQG approach validated with the reanalysis outputs to satellite-derived sea level anomalies and validate the results against in-situ measurements. Due to lower water column stratification, the SQG approach’s performance is better in fall and winter than in spring and summer. Our results demonstrate that using surface information from SSH or surface velocities, combined with information on the stratification of the water column, it is possible to effectively reconstruct the upper ocean dynamics in the Arctic and Subarctic Seas up to 400 m. Future remote sensing missions in the Arctic Ocean, such as SWOT, Seastar, WaCM, CIMR, and CRISTAL, will produce enhanced SSH and surface velocity observations, allowing SQG schemes to characterize upper ocean 3D mesoscale dynamics up to 400 m with higher resolutions and lower uncertainties.
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Zhang, Xiaolin, and Allan J. Clarke. "On the Dynamical Relationship between Equatorial Pacific Surface Currents, Zonally Averaged Equatorial Sea Level, and El Niño Prediction." Journal of Physical Oceanography 47, no. 2 (February 2017): 323–37. http://dx.doi.org/10.1175/jpo-d-16-0193.1.
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AbstractPrevious work has shown that for large zonal scales and low frequencies, wind-forced sea level, even near the equator, can be described by wind-forced long Rossby waves. In the eastern equatorial Pacific where the interannual wind forcing is small, these waves are essentially locally unforced and propagate westward from the boundary. At the boundary the waves’ sea level is in phase because of geostrophy and no normal flow to the boundary. However, because the waves propagate more slowly with increasing latitude, west of the boundary lag increases as latitude increases. Consequently a northward sea level gradient is like a time derivative, and the zonal geostrophic flow is like a time derivative of the sea level. This implies that the equatorial flow should lead the equatorial sea level by about 9 months on El Niño time scales. However, analysis shows that when dissipation of the large-scale flow is taken into account, this lead is reduced to about 3 months. This lead time is approximately the dissipation time scale of the second vertical mode, which dominates the zonal surface flow. Since the eastern equatorial Pacific sea level ηE is proportional to eastern equatorial thermocline displacement and El Niño, the zonal equatorial flow leads El Niño indices. Analysis also shows that the zonally averaged equatorial Pacific sea level leads El Niño and that this lead is associated with the geostrophic zonal velocity and the long Rossby wave physics.
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Toolsee, Tesha, and Tarron Lamont. "Long-Term Trends and Interannual Variability of Wind Forcing, Surface Circulation, and Temperature around the Sub-Antarctic Prince Edward Islands." Remote Sensing 14, no. 6 (March 9, 2022): 1318. http://dx.doi.org/10.3390/rs14061318.
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In the Southern Ocean, the sub-Antarctic Prince Edward Islands (PEIs) play a significant ecological role by hosting large populations of seasonally breeding marine mammals and seabirds, which are particularly sensitive to changes in the surrounding ocean environment. In order to better understand climate variability at the PEIs, this study used satellite and reanalysis data to examine the interannual variability and longer-term trends of Sea Surface Temperature (SST), wind forcing, and surface circulation. Long-term trends were mostly weak and statistically insignificant, possibly due to the restricted length of the data products. While seasonal fluctuations accounted for a substantial portion (50–70%) of SST variability, the strongest variance in wind speed, wind stress curl (WSC), and currents occurred at intra-annual time scales. At a period of about 1 year, SST and geostrophic current variability suggested some influence of the Southern Annular Mode, but correlations were weak and insignificant. Similarly, correlations with El Niño Southern Oscillation variability were also weak and mostly insignificant, probably due to strong local and regional modification of SST, wind, and current anomalies. Significant interannual and decadal-scale variability in SST, WSC, and geostrophic currents, strongest at periods of 3–4 and 7–8 years, corresponded with the variability of the Antarctic Circumpolar Wave. At decadal time scales, there was a strong inverse relationship between SST and geostrophic currents and between SST and wind speed. Warmer-than-usual SST between 1990–2001 and 2009–2020 was related to weaker currents and wind, while cooler-than-usual periods during 1982–1990 and 2001–2009 were associated with relatively stronger winds and currents. Positioned directly in the path of passing atmospheric low-pressure systems and the Antarctic Circumpolar Current, the PEIs experience substantial local and regional atmospheric and oceanic variability at shorter temporal scales, which likely mutes longer-term variations that have been observed elsewhere in the Southern Ocean.
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Flatau, Maria K., Lynne Talley, and Pearn P. Niiler. "The North Atlantic Oscillation, Surface Current Velocities, and SST Changes in the Subpolar North Atlantic." Journal of Climate 16, no. 14 (July 15, 2003): 2355–69. http://dx.doi.org/10.1175/2787.1.
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Abstract Changes in surface circulation in the subpolar North Atlantic are documented for the recent interannual switch in the North Atlantic Oscillation (NAO) index from positive values in the early 1990s to negative values in 1995/96. Data from Lagrangian drifters, which were deployed in the North Atlantic from 1992 to 1998, were used to compute the mean and varying surface currents. NCEP winds were used to calculate the Ekman component, allowing isolation of the geostrophic currents. The mean Ekman velocities are considerably smaller than the mean total velocities that resemble historical analyses. The northeastward flow of the North Atlantic Current is organized into three strong cores associated with topography: along the eastern boundary in Rockall Trough, in the Iceland Basin (the subpolar front), and on the western flank of the Reykjanes Ridge (Irminger Current). The last is isolated in this Eulerian mean from the rest of the North Atlantic Current by a region of weak velocities on the east side of the Reykjanes Ridge. The drifter results during the two different NAO periods are compared with geostrophic flow changes calculated from the NASA/Pathfinder monthly gridded sea surface height (SSH) variability products and the Advanced Very High Resolution Radiometer (AVHRR) SST data. During the positive NAO years the northeastward flow in the North Atlantic Current appeared stronger and the circulation in the cyclonic gyre in the Irminger Basin became more intense. This was consistent with the geostrophic velocities calculated from altimetry data and surface temperature changes from AVHRR SST data, which show that during the positive NAO years, with stronger westerlies, the subpolar front was sharper and located farther east. SST gradients intensified in the North Atlantic Current, Irminger Basin, and east of the Shetland Islands during the positive NAO phase, associated with stronger currents. SST differences between positive and negative NAO years were consistent with changes in air–sea heat flux and the eastward shift of the subpolar front. SST advection, as diagnosed from the drifters, likely acted to reduce the SST differences.
