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Piñeiro, S., González-Pola, C., Fernández-Díaz, J. M., Naveira-Garabato, A. C., Sánchez-Leal, R., Puig, P., et al. (2021). Persistent, depth-intensified mixing during the Western Mediterranean Transition's initial stages. Journal of Geophysical Research: Oceans, 126, e2020JC016535. https://doi.org/10.1029/2020JC016535. © 2020. American Geophysical Union. All Rights Reserved., © 2020. American Geophysical Union. All Rights Reserved. Major deep-convection activity in the northwestern Mediterranean during winter 2005 triggered the formation of a complex anomalous deep-water structure that substantially modified the properties of the Western Mediterranean deep layers. Since then, evolution of this thermohaline structure, the so-called Western Mediterranean Transition (WMT), has been traced through a regularly sampled hydrographic deep station located on the outer continental slope of Minorca Island. A rapid erosion of the WMT's near-bottom thermohaline signal was observed during 2005–2007. The plausible interpretation of this as local bottom-intensified mixing motivates this study. Here, the evolution of the WMT structure through 2005–2007 is reproduced by means of a one-dimensional diffusion model including double-diffusive mixing that allows vertical variation of the background mixing coefficient and includes a source term to represent the lateral advection of deep-water injections from the convection area. Using an optimization algorithm, a best guess for the depth-dependent background mixing coefficient is obtained for the study period. WMT evolution during its initial stages is satisfactorily reproduced using this simple conceptual model, indicating that strong depth-intensified mixing (K ∞ (z) ≈ 22 × 10−4 m2 s−1; z ⪆ 1,400 dbar) is a valid explanation for the observations. Extensive hydrographic and current observations gathered over the continental slope of Minorca during winter 2018, the first deep-convective winter intensively sampled in the region, provide evidence of topographically localized enhanced mixing concurrent with newly formed dense waters flowing along-slope toward the Algerian sub-basin. This transport-related boundary mixing mechanism is suggested to be a plausible source of the water-mass transformations observed during the initial stages of the WMT off Minorca., CTM2014-54374-R. BES-2015-074316., Estudio de la anomalía termohalina en las aguas profundas del Mediterráneo Occidental y su relación con las oscilaciones climáticas, ATHAPOC, SI

How flow mixing process influences the wetlands of the Yangtze Estuary is still poorly understood. Hydrological fieldwork was conducted on five vessel-anchored sites (S1-S2; M1-M2-M3) near the major wetlands of the Yangtze Estuary offshore (121°57′-122°30′E, 30°50′-31°23′N) on May 8-11 2004, to examine the dynamics of neap tides in the non-flood season of the Yangtze (∼24, 700 m3s-1 at Datong) and their impacts on adjacent wetlands. Based on the measurement of the acoustic Doppler current profiler (aDcp), direct-reading current meter and optical backscattering sensor (OBS-3A), two flow patterns were revealed: 1) well mixing flow pattern caused by the turbulent tidal currents, and 2) weak mixing flow pattern resulted by the estuarine circulation in the North Port of the estuary. The characteristics of such different flow patterns were analyzed and resultant sediment dispersals were discussed in relation to the adjacent wetlands. It is suggested that the estuarine circulation might be the important process to nourish the eastern wetland of the Yangtze Estuary which has been neglected before.

Observations from five different systems provide a robust picture of the structure and variability of the Antilles Current, an important contributor to the oceanic flux budget, at 26.5°N during 2005–2015. The analysis includes three direct measurement technologies (current meters, shipboard acoustic Doppler current profilers, and lowered acoustic Doppler current profilers) and two geostrophy‐based measurement technologies (conductivity‐temperature‐depth profilers and pressure‐equipped inverted echo sounders). The direct systems are shown to produce weaker, and less variable, Antilles Current transport estimates than the geostrophy‐based systems. The record‐length‐mean geostrophic estimate for the Antilles Current is 4.7 Sverdrups (Sv; 1 Sv = 106 m3/s), and the daily temporal standard deviation is 7.5 Sv. The variations of the Antilles Current transport exceed those of the entire basin‐wide meridional overturning circulation, illustrating the impact of this unusual current. Seasonal variability shows a maximum northward transport in August–September; however, the seasonal component of the variability is weak, and aliasing of higher frequencies is still a problem even with 10.5 years of data. The dominant time scales of variability in the spectra are at 70 and 180 days, and there is indication of westward propagation of Rossby Wave‐like features into the region at a speed of 9 cm/s. There is no significant correlation between the Antilles Current transport variations and those of the Florida Current at 27°N, in phase or at lags/leads of up to 5 years, likely reflecting the varying coastal wave/wall jet time scales for information to pass from the basin interior through the Bahamas Islands.

The Antarctic Slope Current (ASC), defined here as the region of westward flow along the continental slope off Antarctica, forms the southern limb of the subpolar gyres. It regulates the exchange of water across the shelf break, and provides a path for interbasin westward transport. Despite its significance, the ASC remains largely unobserved around most of the Antarctic continent. Here we present direct velocity observations from a 17-month current meter moored array deployed across the continental slope between the 1000 and the 4200 m isobaths, in the southeastern Indian Ocean near 113°E. The observed time-mean flow consists of a surface intensified jet associated with the Antarctic Slope Front (ASF) and a broader bottom intensified westward flow that extends out to approximately the 4000 m isobath and is strongest along the upper slope. The time-mean transport of the ASC is -29.2 Sv. Fluctuations in the transport are large, typically exceeding the mean by a factor of 2. They are mainly due to changes in the northward extent of the current over the lower slope. However, seasonal changes in the wind also drive variations in the transport of the ASF and the flow in the upper slope. Both mean and variability are largely barotropic, thus invisible to traditional geostrophic methods. This article is protected by copyright. All rights reserved.

The eye of Hurricane Isaac passed through the center of an array of six deep water water-column current meter moorings deployed in the northern Gulf of Mexico. The trajectory of the hurricane provided for a unique opportunity to quantify differences in the full water-column oceanic response to a hurricane to the left and right of the hurricane trajectory. Prior to the storm passage, relative vorticity on the right side of the hurricane was strongly negative, while on the left, relative vorticity was positive. This resulted in an asymmetry in the near-inertial frequencies oceanic response at depth and horizontally. A shift in the response to a slightly larger inertial frequencies ∼1.11f was observed and verified by theory. Additionally, the storm passage coincided with an asymmetric change in relative vorticity in the upper 1000 m, which persisted for ∼15 inertial periods. Vertical propagation of inertial energy was estimated at 29 m/d, while horizontal propagation at this frequency was approximately 5.7 km/d. Wavelet analysis showed two distinct subinertial responses, one with a period of 2–5 days and another with a period of 5–12 days. Analysis of the subinertial bands reveals that the spatial and temporal scales are shorter and less persistent than the near-inertial variance. As the array is geographically located near the site of the Deep Water Horizon oil spill, the spatial and temporal scales of response have significant implications for the fate, transport, and distribution of hydrocarbons following a deep water spill event.

Long-term monitoring over one year revealed high temporal variability of gas emissions at a cold seep in 1250 m water depth offshore Vancouver Island, British Columbia. Data from the North East Pacific Time series Underwater Networked Experiment observatory operated by Ocean Networks Canada were used. The site is equipped with a 260 kHz Imagenex sonar collecting hourly data, conductivity-temperature-depth sensors, bottom pressure recorders, current meter, and an ocean bottom seismograph. This enables correlation of the data and analyzing trigger mechanisms and regulating criteria of gas discharge activity. Three periods of gas emission activity were observed: (a) short activity phases of few hours lasting several months, (b) alternating activity and inactivity of up to several day-long phases each, and (c) a period of several weeks of permanent activity. These periods can neither be explained by oceanographic conditions nor initiated by earthquakes. However, we found a clear correlation of gas emission with bottom pressure changes controlled by tides. Gas bubbles start emanating during decreasing tidal pressure. Tidally induced pressure changes also influence the subbottom fluid system by shifting the methane solubility resulting in exsolution of gas during falling tides. These pressure changes affect the equilibrium of forces allowing free gas in sediments to emanate into the water column at decreased hydrostatic load. We propose a model for the fluid system at the seep, fueled by a constant sub-surface methane flux and a frequent tidally controlled discharge of gas bubbles into the ocean, transferable to other gas emission sites in the world's oceans.

