Inverted Echo Sounders Research Articles

Measurements of Sea Surface Height Variability in the Eastern South Atlantic from Pressure Sensor–Equipped Inverted Echo Sounders: Baroclinic and Barotropic Components

Abstract Variability in sea surface height (SSH) can be decomposed into two contributions: one from changes in mass in the water column (barotropic) and the other from purely steric changes (baroclinic). Both contributions can be determined from data recorded by a pressure sensor–equipped inverted echo sounder (PIES). PIES data from the Agulhas South Atlantic Thermohaline Experiment (ASTTEX) were used, collected in the Cape Basin off South Africa, along 1000 km of an eddy corridor where Agulhas eddies carry cores of warm, salty Indian Ocean waters into the South Atlantic. The paper presents in detail the method used to convert PIES measurements into barotropic, baroclinic, and total SSH, and discusses the error budget. The baroclinic contribution is geopotential height (reference 4500 dbar), which can be determined from the measured vertical acoustic travel time together with a lookup curve based on the regional hydrography. The main error source is scatter about this curve that depends on the extent to which water masses advecting along each geopotential streamline may derive from different ocean regions. The barotropic contribution can be determined from the bottom pressure measurements of the mass variation in the water column and has an uncertainty due to sensor calibration drift in two years corresponding to 1-cm water column height. The barotropic component accounts for 20% of the overall SSH variance and 47% during large signal intervals exceeding 15 cm. PIES data demonstrate via the two measurements that barotropic and baroclinic contributions may work independently or in concert in different mesoscale eddies. The combined structure need not be equivalent barotropic. In particular, deep barotropic eddies exhibit mesoscale spatiotemporal scales and may or may not be systematically tilted or aligned in space or time relative to baroclinic eddies.

Strong bottom currents and cyclogenesis in Drake Passage

Observations from 38 bottom‐moored Current and Pressure Recording Inverted Echo Sounders (CPIES) deployed in Drake Passage during the 2007–2008 International Polar Year provide unprecedented coverage of near‐bottom currents and pressures spanning the entire Antarctic Circumpolar Current. Year‐long‐mean currents exceed 10 cm s−1 north of the Polar Front, and mean directions are not, in general, aligned with the surface fronts. Topographic steering is most evident at the continental margins. Deep eddy kinetic energy (EKE) is maximum at about 200 cm2 s−2 between the Subantarctic and Polar Fronts, coinciding with the location but about one quarter of the value of a maximum in surface EKE. Multiple high‐speed current events, with peak speeds of 60–70 cm s−1 and lasting 30 to 70 days, are coherent across sites separated by 45 km. The observed spinup of eddies coinciding with meanders in the surface fronts is consistent with deep cyclogenesis.

Acoustical Measurement of Nonlinear Internal Waves Using the Inverted Echo Sounder

Abstract The performance of pressure sensor–equipped inverted echo sounders for monitoring nonlinear internal waves is examined. The inverted echo sounder measures the round-trip acoustic travel time from the sea floor to the sea surface and thus acquires vertically integrated information on the thermal structure, from which the first baroclinic mode of thermocline motion may be inferred. This application of the technology differs from previous uses in that the wave period (∼30 min) is short, requiring a more rapid transmission rate and a different approach to the analysis. Sources of error affecting instrument performance include tidal effects, barotropic adjustment to internal waves, ambient acoustic noise, and sea surface roughness. The latter two effects are explored with a simulation that includes surface wave reconstruction, acoustic scattering based on the Kirchhoff approximation, wind-generated noise, sound propagation, and the instrument’s signal processing circuitry. Bias is introduced as a function of wind speed, but the simulation provides a basis for bias correction. The assumption that the waves do not significantly affect the mean stratification allows for a focus on the dynamic response. Model calculations are compared with observations in the South China Sea by using nearby temperature measurements to provide a test of instrument performance. After applying corrections for ambient noise and surface roughness effects, the inverted echo sounder exhibits an RMS variability of approximately 4 m in the estimated depth of the eigenfunction maximum in the wind speed range 0 ≤ U10 ≤ 10 m s−1. This uncertainty may be compared with isopycnal excursions for nonlinear internal waves of 100 m, showing that the observational approach is effective for measurements of nonlinear internal waves in this environment.