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Haryanto, Yosafat Donni, Nelly Florida Riama, Dendi Rona Purnama, Nindya Pradita, Saveira Fairuz Ismah, Arief Wibowo Suryo, Muhammad Fadli, Nugroho Dwi Hananto, Shujiang Li, and R. Dwi Susanto. "EFFECT OF MONSOON PHENOMENON ON SEA SURFACE TEMPERATURES IN INDONESIAN THROUGHFLOW REGION AND SOUTHEAST INDIAN OCEAN." Journal of Southwest Jiaotong University 56, no. 6 (December 24, 2021): 914–23. http://dx.doi.org/10.35741/issn.0258-2724.56.6.80.
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Indonesia is influenced by two types of monsoons, namely, the Asian and Australian monsoons. The differences in conditions occurring during these monsoon phenomena can affect sea surface temperatures (SSTs). This study aims to determine the effect of these monsoons on the SSTs in the southeast Indian Ocean and the Indonesian throughflow (ITF) region. SST and geostrophic current data obtained from Copernicus Marine Service and surface wind speed and direction data from the European Center for Medium-Range Weather Forecasts (ECMWF) from March 2019 to February 2020 were statistically and descriptively analyzed. Observational conductivity–temperature–depth (CTD) data obtained in December 2019 were used to identify statistical errors in the Copernicus Marine Service SST data. The results of the SST data verification show a 0.85°C RMSE and 0.6°C MAE; they are significantly correlated at 0.82 with a 95% confidence level. The results of this study generally show that geostrophic currents move to the east, and SST tends to be warmer during the Asian monsoon period than during the Australian monsoon period, which has a cooler SST (with geostrophic currents moving to the northwest). Specifically, the SST conditions in the ITF region and southeast Indian Ocean cool from the MAM period. This cooling period intensifies during the JJA period and subsides in the SON period. The Australian monsoon, which is dominant during the DJF period, causes warmer-than-average SST conditions in the northern part of Indonesia, particularly the northern part of the ITF.
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McPhee, Miles G. "Intensification of Geostrophic Currents in the Canada Basin, Arctic Ocean." Journal of Climate 26, no. 10 (May 8, 2013): 3130–38. http://dx.doi.org/10.1175/jcli-d-12-00289.1.
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Abstract Continuous sampling of upper-ocean hydrographic data in the Canada Basin from various sources spanning from 2003 through 2011 provides an unprecedented opportunity to observe changes occurring in a major feature of the Arctic Ocean. In a 112-km-radius circle situated near the center of the traditional Beaufort Gyre, geopotential height referenced to 400 dbar increased by about 0.3 gpm from 2003 to 2011, and by the end of the period had increased by about 65% from the climatological value. Near the edges of the domain considered, the anomalies in dynamic height are much smaller, indicating steeper gradients. A rough dynamic topography constructed from profiles collected between 2008 and 2011 shows the center of the gyre to have shifted south by about 2° in latitude, along the 150°W meridian. Geostrophic currents are much stronger on the periphery of the gyre, reaching amplitudes 5–6 times higher than climatological values at grid points just offshore from the Beaufort and Chukchi shelf slopes. Estimates of residual buoy drift velocity after removing the expected wind-driven component are consistent with surface geostrophic currents calculated from hydrographic data. A three-decade time series of integrated ocean surface stress curl during late summer near the center of the Beaufort Gyre shows a large increase in downward Ekman pumping on decadal scales, emphasizing the importance of atmospheric forcing in the recent accumulation of freshwater in the Canada Basin. Geostrophic current intensification appears to have played a significant role in the recent disappearance of old ice in the Canada Basin.
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İnan, Timur, and Ahmet Fevzi BABA. "Prediction of geostrophic currents using big weather data archive and neural networks for the Aegean Sea." Global Journal of Computer Sciences: Theory and Research 9, no. 1 (April 30, 2019): 10–20. http://dx.doi.org/10.18844/gjcs.v9i1.4091.
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Prediction of sea and weather environment variables like wind speed, wind direction, wave height, wave direction, sea surface current direction and magnitude has always been an important subject in marine engineering as they effect on ship speed and effect the time of arrival to destination point as well. In this study, we propose a neural network that can predict the latitudinal and longitudinal components of sea surface currents in the Aegean Sea. The system can predict the sea surface currents components using the wind components which are gathered from the INMARSAT weather report system. The neural network is trained using the historical data which is gathered from UCAR historical weather database and historical surface current data which is gathered from IFREMER database.
Keywords: Sea surface current, weather report, prediction, neural network, big data archive.
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Takatama, Kohei, and Niklas Schneider. "The Role of Back Pressure in the Atmospheric Response to Surface Stress Induced by the Kuroshio." Journal of the Atmospheric Sciences 74, no. 2 (February 1, 2017): 597–615. http://dx.doi.org/10.1175/jas-d-16-0149.1.
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Abstract The effect of ocean current drag on the atmosphere is of interest as a test case for the role of back pressure, because the response is independent of the thermally induced modulation of the boundary layer stability and hydrostatic pressure. The authors use a regional atmospheric model to investigate the impact of drag induced by the Kuroshio in the East China Sea on the overlying winter atmosphere. Ocean currents dominate the wind stress curl compared to the impacts of sea surface temperature (SST) fronts. Wind stress convergences and divergences are weakly enhanced even though the ocean current is almost geostrophic. These modifications change the linear relationships (coupling coefficients) between the wind stress curl/divergence and the SST Laplacian, crosswind, and downwind gradients. Clear signatures of the ocean current impacts are found beyond the sea surface: sea surface pressure (back pressure) decreases near the current axis, and precipitation increases over the downwind region. However, these responses are very small despite strong Ekman pumping due to the current. A linear reduced gravity model is used to explain the boundary layer dynamics. The linear vorticity equation shows that the oceanic influence on wind stress curl is balanced by horizontal advection decoupling the boundary layer from the interior atmosphere. Spectral transfer functions are used to explain the general response of back pressure to geostrophic ocean currents and sea surface height.
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Arnault, Sabine. "Tropical Atlantic geostrophic currents and ship drifts." Journal of Geophysical Research 92, no. C5 (1987): 5076. http://dx.doi.org/10.1029/jc092ic05p05076.
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Isern-Fontanet, Jordi, Emilio García-Ladona, José Antonio Jiménez-Madrid, Estrella Olmedo, Marcos García-Sotillo, Alejandro Orfila, and Antonio Turiel. "Real-time Reconstruction of Surface Velocities from Satellite Observations in the Alboran Sea." Remote Sensing 12, no. 4 (February 22, 2020): 724. http://dx.doi.org/10.3390/rs12040724.