We provide the first observational estimate of the circumpolar distribution of cross‐stream eddy diffusivity at 1000 m in the Southern Ocean using Argo float trajectories. We show that Argo float trajectories, from the float surfacing positions, can be used to estimate lateral eddy diffusivities in the ocean and that these estimates are comparable to those obtained from RAFOS floats, where they overlap. Using the Southern Ocean State Estimate (SOSE) velocity fields to advect synthetic particles with imposed behavior that is "Argo‐like" and "RAFOS‐like" diffusivity estimates from both sets of synthetic particles agreed closely at the three dynamically very different test sites, the Kerguelen Island region, the Southeast Pacific Ocean, and the Scotia Sea, and support our approach. Observed cross‐stream diffusivities at 1000 m, calculated from Argo float trajectories, ranged between 300 and 2500 m2 s−1, with peaks corresponding to topographic features associated with the Scotia Sea, the Kerguelen Plateau, the Campbell Plateau, and the Southeast Pacific Ridge. These observational estimates agree with previous regional estimates from the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) near the Drake Passage, and other estimates from natural tracers (helium), inverse modeling studies, and current meter measurements. These estimates are also compared to the suppressed eddy diffusivity in the presence of mean flows. The comparison suggests that away from regions of strong topographic steering suppression explains both the structure and magnitude of eddy diffusivity but that eddy diffusivities in the regions of topographic steering are greater than what would be theoretically expected and the ACC experiences localized enhanced cross‐stream mixing in these regions.

The circum-Pacific trench slope system is one of the most spectacular topographic features of the Earth. We report the relatively frequent occurrence of upslope passages (∼50 times a year) of internal bores in the Japan Trench system. Observations were made using a triaxial accelerometer, a tiltmeter, and a current meter at the seafloor, ∼3400 m deep, during 10 months from 2013 to 2014. We detected 42 tilt events from the records of the accelerometer and confirmed their consistency with the tiltmeter records. The tilt occurred always landward by 1–10 µrad with a rise time on the order of 1000 s and return to the original with a much longer recovery time. The current and temperature records available for the first two events indicated that landward tilting was associated with an upslope current with a speed of several cm/s and a temperature drop of several tens of m °C. The temperature remained cold even after the current had diminished. All of these observations implied the frontal passages of upslope advancing bores, which caused the instrument to be tilted landward. The observational site was located at the trench slope break, separating the trench slope (sloping supercritically or near critically for the M2 internal tide) from the fore-arc basin (sloping subcritically). This unique locality suggested the internal tide origin of the observed bores, although other possibilities could not be precluded. Bores generated on the trench slope may play a role for upslope transportation of suspended sediments stirred up by deep submarine landslides.

Empirically quantifying tidally-influenced river discharge is typically laborious, expensive, and subject to more uncertainty than estimation of upstream river discharge. The tidal stage-discharge relationship is not monotonic nor necessarily single-valued, so conventional stage-based river rating curves fail in the tidal zone. Herein, we propose an expanded rating curve method incorporating stage-rate-of-change to estimate river discharge under tidal influences across progressive, mixed, and standing waves. This simple and inexpensive method requires (1) stage from a pressure transducer, (2) flow direction from a tilt current meter, and (3) a series of ADP surveys at different flow rates for model calibration. The method was validated using excerpts from 12 tidal USGS gauging stations during baseflow conditions. USGS gauging stations model discharge using a different more complex and expensive method. Comparison of new and previous models resulted in good R2 correlations (min 0.62, mean 0.87 with S.D. 0.10, max 0.97). The method for modeling tidally-influenced discharge during baseflow conditions was applied de novo to eight intertidal stations in the Mission and Aransas Rivers, Texas, USA. In these same rivers, the model was further expanded to identify and estimate tidally-influenced stormflow discharges. The Mission and Aransas examples illustrated the potential scientific and management utility of the applied tidal rating curve method for isolating transient tidal influences and quantifying baseflow and storm discharges to sensitive coastal waters.

The regional INDESO model configured in the Indonesian seas from 2008 to 2016 is used to study the mechanisms responsible for the variability of the currents in the Sulawesi Sea of the Indonesian seas. The model simulation compares reasonably with the seasonal to interannual variability of the moored current meter observations in the upper 350m or so of the Maluku Channel during 2015 and 2016. The interannual variability of the currents in the eastern Sulawesi Sea in the model is found to be associated with both the Pacific and Indian Ocean remote forcing. Lag correlation analysis and a theoretical linear wave model simulation suggest that both the equatorial Kelvin waves from the Indian Ocean and the coastally trapped Kelvin waves from the western Pacific along the Philippine coast can propagate through the Indonesian seas and arrive at the Maluku Channel. In particular, from mid-2015 to 2016 the Indian Ocean Kelvin waves are found to significantly impact the sea level anomaly variability in the Maluku Channel. The results indicate the importance of Indo-Pacific planetary waves to the interannual variability of the currents in the Sulawesi Sea at the entrance of the Indonesian seas. Plain Language Summary The Indonesian seas provide a tropical connection for the climate systems over the tropical Pacific and Indian Oceans, the dynamics of which are not clear so far. This study uses a high-resolution computer model to simulate the currents measured by an acoustic current meter attached to a deep ocean mooring in the Maluku Channel of the northeastern Indonesian seas. Based on the successful simulation, the dynamics of the interannual variations of the circulation in the Maluku Channel are studied using the model and a simplified theoretical linear model. The interannual currents and sea level variability in the Maluku Channel are found to be affected by both the Indian Ocean Kelvin waves and the Philippine coastal Kelvin waves. It is found that the Indian Ocean Kelvin waves dominate the sea level variability in the Maluku Channel from mid-2015 to 2016. In early 2015, the sea level variability in the Maluku Channel is dominated by the coastal Kelvin waves from the east Philippine coasts. The results are important for the understanding of the circulation at the entrance of the Indonesian seas and of the Indo-Pacific interactions through the propagation of the planetary waves.

© 2015. American Geophysical Union. All Rights Reserved. Lake Baikal with a depth of 1637 m is characterized by deep water intrusions that bridge the near surface layer to the hypolimnion. These episodic events transfer heat and oxygen over large vertical scales and maintain the permanent temperature stratified deep water status of the lake. Here we evaluate a series of intrusion events that reached the bottom of the lake in terms of the stratification and the wind conditions under which they occurred and provide a new insight into the triggering mechanisms. We make use of long term temperature and current meter data (2000 2013) recorded in the South Basin of the lake combined with wind data produced with a regional downscaling of the global NCEP RA1 reanalysis product. A total of 13 events were observed during which near surface cold water reached the bottom of the South Basin at 1350 m depth. We found that the triggering mechanism of the events is related to the time of the year that they take place. We categorized the events in three groups: (1) winter events observed shortly before the complete ice cover of the lake that are triggered by Ekman coastal downwelling (2) under ice events and (3) spring events that show no correlation to the wind conditions and are possibly connected to the increased spring outflow of the Selenga River. Key Points: Mooring and wind data from Lake Baikal are analyzed to explain deep water renewal Wind and seasonal stratification are important for deep water volume replacements Three different types of deep water renewal events were found in different seasons

