Changsheng Chen

Comparison of winter and summer hydrographic observations in the Yellow and East China Seas and adjacent Kuroshio during 1986

Abstract Two regional hydrographic surveys conducted in January and July 1986, aboard the R.V. Thompson and R.V. Washington illustrate the seasonal change in water properties from winter to summer in the Yellow and East China Seas (YECS) and adjacent Kuroshio. In January 1986, water over the shelf in the YECS was locally well mixed in the vertical, and the horizontal distribution of water properties was dominated by a large tongue or plume of relatively fresh Yellow Sea Cold Water (YSCW) flowing southeastward along the Chinese margin into the East China Sea. To the east of this plume, along the Korean margin, was found the more saline Yellow Sea Warm Water (YSWW). The Kuroshio front in the East China Sea was located at the shelf break, separating the warmer, more saline Kuroshio water from the relatively well-mixed cooler, less saline coastal water. Evidence of mixing between these two water masses was observed but limited to near the shelf break. In July 1986, water over the shelf in the YECS was strongly stratified everywhere except within tidally mixed areas near the coast. The surface water distribution in the YECS was dominated by a bubble or lens of Changjiang dilute water located to the northeast of the Changjiang mouth, and the bottom YSCW intensified and extended southward to the shelf break. The relatively fresh coastal water from the East China Sea shelf extended far past the shelf break over the Kuroshio near the surface, and in turn, Kuroshio water intruded onto the shelf near the bottom. Mixing between the Kuroshio and coastal water was found over much of the mid- and outer shelf and upper slope, spanning a cross-stream distance of 75 km. The seasonal freshening due to the Changjiang discharge contributed directly to the summer increase in freshwater transport in the upper Kuroshio. In addition, evidence of deep vertical mixing within the Kuroshio itself was found near 32.0°N, 128.2°E, most likely due to a mesoscale eddy found near there and internal tidal mixing over the slope.

Plankton production in tidal fronts: A model of Georges Bank, in summer

A two-dimensional (x,z) coupled physical-biological model of the plankton on Georges Bank during the summer was developed. The physical portion included a primitive-equation turbulence-closure model with topography-following a coordinate. The biological model was a simple N-P-Z model. Tidal forcing at the model boundary generated a well-mixed region on the top of the bank, and strong tidal fronts at the bank edges. Biological fields were homogenized on the bank, while pronounced phytoplankton patches and horizontal gradients in properties developed in the fronts. The biomasses and fluxes of biological variables in the model agreed well with field estimates from Georges Bank. The phytoplankton in the well-mixed region of the bank were found to be nutrient replete, with f ratios of about 0.3. Values up to 0.7 were found for the f ratios in the fronts, where phytoplankton patches were supported by vertical fluxes of nutrients from below the euphoric zone. While the patterns of patchiness in the fronts were stable between tidal periods, the structure of patches and fluxes changed dramatically during a tidal cycle. Enhanced vertical mixing and horizontal gradients formed during a brief period of the tide, accounting for much of the cross-frontal nutrient flux. Sampling in such a dynamic system would be very difficult, and probably miss the essential features.

A Numerical Study of Stratified Tidal Rectification over Finite-Amplitude Banks. Part I: Symmetric Banks

Abstract Tidal rectification over a two-dimensional finite-amplitude symmetrical bank is studied using the Blumberg and Mellor primitive equation coastal ocean circulation model (ECOM-si). In the homogeneous case, the nonlinear interaction of tidal currents with the variable bottom topography generates an along-isobath residual circulation over the bank, which tends to increase as either the slope or height of the bank is increased. In the stratified case, internal waves at tidal and higher frequencies are generated over the sloping sides of the bank. Tidal mixing occurs in the bottom boundary layer, leading to horizontal tidal mixing fronts. The resulting stratified tidal rectification associated with the tidal mixing front, the generation of internal tides, and the modification of internal friction due to stratification leads to a subsurface intensification of the along-isobath residual current at the front and at the top of the bottom mixed layer over the slope, and a cross-bank double cell circulation...