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Surface currents in the Alboran Sea are characterized by a very fast evolution that is not well captured by altimetric maps due to sampling limitations. On the contrary, satellite infrared measurements provide high resolution synoptic images of the ocean at high temporal rate, allowing to capture the evolution of the flow. The capability of Surface Quasi-Geostrophic (SQG) dynamics to retrieve surface currents from thermal images was evaluated by comparing resulting velocities with in situ observations provided by surface drifters. A difficulty encountered comes from the lack of information about ocean salinity. We propose to exploit the strong relationship between salinity and temperature to identify water masses with distinctive salinity in satellite images and use this information to correct buoyancy. Once corrected, our results show that the SQG approach can retrieve ocean currents slightly better to that of near-real-time currents derived from altimetry in general, but much better in areas badly sampled by altimeters such as the area to the east of the Strait of Gibraltar. Although this area is far from the geostrophic equilibrium, the results show that the good sampling of infrared radiometers allows at least retrieving the direction of ocean currents in this area. The proposed approach can be used in other areas of the ocean for which water masses with distinctive salinity can be identified from satellite observations.
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Cunningham, Stuart, and Marko Pavic. "Surface geostrophic currents across the Antarctic circumpolar current in Drake Passage from 1992 to 2004." Progress in Oceanography 73, no. 3-4 (May 2007): 296–310. http://dx.doi.org/10.1016/j.pocean.2006.07.010.
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Sánchez-Reales, J. M., M. I. Vigo, S. Jin, and B. F. Chao. "Global Surface Geostrophic Ocean Currents Derived from Satellite Altimetry and GOCE Geoid." Marine Geodesy 35, sup1 (December 1, 2012): 175–89. http://dx.doi.org/10.1080/01490419.2012.718696.
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Alvarez, A., J. Chiggiato, and K. Schroeder. "Mapping sub-surface geostrophic currents from altimetry and a fleet of gliders." Deep Sea Research Part I: Oceanographic Research Papers 74 (April 2013): 115–29. http://dx.doi.org/10.1016/j.dsr.2012.10.014.
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Doglioni, Francesca, Robert Ricker, Benjamin Rabe, Alexander Barth, Charles Troupin, and Torsten Kanzow. "Sea surface height anomaly and geostrophic current velocity from altimetry measurements over the Arctic Ocean (2011–2020)." Earth System Science Data 15, no. 1 (January 12, 2023): 225–63. http://dx.doi.org/10.5194/essd-15-225-2023.
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Abstract. Satellite altimetry missions flying over the ice-covered Arctic Ocean have opened the possibility of further understanding changes in the ocean beneath the sea ice. This requires complex processing of satellite signals emerging from the sea surface in leads within the sea ice, with efforts to generate consistent Arctic-wide datasets of sea surface height ongoing. The aim of this paper is to provide and assess a novel gridded dataset of sea surface height anomaly and geostrophic velocity, which incorporates both the ice-covered and open ocean areas of the Arctic. Data from the CryoSat-2 mission in the period 2011–2020 were gridded at monthly intervals, up to 88∘ N, using the Data-Interpolating Variational Analysis (DIVA) method. To examine the robustness of our results, we compare our dataset to independent satellite data, mooring time series and Arctic-wide hydrographic observations. We find that our dataset is well correlated with independent satellite data at monthly timescales. Comparisons to in situ ocean observations show that our dataset provides reliable information on the variability of sea surface height and surface geostrophic currents over geographically diverse regions of the Arctic Ocean and different dynamical regimes and sea ice states. At all comparison sites we find agreement with in situ observed variability at seasonal to interannual timescales. Furthermore, we find that our geostrophic velocity fields can resolve the variability of boundary currents wider than about 50 km, a result relevant for studies of Arctic Ocean circulation. Additionally, large-scale seasonal features emerge. Sea surface height exhibits a wintertime Arctic-wide maximum, with the highest amplitude over the shelves. Also, we find a basin-wide seasonal acceleration of Arctic slope currents in winter. We suggest that this dataset can be used to study not only the large-scale sea surface height and circulation, but also the regionally confined boundary currents. The dataset is available in netCDF format from PANGAEA at https://doi.org/10.1594/PANGAEA.931869 (Doglioni et al., 2021d).
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Helber, Robert W., Robert H. Weisberg, Fabrice Bonjean, Eric S. Johnson, and Gary S. E. Lagerloef. "Satellite-Derived Surface Current Divergence in Relation to Tropical Atlantic SST and Wind." Journal of Physical Oceanography 37, no. 5 (May 1, 2007): 1357–75. http://dx.doi.org/10.1175/jpo3052.1.
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Abstract The relationships between tropical Atlantic Ocean surface currents and horizontal (mass) divergence, sea surface temperature (SST), and winds on monthly-to-annual time scales are described for the time period from 1993 through 2003. Surface horizontal mass divergence (upwelling) is calculated using surface currents estimated from satellite sea surface height, surface vector wind, and SST data with a quasi-linear, steady-state model. Geostrophic and Ekman dynamical contributions are considered. The satellite-derived surface currents match climatological drifter and ship-drift currents well, and divergence patterns are consistent with the annual north–south movement of the intertropical convergence zone (ITCZ) and equatorial cold tongue evolution. While the zonal velocity component is strongest, the meridional velocity component controls divergence along the equator and to the north beneath the ITCZ. Zonal velocity divergence is weaker but nonnegligible. Along the equator, a strong divergence (upwelling) season in the central/eastern equatorial Atlantic peaks in May while equatorial SST is cooling within the cold tongue. In addition, a secondary weaker and shorter equatorial divergence occurs in November also coincident with a slight SST cooling. The vertical transport at 30-m depth, averaged across the equatorial Atlantic Ocean between 2°S and 2°N for the record length, is 15(±6) × 106 m3 s−1. Results are consistent with what is known about equatorial upwelling and cold tongue evolution and establish a new method for observing the tropical upper ocean relative to geostrophic and Ekman dynamics at spatial and temporal coverage characteristic of satellite-based observations.
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Chu, Peter C. "Technical note: Two types of absolute dynamic ocean topography." Ocean Science 14, no. 5 (September 4, 2018): 947–57. http://dx.doi.org/10.5194/os-14-947-2018.
