Eddy-like Features Research Articles

Typhoon-Triggered Phytoplankton Bloom and Associated Upper-Ocean Conditions in the Northwestern Pacific: Evidence from Satellite Remote Sensing, Argo Profile, and an Ocean Circulation Model

A long-lasting phytoplankton bloom, characterized by high chlorophyll-a (Chl-a) concentrations in an eddy-like feature, was detected in MODIS satellite imagery of the northwestern Pacific following the passage of Typhoon Krosa in August 2019. Satellite datasets, Argo measurements, and regional ocean models were analyzed to determine the occurrence and abundance of high-Chl-a concentrations and the upper-ocean conditions associated with them before and after the passage of the typhoon. Remote sensing data revealed that the typhoon triggered sharp increases in surface Chl-a concentrations more than five times the pre-typhoon average, which lasted for two weeks. The elevated post-typhoon concentrations coincided with a pre-existing oceanic cyclone that was detected as an altimetry-based sea surface height anomaly. The typhoon looped around the oceanic cyclone and lingered for two days at slow speeds (less than 2 m/s), producing an unusual sea-surface cooling of up to approximately 9 °C in the cyclonic eddy region. Our model successfully captured the typhoon-induced cold-core cyclonic circulation, which corresponded to the region of high Chl-a concentration. Model–data comparisons revealed that the looping motion of the slow-moving typhoon enhanced the pre-existing cyclonic circulation, resulting in strong vertical mixing and upwelling, consequently initiating a phytoplankton bloom due to increased nutrient supply to the euphotic zone.

Strong vorticity fluctuations and antiferromagnetic correlations in axisymmetric fluid equilibria

The macroscale structure and microscale fluctuation statistics of late-time asymptotic steady state flows in cylindrical geometries is studied using the methods of equilibrium statistical mechanics. The axisymmetric assumption permits an effective two-dimensional description in terms of the (toroidal) flow field $\sigma$ about the cylinder axis and the vorticity field $\xi$ that generates mixing within the (poloidal) planes of fixed azimuth. As for a number of other 2D fluid systems, extending the classic 2D Euler equation, the flow is constrained by an infinite number of conservation laws, beyond the usual kinetic energy and angular momentum. All must be accounted for in a consistent equilibrium description. It is shown that the most directly observable impact of the conservation laws is on $\sigma$, which displays interesting large-scale radius-dependent flow structure. However, unlike in some previous treatments, we find that the thermodynamic temperature is always positive. As a consequence, except for an infinitesimal boundary layer that maintains the correct (conserved) value of the overall poloidal circulation, the impact on $\xi$ resides in the statistics of the strongly fluctuating, fine-scale mixing, where it is sensitive to `antiferromagnetic' microscale correlations that help maintain the analogue of local charge neutrality. The poloidal flow is macroscopically featureless, displaying no large scale circulating jet- or eddy-like features (which typically emerge as negative temperature states in analogous Euler and quasigeostrophic equilibria).

Lagrangian Approach for a New Separation Index of the East Korea Warm Current

In this study, a separation index for the East Korea Warm Current (EKWC) is defined by Lagrangian particle tracking using surface geostrophic currents based on satellite-observed absolute dynamic topography. After determining the main stream from the inflow through the western channel of the Korea Strait, the separation index is obtained by averaging the latitude of the main particle trajectories crossing the meridian along 130.5°E. Although the separation index is obtained from coastal velocity, it describes well the surface circulation patterns in the Ulleung Basin. When it is low (i.e., the EKWC separates at low latitude), the flow toward the Japanese coast is the strongest and the EKWC is weak. In the middle phase of the separation index, a large portion of the EKWC meanders and forms the Ulleung Warm Eddy-like feature. When the separation index is high, the EKWC stretches further northward to almost 40°N. Therefore, the separation index could be used as a simple representation of the surface circulation pattern in the East/Japan Sea.

