Dong-Ping Wang

Transports and tidal current estimates in the Taiwan Strait from shipboard ADCP observations (1999–2001)

Tidal and mean flows in the Taiwan Strait are obtained from analysis of 2.5 years (1999–2001) of shipboard ADCP data using a spatial least-squares technique. The average tidal current amplitude is 0.46 ms � 1 , the maximum amplitude is 0.80 ms � 1 at the northeast and southeast entrances and the minimum amplitude is 0.20 ms � 1 in the middle of the Strait. The tidal current ellipses derived from the shipboard ADCP data compare well with the predictions of a high-resolution regional tidal model. For the mean currents, the average velocity is about 0.40 ms � 1 . The mean transport through the Strait is northward (into the East China Sea) at 1.8 Sv. The transport is related to the along Strait wind by a simple regression, transport (Sv)=2.42 + 0.12 � wind (ms � 1 ). Using this empirical formula, the maximum seasonal transport is in summer, about 2.7 Sv, the minimum transport is in winter, at 0.9 Sv, and the mean transport is 1.8 Sv. For comparison, this result indicates that the seasonal amplitude is almost identical to the classical estimate by Wyrtki (Physical oceanography of the southeast Asian waters, scientific results of marine investigations of the South China Sea and Gulf of Thailand, 1959–1961. Naga Report 2, Scripps Institute of Oceanography, 195 pp.) based on the mass balance in the South China Sea, while the mean is close to the recent estimate by Isobe [Continental Shelf Research 19 (1999) 195] based on the mass balance in the East China Sea. 2003 Elsevier Science B.V. All rights reserved.

Evolution of the Macondo well blowout: Simulating the effects of the circulation and synthetic dispersants on the subsea oil transport

During the Deepwater Horizon incident, crude oil flowed into the Gulf of Mexico from 1522 m underwater. In an effort to prevent the oil from rising to the surface, synthetic dispersants were applied at the wellhead. However, uncertainties in the formation of oil droplets and difficulties in measuring their size in the water column, complicated further assessment of the potential effect of the dispersant on the subsea-to-surface oil partition. We adapted a coupled hydrodynamic and stochastic buoyant particle-tracking model to the transport and fate of hydrocarbon fractions and simulated the far-field transport of the oil from the intrusion depth. The evaluated model represented a baseline for numerical experiments where we varied the distributions of particle sizes and thus oil mass. The experiments allowed to quantify the relative effects of chemical dispersion, vertical currents, and inertial buoyancy motion on oil rise velocities. We present a plausible model scenario, where some oil is trapped at depth through shear emulsification due to the particular conditions of the Macondo blowout. Assuming effective mixing of the synthetic dispersants at the wellhead, the model indicates that the submerged oil mass is shifted deeper, decreasing only marginally the amount of oil surfacing. In this scenario, the oil rises slowly to the surface or stays immersed. This suggests that other mechanisms may have contributed to the rapid surfacing of oil-gas mixture observed initially. The study also reveals local topographic and hydrodynamic processes that influence the oil transport in eddies and multiple layers. This numerical approach provides novel insights on oil transport mechanisms from deep blowouts and on gauging the subsea use of synthetic dispersant in mitigating coastal damage.

A study of the omega equation for diagnosing vertical motions at ocean fronts

Estimation of vertical velocity is a key issue for understanding ocean physics and transport of biogeochemical tracers. We examine the accuracy of estimating vertical velocity in fronts with the omega equation. The diagnostic performance of the omega equation is evaluated by using vertical velocities obtained from simulation of frontal instabilities in a primitive equation model as a reference. We use two traditional quasigeostrophic methods in which the flow is either a geostrophic flow computed from density or a nondivergent flow derived from vorticity and also test two new formulations: a quasigeostrophic method using the total flow field and the semigeostrophic omega equation. Results show that all four formulations correctly diagnose the vertical velocity pattern. However, estimates provided by the traditional quasi-geostrophic formulations have systematic bias. In contrast, the two new techniques, which are practically equivalent, produce unbiased vertical velocity diagnostic at fronts. These results point out the importance of including higher order dynamics than quasigeostrophy to take into account the ageostrophic advection in the front. Since adequate filtering of ADCP data is not yet available to obtain a suitable total flow, the semigeostrophic omega equation is proposed as the most valuable tool to compute vertical velocities from high resolution density measurements.