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Abstract. Two types of marine geoid exist with the first type being the average level of sea surface height (SSH) if the water is at rest (classical definition), and the second type being satellite-determined with the condition that the water is usually not at rest. The differences between the two are exclusion (inclusion) of the gravity anomaly and non-measurable (measurable) in the first (second) type. The associated absolute dynamic ocean topography (referred to as DOT), i.e., SSH minus marine geoid, correspondingly also has two types. Horizontal gradients of the first type of DOT represent the absolute surface geostrophic currents due to water being at rest on the first type of marine geoid. Horizontal gradients of the second type of DOT represent the surface geostrophic currents relative to flow on the second type of marine geoid. Difference between the two is quantitatively identified in this technical note through comparison between the first type of DOT and the mean second type of DOT (MDOT). The first type of DOT is determined by a physical principle that the geostrophic balance takes the minimum energy state. Based on that, a new elliptic equation is derived for the first type of DOT. The continuation of geoid from land to ocean leads to an inhomogeneous Dirichlet boundary condition with the boundary values taking the satellite-observed second type of MDOT. This well-posed elliptic equation is integrated numerically on 1∘ grids for the world oceans with the forcing function computed from the World Ocean Atlas (T, S) fields and the sea-floor topography obtained from the ETOPO5 model of NOAA. Between the first type of DOT and the second type of MDOT, the relative root-mean square (RRMS) difference (versus RMS of the first type of DOT) is 38.6 % and the RMS difference in the horizontal gradients (versus RMS of the horizontal gradient of the first type of DOT) is near 100 %. The standard deviation of horizontal gradients is nearly twice larger for the second type (satellite-determined marine geoid with gravity anomaly) than for the first type (geostrophic balance without gravity anomaly). Such a difference needs further attention from oceanographic and geodetic communities, especially the oceanographic representation of the horizontal gradients of the second type of MDOT (not the absolute surface geostrophic currents).
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Trossman, David S., Lu Anne Thompson, Kathryn A. Kelly, and Young-Oh Kwon. "Estimates of North Atlantic Ventilation and Mode Water Formation for Winters 2002–06." Journal of Physical Oceanography 39, no. 10 (October 1, 2009): 2600–2617. http://dx.doi.org/10.1175/2009jpo3930.1.
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Abstract Lagrangian estimates for ventilation rates in the Gulf Stream Extension using Argo and World Ocean Circulation Experiment/Atlantic Climate and Circulation Experiment (WOCE/ACCE) float data, scatterometer (QuikSCAT) wind stress satellite observations, and altimetric [Archiving, Validation, and Interpretation of Satellite Oceanographic data (AVISO)] sea surface height (SSH) satellite observations from 2002 to 2006 are presented. Satellite winds and estimates of surface geostrophic currents allow the inclusion of the effects of currents on wind stress as well as their impact on the Ekman pumping. The presence of large surface geostrophic currents decreases the total Ekman pumping, contributing up to 20% where the Gulf Stream makes its two sharpest turns, and increases the total Ekman pumping by 10% or less everywhere else. The ageostrophic currents may be as large as 15% of the geostrophic currents, but only in proximity of the Gulf Stream. Using currents and mixed layer depths (MLDs) that are either climatological or vary from year to year, obducted water tends to originate along the Gulf Stream, while subducted water tends to originate to its south. However, using time-varying MLDs for each year, subduction varies significantly, sometimes oppositely from obduction. The 18° Water (EDW) subducts in different locations and is distributed differently each year but tends to be located in the Sargasso Sea. Vertical pumping is the only dominant factor in ventilation closer to the coast where MLDs are shallower and lighter parcels are subducted. Vertical pumping contributes up to 20% of the several hundreds of ventilated meters per year around the Gulf Stream and less elsewhere. Using a temperature- or density-based criterion for estimating the MLDs, especially along the coasts and north of 45°N, obduction estimates differ by up to 25%. The horizontal and temporal structure of the MLDs is the primary factor that controls the tens of sverdrups of ventilation (and a few sverdrups of EDW subduction).
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Ciani, Daniele, Marie-Hélène Rio, Bruno Buongiorno Nardelli, Hélène Etienne, and Rosalia Santoleri. "Improving the Altimeter-Derived Surface Currents Using Sea Surface Temperature (SST) Data: A Sensitivity Study to SST Products." Remote Sensing 12, no. 10 (May 17, 2020): 1601. http://dx.doi.org/10.3390/rs12101601.
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Measurements of ocean surface topography collected by satellite altimeters provide geostrophic estimates of the sea surface currents at relatively low resolution. The effective spatial and temporal resolution of these velocity estimates can be improved by optimally combining altimeter data with sequences of high resolution interpolated (Level 4) Sea Surface Temperature (SST) data, improving upon present-day values of approximately 100 km and 15 days at mid-latitudes. However, the combined altimeter/SST currents accuracy depends on the area and input SST data considered. Here, we present a comparative study based on three satellite-derived daily SST products: the Remote Sensing Systems (REMSS, 1/10 ∘ resolution), the UK Met Office OSTIA (1/20 ∘ resolution), and the Multiscale Ultra-High resolution SST (1/100 ∘ resolution). The accuracy of the marine currents computed with our synergistic approach is assessed by comparisons with in-situ estimated currents derived from a global network of drifting buoys. Using REMSS SST, the meridional currents improve up to more than 20% compared to simple altimeter estimates. The maximum global improvements for the zonal currents are obtained using OSTIA SST, and reach 6%. Using the OSTIA SST also results in slight improvements (≃1.3%) in the zonal flow estimated in the Southern Ocean (45 ∘ S to 70 ∘ S). The homogeneity of the input SST effective spatial resolution is identified as a crucial requirement for an accurate surface current reconstruction. In our analyses, this condition was best satisfied by the lower resolution SST products considered.
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Poulain, Pierre-Marie, Luca Centurioni, Tamay Özgökmen, Daniel Tarry, Ananda Pascual, Simon Ruiz, Elena Mauri, Milena Menna, and Giulio Notarstefano. "On the Structure and Kinematics of an Algerian Eddy in the Southwestern Mediterranean Sea." Remote Sensing 13, no. 15 (August 3, 2021): 3039. http://dx.doi.org/10.3390/rs13153039.
