Chen Changsheng

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Chen

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Changsheng

ORCID

0000-0001-8715-6101

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Process modeling studies of physical mechanisms of the formation of an anticyclonic eddy in the central Red Sea

(John Wiley & Sons, 2014-02-25) Chen, Changsheng ; Li, Ruixiang ; Pratt, Lawrence J. ; Limeburner, Richard ; Beardsley, Robert C. ; Bower, Amy S. ; Jiang, Houshuo ; Abualnaja, Yasser ; Xu, Qichun ; Lin, Huichan ; Liu, Xuehai ; Lan, Jian ; Kim, Taewan

Surface drifters released in the central Red Sea during April 2010 detected a well-defined anticyclonic eddy around 23°N. This eddy was ∼45–60 km in radius, with a swirl speed up to ∼0.5 m/s. The eddy feature was also evident in monthly averaged sea surface height fields and in current profiles measured on a cross-isobath, shipboard CTD/ADCP survey around that region. The unstructured-grid, Finite-Volume Community Ocean Model (FVCOM) was configured for the Red Sea and process studies were conducted to establish the conditions necessary for the eddy to form and to establish its robustness. The model was capable of reproducing the observed anticyclonic eddy with the same location and size. Diagnosis of model results suggests that the eddy can be formed in a Red Sea that is subject to seasonally varying buoyancy forcing, with no wind, but that its location and structure are significantly altered by wind forcing, initial distribution of water stratification and southward coastal flow from the upstream area. Momentum analysis indicates that the flow field of the eddy was in geostrophic balance, with the baroclinic pressure gradient forcing about the same order of magnitude as the surface pressure gradient forcing.
Zonal surface wind jets across the Red Sea due to mountain gap forcing along both sides of the Red Sea

(American Geophysical Union, 2009-10-10) Jiang, Houshuo ; Farrar, J. Thomas ; Beardsley, Robert C. ; Chen, Ru ; Chen, Changsheng

Mesoscale atmospheric modeling over the Red Sea, validated by in-situ meteorological buoy data, identifies two types of coastal mountain gap wind jets that frequently blow across the longitudinal axis of the Red Sea: (1) an eastward-blowing summer daily wind jet originating from the Tokar Gap on the Sudanese Red Sea coast, and (2) wintertime westward-blowing wind-jet bands along the northwestern Saudi Arabian coast, which occur every 10–20 days and can last for several days when occurring. Both wind jets can attain wind speeds over 15 m s−1 and contribute significantly to monthly mean surface wind stress, especially in the cross-axis components, which could be of importance to ocean eddy formation in the Red Sea. The wintertime wind jets can cause significant evaporation and ocean heat loss along the northeastern Red Sea coast and may potentially drive deep convection in that region. An initial characterization of these wind jets is presented.
Using MM5 to hindcast the ocean surface forcing fields over the Gulf of Maine and Georges Bank Region

(American Meteorological Society, 2005-02) Chen, Changsheng ; Beardsley, Robert C. ; Hu, Song ; Xu, Qichun ; Lin, Huichan

The fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) is applied to the Gulf of Maine/Georges Bank (GoM/GB) region. This model is configured with two numerical domains with horizontal resolutions of 30 and 10 km, respectively, and driven by the NCAR-Eta weather model through a nested grid approach. Comparison of model-computed winds, wind stress, and heat flux with in situ data collected on moored meteorological buoys in the western GoM and over GB in 1995 shows that during the passage of atmospheric fronts over this region, MM5 provides a reasonable prediction of wind speed but not wind direction, and provides a relatively accurate estimation of longwave radiation but overestimates sensible and latent fluxes. The nudging data assimilation approach with inclusion of in situ wind data significantly improves the accuracy of the predicted wind speed and direction. Incorporation of the Fairall et al. air–sea flux algorithms with inclusion of Advanced Very High Resolution Radiometer (AVHRR)-derived SST improves the accuracy of the predicted latent and sensible heat fluxes in the GoM/GB region for both stable and unstable weather conditions.
A dike–groyne algorithm in a terrain-following coordinate ocean model (FVCOM) : development, validation and application

( 2012-01) Ge, Jianzhong ; Chen, Changsheng ; Qi, Jianhua ; Ding, Pingxing ; Beardsley, Robert C.