The Kuroshio Extension and its recirculation gyres

This paper reports on the strength and structure of the Kuroshio Extension and its recirculation gyres. In the time average, quasi-permanent recirculation gyres are found to the north and south of the Kuroshio Extension jet. The characteristics of these recirculations gyres are determined from the combined observations from the Kuroshio Extension System Study (KESS) field program (June 2004–June 2006) and include current meters, pressure and current recording inverted echo sounders, and subsurface floats. The position and strength of the recirculation gyres simulated by a high-resolution numerical model are found to be consistent with the observations. The circulation pattern that is revealed is of a complex system of multiple recirculation gyres that are embedded in the crests and troughs of the quasi-permanent meanders of the Kuroshio Extension. At the location of the KESS array, the Kuroshio Extension jet and its recirculation gyres transport of about 114 Sv. This represents a 2.7-fold increase in the transport of the current compared to the Kuroshio's transport at Cape Ashizuri before it separates from the coast and flows eastward into the open ocean. This enhancement in the current's transport comes from the development of the flanking recirculation gyres. Estimates from an array of inverted echo sounders and a high-resolution ocean general circulation model are of similar magnitude.

Stream-coordinate structure and variability of the Kuroshio Extension

Fine horizontal-scale surveys performed during the Kuroshio Extension System Study in May 2004 provide near-synoptic ADCP and CTD data along cross-jet transects just up-stream of the first meander trough of the Kuroshio Extension. An array of current- and pressure-recording inverted echo sounders (CPIES) deployed over a ∼ 600 × 600 km region centered on the first meander trough provide time series of bottom pressure and currents as well as acoustic travel time ( τ ), which is converted via the gravest empirical mode method to profiles of temperature, salinity, and specific volume anomaly. These datasets are used here to determine the mean velocity, hydrographic, and potential vorticity structure of the Kuroshio Extension in its “weakly meandering” state in a stream-coordinate system, which avoids the lateral smearing of the jet structure that results from an Eulerian approach. Maximum surface down-stream velocities range from 1 to 2 m/s, averaging around 1.4 m/s. Down-stream velocities reach the bottom south of the core with average magnitudes of 1–5 cm/s but vary in magnitude and direction depending upon the presence of deep barotropic eddies. In the first meander crest, mean cross-stream flux ( O (0.5–2 cm/s)) is towards the cyclonic side, while entering the trough it is towards the anticyclonic side. However, cross-stream flows appear to be event-driven, with fluctuations in steepness of the meander pattern driven by jet instability. Transports above 5300 dbar decrease west to east from 138 Sv at 143 . 75 ∘ E in the first crest, to 124 Sv at the trough and 75 Sv entering the second crest at 148 . 5 ∘ E . The mean from the surveys shows relative vorticity ( ζ ) making contributions as high as 72% of f on the cyclonic side and - 41 % of f on the anticyclonic side, while the “twisting” term due to vertical shear and horizontal density gradients reaches a maximum in the mean of 45% of f north of the core. Comparison of the structure in crests and troughs from the CPIES dataset reveals differences in the distribution of relative vorticity across the core. Both datasets suggest the presence of four isopycnal potential vorticity gradient layers, where strong cross-stream gradients in shallower layers represent a barrier to cross-stream flow. Comparison of the Kuroshio Extension to the Gulf Stream suggests that mean velocities and gradients are generally about 30% weaker in the Kuroshio Extension, and the strong jet structure penetrates to about 25% greater depths in the Gulf Stream.