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An Algerian Eddy, anticyclonic vortex generated by the instability of the Algerian Current in the southwestern Mediterranean Sea, is studied using data provided by drifters (surface currents), Argo floats (temperature and salinity profiles), environmental satellites (absolute dynamic topography maps and ocean color images) and operational oceanography products. The eddy was generated in May 2018 and lasted as an isolated vortex until November 2018. Its morphology and kinematics are described in June–July 2018 when drifters were trapped in its core. During that period, the eddy was slowly moving to the NE (~2 km/day), with an overall diameter of about 200 km (slowly growing with time) and maximal surface swirl velocity of ~50 cm/s at a radius of ~50 km. Geostrophic currents derived from satellite altimetry data compare well with low-pass filtered drifter velocities, with only a slight overestimation, which is expected as its maximum vorticity corresponds to a small Rossby number of ~0.6. Satellite ocean color images and some drifters show that the eddy has an elliptical spiral structure. The looping tracks of the drifters trapped in the eddy were analyzed using two statistical methods: least-squares ellipse fitting and wavelet ridge analysis, revealing a typical eccentricity of about 0.5, a wide range of inclination and a rotation period between 3 and 10 days. Clusters of drifters on the northeastern limb of the eddy were also considered to estimate divergence and vorticity. The results indicate convergence (divergence) and downwelling (upwelling) at scales of 20–50 km near the northeastern (northwestern) edge of the eddy, in agreement with the quasi-geostrophic theory. Vertically, the eddy extends mostly down to 250 m depth, with a warm, low-salinity and low-density signature and with geostrophic currents near 50 cm/s in the top layer (down to ~80 m) reducing to less than 10 cm/s near 250 m. Near the surface, colder water is advected into it.
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Chen, Yu-Ru, Jeffrey D. Paduan, Michael S. Cook, Laurence Zsu-Hsin Chuang, and Yu-Jen Chung. "Observations of Surface Currents and Tidal Variability Off of Northeastern Taiwan from Shore-Based High Frequency Radar." Remote Sensing 13, no. 17 (August 30, 2021): 3438. http://dx.doi.org/10.3390/rs13173438.
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A network of high-frequency radars (HFRs) has been deployed around Taiwan. The wide-area data coverage is dedicated to revealing near real-time sea-surface current information. This paper investigates three primary objectives: (1) describing the seasonal current synoptic variability; (2) determining the influence of wind forcing; (3) describing the tidal current field pattern and variability. Sea surface currents derived from HFR data include both geostrophic components and wind-driven components. This study explored vector complex correlations between the HFR time series and wind, which was sufficient to identify high-frequency components, including an Ekman balance among the surface currents and wind. Regarding the characteristics of mesoscale events and the tidal field, a year-long high-resolution surface dataset was utilized to observe the current–eddy–tide interactions over four seasons. The harmonic analysis results derived from surface currents off of northeastern Taiwan during 2013 are presented. The results agree well with the tidal parameters estimated from tide-gauge station observations. The analysis shows that this region features a strong, mixed, mainly semidiurnal tide. Continued monitoring by a variety of sensors (e.g., satellite and HFR) would improve the understanding of the circulation in the region.
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Stramska, Malgorzata, Andrzej Jankowski, and Agata Cieszyńska. "Surface currents in the Porsanger fjord in northern Norway." Polish Polar Research 37, no. 3 (September 1, 2016): 337–60. http://dx.doi.org/10.1515/popore-2016-0018.
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Abstract We describe surface currents in the Porsanger fjord (Porsangerfjorden) located in the European Arctic in the vicinity of the Barents Sea. Our analysis is based on surface current data collected in the summer of 2014 using High Frequency (WERA, Helzel Messtechnik GmbH) radar system. One of our objectives was to separate out the tidal from the nontidal components of the currents and to determine the most important tidal constituents. Tides in the Porsanger fjord are substantial, with tidal range on the order of about 3 m. Tidal analysis attributes to tides about 99% of variance in sea level time series recorded in Honningsvaag. The most important tidal component in sea level data is the M2 component, with amplitude of ~90 cm. The S2 and N2 constituents (amplitude of ~20 cm) also play a significant role in the semidiurnal sea level oscillations. The most important diurnal component is K1 with amplitude of about 8 cm. The most important tidal component in analyzed surface currents records is the M2 component. The second most important component is the S2. Our results indicate that in contrast to sea level, only about 10-30% of variance in surface currents can be attributed to tidal currents. This means that about 70-90% of variance is due to wind-induced and geostrophic currents.
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Benilov, E. S., and P. V. Sakov. "Dynamics of large-amplitude geostrophic flows over bottom topography." Nonlinear Processes in Geophysics 4, no. 1 (March 31, 1997): 55–62. http://dx.doi.org/10.5194/npg-4-55-1997.
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Abstract. We examine the interaction of near-surface and near- bottom flows over bottom topography. A set of asymptotic equations for geostrophic currents in a three-layer fluid is derived. The depths of the active (top/bottom) layers are assumed small, the slope of the bottom is weak, the interfacial displacement is comparable to the depths of the thinner layers. Using the equations derived, we examine the stability of parallel flows and circular eddies. It is demonstrated that eddies with non-zero near-surface component are always unstable; eddies localized in the near-bottom layer may be stable subject to additional restrictions imposed on their horizontal profiles and bottom topography.
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Zhang, Weifeng G., John L. Wilkin, and Robert J. Chant. "Modeling the Pathways and Mean Dynamics of River Plume Dispersal in the New York Bight." Journal of Physical Oceanography 39, no. 5 (May 1, 2009): 1167–83. http://dx.doi.org/10.1175/2008jpo4082.1.
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Abstract This study investigates the dispersal of the Hudson River outflow across the New York Bight and the adjacent inner- through midshelf region. Regional Ocean Modeling System (ROMS) simulations were used to examine the mean momentum dynamics; the freshwater dispersal pathways relevant to local biogeochemical processes; and the contribution from wind, remotely forced along-shelf current, tides, and the topographic control of the Hudson River shelf valley. The modeled surface currents showed many similarities to the surface currents measured by high-frequency radar [the Coastal Ocean Dynamics Applications Radar (CODAR)]. Analysis shows that geostrophic balance and Ekman transport dominate the mean surface momentum balance, with most of the geostrophic flow resulting from the large-scale shelf circulation and the rest being locally generated. Subsurface circulation is driven principally by the remotely forced along-shelf current, with the exception of a riverward water intrusion in the Hudson River shelf valley. The following three pathways by which freshwater is dispersed across the shelf were identified: (i) along the New Jersey coast, (ii) along the Long Island coast, and (iii) by a midshelf offshore pathway. Time series of the depth-integrated freshwater transport show strong seasonality in dispersal patterns: the New Jersey pathway dominates the winter–spring seasons when winds are downwelling favorable, while the midshelf pathway dominates summer months when winds are upwelling favorable. A series of reduced physics simulations identifies that wind is the major force for the spreading of freshwater to the mid- and outer shelf, that remotely forced along-shelf currents significantly influence the ultimate fate of the freshwater, and that the Hudson River shelf valley has a modest dynamic effect on the freshwater spreading.