Abundance, distribution and population structure of the copepod Calanus finmarchicus in a springtime right whale feeding area in the southwestern Gulf of Maine

Abstract Springtime aggregations of the planktivorous right whale ( Eubalaena glacialis ) occur in the northern Great South Channel region of the southwestern Gulf of Maine, where they feed upon dense concentrations of the copepod Calanus finmarchicus . This association was studied during the multidisciplinary South Channel Ocean Productivity Experiment (SCOPEX) in 1988 and 1989. The spatial and temporal variability of the abundance, geographic distribution, and population structure of these copepods were analyzed using data from 99 vertically-stratified or horizontally-sequenced MOCNESS plankton tows. Higher water column abundances and higher relative proportion of older copepod lifestages occurred near feeding whales compared to sites without whales, but total water column copepod biomass and Calanus abundance did not always differ between these types of locations. This suggests that the whales seek out aggregations of older copepod lifestages rather than simply the most dense aggregations. Other factors (and perhaps an element of chance) may influence which specific patches, among all patches potentially suitable in terms of copepod abundance and age composition, the whales utilize at a particular time. The times and locations of the highest Calansus water column abundances varied between years, as did the presence of feeding whales, probably because of year-to-year differences in the springtime temperature cycle and current strength. A temporal progression of lifestages occurred within the region in both years during the roughly 3-week duration of each survey, indicative of a growing rather than a diapausing population, at least up to the copepodite 4 (C4) stage. Due in part to a delay in the springtime warming in 1989 compared to 1988, the copepod development cycle, which is largely driven by in situ temperature, was delayed about 1–2 weeks in 1989. Peak abundances of younger Calanus were found in the northwestern part of the region each year, whereas peak abundances of older Calanus were found in the southwestern and northeastern part. This was probably due to the advection of maturing copepods by the regional circulation, especially the near-surface current associated with the movement of the low-salinity surface plume which forms each spring off Cape Cod. The copepod development cycle occurs within a moving frame of reference (i.e. the water itself); thus, peak abundances of the older copepods (those fed on by the whales) occurred later in the spring and further downstream in 1989 (when there were colder springtime temperatures and faster currents) than in 1988 (when the springtime temperatures were warmer and currents slower). Maximum Calanus abundances and biomass and water-colum abundances of older copepodite stages were significantly higher (about double) in 1989 than in 1988, both in the region as a whole and at sites where whales were feeding. Maximum concentrations from the MOCNESS tows were 13,300 m −3 in 1988 and 30,800 m −3 in 1989; however, a thin, visibly-red surface patch of Calanus , sampled in 1989 by a bucket, had a concentration of 331,000 m −3 . Copepods were also more aggregated in the vertical (i.e. more highly concentrated at the depth of maximum abundance) in 1989 than in 1988, and samples from whale-feeding areas were more homogeneous in composition (higher proportion of Calanus relative to all zooplankton) in 1989. At smaller spatial and temporal scales, abundances varied by a factor of 1–890 X in samples from horizontal tows spanning about 0.5–1.5 km and by a factor of 1–50 X over 24 h in the same geographic location in whale-feeding areas. Some of this variability was probably due to advection by the semidiurnal tidal currents. Near feeding whales, the copepod spatial distribution was patchy on small scales (with an estimated mean patch “size” of about 500 m), but the patchiness varied in texture interannually. Copepod abundances were much lower in early spring (March 1988) than in later spring (May 1988), with the March population structure dominated by adult females and the May population dominated by copepodite 4 and 5 stages (C4 and C5).