Modulation of the inflow into the Caribbean Sea by North Brazil Current rings

A two-year long record from a triangular mooring array between the Lesser Antilles islands Tobago, Barbados, and St. Lucia is used to investigate the inflow into the Caribbean Sea, the amount of South Atlantic Water (SAW) carried with the inflow, and the role of North Brazil Current (NBC) rings in the observed variability. The data set consists of time series from temperature/conductivity recorders and current meters in the moorings, bottom-mounted inverted echo sounders at the Tobago and St. Lucia mooring positions, and supplementary shipboard measurements. The acoustic travel time measurements of the inverted echo sounders and the conductivity/temperature time series are used for continuous estimation of dynamic height profiles and geostrophic currents between the surface and 1000 dbar as well as the amount of SAW found at the mooring positions. The observations show a domination of intraseasonal variability between 0 and 15 Sv, superimposed on the long-term fluctuations. With time scales of one to three months, these represent the signature of the NBC rings. The baroclinic transport time series shows nine periods of increased variability, indicative of the rings interacting with the Lesser Antilles island arc; with the exception of one, these periods were associated with corresponding sea surface height anomalies. No marked seasonality was observed in the transport variability or the ring frequency. The arrival of individual rings leads to a weakening of the inflow into the Caribbean. Nevertheless, the rings carry large amounts of SAW into the area, and the immediate increase of the transport towards the end of a ring event suggests a subsequent flow of this SAW-rich water into the Caribbean. At St. Lucia, rings sometimes cause a short-term decrease of SAW content, indicative of an influx of northern hemispheric water and a blocking situation. The average transport of SAW into the Caribbean south of St. Lucia during the observations amounted to 5.5 Sv, with no significant seasonal cycle, but a small positive trend in SAW fraction as well as in transport of about 15% and 1 Sv, respectively; a corresponding trend in the baroclinic volume transport was not observed.

Coupled patterns between fields of dynamic height and bottom pressure in the Japan/East Sea

Vertical coupling between shallow and deep mesoscale eddy processes in the southwestern Japan/East Sea is investigated using acoustic travel-time and bottom pressure measurements from a two-year deployment of an array of pressure-gauge-equipped inverted echo sounders. Canonical correlation analysis is applied to the simultaneously measured data to extract the coupled modes. The upper layer exhibits a migration of the Ulleung Warm Eddy, and correspondingly the deep potential vorticity in the Ulleung Basin adjusts in magnitude and sense to conserve potential vorticity. Coupling between meanders of strong upper currents and abyssal flows has been observed previously in the Gulf Stream during the Synoptic Ocean Prediction experiment. The evidence from our study strongly suggests that such coupling occurs also under much weaker flows.

Accuracy in Mooring Motion Temperature Corrections

Abstract Moored temperature sensors, whether fixed or profiling, routinely need to be corrected to remove the signals associated with the vertical motion of the sensors when the moorings “blow over” in strong flow events (for profiling sensors the problems occur only at the upper end of the profiling range). Hydrographic data are used to estimate the accuracy with which moored temperature sensors in the Gulf Stream can be corrected for mooring motion aliasing using standard correction techniques, and the implications for other ocean regions are discussed. Comparison with hydrographic data and coincident inverted echo sounder (IES) data from the Synoptic Ocean Prediction Experiment (SYNOP) shows that the errors inherent in mooring motion corrected temperatures during significant pressure deflections are potentially 2–3 times as large as previous estimates based on a smaller dataset of observations in the Kuroshio at approximately the same latitude in the Pacific. For sensors with a nominal level of 400 dbar and a typical root-mean-square pressure deflection of 150 dbar, accuracy limits of up to 0.7°C on the “corrected” temperatures are applicable. Deeper sensors typically have smaller accuracy bounds. There is a suggestion that the presence of a mode water layer near the nominal depth of the shallowest sensor can result in much higher errors in mooring motion corrected temperature data. The accuracy estimates derived herein should apply not only to moorings deployed in the Gulf Stream but also to all currents that exhibit similar velocity amplitudes and thermal gradients such as the Agulhas or Kuroshio.

