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W. J. Teague
,
P. Pistek
,
G. A. Jacobs
, and
H. T. Perkins

Abstract

TOPEX/Poseidon (T/P) can provide accurate tide data in the Yellow and Bohai Seas. The accuracy of T/P as a measurement tool for tides in the coastal seas can be evaluated through comparison to data from in situ pressure gauges and coastal tide stations. The effort here is concentrated on the broad continental shelf region of the Yellow and Bohai Seas. Tide coefficients are derived from five years of T/P data for M2, S2, K1, and O1 constituents. These constituents are responsible for about 80% of the tidal variability in the Yellow Sea. The T/P data are compared with two moored pressure gauges deployed along ground tracks and eight coastal tide gauges near the ground tracks. In addition, for internal consistency, the T/P data are compared at seven crossover points. Comparisons with tidal stations are quite good in the Yellow Sea, but not as good in the Bohai Sea. Comparisons of moored pressure gauges with T/P are better than coastal tide station comparisons in terms of tidal amplitude and phase. Crossover point comparisons are best in the Yellow Sea. The T/P data dropouts degrade the coefficient determination within 20 to 35 km of the coast.

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D. N. Fox
,
W. J. Teague
,
C. N. Barron
,
M. R. Carnes
, and
C. M. Lee

Abstract

The Modular Ocean Data Assimilation System (MODAS) is used by the U.S. Navy for depiction of three-dimensional fields of temperature and salinity over the global ocean. MODAS includes both a static climatology and a dynamic climatology. While the static climatology represents the historical averages, the dynamic climatology assimilates near-real-time observations of sea surface height and sea surface temperature and provides improved temperature and salinity fields. The methodology for the construction of the MODAS climatology is described here. MODAS is compared with Levitus and Generalized Digital Environmental Model climatologies and with temperature and salinity profiles measured by SeaSoar in the Japan/East Sea to illustrate MODAS capabilities. MODAS with assimilated remotely sensed data is able to portray time-varying dynamical features that cannot be represented by static climatologies.

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D. A. Mitchell
,
M. Wimbush
,
D. R. Watts
, and
W. J. Teague

Abstract

The standard gravest empirical mode (GEM) technique for utilizing hydrography in concert with integral ocean measurements performs poorly in the southwestern Japan/East Sea (JES) because of a spatially variable seasonal signal and a shallow thermocline. This paper presents a new method that combines the U.S. Navy's Modular Ocean Data Assimilation System (MODAS) static climatology (which implicitly contains the mean seasonal signal) with historical hydrography to construct a “residual GEM” from which profiles of such parameters as temperature (T) and specific volume anomaly (δ) can be estimated from measurements of an integral quantity such as geopotential height or acoustic echo time (τ). This is called the residual GEM technique. In a further refinement, sea surface temperature (SST) measurements are included in the profile determinations. In the southwestern JES, profiles determined by the standard GEM technique capture 70% of the T variance and 64% of the δ variance, while the residual GEM technique using SST captures 89% of the T variance and 84% of the δ variance. The residual GEM technique was applied to optimally interpolated τ measurements from a two-dimensional array of pressure-gauge-equipped inverted echo sounders moored from June 1999 to July 2001 in the southwestern JES, resulting in daily 3D estimated fields of T and δ throughout the region. These estimates are compared with those from direct measurements and good agreement is found between them.

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D. A. Mitchell
,
W. J. Teague
,
M. Wimbush
,
D. R. Watts
, and
G. G. Sutyrin

Abstract

Current and temperature patterns in the Ulleung Basin of the Japan/East Sea are examined using acoustic travel-time measurements from an array of pressure-gauge-equipped inverted echo sounders moored between June 1999 and July 2001. The focus here is the formation and behavior of a persistent cold eddy observed south of Dok Island, referred to as the Dok Cold Eddy (DCE), and meandering of the Subpolar Front. The DCE is typically about 60 km in diameter and originates from the pinching off of a Subpolar Front meander between Ulleung and Dok Islands. After formation, the DCE dwells southwest of Dok Island for 1–6 months before propagating westward toward Korea, where it deflects the path of the East Korean Warm Current (EKWC). Four such DCE propagation events between January and June 2000 each deflected the EKWC, and after the fourth deflection the EKWC changed paths and flowed westward along the Japanese shelf as the “Offshore Branch” from June through November 2000. Beginning in March 2001, a deep, persistent meander of the Subpolar Front developed and oscillated with a period near 60 days, resulting in the deformation and northwestward displacement of the Ulleung Eddy. Satellite-altimeter data suggest that the Ulleung Eddy may have entered the northern Japan/East Sea. The evolution of this meander is compared with thin-jet nonlinear dynamics described by the modified Korteweg–deVries equation.