Lagrangian Coherent Structures in a coastal upwelling environment

A unique spatiotemporal perspective of evolving surface currents off the northern California coast is provided with Lagrangian Coherent Structures (LCSs) determined from attracting Finite-Time Lyapunov Exponents (FTLEs). The FTLEs are calculated from hourly 2-D surface current velocities obtained with HF radars. Attracting FTLE field maxima can identify confluence and shear in flows which can be useful in mapping dynamics associated with fronts. FTLE and SST fields are compared during three time periods in 2009: late March, early September, and late September. During strong upwelling-favorable winds in late March the FTLE and SST fields were not strongly related indicating that frontal dynamics were not strongly influencing surface circulation. Exceptions to this occurred when FTLE ridges calculated from a shorter integration period captured the evolution of a cold water filament and when a FTLE ridge migrated offshore along with an upwelling front. During the two September cases an improved relationship between the FTLE and SST fields was found although occasionally they became shifted. The shifts occurred when the FTLE integration period spanned backwards in time through periods of relaxed and upwelling-favorable winds. This suggests that frontal dynamics captured by LCSs during relaxed winds can subsequently become advected after the onset of wind-forcing by a surface mixed layer decoupled from stably stratified water below the surface mixed layer. Additionally, the LCSs were found to be useful in mapping the origin and destination of surface trajectories, the confluence associated with a persistent eddy-like feature, and retention zones off coastal promontories.

Optical polarization map of the Polaris Flare with RoboPol

The stages before the formation of stars in molecular clouds are poorly understood. Insights can be gained by studying the properties of quiescent clouds, such as their magnetic field structure. The plane-of-the-sky orientation of the field can be traced by polarized starlight. We present the first extended, wide-field ($\sim$10 $\rm deg^2$) map of the Polaris Flare cloud in dust-absorption induced optical polarization of background stars, using the RoboPol polarimeter at the Skinakas Observatory. This is the first application of the wide-field imaging capabilities of RoboPol. The data were taken in the R-band and analysed with the automated reduction pipeline of the instrument. We present in detail optimizations in the reduction pipeline specific to wide-field observations. Our analysis resulted in reliable measurements of 641 stars with median fractional linear polarization 1.3%. The projected magnetic field shows a large scale ordered pattern. At high longitudes it appears to align with faint striations seen in the Herschel-SPIRE map of dust emission (250 $\mu m$), while in the central 4-5 deg$^2$ it shows an eddy-like feature. The overall polarization pattern we obtain is in good agreement with large scale measurements by Planck of the dust emission polarization in the same area of the sky.

Shelf circulation prior to and post a cold front event measured from vessel-based acoustic Doppler current profiler

Shelf circulation impacted by a shift in wind regime during the passage of an atmospheric cold front system is studied with a field survey over the mid-shelf of the South Atlantic Bight between Oct 4 and 9, 2004. Weak southerly winds preceded the cold front for a few days, followed by a rapid shift in wind direction and strengthening of northeasterly winds over a few more days. More than 93h of acoustic Doppler current profiler (ADCP) data were obtained along an equilateral triangle of 105km in perimeter, which was continuously occupied for 11 times. A harmonic analysis was applied to extract tidal and subtidal wind-driven flow components by collapsing the 93hour data into one M2 tidal period. It was found that the cross-shelf flow was barely affected by the wind while the along-shelf flow responded with a spatially uniform and almost steadily increasing mean flow velocity, superimposed on an oscillatory tidal current. The wind induced along-shelf transport was estimated to be ~0.3Sv over the inner and middle shelf. The net cross-shelf transport was negligible. Apparently, the northeasterly wind causes an along-shelf current which was subject to Coriolis force that sets up an increasing coastal sea level pressure gradient as the water kept piling up against the coast, which was confirmed by tide gauge data. The observations found that the flow field prior to the strong winds had more complicated structures including eddy-like features, while after the strong northeasterly winds, the flow became eddy free and uniform in space. A theoretical model solved by a Laplace Transform was used to examine the wind-driven flow mechanism and the results were compared with the observations of net along-shelf flow velocity.