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THOMAS, P. J., and P. F. LINDEN. "Rotating gravity currents: small-scale and large-scale laboratory experiments and a geostrophic model." Journal of Fluid Mechanics 578 (April 26, 2007): 35–65. http://dx.doi.org/10.1017/s0022112007004739.
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Laboratory experiments simulating gravity-driven coastal surface currents produced by estuarine fresh-water discharges into the ocean are discussed. The currents are generated inside a rotating tank filled with salt water by the continuous release of buoyant fresh water from a small source at the fluid surface. The height, the width and the length of the currents are studied as a function of the background rotation rate, the volumetric discharge rate and the density difference at the source. Two complementary experimental data sets are discussed and compared with each other. One set of experiments was carried out in a tank of diameter 1 m on a small-scale rotating turntable. The second set of experiments was conducted at the large-scale Coriolis Facility (LEGI, Grenoble) which has a tank of diameter 13 m. A simple geostrophic model predicting the current height, width and propagation velocity is developed. The experiments and the model are compared with each other in terms of a set of non-dimensional parameters identified in the theoretical analysis of the problem. These parameters enable the corresponding data of the large-scale and the small-scale experiments to be collapsed onto a single line. Good agreement between the model and the experiments is found.
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Wolff, Jörg-Olaf, and John A. T. Bye. "Drift patterns in an Antarctic channel from a quasi-geostrophic model with surface friction." Annals of Glaciology 27 (1998): 501–6. http://dx.doi.org/10.3189/1998aog27-1-501-506.
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The surface layer of the Southern Ocean is subject to the action of wind, waves and currents. We present solutions from a fine-resolution quasi-geostrophic model with surface friction, which is driven by a specified mean and fluctuating wind field, and predicts the surface current, and also the surface Stokes drift due to the wavefield. The resulting flow patterns control the dispersion of particles at the sea surface, and, using a proven Lagrangian algorithm, batches of particles of specified draught can be injected into the flow at various locations and tracked. The simulated patterns are compared with historical data on dispersion and with drift-card and satellite-drogue studies in the Southern Ocean, iceberg tracking and other studies to show the relative importance of dispersion by synoptic variability in the atmosphere and mesoscale eddies in the ocean.
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Cronin, Meghan F., and William S. Kessler. "Near-Surface Shear Flow in the Tropical Pacific Cold Tongue Front*." Journal of Physical Oceanography 39, no. 5 (May 1, 2009): 1200–1215. http://dx.doi.org/10.1175/2008jpo4064.1.
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Abstract Near-surface shear in the Pacific cold tongue front at 2°N, 140°W was measured using a set of five moored current meters between 5 and 25 m for nine months during 2004–05. Mean near-surface currents were strongly westward and only weakly northward (∼3 cm s−1). Mean near-surface shear was primarily westward and, thus, oriented to the left of the southeasterly trades. When the southwestward geostrophic shear was subtracted from the observed shear, the residual ageostrophic currents relative to 25 m were northward and had an Ekman-like spiral, in qualitative agreement with an Ekman model modified for regions with a vertically uniform front. According to this “frontal Ekman” model, the ageostrophic Ekman spiral is forced by the portion of the wind stress that is not balanced by the surface geostrophic shear. Analysis of a composite tropical instability wave (TIW) confirms that ageostrophic shear is minimized when winds blow along the front, and strengthens when winds blow oblique to the front. Furthermore, the magnitude of the near-surface shear, both in the TIW and diurnal composites, was sensitive to near-surface stratification and mixing. A diurnal jet was observed that was on average 12 cm s−1 stronger at 5 m than at 25 m, even though daytime stratification was weak. The resulting Richardson number indicates that turbulent viscosity is larger at night than daytime and decreases with depth. A “generalized Ekman” model is also developed that assumes that viscosity becomes zero below a defined frictional layer. The generalized model reproduces many of the features of the observed mean shear and is valid both in frontal regions and at the equator.
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Venaille, A. "Bottom-trapped currents as statistical equilibrium states above topographic anomalies." Journal of Fluid Mechanics 699 (April 17, 2012): 500–510. http://dx.doi.org/10.1017/jfm.2012.146.
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AbstractOceanic geostrophic turbulence is mostly forced at the surface, yet strong bottom-trapped flows are commonly observed along topographic anomalies. Here we consider the case of a freely evolving, initially surface-intensified velocity field above a topographic bump, and show that the self-organization into a bottom-trapped current can result from its turbulent dynamics. Using equilibrium statistical mechanics, we explain this phenomenon as the most probable outcome of turbulent stirring. We compute explicitly a class of solutions characterized by a linear relation between potential vorticity and streamfunction, and predict when the bottom intensification is expected. Using direct numerical simulations, we provide an illustration of this phenomenon that agrees qualitatively with theory, although the ergodicity hypothesis is not strictly fulfilled.
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Ho, Chia-Ying, Tien-Hsi Fang, Cheng-Han Wu, and Hung-Jen Lee. "Interplay between Asian Monsoon and Tides Affects the Plume Dispersal of the New Hu-Wei River off the Coast of Midwest Taiwan." Water 14, no. 2 (January 7, 2022): 152. http://dx.doi.org/10.3390/w14020152.
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In the coupled estuary–shelf system, plumes originating from the New Hu-Wei and Choshui rivers, consisting of many terrestrial materials, could contaminate the water of the Mailiao industrial harbor. To determine the contribution of the two rivers to pollution, the interaction between river-forced, tide-generating, and monsoon-driven water motions in and around the Mailiao industrial zone harbor was examined by performing a series of numerical model experiments. We used a three-dimensional general circulation model to examine the interplay between Asian monsoon-driven, river-forced, and tide-induced water motions, one of which could primarily affect the plume. The model-derived results for different river discharges revealed that almost all of the ammonium entering the harbor had a slope-positive trend, with oscillations in response to flood–ebb tidal cycles. The ammonium increased with time and flux, except for the 10 m3/s flux. Although the river discharge flux exceeded 200 m3/s, the ammonium entering the harbor was the same as that of the 200 m3/s flux; the ammonium concentration did not increase significantly with time after the flux exceeded 200 m3/s. In addition, irrespective of flood or ebb tidal currents being suppressed by strong Asian monsoons, this mechanism avoided contaminating the water quality of the harbor while northeasterly winds prevailed. By contrast, the southwesterly monsoon drove the geostrophic current northward along the coast; concurrently, the coastal sea level increased to form the surface isobar slope up toward the coast, producing a secondary flow to accelerate geostrophic alongshore currents. The northward geostrophic currents compressed the plumes shoreward, forming a relatively narrow-band plume; the coupling model demonstrated that the southwesterly monsoon-driven current pushed plumes favorably along the west pier into the harbor.