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W. J. Teague
,
G. A. Jacobs
,
H. T. Perkins
,
J. W. Book
,
K-I. Chang
, and
M-S. Suk

Abstract

High resolution, continuous current measurements made in the Korea/Tsushima Strait between May 1999 and March 2000 are used to examine current variations having time periods longer than 2 days. Twelve bottom-mounted acoustic Doppler current profilers provide velocity profiles along two sections: one section at the strait entrance southwest of Tsushima Island and the second section at the strait exit northeast of Tsushima Island. Additional measurements are provided by single moorings located between Korea and Tsushima Island and just north of Cheju Island in Cheju Strait. The two sections contain markedly different mean flow regimes. A high velocity current core exists at the southwestern section along the western slope of the strait for the entire recording period. The flow directly downstream of Tsushima Island contains large variability, and the flow is disrupted to such an extent by the island that a countercurrent commonly exists in the lee of the island. The northeastern section is marked by strong spatial variability and a large seasonal signal but in the mean consists of two localized intense flows concentrated near the Korea and Japan coasts. Peak nontidal currents exceed 70 cm s−1 while total currents exceed 120 cm s−1. The estimated mean transport calculated from the southwest line is 2.7 Sv (Sv ≡ 106 m3 s−1). EOF analyses indicate total transport variations in summer are due mainly to transport variations near the Korea coast. In winter, contributions to total transport variations are more uniformly distributed across the strait.

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C. A. Luecke
,
H. W. Wijesekera
,
E. Jarosz
,
D. W. Wang
,
J. C. Wesson
,
S. U. P. Jinadasa
,
H. J. S. Fernando
, and
W. J. Teague

Abstract

Long-term measurements of turbulent kinetic energy dissipation rate (ε), and turbulent temperature variance dissipation rate (χ T ) in the thermocline, along with currents, temperature, and salinity were made at two subsurface moorings in the southern Bay of Bengal (BoB). This is a part of a major international program, conducted between July 2018 and June 2019, for investigating the role of the BoB on the monsoon intraseasonal oscillations. One mooring was located on the typical path of the Southwest Monsoon Current (SMC), and the other was in a region where the Sri Lanka dome is typically found during the summer monsoon. Microstructure and finescale estimates of vertical diffusivity revealed the long-term subthermocline mixing patterns in the southern BoB. Enhanced turbulence and large eddy diffusivities were observed within the SMC during the passage of a subsurface-intensified anticyclonic eddy. During this time, background shear and strain appeared to influence high-frequency motions such as near-inertial waves and internal tides, leading to increased mixing. Near the Sri Lanka dome, enhanced dissipation occurred at the margins of the cyclonic feature. Turbulent mixing was enhanced with the passage of Rossby waves and eddies. During these events, values of χ T exceeding 10−4 °C2 s−1 were recorded concurrently with ε values exceeding 10−5 W kg−1. Inferred diffusivity peaked well above background values of 10−6 m2 s−1, leading to an annually averaged diffusivity near 10−4 m2 s−1. Turbulence appeared low throughout much of the deployment period. Most of the mixing occurred in spurts during isolated events.

Free access
C. A. Luecke
,
H. W. Wijesekera
,
E. Jarosz
,
D. W. Wang
,
T. G. Jensen
,
S. U. P. Jinadasa
,
H. J. S. Fernando
, and
W. J. Teague

Abstract

The formation of a sharp oceanic front located south-southeast of Sri Lanka during the southwest monsoon is examined through in situ and remote observations and high-resolution model output. Remote sensing and model output reveal that the front extends approximately 200 km eastward from the southeast coast of Sri Lanka toward the southern Bay of Bengal (BoB). This annually occurring front is associated with the boundary between the southwest monsoon current with high-salinity water to the south, and a weak flow field comprised of relatively fresh BoB water to the north. The front contains a line of high chlorophyll extending from the coastal upwelling zone, often for several hundred kilometers. Elevated turbulent diffusivities ∼10−2 m2 s−1 along with large diapycnal fluxes of heat and salt were found within the front. The formation of the front and vertical transports are linked to local wind stress curl. Large vertical velocities (∼50 m day−1) indicate the importance of ageostrophic, submesoscale processes. To examine these processes, the Ertel potential vorticity (PV) was computed using the observations and numerical model output. The model output shows a ribbon of negative PV along the front between the coastal upwelling zone and two eddies (Sri Lanka Dome and an anticyclonic eddy) typically found in the southern BoB. PV estimates support the view that the flow is susceptible to submesoscale instabilities, which in turn generate high vertical velocities within the front. Frontal upwelling and heightened mixing show that the seasonal front is regionally important to linking the fresh surface water of the BoB with the Arabian Sea.

Significance Statement

Within the ocean, motions span extraordinarily wide ranges of sizes and time scales. In this study we focus on a narrow, intensified feature called a front. This front occurs in the southern Bay of Bengal during the summer monsoon and forms a boundary between fresher water to the north and saltier water to the south. Features such as this are difficult to study, however, by combining observations made from ships and satellites with output from numerical models of the ocean, we are able to better understand the front. This is important because fronts like the one studied here play a role in determining the pathways of heat within the ocean, which, in turn, may feedback into the atmosphere and weather patterns.