Beaufort shelf break eddies and shelf-basin exchange of Pacific summer water in the western Arctic Ocean detected by satellite and modeling analyses

[1] Mesoscale eddies and shelf-basin exchange of Pacific summer water in the western Arctic Ocean are examined using satellite data sets and an eddy-resolving coupled sea ice–ocean model. Several surface eddy-like features along the Beaufort shelf break are detected by the Moderate-Resolution Imaging Spectroradiometer (MODIS) sea surface temperature and the Global Imager (GLI) radiance image. The QuikSCAT sea wind vectors indicate that summertime shelf-wide wind is an important factor for interannual variations in the eddy properties. A realistic numerical experiment reveals that the origin of shelf break warm eddies and timing of their generation can be classified into three types. Type I eddies are frequently produced in the vicinity of the Barrow Canyon throughout the summer season by a combination of outflow of Pacific summer water with a low potential vorticity from the Barrow Canyon and lateral velocity shear of the canyon jet. Surface wind variability modulates the timing of generation. Type II and Type III eddies originate from an eastward current along the Beaufort shelf break, although origin depth and background hydrographic structure differ between them. Type II eddies are spawned from the shelf slope at middepth during early summer. Type III eddies are generated from the surface isopycnal front over the shelf break during late summer. The shift from Type II eddies to Type III eddies is caused by surface buoyancy input in the upstream region. A sensitivity experiment using different atmospheric forcing data suggest that the mechanism controlling Pacific water transport from the Chukchi shelf to the Canada Basin differs depending on surface wind fields in the shelf region. The mesoscale eddies primarily induce the shelf-to-basin transport under weak or westerly wind conditions during summer, while wind-driven Ekman transport is a major driver in the easterly wind regime. Year 2003 (2007) corresponds to the former (latter) case.

Physical controls and mesoscale variability in the Labrador Sea spring phytoplankton bloom observed by Seaglider

We investigated the 2005 spring phytoplankton bloom in the Labrador Sea using Seaglider, an autonomous underwater vehicle equipped with hydrographic, bio-optical and oxygen sensors. The Labrador Sea blooms in distinct phases, two of which were observed by Seaglider: the north bloom and the central Labrador Sea bloom. The dominant north bloom and subsequent zooplankton growth are enabled by the advection of low-salinity water from West Greenland in the strong and eddy-rich separation of the boundary current. The glider observed high fluorescence and oxygen supersaturation within haline-stratified eddy-like features; higher fluorescence was observed at the edges than centers of the eddies. In the central Labrador Sea, the bloom occurred in thermally stratified water. Two regions with elevated subsurface chlorophyll were also observed: a 5 m thin-layer in the southwest Labrador Current, and in the Labrador shelf-break front. The thin layer observations were consistent with vertical shearing of an initially thicker chlorophyll patch. Observations at the front showed high fluorescence down to 100 m depth and aligned with the isopycnals defining the front. The high-resolution Seaglider sampling across the entire Labrador Sea provides first estimates of the scale dependence of coincident biological and physical variables.

Seasonal hydrodynamical features on the continental shelf of Colima (west coast of Mexico)

Four high-resolution hydrographic surveys were carried out in 2002 on the continental shelf of Colima, Pacific coast of Mexico. The data collected revealed the presence of cyclonic and anticyclonic eddy-like features covering the whole extension of the shelf, which is a rather different pattern from the along-shore general circulation described in previous works. Despite the apparent complexity of the circulation features, the overall shelf-slope exchange pattern is similar in autumn and winter and the opposite in spring. In summer the exchanges also have a similar distribution to autumn and winter, but they are restricted to the upper layers. This suggests that the observed circulation features could result from the interaction between the large-scale seasonal circulation and some kind of local forcing.

A field study of the behaviour of an anticyclonic eddy on the Catalan continental shelf (NW Mediterranean)

Through the analysis of satellite thermal images, mesoscale anticyclonic eddies have been observed to recurrently drift along the NW Mediterranean coasts. The presence of these eddies is known to cause transient “inversions” of the shelf–slope current. By means of an exhaustive analysis of sea surface temperature images a group of anticyclonic structures were monitored in September 2001. One of these eddy-like features was tracked and intensely surveyed by means of conductivity–temperature–depth (CTD) casts and repeated fast surveys with N ν-Shuttle (oscillating CTD) and Acoustic Doppler Current Profiler (ADCP) measurements during the ET2001 oceanographic cruise. A well developed anticyclonic eddy was surveyed over the Catalan continental shelf, 60 km north of Barcelona. In situ velocity data revealed that the anticyclonic motion was characterized by surface velocities of 50 cm/s and a diameter of about 45 km. The signal of the eddy was found down to 100 m depth, well below the seasonal thermocline. The passage of the eddy modified the local flow, involving advection and subduction of surrounding waters.