Observation of baroclinic eddies southeast of Okinawa Island

In the region southeast of Okinawa, during May to July 2001, a cyclonic and an anticyclonic eddy were observed from combined measurements of hydrocasts, an upward-looking moored acoustic Doppler current profiler (MADCP), pressure-recording inverted echo sounders (PIESs), satellite altimetry, and a coastal tide gauge. The hydrographic data showed that the lowest/highest temperature (T) and salinity (S) anomalies from a 13-year mean for the same season were respectively −3.0/+2.5°C and −0.20/+0.15 psu at 380/500 dbar for the cyclonic/anticyclonic eddies. From the PIES data, using a gravest empirical mode method, we estimated time-varying surface dynamic height (D) anomaly referred to 2000 dbar changing from −20 to 30 cm, and time-varying T and S anomalies at 500 dbar ranging through about ±2 and°C ±0.2 psu, respectively. The passage of the eddies caused variations of both satellite-measured sea surface height anomaly (SSHA) and tide-gauge-measured sea level anomaly to change from about −20 to 30 cm, consistent with the D anomaly from the PIESs. Bottom pressure sensors measured no variation related to these eddy activities, which indicated that the two eddies were dominated by baro-clinicity. Time series of SSHA map confirmed that the two eddies, originating from the North Pacific Subtropical Countercurrent region near 20°–30°N and 150°–160°E, traveled about 3000 km for about 18 months with mean westward propagation speed of about 6 cm/s, before arriving at the region southeast of Okinawa Island.

Structure, transport and potential vorticity of the Gulf Stream at 68°W: Revisiting older data sets with new techniques

The stream-coordinates mean structure of the Gulf Stream at 68°W is derived using new methods for both defining stream coordinates and interpreting bottom pressure and inverted echo sounder travel times collected during the extensive Synoptic Ocean Prediction experiment. These new analyses provide pictures of the vertical structure of Gulf Stream flows that are demonstrably dynamically consistent with the density field at all depths, in contrast to previous work that relies on simple vertical interpolations to fill gaps between sparse current meter measurements. This new view of the Gulf Stream suggests a slightly higher total mean transport, with the increases coming from both baroclinic and barotropic components, and slightly stronger recirculation cells, particularly on the southern side. The recirculation of the Gulf Stream appears to have a weak baroclinic component, perhaps 10% of the total. A significant advantage of the methodology is the ability to obtain sensible vertical and horizontal gradients of currents and density so that the vertical and cross-stream structures of the components of the mean potential vorticity can be clearly imaged. One new feature from this calculation is that the along-stream gradient of the cross-stream velocity, a term that is often ignored in potential vorticity analyses, is non-negligible (though small) and is asymmetric about the current axis. Both the derived structure and implied dynamics of the circulation can be significantly altered by small changes to the method of calculating daily stream coordinates, e.g., by carefully filtering out observations in rings or not. Arrays of pressure-equipped inverted echo sounders provide the opportunity (at reasonable cost) for properly defining the stream coordinates of energetic jets such as the Gulf Stream.

Boils and Turbulence in a Weakly Stratified Shallow Tidal Sea

Abstract Measurements of turbulence are made in a weakly but variably stratified region of tidal straining in the eastern Irish Sea using turbulence sensors profiling vertically through the water column on the Fast Light Yo-yo (FLY) profiler and horizontally on the Autonomous Underwater Vehicle (AUV) Autosub. The tidal currents exceed 1 m s−1 at the location of the measurements in water of a depth of about 43.5 m, and result in turbulence extending from the seabed to the surface with a cycle period that is half that of the tides, as previously observed. The time of onset of enhanced turbulence that is measured by the sensors on FLY and Autosub as the speed of tidal currents increases are in good agreement, as are their mean levels. Boils on the sea surface are identified using the Autonomously Recording Inverted Echo Sounder, version 2 (ARIES II), a two-beam upward-pointing side-scan sonar mounted on a rig resting on the seabed. The boils have mean horizontal dimensions of about 25 m. They are continually present when turbulence within the water column near the surface is large, typically when the rate of dissipation of turbulent kinetic energy per unit mass ɛ exceeds about 3 × 10−6 W kg−1, compared to the rate of about 1 × 10−7 W kg−1 near times of slack tide when no boils are observed. The top of a region with a relatively high ɛ gradually extends upward from the seabed as the tidal flow increases and, in stably stratified conditions resulting from tidal straining during ebb tides, can be detected in the ARIES II sonographs as it approaches the sea surface, although only after sound reflection from the sea surface. Upward-moving bursts of enhanced turbulence below the surface with horizontal dimensions of about 5–9 m in the direction of the tidal flow are identified in the AUV record in periods of high tidal flow using conditional sampling. The bursts result in slight upward displacements of the AUV from its set operational depth. Boils first appear at times before the region of generally enhanced turbulence reaches the surface: large localized upward-moving bursts appear to precede the main advancing region of small-scale turbulence.