Open access
H. W. Wijesekera
,
W. J. Teague
,
D. W. Wang
,
E. Jarosz
,
T. G. Jensen
,
S. U. P. Jinadasa
,
H. J. S. Fernando
, and
Z. R. Hallock

Abstract

High-resolution currents and hydrographic fields were measured at six deep-water moorings in the southern Bay of Bengal (BoB) by the Naval Research Laboratory as part of an international effort focused on the dynamics of the Indian Ocean. Currents, temperature, and salinity were sampled over the upper 500 m for 20 months between December 2013 and August 2015. One of the major goals is to understand the space–time scales of the currents and physical processes that contribute to the exchange of water between the BoB and the Arabian Sea. The observations captured Southwest and Northeast Monsoon Currents, seasonally varying large eddies including a cyclonic eddy, the Sri Lanka dome (SLD), and an anticyclonic eddy southeast of the SLD. The observations further showed intraseasonal oscillations with periods of 30–70 days, near-inertial currents, and tides. Monthly averaged velocities commonly exceeded 50 cm s−1 near the surface, and extreme velocities exceeded 150 cm s−1 during the southwest monsoon. Tides were small and dominated by the M2 component with velocities of about 3 cm s−1. The average transport into the BoB over the measurement period was 2 Sv (1 Sv ≡ 106 m3 s−1) but likely exceeded 15 Sv during summer of 2014. This study suggests the water exchange away from coastal boundaries, in the interior of the BoB, may be largely influenced by the location and strength of the two eddies that modify the path of the Southwest Monsoon Current. In addition, there is a pathway below 200 m for transport of water into the BoB throughout the year.

Full access
Kuh Kim
,
Sang Jin Lyu
,
Young-Gyu Kim
,
Byung Ho Choi
,
Keisuke Taira
,
Henry T. Perkins
,
William J. Teague
, and
Jeffrey W. Book

Abstract

Voltage induced by the Tsushima Current on an abandoned submarine telephone cable between Pusan, Korea, and Hamada, Japan, has been measured since March 1998 in order to monitor the volume transport through the Korea Strait. Voltage has a good linear relationship with the transport measured by bottom-mounted acoustic Doppler current profilers (ADCPs) along a section spanning the Korea Strait. The linear conversion factor is estimated to be Λ0 = (8.06 ± 0.63) × 106 m3 s−1 V−1 with the reference voltage of V 0 = 0.48 ± 0.07 V. The voltage-derived transport reveals various temporal variations that have not been known previously. Measurement of the cable voltage provides a reliable means for continuous monitoring of the volume transport of the Tsushima Current, which determines the major surface circulation and hydrography in the East Sea.

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Hemantha W. Wijesekera
,
Conrad A. Luecke
,
David W. Wang
,
Ewa Jarosz
,
Sergio DeRada
,
William J. Teague
,
Kyung-Il Chang
,
Jae Hak Lee
,
Hong-Sik Min
, and
SungHyun Nam

Abstract

Small-scale processes at the southwestern boundary of the Ulleung Basin (UB) in the Japan/East Sea (JES) were examined using combined ship-based and moored observations along with model output. Model results show baroclinic semidiurnal tides are generated at the shelf break and corresponding slope connecting the Korea/Tsushima Strait with the UB and propagate into the UB with large barotropic-to-baroclinic energy conversion over the slope. Observations show high-frequency internal wave packets and indicate strong velocity shear and energetic turbulence associated with baroclinic tides in the stratified bottom layer. Solitary-like waves with frequencies from 0.2N to 0.5N (buoyancy frequency N) were found at the edge of the shelf break with supercritical flow. For subcritical flow, a hydraulic jump formed over the shelf break with weakly dispersive internal lee waves with frequencies varying from 0.5N to N. These high-frequency lee waves were trapped in the stratified bottom layer, with wave stress similar to the turbulent stress near the bottom. The power loss due to turbulent bottom drag can be an important factor for energy loss associated with the hydraulic jump. Turbulent kinetic energy dissipation rates of ∼10−4 W kg−1 were found. Large downward heat and salt fluxes below the high-salinity core mix warm/salty Tsushima Current Water with cold/low-salinity JES Intermediate Water. Mixing over the shelf break could be very important to the JES circulation since the calculated diapycnal upwelling (1–6 m day−1) at the shelf break and slope is substantially greater than the basin-averaged estimate from chemical tracers and modeling studies.

Significant Statement

The Japan/East Sea (JES) is a marginal sea, enclosed by Japan, Korea, and Russia. This study describes mixing processes over the shelf break connecting the northern Korea/Tsushima Strait (KTS) with the southern Ulleung Basin (UB), where the warm, high-salinity Kuroshio water carried by the Tsushima Current interacts with southward-flowing subsurface water masses in the JES. Our analysis suggests that the shelf break and slope between the KTS and the UB are vital areas for water-mass exchange in the southern JES. The enhanced mixing at the shelf break may impact water masses and circulation over the entire JES.

Open access