A dike-groyne module is developed and implemented into the unstructured-grid, three dimensional primitive equation Finite-Volume Coastal Ocean Model (FVCOM) for the study of the hydrodynamics around human-made construction in the coastal area. The unstructured-grid finite-volume flux discrete algorithm makes this module capable of realistically including narrow-width dikes and groynes with free exchange in the upper column and solid blocking in the lower column in a terrain-following coordinate system. This algorithm used in the module is validated for idealized cases with emerged and/or submerged dikes and a coastal seawall where either analytical solutions or laboratory experiments are available for comparison. As an example, this module is applied to the Changjiang Estuary where a dike-groyne structure was constructed in the Deep Waterway channel in the inner shelf of the East China Sea (ECS). Driven by the same forcing under given initial and boundary conditions, a comparison was made for model-predicted flow and salinity via observations between dike-groyne and bed-conforming slope algorithms. The results show that with realistic resolution of water transport above and below the dike-groyne structures, the new method provides more accurate results. FVCOM with this MPI-architecture parallelized dike-groyne module provides a new tool for ocean engineering and inundation applications in coastal regions with dike, seawall and/or dam structures.
Reply to comment on “Current separation and upwelling over the southeast shelf of Vietnam in the South China Sea”

(John Wiley & Sons, 2013-03-31) Chen, Changsheng ; Lai, Zhigang ; Beardsley, Robert C. ; Xu, Qichun ; Lin, Huichan ; Viet, Nguyen Trung ; Yang, Ding
Extratropical storm inundation testbed : intermodel comparisons in Scituate, Massachusetts

(John Wiley & Sons, 2013-10-07) Chen, Changsheng ; Beardsley, Robert C. ; Luettich, Richard A. ; Westerink, Joannes J. ; Wang, Harry ; Perrie, Will ; Xu, Qichun ; Donahue, Aaron S. ; Qi, Jianhua ; Lin, Huichan ; Zhao, Liuzhi ; Kerr, Patrick C. ; Meng, Yanqiu ; Toulany, Bash

The Integrated Ocean Observing System Super-regional Coastal Modeling Testbed had one objective to evaluate the capabilities of three unstructured-grid fully current-wave coupled ocean models (ADCIRC/SWAN, FVCOM/SWAVE, SELFE/WWM) to simulate extratropical storm-induced inundation in the US northeast coastal region. Scituate Harbor (MA) was chosen as the extratropical storm testbed site, and model simulations were made for the 24–27 May 2005 and 17–20 April 2007 (“Patriot's Day Storm”) nor'easters. For the same unstructured mesh, meteorological forcing, and initial/boundary conditions, intermodel comparisons were made for tidal elevation, surface waves, sea surface elevation, coastal inundation, currents, and volume transport. All three models showed similar accuracy in tidal simulation and consistency in dynamic responses to storm winds in experiments conducted without and with wave-current interaction. The three models also showed that wave-current interaction could (1) change the current direction from the along-shelf direction to the onshore direction over the northern shelf, enlarging the onshore water transport and (2) intensify an anticyclonic eddy in the harbor entrance and a cyclonic eddy in the harbor interior, which could increase the water transport toward the northern peninsula and the southern end and thus enhance flooding in those areas. The testbed intermodel comparisons suggest that major differences in the performance of the three models were caused primarily by (1) the inclusion of wave-current interaction, due to the different discrete algorithms used to solve the three wave models and compute water-current interaction, (2) the criterions used for the wet-dry point treatment of the flooding/drying process simulation, and (3) bottom friction parameterizations.
FVCOM validation experiments : comparisons with ROMS for three idealized barotropic test problems

(American Geophysical Union, 2008-07-26) Huang, Haosheng ; Chen, Changsheng ; Cowles, Geoffrey W. ; Winant, Clinton D. ; Beardsley, Robert C. ; Hedstrom, Kate S. ; Haidvogel, Dale B.