Near‐inertial motion on the shelf‐slope front off northeast Spain

Near-inertial motion on the shelf-slope front off northeast Spain was monitored using surface drifters and moored current meters. On the shelf, strong inertial currents were generated by a wind burst. The inertial current amplitude was about 70 cm/s at the surface, 30 cm/s at the base of the mixed layer, and 10 cm/s in the interior. The observed near-inertial frequency on the shelf was about 10% lower than the local inertial frequency, suggesting that the near-inertial motion was embedded in region of strong anticyclonic shear. Also, the phase of near-inertial motion increased through the water column, indicating that the energy propagation was downward. By contrast, the surface inertial currents were only about 10 cm/s in the center of the shelf-slope front. Indirect evidence suggests that the observed small surface inertial currents were the result of rapid downward transfer of near-inertial energy in the front.

Software, data and modelling news: sbPOM: A parallel implementation of Princenton Ocean Model

This paper presents the Stony Brook Parallel Ocean Model (sbPOM) for execution on workstations, Linux clusters and massively parallel supercomputers. The sbPOM is derived from the Princenton Ocean Model (POM), a widely used community ocean circulation model. Two-dimensional data decomposition of the horizontal domain is used with a halo of ghost cells to minimize communication between processors. Communication consists of the exchange of information between neighbor processors based on the Message Passing Interface (MPI) standard interface. The Parallel-NetCDF library is also implemented to achieve a high efficient input and output (I/O). Parallel performance is tested on an IBM Blue Gene/L massively parallel supercomputer, and efficiency using up to 2048 processors remains very good.

Near-inertial motions in the coastal ocean

Internal-inertial waves are frequently observed in the upper ocean and below the thermocline. A three-dimensional general circulation model with turbulence-closure mixed layer is used to study the generation and propagation of near-inertial motion below the mixed layer. In particular, the problem of effect of a coastal wall on the wind induced inertial-internal wave field is re-examined using a fully nonlinear model. Responding to a wind pulse, a sharp wavefront propagates offshore. After the wavefront passage, strong near-inertial internal waves, marked by the tilting velocity isolines and the interface oscillations, are generated. The predicted near-inertial motion is consistent with the wave dispersion relation. Downward energy propagation occurs after the wavefront passage, and both kinetic and potential energy are strongly modified. After several inertial periods, the kinetic energy in the upper layer can be completely removed. The theoretical results, which are supported by observations, indicate that internal-inertial wave are important for mixing in the upper coastal ocean.

Multivariate objective analysis of the coastal circulation of Barbados, West Indies: implication for larval transport

A multivariate spatial objective analysis (MVOA) assimilating high spatio-temporal resolution of hydrographic (CTD) and acoustic (ADCP) observations near Barbados provided a comprehensive view of the local surface circulation (0–100 m) during early spring of two consecutive years (1996 and 1997). Significant submesoscale fluctuations of the velocity and salinity fields exhibit a very dynamic environment. In the middle of each cruise, lowsalinity water originating from the Amazon and entrained by a North Brazil Current Ring (NBCR) intruded from offshore and persisted on the west coast of Barbados throughout the rest of the survey. Principal component analysis (PCA) of velocity relative to the vertical structure and temporal factors in the study area demonstrated that the local circulation was mostly baroclinic and was dominated by a strong salinity front impinging on the island and large amplitude current reversals with a periodicity of ca. 20 d. During transition times, indicated by a change of the sign of the amplitude of the empirical orthogonal function (EOF), the flow became barotropic. This situation produced strong southward currents followed by the onset of vertical velocity shear. Most of the flow variability occurred in the upper 40 m of the water column, which was also found to be the depth of penetration of the low-salinity lenses. These results indicate that the NBCR structure was retained during both intrusions. Lagrangian trajectories using the MVOA currents were found to be consistent with in situ drifter trajectories, suggesting that the analyzed flow field is representative of the near-shore circulation. Tracking of particles released in the surface layer (0–20 m) from the reef shows a maximum residence time of 18 d indicating the possibility of larval retention within the island-scale flow field. Finally, our results suggest that MVOA, within its limitations, is a powerful tool that can be applied elsewhere to infer circulation and larval transport, even in situations when forcing is unknown. r 2002 Published by Elsevier Science Ltd.