The Cold-Water Pathway during an Upwelling Event on the New Jersey Shelf

Abstract A comprehensive dataset obtained in summer of 1996 on the New Jersey shelf off Atlantic City is analyzed to determine the pathway for cold water during a period of sustained upwelling. The data include shipboard CTD/ADCP surveys, time series from three across-shelf lines of moorings, and remote sensing. An upwelling event that occurred from 30 June through 10 July 1996 is studied. The event comprised three stages: first, the cold water was upwelled through the bottom layer; second, the onshore flow concentrated in the pycnocline; and third, mesoscale eddylike features developed. During the first upwelling-favorable wind pulse, which lasted approximately 2 days, a northward alongshelf current was generated and the pycnocline outcropped at the surface approximately 5 km off the coast. The northward current occupied the whole water column, and cold water was brought to the coast through the near-bottom layer, in the manner of Ekman dynamics. After that, an alongshelf pressure gradient was set, which...

Ocean structure and climate (Eastern North Atlantic): in situ measurement and remote sensing (altimeter)

Structure and climate of the east North Atlantic are appraised within a framework of in situ measurement and altimeter remote sensing from 0°–60°N. Long zonal expendable bathythermograph/conductivity–temperature–depth probe sections show repeating internal structure in the North Atlantic Ocean. Drogued buoys and subsurface floats give westward speeds for eddies and wavelike structure. Records from longterm current meter deployments give the periodicity of the repeating structure. Eddy and wave characteristics of period, size or wavelength, westward propagation speed, and mean currents are derived at 20°N, 26°N, 32·5°N, 36°N and 48°N from in situ measurements in the Atlantic Ocean. It is shown that ocean wave and eddy-like features measured in situ correlate with altimeter structure. Interior ocean wave crests or cold dome-like temperature structures are cyclonic and have negative surface altimeter anomalies; mesoscale internal wave troughs or warm structures are anticyclonic and have positive surface height anomalies. Along the Eastern Boundary, flows and temperature climate are examined in terms of sla and North Atlantic Oscillation (NAO) Index. Longterm changes in ocean climate and circulation are derived from sla data. It is shown that longterm changes from 1992 to 2002 in the North Atlantic Current and the Subtropical Gyre transport determined from sla data correlate with winter NAO Index such that maximum flow conditions occurred in 1995 and 2000. Minimum circulation conditions occurred between 1996–1998. Years of extreme negative winter NAO Index result in enhanced poleward flow along the Eastern Boundary and anomalous winter warming along the West European Continental Slope as was measured in 1990, 1996, 1998 and 2001.

SST Observations of the Agulhas and East Madagascar Retroflections by the TRMM Microwave Imager

The retroflections of the East Madagascar Current and Agulhas Current are complex rapidly evolving systems, the latter controlling the passage of warm salty water from the Indian Ocean to the Atlantic. The TRMM Microwave Imager (TMI) provides frequent observations of sea surface temperature through clouds, allowing one to monitor the evolution of these systems. The authors develop a simple feature-tracking system that obviates the need for user intervention, and use its results to guide more focused studies. In the period 1997–99, westward progradation of the Agulhas retroflection (associated with ring shedding) is observed about eight times per year, agreeing with previous estimates from infrared data, and many rings move westward or northwestward. However, this behavior is seen to change in the 2000–01 time period, with the Agulhas retroflection occurring farther to the east. A few Natal pulses are seen, but cannot be linked conclusively to the spawning of rings due to TMI's limited latitudinal coverage. The majority of features originating at the East Madagascar retroflection appear to migrate southwestward. A new observation from the data is that, although the first northward meander of the Agulhas Return Current is constrained by bathymetry, its position does vary intermittently, remaining fixed in a given location for up to six months at a time. Southward propagation of features is noted along two ridges: although eddies have been found before along the eastern slope of the Mozambique Ridge, the new results for the Madagascar Ridge indicate an extra pathway for the eddies. Eddylike features are also found leading from the Agulhas Return Current back toward the Agulhas Current. The narrow “corridor” of these features suggests that it is controlled by the gyre recirculation in the southwest Indian Ocean.