Three year long current-meter arrays were deployed in the Malvinas Current at 418S below a satellite altimeter track at about 10 years intervals. Surface geostrophic velocities (SGV) derived from satel- lite altimetric data are compared with the in situ velocities at the upper current meter (􏰁300 m). Multisatel- lite gridded SGV compare better with in situ observations than along-track SGV. In spite of the proximity of the moorings to the complex Brazil-Malvinas Confluence (BMC) region, satellite SGV are significantly corre- lated with the 20 day low-passed in situ velocities (0.85 for along-isobaths velocities, 0.8 for cross-isobaths velocities). The recent in situ measurement period (2014?2015) stands out in the altimetry record with a long-lasting (4 months) high level of eddy kinetic energy at the mooring site and a southernmost location of the Subantarctic Front (SAF). The first two modes of variations of sea level anomaly (SLA) over the BMC remarkably match the first two modes of the low-passed in situ velocities. The first mode is associated with a latitudinal migration of the SAF, and the second with a longitudinal displacement of the Brazil Current overshoot. The two modes dominate the 24 year long record of SLA in the BMC, with energy peaks at the annual and semiannual periods for the first mode and at 3?5 months for the second mode. The SLA over the Southwest Atlantic was regressed onto the two confluence modes of SLA variations and showed remarkable standing wave train like structures in the Argentine Basin. Fil: Ferrari, Ramiro. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; Argentina Fil: Artana, Camila Indira. Universite de Paris VI; Francia Fil: Saraceno, Martin. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; Argentina Fil: Piola, Alberto Ricardo. Ministerio de Defensa. Armada Argentina. Servicio de Hidrografía Naval; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Provost, Christine. Universite de Paris VI; Francia

Satellite observations have revolutionized oceanography, capturing diverse phenomena over much of the globe. However, it remains to understand how surface fields, like sea surface height, reflect the motion occurring at depth. The vertical structure of ocean eddies is often expressed in terms of “baroclinic modes,” which are basis functions derived assuming a flat ocean bottom. Bathymetry alters the modes though, weakening the bottom velocities. Using analytical solutions, we demonstrate that with realistic bathymetry and/or bottom friction, the bottom velocities are nearly zero. The resulting “surface modes” should be ubiquitous in the ocean. This would explain the dominant mode of variability obtained from globally distributed current meter data and is consistent with energy spectra derived from sea surface height data. The results yield a simple way to infer subsurface velocities from satellite data and suggest that ocean analyses should be made in terms of surface modes and topographic waves. This research was originally published in Geophysical Research Letters. © 2017 American Geophysical Union

Theory and numerical simulations suggest that topographic interactions are central to the dynamics of the Antarctic Circumpolar Current (ACC), but few observations are available to test these ideas. We use direct velocity measurements, satellite altimetry, and an ocean state estimate to investigate the interaction of the ACC with the Macquarie Ridge. Satellite altimeter data show that the Subantarctic Front crosses the ridge through a gap immediately north of Macquarie Island. Yearlong current meter records reveal strong deep mean flow (> 0.2 m s−1 at 3000 m) and substantial transport (52 ± 8 × 106 m3 s−1) in the 50 km wide gap. The ACC becomes much more barotropic at the ridge. Acceleration of the deep jet is balanced by the ageostrophic along-gap pressure gradient, convergence of zonal momentum by the mean vertical velocity, and dissipation. The study helps explain how the ACC negotiates large topographic obstacles and highlights the role of local, nonlinear processes in the dynamical balance of the ACC.

International audience; A 20 year long volume transport time series of the Antarctic Circumpolar Current across the Drake Passage is estimated from the combination of information from in situ current meter data (2006–2009) and satellite altimetry data (1992–2012). A new method for transport estimates had to be designed. It accounts for the dependence of the vertical velocity structure on surface velocity and latitude. Yet unpublished velocity profile time series from Acoustic Doppler Current Profilers are used to provide accurate vertical structure estimates in the upper 350 m. The mean cross-track surface geostrophic velocities are estimated using an iterative error/correction scheme to the mean velocities deduced from two recent mean dynamic topographies. The internal consistency and the robustness of the method are carefully assessed.Comparisons with independent data demonstrate the accuracy of the method. The full-depth volume transport has a mean of 141 Sv (standard error of the mean 2.7 Sv), a standard deviation (std) of 13 Sv, and a range of 110 Sv. Yearly means vary from 133.6 Sv in 2011 to 150 Sv in 1993 and standard deviations from 8.8 Sv in 2009 to 17.9 Sv in 1995. The canonical ISOS values (mean 133.8 Sv, std 11.2 Sv) obtained from a year-long record in 1979 are very similar to those found here for year 2011 (133.6 Sv and 12 Sv). Full-depth transports and transports over 3000 m barely differ as in that particular region of Drake Passage the deep recirculations in two semiclosed basins have a close to zero net transport.

Spatial and temporal variations of the semidiurnal tidal currents, especially the baroclinic components, in the deep ocean near a segment of the East Pacific Rise (EPR) are examined using observations from a set of current meter moorings. Over the ridge crest, the baroclinic semidiurnal tidal current is dominated by the zonal component and is coherent along the observed ridge segment. In the direction across the ridge crest, the baroclinic semidiurnal tidal current is not coherent and its magnitude decreases away from the ridge crest. Vertically, the baroclinic semidiurnal tidal current covaries in a few hundred meters near the seafloor. Both the semidiurnal kinetic energy (KE) and shear show significant subinertial temporal variations. By comparing with background forcings, the author found that the subinertial variation of the semidiurnal tides over the ridge crest is related to both the spring-neap cycles and the eddy-induced low-frequency flows in the deep ocean, particularly their cross-ridge components. The observation that the baroclinic tidal currents at the axial stations flowing perpendicularly to the ridge crest suggests that the baroclinic semidiurnal tides at the axial stations are generated locally. It is therefore probable that the subinertial variation of the baroclinic semidiurnal tides near the EPR is due to the eddy-modulated internal tide generation. Since the mesoscale eddies in the eastern tropical Pacific vary with the state of the climate, the observed eddy-modulated internal tide generation connects climate change and variability to the physical and biogeochemical dynamics in the deep ocean and implies an unexplored feedback mechanism potentially affecting the climate system.

[1] Satellite-derived surface height fields reveal that variability in the central and southern basins of the Caspian Sea is correlated with topography. Consistently, empirical orthogonal functions from current meter data from the southern basin are aligned with the isobaths. In addition, the gravest mode, which accounts for over 80% of the variance, has an equivalent barotropic structure in the vertical. To what extent this variability can be modeled using a linear analytical model is examined. The latter assumes equivalent barotropic flow aligned with the geostrophic contours, which in turn are dominated by the topography. With ECMWF winds and ETOPO2 topography, the model yields surface height deviations which are significantly correlated with satellite-derived estimates on seasonal and longer time scales in the central basin. The model is somewhat less successful in the southern basin, where the stratification is stronger. Nevertheless, the results are encouraging, given the extreme simplicity of the model.

[1] We apply several skill tests to assess tidal currents within a three-dimensional, eddy resolving, global ocean circulation model compared to over 5000 observational velocity records spanning 40 years. We examine the skill of the HYbrid Coordinate Ocean Model (HYCOM) on a regional, basin, and global scale and in deep versus shallow water. On a global scale, we examine the model tidal kinetic energy (KE) compared to the tidal KE estimated from the observational velocity records. We examine the vertical structure of the model tidal KE by averaging over predetermined depth bins. We also investigate the ability of the model to satisfy the 95% confidence intervals of the individual tidal ellipse parameters. On a basin scale, we determine if any bias exists in model performance with regards to a particular part of the global ocean and further investigate if any variability of model skill exists within the ocean basins by testing the model against smaller subsets of the observations. Our results show that the skill of the nondata assimilative HYCOM is comparable to the skill of the altimetric-constrained model TPXO7.2. HYCOM is shown to have up to 20% higher skill in resolving the Greenwich phase of the tides on a global basis and demonstrates moderate skill in replicating the vertical structure of the tidal currents as represented by the current meters. HYCOM demonstrates up to 20% higher skill than TPXO7.2 for some ocean basins and some ocean regions but exhibits up to 20% weaker skill in the Southern Ocean.