Variations of Kuroshio geostrophic transport south of Japan estimated from long-term IES observations

Two inverted echo sounders were maintained on coastal and offshore sides of the Kuroshio south of Japan from October 1993 to July 2004. Applying the gravest empirical mode method, we obtained a time series of geostrophic transport. Estimated transports generally agree well with geostrophic transports estimated from hydrography. Their agreement with the hydrographic transports is better than that of transports estimated from satellite altimetry data. The geostrophic transport is expressed as the surface transport per unit depth multiplied by the equivalent depth. The geostrophic transport varies mostly with the surface transport and fractionally with the equivalent depth. Seasonal variation of the geostrophic transport has a minimum in March and a maximum in September, with a range of about one fifth of the total transport.

The use of moored inverted echosounders for monitoring near‐surface processes

The near‐surface ocean is a difficult place to conduct measurements of physical and biological processes. High‐frequency inverted echo‐sounders (IES), moored a short distance below the surface, provide a simple means for sustained, high‐resolution monitoring. In particular, long duration deployments are useful in capturing transient events, such as storms or solitary wave trains, and placing their occurrence within the context of longer term trends. Lessons learned from use of these moored IES systems can be applied to modern ocean observatories. A number of long duration deployments of a self‐contained 200kHz IES were made in the late 1990's. Three specific examples will be discussed. Firstly, an investigation of calibrated volume scattering from near‐surface bubbles in the NE Pacific Ocean showed the frequent occurrence of vertical, plume‐like structures drawn downwards up to 25 m. Within the plumes, the backscatter cross‐section exhibited an exponential decay with depth, with e‐folding scale in the range 0.5 to 2.5 m. Secondly, imaging of near‐surface internal solitary waves was performed from a mooring near the Oregon coast, supplementing measurements made with nearby temperature sensors. These measurements showed some suggestion of scattering by turbulence. Thirdly, through the use of both backscatter intensity and echo statistics, a moored IES demonstrated quantitative monitoring of migratory zooplankton and fish populations.

Observations of Kuroshio flow variations in the East China Sea

Kuroshio velocity structure and transport in the East China Sea (ECS) were investigated as part of a 23‐month study using inverted echo sounders and acoustic Doppler current profilers (ADCPs) along the regularly sampled PN‐line. Flow toward the northeast is concentrated near the continental shelf with the mean surface velocity maximum located 30 km offshore from the shelf break (taken as the 170 m isobath). There are two regions of southwestward flow: a deep countercurrent over the continental slope beneath the Kuroshio axis and a recirculation offshore which extends throughout the whole water column. There is a bimodal distribution to the depth of maximum velocity with occurrence peaks at the surface and 210 dbar. When the maximum velocity is located within the top 80 m of the water column, it ranges between 0.36 m/s and 2.02 m/s; when the maximum velocity is deeper than 80 m, it ranges between 0.31 m/s and 1.11 m/s. The 13‐month mean net absolute transport of the Kuroshio in the ECS is 18.5 ± 0.8 Sv (standard deviation, σ = 4.0 Sv). The mean positive and negative portions of this net flow are 24.0 ± 0.9 Sv and −5.4 ± 0.3 Sv, respectively.

