Jeffrey W. Book

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...

Simulation of the northern Adriatic circulation during winter 2003

[1] Numerical simulations of the Adriatic Sea were conducted with the Navy Coastal Ocean Model during the Adriatic Circulation Experiment in the fall and winter of 2002/2003, and results were compared with observations. The ocean model used a 1-km resolution grid over the entire Adriatic Sea. Model forcing included atmospheric fluxes from the Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS), tides, boundary conditions from a global model, and freshwater river and runoff inflows. Model tidal elevation showed good agreement with International Hydrographic Organization station data, and model tidal currents showed good agreement with tidal currents determined from acoustic Doppler current profiler (ADCP) measurements. Detided model currents showed good agreement with ADCP currents with rms errors along the principal variance axes ranging from 6 to 12.9 cm/s and correlations ranging from 0.16 to 0.81. Correlations between model and ADCP currents along the minor variance axes were generally low. Comparison of the model-simulated temperature and salinity profiles during January and February with conductivity-temperature-depth measurements indicated that the model captured some of the spatial structure of the observed fields. The model response to several bora wind events in January and February showed a recurring pattern of cyclonic and anticyclonic gyres that generally agreed with observations and reflected the pattern of wind stress curl from the COAMPS wind stress forcing. During strong bora forcing, two large cyclonic circulation gyres form in the northern Adriatic with a smaller anticyclonic circulation between them near the Istrian Peninsula. Additionally, a couple of large meanders frequently occur within the Eastern Adriatic Current southeast of Kvarner Bay, and these meanders sometimes close to form small gyres.

Model-based directed drifter launches in the Adriatic Sea: Results from the DART experiment

Abstract : A high-resolution numerical model of the Adriatic Sea is used to predict Lagrangian coherent structure boundaries, quantified by finite-size Lyapunov exponents (FSLE), for flow features in the region of the Gargano Peninsula during the course of the Dynamics of the Adriatic in Real Time (DART) observational program. FSLE fields computed from two-day model forecasts of the surface velocity indicate distinct regions of high relative drifter dispersion. Model predictions of such regions located on available ship-tracks were used to direct the launching of pairs of surface drifters on three days during March 2006, with the goal of maximizing coverage of the sampling area. For two of the three launches, the observed trajectories separated at locations and along directions closely approximated by those predicted from the model FSLE fields. The third case acted as an inadvertent control experiment. Model predictions at release-time showed minimal FSLE structure at the launch locations and the observed drifter pair advected in a coherent fashion for two days. While there are considerable differences between individual drifter observations and trajectory envelopes computed from ensembles of synthetic drifters, the experiment confirms the models ability to approximate the location and shape of energetic flow features controlling the near-time fate of quasi-Lagrangian particles. Overall, the combined use of FSLEs with realistic coastal circulation models appears to be a promising avenue to aid real-time-directed drifter launches in observational programs.

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.

Dynamics of the circulation in the Sea of Marmara: numerical modeling experiments and observations from the Turkish straits system experiment

During September 2008 and February 2009, the NR/V Alliance extensively sampled the waters of the Sea of Marmara within the framework of the Turkish Straits System (TSS) experiment coordinated by the NATO Undersea Research Centre. The observational effort provided an opportunity to set up realistic numerical experiments for modeling the observed variability of the Marmara Sea upper layer circulation at mesoscale resolution over the entire basin during the trial period, complementing relevant features and forcing factors revealed by numerical model results with information acquired from in situ and remote sensing datasets. Numerical model solutions from realistic runs using the Regional Ocean Modeling System (ROMS) produce a general circulation in the Sea of Marmara that is consistent with previous knowledge of the circulation drawn from past hydrographic measurements, with a westward meandering current associated with a recurrent large anticyclone. Additional idealized numerical experiments illuminate the role various dynamics play in determining the Sea of Marmara circulation and pycnocline structure. Both the wind curl and the strait flows are found to strongly influence the strength and location of the main mesoscale features. Large displacements of the pycnocline depth were observed during the sea trials. These displacements can be interpreted as storm-driven upwelling/downwelling dynamics associated with northeasterly winds; however, lateral advection associated with flow from the Straits also played a role in some displacements.

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.