[1] High resolution high frequency (HF) radar observations of surface currents in the Gulf of Trieste (GoT) are presented and compared to moored subsurface current measurements and to high-resolution simulations from the Northern Adriatic Princeton Ocean Model (NAPOM) oceanographic model. Dominant circulation modes of the GoT were resolved and numerical model capabilities in reconstructing them were assessed. The time frame covers March 2011 through October 2012. NAPOM reconstructs the dominant surface circulation features observed by the radar, such as the general basin-wide cyclonic circulation scheme and the coastal jet outflowing the GoT, but is in general less energetic than radar currents. Comparisons between radar, ADCP, and model currents suggest that the model underestimates originate within the low-frequency, diurnal, semidiurnal, and inertial bands, and that both radar and model currents do not reproduce the diurnal tidal ellipse structure observed by the moored current meter. However, radar-model results for the semidiurnal tides are spatially consistent. Using empirical orthogonal function (EOF) decomposition, the coherent spatial scales and corresponding temporal scales were extracted. Findings suggest that HF radar current observations complement model simulations in regions of enhanced topographic variability where variance of model currents at the surface is distorted by the effects of the sigma layer grid. On the contrary, model results complement radar observations in areas with poor radar coverage, and furthermore provide spatial and temporal continuity of ocean state forecasts.

Deep current meter data and output from two high-resolution global ocean circulation models are used to determine the prevalence and location of strong bottom currents in the greater Agulhas Current system. The two models and current meter data are remarkably consistent, showing that benthic storms, with bottom currents greater than 0.2 m s(-1), occur throughout the Agulhas retroflection region south of Africa more than 20% of the time. Furthermore, beneath the mean Agulhas Current core and the retroflection front, bottom currents exceed 0.2 m s(-1) more than 50% of the time, while away from strong surface currents, bottom currents rarely exceed 0.2 m s(-1). Implications for sediment transport are discussed and the results are compared to atmospheric storms. Benthic storms of this strength (0.2 m s(-1)) are comparable to a 9 m s(-1) (Beaufort 5) windstorm, but scaling shows that benthic storms may be less effective at lifting and transporting sediment than dust storms.

[1] We investigate whether the Quasi-geostrophic (QG) modes and the Surface Quasi-geostrophic (SQG) solutions are consistent with the vertical structure of the subinertial variability off southeast Brazil. The first-order empirical orthogonal function (EOF) of current meter time series is reconstructed using different QG mode combinations; the first EOF is compared against SQG solutions. At two out of three moorings, the traditional flat-bottom barotropic (BT) and first baroclinic (BC1) mode combination fails to represent the observed sharp near-surface decay, although this combination contains up to 78% of the depth-integrated variance. A mesoscale broad-band combination of flat-bottom SQG solutions is consistent with the near-surface sharp decay, accounting for up to 85% of the first EOF variance. A higher-order QG mode combination is also consistent with the data. Similar results are obtained for a rough topography scenario, in which the velocity vanishes at the bottom. The projection of the SQG solutions onto the QG modes confirms that these two models are mutually dependent. Consequently, as far as the observed near-surface vertical structure is concerned, SQG solutions and four-QG mode combination are indistinguishable. Tentative explanations for such vertical structures are given in terms of necessary conditions for baroclinic instability. “Charney-like” instabilities, or, surface-intensified “Phillips-like” instabilities may explain the SQG-like solutions at two moorings; traditional “Phillips-like” instabilities may rationalize the BT/BC1 mode representation at the third mooring. These results point out to the presence of a richer subinertial near-surface dynamics in some regions, which should be considered for the interpretation and projection of remotely sensed surface fields to depth.

The Antarctic Circumpolar Current is an important component of the global climate system connecting the major ocean basins as it flows eastward around Antarctica, yet due to the paucity of data it remains unclear how much water is transported by the current. Between 2007 and 2011 flow through Drake Passage was continuously monitored with a line of moored instrumentation with unprecedented horizontal and temporal resolution. Annual mean near-bottom currents are remarkably stable from year to year. The mean depth-independent, or barotropic transport, determined from the near-bottom current meter records was 45.6 Sv with an uncertainty of 8.9 Sv. Summing the mean barotropic transport with the mean baroclinic transport relative to zero at the seafloor of 127.7 Sv gives a total transport through Drake Passage of 173.3 Sv. This new measurement is 30% larger than the canonical value often used as the benchmark for global circulation and climate models. Fil: Donohue, K. A.. University Of Rhode Island; Estados Unidos Fil: Tracey, K. L.. University Of Rhode Island; Estados Unidos Fil: Watts, D. R.. University Of Rhode Island; Estados Unidos Fil: Chidichimo, María Paz. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Ministerio de Defensa. Armada Argentina. Servicio de Hidrografía Naval; Argentina. Universidad de Buenos Aires; Argentina Fil: Chereskin, T. K.. University of California at San Diego. Scripps Institution of Oceanography; Estados Unidos

On March 11, 2011 at 5:46:23 UTC (March 10 11:46:23 PM Galapagos Local Time), the Mw 9.0 Great East Japan Earthquake occurred near the Tohoku region off the east coast of Japan, spawning a Pacific-wide tsunami. Approximately 12,000 km away, the Galapagos Islands experienced moderate tsunami impacts, including flooding, structural damage, and strong currents. In this paper, we present observations and measurements of the tsunami effects in the Galapagos, focusing on the four largest islands in the archipelago; (from west to east) Isabela, Santiagio, Santa Cruz, and San Cristobal. Access to the tsunami affected areas was one of the largest challenges of the field survey. Aside from approximately ten sandy beaches open to tourists, all other shoreline locations are restricted to anyone without a research permit; open cooperation with the Galapagos National Park provided the survey team complete access to the Islands coastlines. Survey locations were guided by numerical simulations of the tsunami performed prior to the field work. This numerical guidance accurately predicted the regions of highest impact, as well as regions of relatively low impact. Tide-corrected maximum tsunami heights were generally in the range of 3–4 m with the highest runup of 6 m measured in a small pocket beach on Isla Isabela. Puerto Ayora, on Santa Cruz Island, the largest harbor in the Galapagos experienced significant flooding and damage to structures located at the shoreline. A current meter moored inside the harbor recorded relatively weak tsunami currents of less than 0.3 m/s (0.6 knot) during the event. Comparisons with detailed numerical simulations suggest that these low current speed observations are most likely the result of data averaging at 20-min intervals and that maximum instantaneous current speeds were considerably larger. Currents in the Canal de Itabaca, a natural waterway between Santa Cruz Island and a smaller island offshore, were strong enough to displace multiple 5.5-ton navigation buoys. Numerical simulations indicate that currents in the Canal de Itabaca exceeded 4 m/s (~8 knots), a very large flow speed for a navigational waterway.

The distribution of deep near-inertial waves (NIWs) is investigated using data mainly from an array of 46 near-bottom acoustic current meter sensors spanning a 600 km × 600 km region as part of the Kuroshio Extension System Study during 2004–2006. The deep NIW distribution is interpreted in the context of both upper-layer and near-bottom mapped circulations. The wintertime-mean mixed-layer NIW energy input, modeled from observed wind stress, has the same range of values north and south of the Kuroshio Extension in this region. Yet, the wintertime-mean deep NIW energy distribution reveals a sharp factor-of-5 decrease from north to south of the Kuroshio jet. This direct observational evidence shows that the Kuroshio Extension blocks the equatorward propagation of NIWs. The NIW energy that does reach the sea floor within the subset of wintertime observations in the subtropical gyre arrives with patchy spatial and temporal distribution. Elevated NIW energy in deep water is associated with anticyclones in the deep barotropic flow and unassociated with upper layer eddies.