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Pedlosky, Joseph. "On the Weakly Nonlinear Ekman Layer: Thickness and Flux." Journal of Physical Oceanography 38, no. 6 (June 1, 2008): 1334–39. http://dx.doi.org/10.1175/2007jpo3830.1.
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Abstract The first-order effects of nonlinearity on the thickness and frictionally driven flux in the Ekman layer are described for the case of an Ekman layer on a solid, flat plate driven by an overlying geostrophic flow as well as the Ekman layer on a free surface driven by a wind stress in the presence of a deep geostrophic current. In both examples, the fluid is homogeneous. Particular attention is paid to the effect of nonlinearity in determining the thickness of the Ekman layer in both cases. An analytical expression for the Ekman layer thickness as a function of Rossby number is given when the Rossby number is small. The result is obtained by insisting that the perturbation expansion of the Ekman problem in powers of the Rossby number remains uniformly valid. There are two competing physical effects. The relative vorticity of the geostrophic currents tends to reduce the width of the layer, but the vertical velocity induced in the layer can fatten or thin the layer depending on the sign of the vertical velocity. The regularized expansion is shown to give, to lowest order, expressions for the flux in agreement with earlier calculations.
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Kil, Bumjun, Jerry D. Wiggert, and Stephan D. Howden. "Evidence That an Optical Tail in the Gulf of Mexico After Tropical Cyclone Isaac was the Result of Offshore Advection of Coastal Water." Marine Technology Society Journal 48, no. 4 (July 1, 2014): 27–35. http://dx.doi.org/10.4031/mtsj.48.4.4.
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AbstractThis study investigates the hypothesis of Acker's Web report in 2013 that an optical tail of high chlorophyll a, observed in the open Gulf of Mexico (GoM) approximately 2 weeks after tropical storm Isaac made landfall in coastal Louisiana, was due to advection of outflowing Mississippi River related with the mesoscale eddy field in the open GoM. By using available in situ data and data from multiple satellites, strong evidence was found to support Acker's hypothesis. Drifting buoy, remotely sensed sea surface salinity, and surface geostrophic current data were used to show that low-salinity water (LSW) was indeed associated with the optical tail. Remotely sensed colored dissolved organic matter indicated that the LSW was of coastal origin, and satellite-observed rain rate indicated that this LSW in the optical tail was not due to local precipitation. The path of freshwater from the Mississippi River Delta to the region offshore in the optical tail was shown to be similar to a simulated trajectory estimated by surface geostrophic currents; likewise, the drifting buoys deployed near the shelf break offshore of the Mississippi River Delta prior to the peak in discharge.
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Wijffels, Susan E., Gary Meyers, and J. Stuart Godfrey. "A 20-Yr Average of the Indonesian Throughflow: Regional Currents and the Interbasin Exchange." Journal of Physical Oceanography 38, no. 9 (September 1, 2008): 1965–78. http://dx.doi.org/10.1175/2008jpo3987.1.
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Abstract Twenty years of monthly or more frequent repeat expendable bathythermograph data are used to estimate the mean geostrophic velocity and transport relative to 750 m of the Indonesian Throughflow (ITF) and its partitioning through the major outflow straits into the Indian Ocean. Ekman transports are estimated from satellite and atmospheric reanalysis wind climatologies. A subsurface maximum near 100 m characterizes the geostrophic ITF, but Ekman flows drive a warm near-surface component as well. A subsurface intensified fresh Makassar Jet feeds the Lombok Strait Throughflow (∼2 Sv; 1Sv ≡ 106 m3 s−1) and an eastward flow along the Nusa Tenggara island chain [the Nusa Tenggara Current (6 Sv)]. This flow feeds a relatively cold 3.0-Sv flow through the Ombai Strait and Savu Sea. About 4–5 Sv pass through Timor Passage, fed by both the Nusa Tenggara Current and likely warmer and saltier flow from the eastern Banda Sea. The Ombai and Timor Throughflow feature distinctly different shear profiles; Ombai has deep-reaching shear with a subsurface velocity maximum near 150 m and so is cold (∼15.5°–17.1°C), while Timor Passage has a surface intensified flow and is warm (∼21.6°–23°C). At the western end of Timor Passage the nascent South Equatorial Current is augmented by recirculation from a strong eastward shallow flow south of the passage. South of the western tip of Java are two mean eastward flows—the very shallow, warm, and fresh South Java Current and a cold salty South Java Undercurrent. These, along with the inflow of the Eastern Gyral Current, recirculate to augment the South Equatorial Current, and greatly increase its salinity compared to that at the outflow passages. The best estimate of the 20-yr-average geostrophic plus Ekman transport is 8.9 ± 1.7 Sv with a transport-weighted temperature of 21.2°C and transport-weighted salinity of 34.73 near 110°E. The warm temperatures of the flow can be reconciled with the much cooler estimates based on mooring data in Makassar Strait by accounting for an unmeasured barotropic and deep component, and local surface heat fluxes that warm the ITF by 2°–4°C during its passage through the region.
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Beron-Vera, Francisco J., María J. Olascoaga, and Gustavo J. Goni. "Surface Ocean Mixing Inferred from Different Multisatellite Altimetry Measurements." Journal of Physical Oceanography 40, no. 11 (November 1, 2010): 2466–80. http://dx.doi.org/10.1175/2010jpo4458.1.
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Abstract Two sea surface height (SSH) anomaly fields distributed by Archiving, Validation, and Interpretation of Satellite Oceanographic (AVISO) Altimetry are evaluated in terms of the effects that they produce on mixing. One SSH anomaly field, tagged REF, is constructed using measurements made by two satellite altimeters; the other SSH anomaly field, tagged UPD, is constructed using measurements made by up to four satellite altimeters. Advection is supplied by surface geostrophic currents derived from the total SSH fields resulting from the addition of these SSH anomaly fields to a mean SSH field. Emphasis is placed on the extraction from the currents of Lagrangian coherent structures (LCSs), which, acting as skeletons for patterns formed by passively advected tracers, entirely control mixing. The diagnostic tool employed to detect LCSs is provided by the computation of finite-time Lyapunov exponents. It is found that currents inferred using UPD SSH anomalies support mixing with characteristics similar to those of mixing produced by currents inferred using REF SSH anomalies. This result mainly follows from the fact that, being more easily characterized as chaotic than turbulent, mixing as sustained by currents derived using UPD SSH anomalies is quite insensitive to spatiotemporal truncations of the advection field.