Mesoscale variability in the boundary currents of the Alaska Gyre

Measurements of sea-surface height anomalies acquired during the GEOSAT, ERS-1, and TOPEX altimeter missions show that the boundary currents of the Alaska gyre exhibit interannual variability with respect to the occurrence, size, and propagation of mesoscale, eddy-like features. Observations and model results suggest that eddies are generated in the Alaska Current during years in which the wind forcing in the eastern Gulf of Alaska promotes strong downwelling along the British Columbia–Alaska coast. Wind forcing conditions that support eddy formation and intensification often occur in years that coincide with El Niño–Southern Oscillation events. Eddy variability is significantly more deterministic in the Alaska Current than in the Alaskan Stream.

Upper-ocean circulation patterns in the northern Gulf of California, expressed in Ers-2 Synthetic Aperture Radar Imagery

ERS-2 Synthetic Aperture Radar (SAR) frames were collected over the northern Gulf of California during winter 1999. The frames, presented here as a single composite image (100 x 300 km), express a mesoscale meander or eddy-like feature, approximately 80 km wide, located close to the peninsula of Baja California. The eddy is expressed as bright, concentric bands that curve in an anticyclonic direction. The location and dimensions of the eddy are in very good agreement with the winter circulation of the northern Gulf of California described by Lavín et al. (1997), from the analysis of direct observations using satellite-tracked drifters and hydrographic surveys. The SAR image also depicts other smaller, cyclonic eddy-like features, located close to the mainland, which may be associated with the general circulation induced by the anticyclonic eddy. SAR images, together with the reduced number of in situ observations, can provide new and valuable information about the mesoscale circulation of the upper ocean in the Gulf of California, even when cloud cover prevents the use of other remote sensors, such as infrared radiometers and visible imaging systems.

The circulation and water masses in the Gulf of the Farallones

Six ADCP and CTD ship surveys of the continental shelf and slope in the vicinity of the Gulf of the Farallones, CA, were conducted in 1990–1992. ADCP data provide much more detail on the structure of the currents over the slope and shelf in the Gulf and reveal a persistent, largely barotropic poleward flow with a complex mesoscale flow field superimposed. The directly measured currents are not well represented by the geostrophic velocity fields derived from hydrographic casts. Important upper-ocean circulation features include: a Slope Countercurrent (SCC), variable shelf circulation, and submesoscale eddy-like features. The SCC was present in all seasons and is believed due to a strong year-round positive wind-stress curl enhanced by Point Reyes. Its flow was poleward throughout the upper 300 m, and often surface intensified. Poleward transport in the upper 400 m was 1–3 Sv, much greater than previous geostrophic estimates for the California Current System constrained to a 500 dbar reference level. The shelf circulation was much more variable than the SCC and generally exhibited a pattern consistent with classic Ekman dynamics, responding to synoptic wind forcing. Submesoscale vortices, or eddies, often dominated the general flow field. These eddies are thought to be generated by the frictional torque associated with current–topography interactions. Their centers typically have a distinct water type associated with either the SCC or the southward-flowing California Current. Higher spiciness anomalies, representing a higher percentage of Pacific Equatorial Water (PEW), were typically found in the core of the SCC or within anticyclonic eddies. Lower (bland) spiciness anomalies, characteristic of a higher percentage of Pacific Subarctic Water (PSAW), were associated with cyclonic eddies. While these circulation features were largely barotropic, the flow also adjusted baroclinically to changes in the density field, as different water types were advected by the general flow field or by mesoscale instabilities in the large-scale boundary currents as they interacted with topography. Despite a seasonal cycle in regional wind and ocean temperature time series, there is no obvious seasonal pattern in the circulation. Most of the temporal variability in the current appears to be due to synoptic and interannual variations in atmospheric forcing. Because of the very dynamic three-dimensional nature of the regional circulation, the Gulf of the Farallones is likely to be a center for active mixing and exchange between the coastal and California Current waters, relative to most US west coast locales.