Observations of semidiurnal currents from high-frequency radio Doppler current meters and moored acoustic Doppler current profilers (ADCPs) in the Kauai Channel, Hawaii, are described and compared with two primitive equation numerical models of the tides. The Kauai Channel, separating the islands of Oahu and Kauai, is a site of strong internal tide generation by the barotropic tides flowing over Kaena Ridge, the subsurface extension of Oahu. The nature and impacts of internal tide generation in the Kauai Channel were intensively studied during the 2002–03 near-field component of the Hawaii Ocean Mixing Experiment. Comparisons of observed coherent (i.e., phase locked to the astronomical forcing) M2 and S2 surface currents with model predictions show good agreement for the phases, indicating propagation of internal tides away from the ridge. Although the predicted M2 and S2 surface currents are similar (except for their magnitudes), as expected for internal waves at periods closer to each other (12.4 and 12 h, respectively) than to the inertial period (33 h), the observed M2 and S2 surface currents differ significantly. The S2 kinetic energy pattern resembles the predicted pattern. In contrast, the observed structure and magnitude of the more important M2 kinetic energy pattern differs significantly from the model predictions. The models predict a band of enhanced M2 surface kinetic energy 30–40 km from the ridge axis, corresponding to the first surface reflection of internal tide beams generated on the ridge flanks. The beams are clearly observed by the moored ADCPs, albeit with weaker amplitudes than predicted. Observations at the surface show an area of enhanced kinetic energy that is 10–20 km farther away from the ridge than predicted, with weaker magnitude. Observed M2 surface currents also exhibit apparent seasonal variability, with magnitudes weaker in spring 2003 than in fall 2002. Complex-demodulated semidiurnal currents exhibit significant temporal variability in amplitude and phase, not only because of the interference between semidiurnal constituents (e.g., the spring–neap cycle) but also on shorter and irregular time scales. The result is that ∼20% of semidiurnal energy is incoherent with astronomical forcing. Furthermore, the temporal variability is not spatially coherent; the spatial patterns of semidiurnal kinetic energy resemble those predicted by the numerical models during the strongest spring tides but differ from them at other times. As a result, M2 and S2 kinetic energy patterns phase locked to the astronomical forcing differ from each other. Some features of the observed spatial pattern and amplitude modulations can be qualitatively reproduced by a simple analytical model of the effects of homogeneous barotropic background currents on internal tide beams.

In this paper the origin of the bi-monthly variability in the Mascarene Basin is reconsidered. Free oscillatory modes of the Mascarene Basin are determined by performing normal mode analysis on the motionless solution in a barotropic shallow-water model with realistic bathymetry. Several modes are identified with monthly to bi-monthly time scales. The mode that agrees best with recent current meter observations can be interpreted as a barotropic Rossby basin mode, confined to the tilted geometry of the Mascarene Basin.

In the ocean surface, there are currents generated by ocean winds. Wind driven-currents are nonstationary phenomena temporally and spatially. Wind driven-currents affect coastal currents. Therefore, the grasps of wind-driven currents are important. It is necessary to observe velocity of currents temporally and spatially due to estimate wind-driven currents. However, it is difficult to observe spatial velocity of currents and ocean winds by current meter and anemometer of stationary type. High frequency (HF) radar can be observed spatial velocity of currents. Although ocean winds cannot be measured directly by HF radar, it is possible to estimate using wave height, wave period, and the Sverdrup-Munk-Bretschneider's (SMB) method.In this study, ocean winds were estimated by obtained wave height and period from HF radar and SMB method. In addition, wind-driven current was presumed using the estimated wind and bulk method. Finally, we developed self-contained estimation system of wind-driven system currents using high frequency radar.

Rosemary Bank is a broadly domed and elongate seamount with a diameter of 70 km, occurring in water depths of between 300 and 2300 m, 120 km west of the UK mainland in the northern Rockall Trough. Recent multibeam bathymetry and sub-bottom profiles, together with pre-existing current meter and CTD data, seismic reflection profiles and seabed core samples were examined in order to evaluate past and present bottom-current pathways and processes. The multibeam data image volcanic parasitic cones, concave slide scars and the terraced slopes of the bank. Bottom-current sedimentation is interpreted as producing a drift–moat complex surrounding the entire seamount and including two sediment wave-fields, developed to the west and east of the bank in water depths of 1500–2000 m. The western drift covers an area of over 1000 km2. Sediment waves to the west of the bank are up to 150 m high with wave lengths of 1.5–2 km. Four 100 m deep, 3 km wide, linear depressions, bisect the waves and are interpreted as 25–30 km long extensions of the moat. Seismic reflection profiles show the main phase of drift construction was during the mid-Miocene to Pliocene with the Pliocene to Holocene being an interval of drift maintenance. Cores from sediments draping over and adjacent to the seamount contain sandy and gravelly contourites interbedded with hemipelagites of late Pleistocene to Holocene age. Current meter and CTD data from the western moat indicate Labrador Sea Water flowing northwest, in contrast to the previously assumed anticlockwise circulation pattern around the seamount.

The near seabed currents at Ormen Lange have significant spatial and temporal variability under strong influence by the rather rough topography that was formed by the Storegga sub-sea slide. Analysis of current meter records in a given location exhibit periods with relatively calm current conditions accompanied by periods of fairly strong currents, consisting of successions of intermittent events. During such events the near seabed measurements show currents exceeding 60 cm/s. The forcing mechanisms behind such events vary but it is evident that currents and internal waves generated by atmospheric forcing probably dominates, although internal pressure gradients associated with non-linear internal waves and bores/hydraulic jumps are contributing as well. The development of individual events is to a great extent controlled by the interaction between the density stratification and the sloping seabed. In-situ measurements have been combined with numerical simulations in a number of studies with the objective to increase the understanding of events giving rise to extreme currents and internal pressure gradients close to the seabed at Ormen Lange.

‘Eleven months’ current meter observations from the deep Bay of Biscay were examined for the residual (incoherent internal tidal; icIT) signal, left after harmonic analysis using eight tidal constituents (large-scale barotropic or coherent baroclinic signal) within the semidiurnal band. This residual signal comprised ∼30% of the total tidal kinetic energy and, due to its flat spectral appearance, it was responsible for typically 5–7 days intermittency. Although icIT was part of the red noise internal wave band continuum, it was not attributable to instrumental noise. It consisted of quasi-harmonics at non-tidal harmonic frequencies having amplitudes larger than N2, the third largest semidiurnal tidal constituent. It is suggested that the kinetic energy at these non-tidal frequencies reflects interaction between semidiurnal tidal motions and the slowly varying background conditions.

Thermocouple/thermistor array packages and an in situ gamma detector were deployed in 1994 at two vent sites on the northern Cleft Segment of the Juan de Fuca Ridge. Continuous records of fluid temperatures were obtained in four separate locations over a period of 5.5 months, and these data were supplemented by current meter observations made ∼2.5 km to the south within the axial valley. Temperatures measured at a location of focused high temperature flow showed that: (1) the maximum temperature in the chimney was stable and did not exhibit tide-related variability; (2) temperatures within the chimney wall were variable on time-scales of minutes, indicating rapid shifts in amounts of cold seawater or hot vent fluid flowing across chimney walls; and (3) the stable maximum temperature within the chimney conduit was ∼9°C less than the maximum fluid temperature recorded at the vent site during the same time interval, and thus stable high temperatures within chimneys are not necessarily indicative of the maximum temperature within the vent structure or the hydrothermal system. Time-series records from areas of diffuse flow indicate modulation of temperature and total radioactivity by tidally induced changes in bottom currents. Spectra of the current meter record and temperature records are similar, with spectral peaks observed at 12.4 h, 16–17 h (inertial peak) and 4–5 days. Phases between maxima in current, tide and temperature records are consistent with temperature changes resulting from periodic shifts in currents from north to south, and the subsequent northward or southward advection of warm fluids venting from multiple local sources. Periodic (12.4 h) variability of temperature was also recorded by a thermocouple buried ∼1 cm within one of the deposits and is likely a result of periodic variations in the temperature at the boundary of the highly conductive sulfide deposit. The time-series results presented demonstrate the need for measuring and considering the effects of local currents when investigating causes of temporal variability within seafloor hydrothermal systems.