The oceanic response of the Turkish Straits System to an extreme drop in atmospheric pressure

Moorings across all four entry/exit sections of the Dardanelles Strait and the Bosphorus Strait simultaneously measured the response of the Turkish Straits System to the passage of a severe cyclonic storm that included an atmospheric pressure drop of more than 30 mbar in less than 48 h. The bottom pressure response at the Aegean Sea side of the Dardanelles Strait was consistent with an inverted barometer response, but the response at the other sections did not follow an inverted barometer, leading to a large bottom pressure gradient through the Turkish Straits System. Upper-layer flow toward the Aegean Sea was reversed by the storm and flow toward the Black Sea was greatly enhanced. Bottom pressure across the Sea of Marmara peaked 6 h after the passage of the storms minimum pressure. The response on the Dardanelles side was a combination of sea elevation and pycnocline depth rise, and the response on the Bosphorus side was an even greater sea elevation rise and a drop in pycnocline depth. The peak in bottom pressure in the Sea of Marmara was followed by another reverse in the flow through the Dardanelles Strait as flow was then directed away from the Sea of Marmara in both straits. A simple conceptual model without wind is able to explain fluctuations in bottom pressure in the Sea of Marmara to a 0.89–0.96 level of correlation. This stresses the importance of atmospheric pressure dynamics in driving the mass flux of the Turkish Strait System for extreme storms.

Low power control systems for microbial fuel cell batteries

The URI Ocean Engineering Department has been working with the US Naval Research Laboratory to develop and test microbial fuel cell battery systems for low power seafloor applications of extended duration. These low power fuel cells utilize graphite electrodes in anoxic sediment coupled to bottle brush graphite electrodes in the water column to make up a battery system. These battery systems typically produce power on the order of 10-50 milliwatts per square meter of electrode area, or as much as ten times more if sediment electrode pore water can be exchanged by means of a pump. As part of this program, testing of these microbial fuel cell batteries has been conducted in the Potomac River Washington D.C., Narragansett Bay Rhode Island, Tuckerton New Jersey, and Monterey Bay California. For these tests it has been necessary to develop long term, low power control and monitoring systems to manage the batteries operation and to record their performance without consuming a significant portion of the meager power produced. This paper details the development and testing of these low power monitoring systems and their results. For this project we have developed several related systems. All of the systems utilize Oopic microcontrollers as the core low power computer. The Oopic consumes about 20 mA at 5 volts when on and none when off. Its programs are stored on EEPROM, and they start from the beginning each time the Oopic is powered up. Notes from the previous time awake can be stored in the EEPROM below the program. The microcontrollers are turned on by an electronic alarm from an 12C real time alarm clock chip with its own multi year lithium battery. The microcontrollers command the shut off of power as part of their program. The real time clock maintains crystal controlled date and time without using any system power. The clock can be programmed to wake up the system at a specific date and time, or at any of numerous intervals. Once awake, the Oopic can check the date, monitor/control the system status and even operate a low power pump. Long term measurements over many months can be logged on a micro SD card by means of a serial file management storage chip which uses only 3 ma at 3.3 volts when not writing, and a momentary 40 mA to write. When the system is asleep none of the subsystems consumes any power. Programmed to awake once an hour for 20 seconds to monitor the system and record the results on the SD card, the average power consumption can be kept to less than 2 milliwatts. The control system can be woken up, report data, accept commands, or be reprogrammed; all through an RS232 serial cable. The system utilizes an RS232 chip that manufactures +/-10 V RS232 signals from +5 V with only a few milliamps of additional power consumption. This chip is capable of communicating over a 100 m long cable at 9600 baud. In one instance in Tuckerton New Jersey near a dock, it was possible to continuously cable the control system ashore where a Netburner network based microcontroller was connected. The Netburner was programmed to provide an FTP and Telnet site, allowing remote monitoring and control from across the country over the internet.

Current-eddy interaction in the Agulhas Return Current region from the seismic oceanography perspective

Interleaving in the Agulhas Return Current (ARC) frontal region is commonly manifested in the form of thermohaline intrusions, as sub-tropical and sub-polar water masses of similar density meet. In Jan/Feb 2012, the Naval Research Laboratory and collaborators carried out a field experiment in which seismic and traditional hydrographic observations were acquired to examine frontal zone mixing processes. The high lateral resolution (10 m) of the seismic observations allowed fine-scale lateral tracking of thermal intrusions, which were corroborated with simultaneous XBT casts. Between seismic deployments both salinity and temperature data were acquired via CTD, Underway-CTD and microstructure profiles. This study focuses on analyzing seismic reflection data in a particular E-W transect where the northward flowing ARC interacted with the southward flowing portion of a large anticyclonic eddy. Strong reflectors were most prominent at the edge of a hyperbolic zone formed between the eddy and ARC, where sub-polar waters interacted with waters of sub-tropical origin on either side. Reflectors were shallow within the hyperbolic zone and extended to 1200 m below the ARC. The nature of the observed reflectors will be determined from comparison of seismic reflection and derived ∂T/∂z fields, and XBT and TS profiles from the available hydrographic data.