Boils and turbulence in a weakly stratified shallow tidal sea

Measurements of turbulence are made in a weakly but variably stratified region of tidal straining in the eastern Irish Sea using turbulence sensors profiling vertically through the water column on the Fast Light Yo-yo (FLY) profiler and horizontally on the Autonomous Underwater Vehicle (AUV) Autosub. The tidal currents exceed 1 m s−1 at the location of the measurements in water of a depth of about 43.5 m, and result in turbulence extending from the seabed to the surface with a cycle period that is half that of the tides, as previously observed. The time of onset of enhanced turbulence that is measured by the sensors on FLY and Autosub as the speed of tidal currents increases are in good agreement, as are their mean levels. Boils on the sea surface are identified using the Autonomously Recording Inverted Echo Sounder, version 2 (ARIES II), a two-beam upward-pointing side-scan sonar mounted on a rig resting on the seabed. The boils have mean horizontal dimensions of about 25 m. They are continually present when turbulence within the water column near the surface is large, typically when the rate of dissipation of turbulent kinetic energy per unit mass ε exceeds about 3 × 10−6 W kg−1, compared to the rate of about 1 × 10−7 W kg−1 near times of slack tide when no boils are observed. The top of a region with a relatively high ε gradually extends upward from the seabed as the tidal flow increases and, in stably stratified conditions resulting from tidal straining during ebb tides, can be detected in the ARIES II sonographs as it approaches the sea surface, although only after sound reflection from the sea surface. Upward-moving bursts of enhanced turbulence below the surface with horizontal dimensions of about 5–9 m in the direction of the tidal flow are identified in the AUV record in periods of high tidal flow using conditional sampling. The bursts result in slight upward displacements of the AUV from its set operational depth. Boils first appear at times before the region of generally enhanced turbulence reaches the surface: large localized upward-moving bursts appear to precede the main advancing region of small-scale turbulence.

Comparison of sea surface dynamic heights estimated from inverted echo sounder data and satellite altimeter data

Sea surface dynamic height (SSDH) anomalies estimated from long-time inverted echo sounder (IES) data using gravest empirical mode method are compared with satellite altimeter data. The IES-derived SSDH anomalies agree well with the altimetric SSDH anomalies, although the IES-derived SSDH variation is slightly underestimated. We calculate power spectra of these SSDH anomalies to discuss time-scale dependency of their differences. The differences between two SSDH anomalies are found mainly caused by components with the periods from 50 days to 100 days, which correspond to meso-scale eddies or small meanders of the Kuroshio. These differences result in smaller variation of the IES-derived SSDH.

On the detectability of internal tides in Drake Passage

The Southern Ocean hosts significant topographic mixing that might be associated with internal tides. Tidal signals are evident in bottom temperature at 1000 m in Drake Passage, suggesting that internal tides with an amplitude of between ∼20 and 200 m may be present. Various necessary conditions for internal tide generation show that the steep topography in and around Drake Passage can initiate internal tides, and recent global tide models have suggested this region to generate very large interface displacements. Here, we present an attempt to detect internal tides in Drake Passage. During the last 10 years, combinations of bottom pressure recorders and inverted echo sounders have been deployed in the region. The bottom pressure recorders measure predominantly the barotropic tide; the inverted echo sounders measure travel time from sea bed to sea surface and therefore are influenced both by sea level (barotropic tide) and internal sound speed (internal tide). By subtracting one from the other, the internal tide should be detectable. Although the technique works successfully around Hawaii, it does not prove the existence of large internal tides in Drake Passage. The detectability of the internal tidal signal in Drake Passage is investigated using a six-layer one-dimensional model to simulate the bottom pressure and travel time signals of a semi-diurnal tide. The temperature and salinity stratification in Drake Passage is sufficiently weak that large vertical excursions are necessary to produce a signal in travel time detectable above the noise in Drake Passage. An internal tide of at least 70 or 20 m in northern and southern Drake Passage, respectively, would be detected. The fact that these are, perhaps surprisingly, not detected by the combination of bottom pressure and travel time, constrains the internal tides in Drake Passage to be ∼20 m in southern Drake Passage, and between 20 and 70 m in northern Drake Passage. The model also predicts that satellite altimetry would not be able to detect internal tides in Drake Passage, but would in the Brazil Basin and Hawaii regions.