International audience; In contrast to a long-standing belief, observations in the Antarctic Circumpolar Current (ACC) show that mean velocity vectors rotate with depth, thus suggesting a possible importance of the time-mean flow for the local poleward heat transport. The respective contributions of the eddy and mean flows to the heat flux across the ACC in Drake Passage (DP) are investigated using recently acquired and historical time series of velocity and temperature from a total of 24 current meter moorings and outputs of a high-resolution (1/12°) model with realistic topography. Only 11 out of the 24 depth-integrated eddy heat flux estimates are found to be significant, and they are poleward. Model depth-integrated eddy heat fluxes have similar signs and amplitudes as the in situ estimates at the mooring sites. They are mostly poleward or nonsignificant, with amplitude decreasing to the south. The cross-stream temperature fluxes caused by the mean flow at the moorings have a sign that varies with location and corresponds to the opposite of the vertical velocity estimates. The depth-integrated temperature fluxes due to the mean flow in the model exhibit small spatial scales and are of opposite sign to the bottom vertical velocities. This suggests that the rotation of the mean velocity vectors with depth is mainly due to bottom topography. The rough hilly topography in DP likely promotes the small-scale vertical velocities and temperature fluxes. Eddy heat fluxes and cross-stream temperature fluxes are integrated over mass-balanced regions defined by the model transport streamlines. The contribution of the mean flow to the ocean heat fluxes across the Southern ACC Front in DP (covering about 4% of the circumpolar longitudes) is about four times as large as the eddy heat flux contribution and the sum of the two represent on the order of 10% of the heat loss to the atmosphere south of 60°S.

Two field studies were conducted across and along the continental shelf, one from February to May 1996 (deployment 1) and the other from July to October 1996 (deployment 2) in part to determine the mass and salt budgets of shelf water from south of Cape Henry to north of Cape Hatteras, the southernmost portion of the Mid-Atlantic Bight. The temporal means of current meter records indicated that most of the water enters the region across its northern boundary near the shelf break as part of a southward, alongshore current and exits the southeast corner as a southeastward flowing current. Estimates of the volume transports indicated that not all the transport across the northern boundary was accounted for by transport across the southern boundary and that the remainder occurred as a broad, diffusive flow across the eastern boundary at the shelf break. Time series of volume transport across northern and southern boundaries were very similar and associated with variations in the alongshore wind stress and sea level, indicative of a geostrophic balance. Examination of the individual current meter records indicated these fluctuations were very barotropic even during deployment 2, which included the stratified summer season. Time series of the volume transport across the eastern boundary at the shelf break strongly mirrored the volume transport across the northern boundary minus that across the southern boundary, suggesting that the inferred eastern boundary transport was real and accommodated whatever the southern boundary could not. The turbulent salt flux across each boundary contributes very little to the net salt flux. The mean and time-dependent salt fluxes show nearly identical patterns as the respective mass fluxes because the salt fluxes are almost governed by current velocity fields. The instantaneous and mean salt fluxes across each boundary were very well approximated by the instantaneous and mean volume transports across the boundary times the deployment average salinity across that boundary, respectively. The Ocean Margins Program (OMP) moored current and salinity observations appear sufficient to make estimates of the mean and time-dependent mass and salt balance.

We investigated ocean surface current variability in the region east of Okinawa Island in the spring of 1998 using in situ and remotely sensed data including surface velocity estimates inferred from HF ocean radars. Most previous studies utilizing HF ocean radars were limited to shallow continental shelves. In contrast, most of the observation area in this study was in the open ocean. During the observation period the HF ocean radars sampled a region where anticyclonic and cyclonic mesoscale eddies were adjacent to each other. The HF-radar-derived ocean currents agree with those measured by a current meter. The surface currents were highly variable in both time and space and were affected by the eddy field in the offshore region. The current field was related to the near-surface water temperature. For example, a local temperature rise was often associated with northeastward flows. The surface currents in regions shallower than about 1000 m had a significant correlation with local wind forcing. Our HF ocean radars often detected a strongly convergent zone, which was different in character from those described in previous studies in that it was in the deep ocean. From other data such as National Oceanic and Atmospheric Administration advanced very high resolution radiometer imagery, we concluded that the convergent zone was due to mesoscale eddy fronts.

Hydrographic data about latitudes 30°–32°S in the Pacific Ocean are presented and used together with direct velocity observations from floats and moorings to investigate the spatial distribution of meridional flow and its associated transports. An initial velocity scheme is developed with reference to the water property distributions that is subsequently adjusted to meet constraints on net mass and dissolved silica transports. The resulting circulation is found to be sensitive to a priori scaling assumptions and differs in detail from past estimates of the flow in this region. Additionally constraining the flow with directly measured time averaged estimates of the velocity from floats and a current meter array significantly reduces the uncertainty in the derived flow field and improves consistency between the velocity and the property distributions. The derived circulation includes a nearly closed bottom-to-deep-water overturning circulation of 16–20 Sv, below a subtropical gyre flow of thermocline and surface waters consisting of nearly distinct Tasman Sea (∼30 Sv) and basin interior (∼10 Sv) circulations and (a prescribed) ∼10±5 Sv of net northward flow to supply the Indonesian and Bering Strait throughflows. On the basis of this circulation scheme, the heat flux divergence for the Pacific north of the P6 line is estimated to be 0.75±0.56 pW (ocean gaining heat from the atmosphere) and the net freshwater divergence is estimated to be 0.1±0.1 Sv (ocean gains fresh water via air-sea exchange and runoff). The former is highly dependent on the strength of the Indonesian throughflow; the latter also depends on the intensity of the lateral subtropical gyre circulation, which is constrained by the float data. Combined with estimates from the Indian and Atlantic Oceans, we find the subtropical Southern Hemisphere oceans transport 0.46±0.38 PW poleward, less than half the poleward heat transport achieved by the subtropical oceans in the Northern Hemisphere.

Natal Pulses are intermittent, solitary meanders on the trajectory of the otherwise remarkably stable northern Agulhas Current. They play disparate roles in the process of inter-ocean exchange. They have been thought to trigger the spawning of Agulhas Rings at the Agulhas Retroflection, but also to generate an upstream retroflection that prevents Agulhas water from reaching the inter-ocean boundary. For the Natal Pulse to be such a control it has to extend to considerable depths. We present the first hard evidence that demonstrates that the Natal Pulse is indeed an inherent property of the Agulhas Current throughout its full depth. Our data comprise Eulerian current meter observations and Lagrangian float trajectories in combination with sea-surface height and sea-surface temperature data. The results reveal the trapping of water within the Natal Pulse, its southward advection at a phase speed of about 11-12 cm s - 1, and rotation periods of 6 days.

A current meter mooring was deployed in the eastern Indonesian archipelago, in the Maluku Sea, between the islands of Sulawesi and Halmahera, to contribute to estimates of the volume of the Indonesian Throughflow, for 13 months (June 1993-July 1994), with meters located at 740, 1250, 1750, and 2240 m below the surface. Hydrocasts for salinity and temperature were made at the time of deployment. The current meter and hydrographic data indicate a predominantly southward flow at the upper three meters throughout the year and a steady northward flow at the bottom meter. T-S and oxygen properties below 1800 m indicate a clockwise circulation in the Maluku Sea, with a vertical velocity of about 2 m/d. If the currents at the mooring, which is located on the western side of the Maluku Sea, are typical of the entire basin width, the average southward through flow between 740 m and 1500 m below the surface, 7 Sv (1 Sv = 106 m3/s ), is comparable in magnitude with estimates of upper layer transport and should be accounted for in estimates of the Indonesian Throughflow.