Effects of small-scale wind on coastal upwelling with application to Point Conception

A three-dimensional, limited-area, ocean general circulation model is used to study the response of the coastal ocean to small-scale wind forcing. The model considers an idealized forcing of a semi-infinite, upwelling-favorable wind with a large positive wind curl at one end. Such a wind pattern is commonly found in the vicinity of capes and points where the synoptic-scale wind is disturbed by coastal mountain ranges. The model study includes both process and hindcast studies. The process study considers the upwelling spinup and relaxation, driven by a positive wind curl. It also examines the model sensitivity to wind patterns and an external pressure force. The model results show that a strong poleward alongshore pressure gradient is set up by a positive wind curl. This pressure force drives an inshore poleward current into the upwelling region during active wind forcing, and it causes a surge of warm water into the upwelling zone during wind relaxation. The hindcast study is applied to the 1983 Organization of Persistent Upwelling Structures (OPUS) observations near Point Conception off central California. The simulation uses an idealized geometry but with observed winds. During OPUS, a sudden reversal of the surface equatorward currents occurred in the midst of a steady upwelling-favorable wind. This unexpected coastal flow feature was faithfully reproduced in the model simulation.

Wavenumber Spectrum in the Gulf Stream from Shipboard ADCP Observations and Comparison with Altimetry Measurements

The wavenumber spectra for velocity and temperature in the Gulf Stream region are calculated from a decade (1994‐2004) of shipboard acoustic Doppler current profiler (ADCP) measurements taken as part of the Oleander Project. The velocity and temperature spectra have comparable magnitude, in terms of the kinetic and potential energy, and both indicate a k 23 slope in the mesoscales. In contrast, the corresponding velocity spectrum determined from satellite altimetry sea surface heights yields a significantly higher energy level and a k 22 slope. The discrepancy between altimeter-derived and directly measured velocity spectra suggests that altimetric velocity probably is contaminated by noise in sea surface height measurement. Also, the k 23 slope, which appears to be in agreement with two-dimensional quasigeostrophic turbulence theory, does not support the contemporary surface quasigeostrophic theory. These results highlight large gaps in the current understanding of the nature of surface geostrophic turbulence.

“Upwelling” and “cyclonic” regimes of the near‐surface circulation in the Santa Barbara Channel

The observed near-surface circulation in the Santa Barbara Channel indicates in particular two patterns: a dominant cyclonic circulation mode and a less frequent upwelling flow mode. To explain the dynamics that may govern these two flow regimes, momentum balance from a hindcast model of currents in the channel, forced by observed hourly winds and hydrographic data, was calculated. The along-channel balance was found to be between wind, which was eastward (i.e., equatorward), sea level tilt, which was westward (i.e., poleward), and Coriolis, which was westward if the wind was (1) intense west and east of the channel and was eastward if the wind was (2) weaker in the east. Wind condition 1 produced southward cross-channel flow in the midchannel, connected by eastward currents upstream (downstream) along the northern (southern) coast of the channel, while wind condition 2 produced northward cross-channel flow connected by cyclonic recirculation in the west and westward inflow in the east. It is suggested that the former corresponds to the dynamical balance that may occur in the upwelling flow mode, while the latter corresponds to the cyclonic circulation mode.