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Mulet, Sandrine, Marie-Hélène Rio, Hélène Etienne, Camilia Artana, Mathilde Cancet, Gérald Dibarboure, Hui Feng, et al. "The new CNES-CLS18 global mean dynamic topography." Ocean Science 17, no. 3 (June 17, 2021): 789–808. http://dx.doi.org/10.5194/os-17-789-2021.
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Abstract. The mean dynamic topography (MDT) is a key reference surface for altimetry. It is needed for the calculation of the ocean absolute dynamic topography, and under the geostrophic approximation, the estimation of surface currents. CNES-CLS mean dynamic topography (MDT) solutions are calculated by merging information from altimeter data, GRACE, and GOCE gravity field and oceanographic in situ measurements (drifting buoy velocities, hydrological profiles). The objective of this paper is to present the newly updated CNES-CLS18 MDT. The main improvement compared to the previous CNES-CLS13 solution is the use of updated input datasets: the GOCO05S geoid model is used based on the complete GOCE mission (November 2009–October 2013) and 10.5 years of GRACE data, together with all drifting buoy velocities (SVP-type and Argo floats) and hydrological profiles (CORA database) available from 1993 to 2017 (instead of 1993–2012). The new solution also benefits from improved data processing (in particular a new wind-driven current model has been developed to extract the geostrophic component from the buoy velocities) and methodology (in particular the computation of the medium-scale GOCE-based MDT first guess has been revised). An evaluation of the new solution compared to the previous version and to other existing MDT solutions show significant improvements in both strong currents and coastal areas.
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Swallow, John, Michèle Fieux, and Friedrich Schott. "The boundary currents east and north of Madagascar: 1. Geostrophic currents and transports." Journal of Geophysical Research 93, no. C5 (1988): 4951. http://dx.doi.org/10.1029/jc093ic05p04951.
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Huang, Chao, and Yongsheng Xu. "Update on the Global Energy Dissipation Rate of Deep-Ocean Low-Frequency Flows by Bottom Boundary Layer." Journal of Physical Oceanography 48, no. 6 (June 2018): 1243–55. http://dx.doi.org/10.1175/jpo-d-16-0287.1.
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AbstractThe global dissipation caused by bottom boundary layer drag is one of the major pathways for the consumption of kinetic energy in the deep ocean. However, the spatial distribution and global integral of the drag dissipation are still debatable. This paper presents an updated estimate of the dissipation rate, using the barotropic component of surface geostrophic currents and 632 in situ velocity measurements. Also, the seafloor roughness is proposed as a parameter of drag efficiency in the parameterized method. The results provide a map of the drag dissipation rate with a global integral of ~0.26 TW. Approximately 66% of this dissipation occurs in the Southern Ocean, which is consistent with the proportion of wind power input into this region. Building upon the work in previous studies on the bottom boundary layer drag, more long-period observations are used, eliminating the influence of the baroclinic contribution to the surface geostrophic currents in the construction of the bottom velocity, and taking topographic roughness into account. The estimates have implications for the maintenance of density structure in the deep ocean and understanding of the kinetic energy budget.
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Ashkenazy, Yosef, and Hezi Gildor. "On the Probability and Spatial Distribution of Ocean Surface Currents." Journal of Physical Oceanography 41, no. 12 (December 1, 2011): 2295–306. http://dx.doi.org/10.1175/jpo-d-11-04.1.
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Abstract Insights into the probability distribution of ocean currents are important for various applications such as the chance to encounter extreme events, which may affect, for example, marine construction, and for estimating the energy that can be extracted from the ocean. In addition, for devising better parameterizations for submesoscale mixing, which present climate models cannot resolve, one should understand the velocity distribution and its relation to the various forcing of surface ocean circulation. Here, the authors investigate the probability distribution of surface currents from the Gulf of Eilat/Aqaba measured by high-frequency radar. Their results show that the distribution of ocean current speeds can be approximated by a Weibull distribution. Moreover, the authors demonstrate the existence of spatial variations of the scale and shape parameters of the Weibull distribution over a relatively small region of only a few kilometers. They use a simple surface Ekman layer model to investigate this spatial variability. They find that, when forced by local winds, this model does not reproduce the observations. The addition of Gaussian noise to the zonal and meridional components of the bottom geostrophic currents has only a slight effect on the surface current distribution. However, noise added to the components of the local wind (mimicking wind gusts) has a much greater effect on the distribution of surface currents, suggesting that wind spatial and temporal variability underlay the observed spatial variability of the parameters of the Weibull distribution.
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Wu, Yang, Xiaoming Zhai, and Zhaomin Wang. "Decadal-Mean Impact of Including Ocean Surface Currents in Bulk Formulas on Surface Air–Sea Fluxes and Ocean General Circulation." Journal of Climate 30, no. 23 (December 2017): 9511–25. http://dx.doi.org/10.1175/jcli-d-17-0001.1.
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The decadal-mean impact of including ocean surface currents in the bulk formulas on surface air–sea fluxes and the ocean general circulation is investigated for the first time using a global eddy-permitting coupled ocean–sea ice model. Although including ocean surface currents in air–sea flux calculations only weakens the surface wind stress by a few percent, it significantly reduces wind power input to both geostrophic and ageostrophic motions, and damps the eddy and mean kinetic energy throughout the water column. Furthermore, the strength of the horizontal gyre circulations and the Atlantic meridional overturning circulation are found to decrease considerably (by 10%–15% and ~13%, respectively). As a result of the weakened ocean general circulation, the maximum northward global ocean heat transport decreases by about 0.2 PW, resulting in a lower sea surface temperature and reduced surface heat loss in the northern North Atlantic. Additional sensitivity model experiments further demonstrate that it is including ocean surface currents in the wind stress calculation that dominates this decadal impact, with including ocean surface currents in the turbulent heat flux calculations making only a minor contribution. These results highlight the importance of properly accounting for ocean surface currents in surface air–sea fluxes in modeling the ocean circulation and climate.