Henry Perkins

Low-Frequency Current Observations in the Korea/Tsushima Strait*

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 b...

Northern Adriatic Response to a Wintertime Bora Wind Event

During winters, the northern Adriatic Sea experiences frequent, intense cold-air outbreaks that drive oceanic heat loss and imprint complex but predictable patterns in the underlying waters. This strong, reliable forcing makes this region an excellent laboratory for observational and numerical investigations of air-sea interaction, sediment and biological transport, and mesoscale wind-driven flow. Narrow sea surface wind jets, commonly known as “bora,” occur when cold, dry air spills through gaps in the Dinaric Alps (the mountain range situated along the Adriatics eastern shore). Horizontal variations in these winds drive a mosaic of oceanic cyclonic and anticyclonic cells that draw coastal waters far into the middle basin. The winds also drive intense cooling and overturning, producing a sharp front between dense, vertically homogenous waters (North Adriatic Dense Water, or NAdDW) in the north and the lighter (colder, fresher), stratified waters of the Po River plume. Once subducted at the front, the NAdDW flows southward in a narrow vein following the isobaths (contours of constant depth) of the Italian coast. In addition to governing the basins general circulation, these processes also influence sediment transport and modulate biological and optical variability

Current and tide observations in the southern Yellow Sea

Determination of mean currents and tides are particularly difficult in coastal regions. Flows are often nongeostrophic and short lived. Bathymetry is frequently not adequately known and can greatly influence coastal dynamics. To better quantify tides and currents in the southern Yellow Sea, three pressure gauges and three acoustic Doppler current profilers (ADCPs) were bottom moored for 4–6 month intervals at depths ranging from 77 to 89 m. Tidal range is over 2 m, and maximum current velocities are between 50 and 80 cm/s at the mooring locations. The present data are analyzed for 13 significant tidal constituents which are found to account for ∼85% of the sea surface height variability. M2 is the dominant constituent, followed by S2 and K1. Flow is dominated by the tides. Mean currents are relatively small, ranging from ∼1.5 cm/s at the northern mooring to ∼4 cm/s at the southern mooring. Delineations of barotropic and depth-dependent currents are made from the ADCP measurements in terms of mean and eddy kinetic energy. Currents are found to be most depth dependent in the near-surface layers. Approximately 85–90% of the eddy kinetic energy is depth independent.

Summer transport through the Tsushima‐Korea Strait

Transport variations through the Korea-Tsushima Strait are examined from two lines of bottom-moored acoustic doppler current profilers (ADCPs) deployed southwest and northeast of Tsushima Island in May 1999. Almost full-depth velocity profiles are measured. An optimal interpolation (OI) scheme is used to interpolate the data spatially and to provide error estimates along each section. The strong northeastward current core through the southern section lies approximately in the center of the strait, and small southwestward flows occur sporadically near both the Korea and Japan coasts. Much of the flow through the northern line occurs near the Korea and Japan coasts, with a weak southwestward mean flow and large variability in the strait center on the leeside of Tsushima Island. The estimated mean transport is 2.9 sverdrups (Sv) through the southern line and 2.5 Sv through the northern line. The northern line does not extend close to the Korea coast where there is significant flow. Expected errors in the transport estimates at any time are about 0.5 Sv RMS for each mooring line. The gradual transport increase through the summer is carried through the center of the southern line and on the Korea side of the strait through the northern line.

Monitoring Volume Transport through Measurement of Cable Voltage across the Korea Strait

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 V0 = 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.

Tide observations in the Korea-Tsushima Strait

Abstract Tides are analyzed in the Korea-Tsushima Strait using measurements from 11 moorings, each containing an acoustic Doppler current profiler (ADCP) and a pressure gauge. These instruments were bottom moored at depths ranging from 59 to 142 m from May 1999 until October 1999 along two lines across the Strait, northeast and southwest of Tsushima Island. Tide amplitudes range over 3 m along the southern line but only range about 0.7 m along the northern line. Maximum total current velocities exceed 100 cm/s in the surface layers and typically exceed 50 cm/s at mid-depths along both lines. These data are analyzed for eight tidal constituents, which are found to account for about 88% of the sea surface height variability along the southern line and 70% along the northern line. M2, S2, K1, and O1 are the dominant constituents. Their amplitudes are generally 10–20% smaller than amplitudes from tide charts. M2 tidal velocities range from 17 to 25 cm/s along the line northeast of Tsushima Island, and are largest at the mooring on the western side of the Strait, nearest to Korea. Southeast of Tsushima Island, either M2 or K1 dominates the tidal contribution to the current, with tidal velocities ranging between 13 and 23 cm/s. Tidal velocities are fairly depth independent at mid-depths but exhibit varying degrees of depth dependence in the near-surface and near-bottom layers. While tidal currents are responsible for about 25% of the eddy kinetic energy in the near surface layer, they account for more than 50% of the eddy kinetic energy at mid-depths and about 70% near the bottom.

Currents in Korea‐Tsushima Strait during summer 1999

Results are presented from continuous current measurements across Korea-Tsushima Strait between May and October 1999. The data are from eleven bottom-mounted Acoustic Doppler Current Profilers that recorded full-depth profiles of currents along two lines, one at each end of the Strait. The two sections show markedly different mean flow regimes. At the southern entrance, the cross-section flow varies smoothly across the channel, showing a broad maximum at mid-channel. The northern section is marked by strong spatial variability but in the mean consists of two streams, one on each side of the strait. Between the two is a regime of highly variable flow with a weak mean, presumably indicating the wake from Tsushima Island. Flow variability in time is described by statistical measures and by representative snapshots.

Inertial oscillations in the Korea Strait

Inertial oscillations (IO) are examined in the Korea Strait based on measurements from 13 acoustic Doppler current profilers covering the time period May 1999 through March 2000. Strong IO responses to wind stress occur during summer. A simple linear model predicts that winter wind stress is expected to generate inertial responses of the same order of magnitude as those in summer. However, the observed winter IO response is much weaker than predicted. During summer, the currents within the mixed layer and below the mixed layer are of comparable amplitude but in opposite directions. The depth at which the currents reverse directions varies throughout the year as the mixed layer deepens from about 40 m during summer to the bottom of the water column in November. During winter, the velocity structure is more uniform in depth with currents in the same direction throughout the water column. One possible explanation for these phenomena is related to the combined effect of the strait boundaries and the strong summer stratification. The stratification prevents the wind stress momentum flux from mixing downward below the thermocline and thus allows the development of a bottom current separate from the surface current. Such a velocity structure is necessary to satisfy the no-flow condition through the land boundaries.

Evaluation of Tides from TOPEX/Poseidon in the Bohai and Yellow Seas *

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.

A Miniature Fluorometer for Oceanographic Applications

Abstract A fluorometer intended for ocean measurements of dye patches achieves small size, low power consumption, and high sensitivity. Its performance is reported using fluorescein sodium as the fluorescent agent, although the principle is applicable to any dye that fluoresces at a wavelength sufficiently different from the excitation wavelength. An implementation in the Naval Research Laboratory’s advanced microstructure profiler is described in detail, and results from static calibration and dynamic response tests are given.