The purpose of this paper is to familiarize readers with major results and findings of the biggest multi-institutional Soviet hydrophysical experiment, "Megapolygon- 87," conducted in the northwestern Pacific Subarctic Frontal Zone in June-October 1987. Six large-scale, high-spatial-resolution conductivity-temperature-depth (CTD) surveys (1653 profiles) and numerous moorings (an array of 177 buoys deployed within a 500 km 3 500 km area) with current meters at four depths provided unique observations of frontal structure, mesoscale and submesoscale eddies, and their evolution and interaction with each other. We discuss errors induced in the velocity records by surface waves and show how they can be corrected to a reasonable value. Joint analysis of current meter and CTD data allowed detection of a number of interesting processes and objects in the study area. Among them are meandering of the Subarctic Front around its branching point with formation of chains of cyclonic and anticyclonic mesoscale eddies and strong baroclinicity of velocity fields with significant difference between the thermocline and abyssal flows. Statistical characteristics of corrected "Megapolygon-87" velocities are compared with preceding measurements in the region.

A mooring with current meters at 400, 700, and 900 m at the 950-m isobath south of the 700-m-deep sill across the Halmahera Sea revealed many signals between June 1993 and July 1994. Strong tidal currents of 50 cm/s dragged the mooring down by as much as 80 m on occasions when the lunar perigees corresponded with new or full moons. At 400 m the nontidal currents were southward at up to 25 cm/s from October to April and northwestward at up to 20 cm/s at other times. At 700-m depth there was a near-continuous nontidal southward flow of 9 cm/s across the sill into the Halmahera Basin, which accords with findings by earlier Dutch and Indonesian workers. The current meter at 900-m depth showed the nontidal flow to be weak (∼1 cm/s) to the west. The southward transport between 350 and 700 m was inferred to reach a maximum of 5 Sv during the NW monsoon, with the annual mean being 1.5 Sv. There was a spring-neap effect on the nontidal currents at 400 m that was most pronounced in the last few months of the mooring's life: these currents changed from ∼10 cm/s to the east during neap tides to ∼20 cm/s to the NNW during spring tides. Temperature and salinity profiles suggest that the waters of the Halmahera Sea are derived in part from the New Guinea Coastal Undercurrent.

Along-shelf transports across three cross-shelf lines on the continental shelf near Cape Hatteras have been calculated from moored current meter data over a continuous 24 month period in 1992–1994. The along-shelf convergence has been used to infer off-shelf export. Transport and transport convergence have been related to wind and Gulf Stream forcing and to variability in sea level at the coast. The along-shelf transport variability is primarily wind-driven and highly correlated with sea level fluctuations at the coast. Both winds and along-shelf transport exhibit a near-annual period variability. Along-shelf transport is not well correlated with Gulf Stream offshore position. Along-shelf transport convergence is highly correlated with Gulf Stream position offshore, with a more shoreward Gulf Stream position leading increased along-shelf convergence by hours to a few days. Long-period variability of 14–16 months and 1–3 months is apparent in both Gulf Stream position and transport convergence. Variability in along-shelf convergence is poorly correlated with wind, wind convergence, or coastal sea level. A likely hypothesis accounting for the observed relationship between Gulf Stream position and along-shelf transport convergence is that the Gulf Stream is directly influencing cross-shelf export processes along the outer boundary of the study site. Despite predominantly convergent flow on the shelf at Cape Hatteras, brief periods of along-shelf divergence and shoreward cross-shelf transport exist (∼10% of the time just north of Cape Hatteras and ∼34% of the time just south of Cape Hatteras during episodes of up to 3–8 days duration). Implied onshore flows of a few cm s−1 are tentatively identified in the moored current meter data for these periods. Satellite imagery for an extended along-shelf divergent period clearly suggests that shelf edge parcels could be advected a significant fraction of the way across the shelf.

The distribution and movement of chloropigments (chlorophylls and associated degradation products) in the bottom boundary layer near Duck, North Carolina, were examined during July and August 1994. Time series of chloropigment fluorescence, current velocity, and surface wave properties were acquired from instruments mounted on a bottom tripod set at 20 m depth. These data were combined with moored current meter measurements, meteorological data, and shipboard surveys in a comparative assessment of physical processes and chloropigment distribution over a wide range of temporal and spatial scales. Two dominant scales of chloropigment variation were observed. On numerous occasions, small-scale (order m) structure in the near-bottom fluorescence field was observed, even in the absence of identifiable structure in the temperature and salinity fields. Over larger time-scales and space scales, variations in fluorescence were related to changes in water mass properties that could be attributed to alternating events of upwelling and downwelling. This view was reinforced by shipboard measurements that revealed correlations between fluorescence and hydrographic fields, both of which were modified by wind-forced upwelling and downwelling and by the advection of low-salinity water from Chesapeake Bay. Local resuspension of sediments did not contribute appreciably to the near-bottom pigment load seen at the tripod, because of low bottom stress. Estimates of chloropigment flux indicated a net shoreward transport of chloropigments in the lower boundary layer. However, the rapid fluctuations of currents and pigment concentrations gave rise to large and frequent variations in chloropigment fluxes, generating uncertainty in extrapolations of this finding to longer timescales.

A meander-formation mechanism of the Kuroshio that involves anticyclonic eddies propagating from the Kuroshio Extension, is presented using the analysis of observations of the TOPEX/POSEIDON (T/P) altimeter data and in-situ data. Support of the mechanism comes also from a numerical simulation. In October 1997, a current meter mooring over the Izu Ridge at 500-m-depth captured large velocity fluctuations of about 50 cm s−1 that were associated with a strong anticyclonic eddy. The observations from T/P show that the anticyclonic eddy propagated westward and collided with the Kuroshio off Kyushu. At collision, the anticyclonic eddy coupled with a cyclonic circulation accompanied by a small meander and formed a vortex pair. They were then advected downstream and finally formed a large-meander-like path off the south coast of Honshu. The meander lasted for a few months. Subsequently, the anticyclonic eddy was detached, and the meander grew smaller in amplitude. Numerical experiments successfully simulate this interaction between the Kuroshio and anticyclonic eddies.

The large scale circulation of the WOCE hydrographic section SR3 between Tasmania and Antarctica is studied through inversion of hydrographic, wind stress and current meter data acquired between 1993 and 1996. These data are assimilated within the framework of a steady state model that is based on hydrostatics, geostrophy, tracer, and potential vorticity conservation. Posterior variances of the major current systems are estimated via the implicit inversion of the Hessian matrix, associated with the assimilation scheme. Analysis of the residuals shows that averaged water mass properties are conserved and that most of the adjustment by the solution is through the uniform vertical displacement of density surfaces within the water column. The inclusion of the tracer and potential vorticity constraints allows parts of the large-scale vertical and meridional circulation to be determined. The upwelling over the Antarctic Divergence is driven mainly by the winds, while the downwelling in the Sub-antarctic Front (SAF) is shown to result from the meridional gradients of the density field caused by the mean Antarctic Circumpolar Current. The shallow meridional and vertical circulation in the SAF is along density surfaces and follows the salinity minimum waters to a depth of 500 dbar. This result suggests a possible mechanism for ventilation of Antarctic Intermediate Water. The total volume transport between Tasmania and Antarctica is estimated as 151±50 Sv, with variations between repeat measurements for this section of about 10 Sv. The cross-section circulation pattern can be qualitatively subdivided into five persistent features and their transports and errors are given.

Between September 1991 and March 1994 the oceanic region off the east coast of Australia at 30°S was the subject of an intense observational program Part of the World Ocean Circulation Experiment, the objective was to improve our understanding of the dynamics of the East Australian Current (EAC) and to measure its volume and energy flux. The two main components were the Pacific Current Meter Array 3 (PCM3) with six moorings spanning a total distance of 120 km, and repeat, high-resolution hydrographic surveys. The long-term average fromPCM3 shows the EAC as a narrow, swift, and highly variable current centered at 40 km from the coast with strong shear in the upper 1000 m and mean southward velocities of 0.6 m s−1 in the core. The measurements also revealed a thick countercurrent underneath the poleward flow. The mean volume transport for the entire section covered by the array (between the coast and 154.4°E) was 22.1±4.6 Sv toward the south with an rms variability of 30 Sv. Combining the data from the PCM3 array with the hydrographic sections generated nine detailed snapshots of the absolute geostrophic current field. The snapshots were used to evaluate the reliability of the PCM3 data for determining volume transports. The rms difference between transports derived from direct current observations and from hydrographie data was 3.8 Sv, indicating that the PCM3 array can be used to describe the current variability despite the sparse spatial distribution of velocity measurements. Transport variability was dominated by periods between 90 and 140 days, which are attributed to eddy shedding in the region. Occasional intense northward flow events were observed where total transports reached up to 50 Sv northward. On those occasions the southward boundary current changed from being surface intensified and mainly baroclinic to northward and more barotropic, related to the northward migration past the array of the EAC coastal separation point.