Spatial length scale analysis of data from observations and a model of the Iceland-Faeroes front

Hydrographic data from the Iceland-Faeroes Ridge region, covering an area of 59°–69° north and 0°–20° west, have been subjected to a spatial data analysis. The analysis consists of the two-dimensional spectral method (2D-FFT) and the empirical eigenfunction method (EOF). Results from the two methods show good agreement indicating that the significant length scales for horizontal variability present in the data have wavelengths of approximately 339 km, 72 km and 37 km. These wavelengths relevant to the Iceland-Faeroes front are interpreted as the sizes respectively of the warm water intrusion from the south of the front with a diameter of half the wavelength (i.e.,⋍169 km), and the meander-like and eddy-like features. Predictions of the same frontal system have also been made using a realistic numerical model to provide hydrographic outputs similar to the observations. The hydrographic outputs from the model have been subjected to the same 2D-FFT method to establish the spatial length scale present in the model predictions. A comparison of results from a spectral data analysis between the field measurements and the model predictions shows that the model can predict the sizes of the meander-like and the eddy-like features quite accurately. However, predictions of frontal orientation, frontal slopes and size of the warm water intrusion still require further study. The work presented also demonstrates the importance of spatial statistics in oceanographic research, particularly in ocean predictability studies.

Summertime thermohaline structure off the Brazil Current Region between Santos (SP) and Rio de Janeiro (RJ)

Within the scope of the MAR-14 Project, part of the Brazil-Germany Bilateral Agreement in Marine Sciences, an oceanographic survey aboard the R/V Victor Hensen was carried out in Brazilian coastal waters between Santos (23o56'S) and Rio de Janeiro (22o54'S), from January 15 to January 22,1991. In this article we report results of preliminary analyses of the hydrographic data collected with CTD, STD, Nansen bottles and XBT's. These preliminary results show intense stratification in the first 200 m depth, and the penetration of the Brazil Current deep into the continental shelf region. Two eddy-like features were detected. The first one, anticyclonic, was located in the northern part of the domain and confined to the uppermost 200 m. The second, a cyclonic vortex, was found a little to the southwest below 200 m and extending downwards to about 800 m depth. Water mass analyses based on T-S diagrams suggest that the interface between the South Atlantic Central Water (SACW) and the Antarctic Intermediate Water (AIW) is located at about 500 m depth. One important aspect of this study is that this was the first time a high resolution survey with a CTD probe was realized along the eastern Brasilian Coast, south of Cabo Frio.

Currents under land-fast ice in the Canadian Arctic Archipelago Part 1: Vertical velocities

During an acoustic Doppler current profiler (ADCP) study of the velocity field under land-fast ice, coincident CTD casts showed three instances of pronounced instability in the water column, immediately above and below the pycnocline. In this paper we demonstrate that the density inversions are associated with the passage of high frequency linear internal waves and finite amplitude waves. Contours of ADCP acoustic return intensity display pronounced vertical eddy-like features which may indicate overturning. A Richardson number calculation showed that even the most highly stratified portion of the pycnocline had sufficient vertical velocity shear to promote dynamic instability. Finally, we calculate that the vertical nutrient flux, resulting from the high frequency internal wave field, could supply a significant portion of the ice-algae nutrient budget

Summertime thermohaline structure off the Brazil Current Region between Santos (SP) and Rio de Janeiro (RJ)

Within the scope of the MAR-14 Project, part of the Brazil-Germany Bilateral Agreement in Marine Sciences, an oceanographic survey aboard the R/V Victor Hensen was carried out in Brazilian coastal waters between Santos (23º56'S) and Rio de Janeiro (22º54'S), from January 15 to January 22,1991. In this article we report results of preliminary analyses of the hydrographic data collected with CTD, STD, Nansen bottles and XBT's. These preliminary results show intense stratification in the first 200 m depth, and the penetration of the Brazil Current deep into the continental shelf region. Two eddy-like features were detected. The first one, anticyclonic, was located in the northern part of the domain and confined to the uppermost 200 m. The second, a cyclonic vortex, was found a little to the southwest below 200 m and extending downwards to about 800 m depth. Water mass analyses based on T-S diagrams suggest that the interface between the South Atlantic Central Water (SACW) and the Antarctic Intermediate Water (AIW) is located at about 500 m depth. One important aspect of this study is that this was the first time a high resolution survey with a CTD probe was realized along the eastern Brasilian Coast, south of Cabo Frio.