Spatial and temporal structure of the Denmark Strait Overflow revealed by acoustic observations

In spite of the fundamental role the Atlantic Meridional Overturning Circulation (AMOC) plays for global climate stability, no direct current measurement of the Denmark Strait Overflow, which is the densest part of the AMOC, has been available until recently that resolve the cross-stream structure at the sill for long periods. Since 1999, an array of bottom-mounted acoustic instruments measuring current velocity and bottom-to-surface acoustic travel times was deployed at the sill. Here, the optimization of the array configuration based on a numerical overflow model is discussed. The simulation proves that more than 80% of the dense water transport variability is captured by two to three acoustic current profilers (ADCPs). The results are compared with time series from ADCPs and Inverted Echo Sounders deployed from 1999 to 2003, confirming that the dense overflow plume can be reliably measured by bottom-mounted instruments and that the overflow is largely geostrophically balanced at the sill.

Synoptic maps of temperature and velocity within the Subantarctic Front south of Australia

From April 1995 to March 1997 a 450‐km array of inverted echo sounders, horizontal electric field recorders, and current meters measured the horizontal and vertical structure of the current, temperature, and salinity fields associated with the Subantarctic Front (SAF) south of Australia. Synoptic maps of the temperature and velocity fields often show the SAF divided into two zonally separated jets, which are also found in the mean. These jets were in close proximity yet directed toward different azimuths. The daily maps also show that the SAF often flowed as a single strong jet, during which times it underwent vigorous meandering, with crest‐to‐trough meridional distances ∼250 km. These meanders stalled within the array and contorted to form “S”‐shaped paths. In one case a cold‐core ring was formed. Propagating meanders have periods of 20–70 days, wavelengths of 240–420 km, and phase speeds of 12–6 km d−1. Deep currents reveal strong cyclones and anticyclones propagating primarily eastward through the array beneath the meandering jet. Thus, at times the deep flow is aligned with the upper flow, whereas at other times, significant barotropic flows cross the upper front. Occasionally, the northern Polar Front was also found within the array. The observed variability illustrates how the interaction of a meandering current with transient features such as meanders and eddies makes the identification of a front difficult when a single isotherm or isopycnal is utilized as the definition.

Currents, Eddies, and a "Fish Story" in the Southwestern Japan/East Sea

Abstract : As part of the Japan/East Sea (JES) initiative supported by the U.S. Office of Naval Research, we conducted an observational experiment to understand the physics of the mesoscale circulation in the Ulleung Basin, located in the south-western corner of the JES. The current passing through the Korea Strait divides upon entering the JES, with portions of the current flowing along the Korean and Japanese coasts. The variability of these currents is especially energetic in our study region. Our objectives were to measure the time-varying currents in the upper and deep levels of the JES. We relate the population density of vertically migrating fish or squid to the time-varying locations of fronts. We moored a two-dimensional array of pressure-gauge-equipped inverted echo sounders (PIES) and deep-recording current meters for 24 months starting June 1999. Figure 1 shows the study region, which spanned roughly a 250-km square between the Republic of Korea and Japan. The inverted echo sounder is a bottom-moored instrument that measures the time required for an acoustic pulse to travel from the seafloor to the sea surface and back. Travel-time variations arise primarily from temperature changes in the overlying water column caused by meandering fronts and eddies. The PIES array was used to map the temperature field at a suite of depth levels and the dynamic method was used to calculate profiles of geostrophic current. Bottom pressure and deep current meters moored 25 m above the seafloor were used together to map the deep circulation.