Near‐inertial oscillations over the Texas‐Louisiana shelf

Wind-induced, near-inertial oscillations over the Texas-Louisiana shelf in spring and summer 1992 are described using the current and wind observations taken during the first year of the Texas-Louisiana Shelf Circulation and Transport Processes Study (LATEX A). Rotary spectral analysis shows clockwise-rotating energy peaks at near-inertial frequencies for records from all current meter moorings after the suppression of principal tidal signals. The vertical structure of near-inertial oscillations is characterized by a first baroclinic mode with a near 180° phase difference between the upper mixed layer and the lower stratified layer. The oscillations are intermittent with a modulation timescale of about 5–10 days. They are surface-intensified and have maximum values near the shelf break, decaying gradually toward the coast but rapidly offshore. Near-inertial oscillations appear to accompany a sudden change of the wind stress during frontal passages. Diagnostic analysis suggests that the large near-inertial oscillations over the LATEX shelf are mainly generated by high-frequency (near-inertial) variation of the wind stress accompanying the passage of atmospheric fronts. When the downward transfer of the near-inertial energy to the deep stratified layer is small, a simple mixed layer model forced by the observed wind stress provides a reasonable prediction of the near-inertial currents in the mixed layer.

Prognostic Modeling Studies of the Keweenaw Current in Lake Superior. Part I: Formation and Evolution

Abstract The formation and evolution of the Keweenaw Current in Lake Superior were examined using a nonorthogonal-coordinate primitive equation numerical model. The model was initialized by the monthly averaged temperaturefield observed in June and September 1973 and run prognostically under different forcing conditions with and without winds. As a Rossby adjustment problem, the model predicted the formation of a well-defined coastal current jet within an inertial period of 16.4 h after the current field adjusted to the initial temperature field. The magnitude and direction of this current jet varied with the cross-shelf temperature gradient and wind velocity. It tended to intensify during northeastward (downwelling favorable) winds, and to lessen, or even reverse, during southwestward to northwestward (upwelling favorable) or southeastward (downwelling favorable) winds. In a case with strong stratification and without external atmospheric forcings, a well-defined clockwise warm-core eddy formed near the ...

Influences of suspended sediments on the ecosystem in Lake Michigan: a 3-D coupled bio-physical modeling experiment

The influence of suspended sediments on the Lake Michigan ecosystem was examined using a 3-dimensional (3-D) coupled biological and physical model developed by Chen et al. (part I). The model was driven by the realistic meteorological forces observed in March 1998, with daily inputs of suspended sediment concentration that were derived from temporally and spatially interpolated satellite imagery. The model results show the significant impact of a seasonally recurring coastal resuspension plume on the spatial and temporal variation of the nutrients and plankton in southern Lake Michigan. The plume-released nutrients played an essential role in maintaining the nutrient level in the lake. Although the growth of phytoplankton in the plume depended on the availability of nutrients and light, the offshore decrease in phytoplankton biomass still satisfied the Sverdrup’s relationship. Cross-shore fluxes of nutrients and phytoplankton were controlled by episodic wind events with a period of 5–7 days: offshore during southward winds and onshore during northward winds. The flux estimates for biological variables suggest that the microbial food web is a key contributor to secondary production in southern Lake Michigan and the lower trophic level food web system could be dynamically divided into two decoupled loops: (1) detritus–bacteria–microzooplankton–large zooplankton; and (2) nutrient–phytoplankton–detritus. The model-predicted spatial distributions of nutrients and phytoplankton were in reasonable agreement with observations taken during the 1998 EEGLE interdisciplinary cruises, suggesting that the model was sufficiently robust to capture the basic characteristics of the Lake Michigan ecosystem during the plume event.