Two three-dimensional finite element models, one diagnostic and one semiprognostic, are used to simulate summer and winter currents for the western continental margin of Vancouver Island, British Columbia. The calculations are forced (or nudged) with seasonal winds, climatological density fields, and elevation-specified boundary conditions that have been adjusted via inversion to more accurately represent the California Undercurrent and estuarine flow in Juan de Puca Strait. Tides are included in the simulations in order to correctly represent turbulent mixing, bottom friction, and the contribution of tidal rectification. The seasonal model currents are shown to compare favorably with multiyear, low-pass filtered current meter observations and to capture strong shears both vertically in Juan de Fuca Strait and horizontally and vertically across the continental shelf and slope. The tidal currents are more accurate than those computed with a previous three-dimensional barotropic model.

The region off Adelie Land is considered as one of the sources of Antarctic Bottom Water. Hydrographic observations were carried out during two cruises in December 1994 and January 1995 and January and February 1996 in this region. Vertical sections along 140°E show that bottom water is colder and fresher than the water above. This bottom water also has higher dissolved oxygen and lower silicate concentrations. The saline bottom water that originated from Ross Sea is not found in these hydrographic data obtained west of 142°E. Current meter moorings were also carried out at three locations on the continental slope in this region. At one of these moorings (139°59′E, 65°10′S, 2665 m deep), data were successfully obtained from January 1995 to March 1996. Three current meters were deployed at 1075, 1778, and 2632 m deep in this mooring. The data show that the average current speed at the lower current meter is 16.2 cm s−1, and it is about 3 times larger than those at the upper two current meters. Also, variability of speed and temperature is largest at the lower current meter. In addition, seasonal variability of speed and temperature is evident only at this current meter. From August to December, speed is larger by 5.7 cm s−1 and temperature is lower by 0.27°C. Also, their variability is larger during the same period. This seasonal variability observed near the bottom suggests seasonal variability of bottom water formation in this region.

Moored observations of currents and temperatures made in the upper 600 rn on eddy-resolving scales over a 2-year period are used to examine the spatial and temporal characteristics of the California Current mesoscale circulation. The observations were made at three principal longitudes: 124oW, 126oW, and 128oW in the vicinity of Point Arena. They bracket the 600-km-wide band of high mesoscale variability found along the eastern boundary of the North Pacific. At all locations, the mesoscale variability was larger than the mean flow, and the spatial modes of variability as determined from empirical orthogonal function analysis consisted of an alongshore mode, a cross-shore mode, and a rotational mode. Observations made near the continental slope (124oW) were dominated by the poleward flowing California Undercurrent, with mesoscale eddies and meanders superposed. The nearshore eddy kinetic energy peaked in a band centered around 60 days. Observations made at 128oW, near the offshore boundary between the energetic mesoscale band and the "eddy desert" of the northeast Pacific, were characterized by small means, fewer eddy events, and a peak in eddy kinetic energy at 120-180 days. The good horizontal resolution of the current meter arrays allowed us to estimate the relative vorticity, horizontal divergence, and Rossby number and therefore to evaluate the relative strength and occurrence of anticyclones and cyclones. We found the mesoscale eddy field to be strongly nonlinear, with Rossby numbers ranging from 0.1 to 0.5. All of the eddies observed at the offshore site were nonlinear, deep, warm anticyclones. Shipboard hydrography revealed the origin of one of these anticyclones to be the California Undercurrent, and this eddy retained its strong anomalies after several months and several hundred kilometers of propagation. Despite the lower incidence of eddies as one moves west from the coast, the eddies that we observed offshore provide evidence for propagation and transport of properties from the coast to the central North Pacific across the California Current System.

Observations of temperature and salinity distributions and currents have been combined with numerical simulations to investigate the response of a coastal embayment to atmospheric cooling during winter. The field experiment, including current meter mooring, conductivity-temperature-depth (CTD) surveys, and weather monitoring, was carried out in Jervis Bay, New South Wales, Australia, during the period of July to October 1996. The bay is small enough that a synoptic CTD survey can be achieved over a period of 6 hours but still large enough that Coriolis effects are important. During a cooling event, vertical convection and surface wind stress combined to produce a well-mixed water column. Continued cooling produced cold, dense water in the shallow regions of the bay and could be identified as a tongue of cold bottom water flowing out of the bay onto the adjacent shelf. The cold outflow produced a surface inflow to the bay of warmer shelf water, causing the bay waters to restratify. The response has been modeled using the three-dimensional Princeton Ocean Model with a prescribed surface heat flux based on meteorological observations. Following a period of cooling, the model produced a stronger anticyclonic gyre at the surface and a weaker cyclonic gyre nearer the bottom. As the cold bottom water flowed out of the bay, warm shelf water entered at the surface, and the anticyclonic gyre was replaced by two counter rotating gyres: cyclonic in the northern half and anticyclonic in the southern half of the bay. In order to achieve a quantitative agreement between the model and observations, including the restratification following the cooling event and flow reversal associated with the change from anticyclonic to cyclonic circulation in the northern half of the bay, the surface heat fluxes needed to be artificially increased to compensate for excessive mixing in the model. The model results predicted a flushing time of order 1 week, depending on the duration and magnitude of the surface cooling and the initial conditions in the bay.

The subsurface oceanic circulation is an important part of the Earth climate system. Subsurface currents traditionally are inferred indirectly from distributions of temperature and dissolved substances, occasionally supplemented by current meter measurements. Neutrally-buoyant floats however, now enable us to obtain for the first time directly measured intermediate depth velocity fields over large areas such as the western South Atlantic. Here, our combined data set provides unprecedented observations and quantification of key flow patterns, such as the Subtropical Gyre return flow (12 Sv; 1 Sverdrup = 10(6)m(3)s(-1)), its bifurcation near the Santos Plateau and the resulting continuous narrow and swift northward intermediate western boundary current (4 Sv). This northward flowing water passes through complex equatorial flows and finally enters into the North Atlantic.

We present a set of current meter measurements collected in the southwest Atlantic from December 1993 to June 1995. Four moorings were set out to monitor the Malvinas (Falkland) Current at 40°–41°S, and one mooring was located within the Brazil-Malvinas Front area at 38°S. The mean flow of the Malvinas Current has an equivalent barotropic structure, whereas the vertical structure of the variable part of the flow is dominated by a barotropic-like empirical mode for regions shallower than 2500 m, surface intensified, which appears as a linear combination of barotropic and baroclinic modes, suggesting mode coupling due to the steep topography. This mode appears to gradually become nearly exactly barotropic toward shallower depths. Higher empirical modes, bottom intensified and associated with higher frequencies, suggest waves trapped by the conjugated effects of topography and stratification. The time variability is discussed in terms of along- and cross-isobath flows. The along-shelf flow features substantial energy in the 50 to 70-day band. There is no evidence of an annual cycle nor of a distinct semiannual cycle, but there is a suggestion of significant energy at periods a little shorter (135 days). The cross-shelf variability is important, with a quite clear annual cycle, associated with excursions of the subantarctic front. Its phase coincides with that of the Brazil Current transport, suggesting that annual migrations of the front are predominately determined by the strength of the latter, whereas the intensity of the Malvinas Current would only have marginal impact, a conclusion that may not hold at other timescales.