Prognostic Modeling Studies of the Keweenaw Current in Lake Superior. Part II: Simulation

The Keweenaw Current, observed along the coast of the Keweenaw Peninsula in Lake Superior during July 1973, was simulated using a 3D, nonorthogonal coordinate transformation, primitive equation coastal ocean model. The model domain covered the entire lake with a high resolution of 250‐600 m in the cross-shelf direction and 4‐6 km in the alongshelf direction along the peninsula. The model was initialized using the monthly averaged temperature field observed in June 1973 and was run prognostically with synoptic wind forcing plus monthly averaged heat flux. Good agreement was found between model-predicted and observed currents at buoy stations near Eagle Harbor. Comparison of the model results with and without inclusion of heat flux suggested that combined wind and heat fluxes played a key role in the intensification of the Keweenaw Current during summer months. The model-predicted relatively strong near-inertial oscillations occurred episodically under conditions of a clockwise-rotating wind. These oscillations intensified at the surface, were weak near the coast, and increased significantly offshore.

The structure of the Kuroshio southwest of Kyushu: velocity, transport and potential vorticity fields

Abstract A triangular CTD/ADCP survey was made across the Kuroshio southwest of Kyu shu abroad the R. V. Thompson during January 1986. Due to relatively poor navigation data, a simple averaging technique has been used to convert the ADCP data taken between CTD station pairs into an average absolute velocity normal to the station pair, with a maximum error varying from about ±5 cm s −1 to less than ±0.1 cm s −1 depending on the type of navigation data. The average ADCP velocity at 60 m (or 10 m over the shelf) was then used as the reference velocity to calculate the absolute geostrophic velocity through the sides of the study triangle. The results show that the ADCP velocity shear was in reasonably good agreement with the geostrophic shear of the Kuroshio. The Kuroshio entered the study triangle as a coherent current and then split around a tall seamount into two branches as it left the area. The volume transport of the Kuroshio southwest of Kyushu in January 1986 was 30.3 ± 2.0 Sv, and the advective temperature transport was 27.6 ± 1.8 × 10 14 W. These values are similar to those reported for the Gulf Stream in the Florida Strait, and a roughly linear correlation exists between temperature and volume transports in both regions. Mass conservation within the study triangle allowed construction of a streamfunction that showed the presence of cyclonic and anti-cyclonic mesoscale eddies to the north and northwest of the core of the Kuroshio. Potential vorticity estimated from the absolute geostrophic velocity field was conserved along streamlines on potential density surfaces except perhaps near a seamount where curvature vorticity must be considered, and the path of the Kuroshio could be traced by the core of maximum potential vorticity. Finally, the Kuroshio was potentially unstable as it flowed along the continental margin in the Okinawa Trough because the gradient of potential vorticity on potential density surfaces changed sign across the Kuroshio. This helps explain the mesoscale frontal and eddy features observed in the cyclonic side of the Kuroshio in the East China Sea.

Variability of currents in late spring in the northern Great South Channel

Abstract The residual flows computed from detided shipboard ADCP data collected in late spring 1988 and 1989 clearly show different circulation patterns in the near-surface and deeper regions in the Great South Channel (GSC). In the upper 50 m, the residual flow in the northern GSC consists of three principal currents: (1) a southward coastal current located along the western flank of the GSC; (2) a broad cyclonic circulation crudely following the local topography in the interior region of the northern GSC; and (3) a northeastward current along the western flank of Georges Bank. Below 50 m, the residual flow tends to be cyclonic along the local 100-m isobath in the northern GSC. These circulation patterns are consistent with the vertical distributions of water properties and the trajectories of satellite-tracked drifters drogued at 5 and 50 m. Comparisons with geostrophic current shears and numerical model results suggest that the residual flow in spring is driven primarily by tidal rectification over the shallower sides of the northern GSC and by buoyancy forcing over the deeper flanks of the GSC. The southward transports of low-salinity plume surface water and Maine Intermediate Water (MIW) were about 0.07 ± 0.03 Sv and 0.31 ± 0.38 Sv in April 1988 and about 0.12 ± 0.06 Sv and 0.66 ± 0.14 Sv in June 1989. The larger transports of low-salinity plume water and MIW found in June 1989 are believed to be due to the increased freshwater river discharge in 1989 and occurrence of a subsurface coastal jet current along the western flank of the GSC.