The unstructured-grid Finite-Volume Coastal Ocean Model (FVCOM) is evaluated using three idealized benchmark test problems: the Rossby equatorial soliton, the hydraulic jump, and the three-dimensional barotropic wind-driven basin. These test cases examine the properties of numerical dispersion and damping, the performance of the nonlinear advection scheme for supercritical flow conditions, and the accuracy of the implicit vertical viscosity scheme in barotropic settings, respectively. It is demonstrated that FVCOM provides overall a second-order spatial accuracy for the vertically averaged equations (i.e., external mode), and with increasing grid resolution the model-computed solutions show a fast convergence toward the analytic solutions regardless of the particular triangulation method. Examples are provided to illustrate the ability of FVCOM to facilitate local grid refinement and speed up computation. Comparisons are also made between FVCOM and the structured-grid Regional Ocean Modeling System (ROMS) for these test cases. For the linear problem in a simple rectangular domain, i.e., the wind-driven basin case, the performance of the two models is quite similar. For the nonlinear case, such as the Rossby equatorial soliton, the second-order advection scheme used in FVCOM is almost as accurate as the fourth-order advection scheme implemented in ROMS if the horizontal resolution is relatively high. FVCOM has taken advantage of the new development in computational fluid dynamics in resolving flow problems containing discontinuities. One salient feature illustrated by the three-dimensional barotropic wind-driven basin case is that FVCOM and ROMS simulations show different responses to the refinement of grid size in the horizontal and in the vertical.
Saltwater intrusion into the Changjiang River : a model-guided mechanism study

(American Geophysical Union, 2009-02-12) Xue, Pengfei ; Chen, Changsheng ; Ding, Pingxing ; Beardsley, Robert C. ; Lin, Huichan ; Ge, Jianzhong ; Kong, Yazhen

The Changjiang River (CR) is divided into a southern branch (SB) and a northern branche (NB) by Chongming Island as the river enters the East China Sea. Observations reveal that during the dry season the saltwater in the inner shelf of the East China Sea flows into the CR through the NB and forms an isolated mass of saltwater in the upstream area of the SB. The physical mechanism causing this saltwater intrusion has been studied using the high-resolution unstructured-grid Finite-Volume Coastal Ocean Model (FVCOM). The results suggest that the intrusion is caused by a complex nonlinear interaction process in relation to the freshwater flux upstream, tidal currents, mixing, wind, and the salt distribution in the inner shelf of the East China Sea. The tidal rectification, resulting from the interaction of the convergence or divergence of tidal momentum flux and bottom friction over abrupt topography, produces a net upstreamward volume flux from NB to SB. With river discharge the net water transport in the NB is driven through a momentum balance of surface elevation gradient forcing, horizontal advection, and vertical diffusion. In the dry season, reducing the surface elevation gradient forcing makes tidal rectification a key process favorable for the saltwater intrusion. A northerly wind tends to enhance the saltwater intrusion by reducing the seaward surface elevation gradient forcing rather than either the baroclinic pressure gradient forcing or the wind-driven Ekman transport. A convergence experiment suggests that high grid resolution (∼100 m or less) is required to correctly resolve the net water transport through the NB, particularly in the narrow channel on the northern coast of Chongming Island.
Effects of interannual environmental variability on the transport-retention dynamics in haddock Melanogrammus aeglefinus larvae on Georges Bank

(Inter-Research, 2013-07-30) Boucher, Jason M. ; Chen, Changsheng ; Sun, Yunfang ; Beardsley, Robert C.

Georges Bank is a region of high biological productivity characterized by a well-defined clockwise tidal rectified circulation gyre. Fluctuations in the year-class strength of haddock Melanogrammus aeglefinus on Georges Bank have been attributed to mortality during the highly vulnerable larval stages. While predation and starvation greatly impact on survival, advection to unfavorable regions may result in greatly reduced numbers of individuals. For successful self-recruitment to occur, individuals must remain within the shoal region of Georges Bank until settlement. An individual-based model (IBM) was utilized to estimate the retention of haddock eggs and larvae on Georges Bank annually for 1995 through 2009. The IBM was driven by the Finite-Volume Community Ocean Model (FVCOM) for the Gulf of Maine domain. Biological components of haddock larvae were omitted to restrict analyses to the impact of circulation on passive transport. Inflow over the Scotian Shelf and tidal interaction patterns were identified as the major drivers of variability in interannual transport-retention dynamics. The simulations indicated that retention rates were highest in 2000 and 2003, and lowest in 1997. The above-average retention in 2000 and 2003 with anomalously large recruitment only in 2003 indicates that high retention appears to be necessary but not sufficient to explain large recruitment events of haddock on Georges Bank.
Observing system simulation experiments of dissolved oxygen monitoring in Massachusetts Bay

(American Geophysical Union, 2012-05-08) Xue, Pengfei ; Chen, Changsheng ; Beardsley, Robert C.

Observing system simulation experiments (OSSEs) were performed in Massachusetts Bay for the design of optimal monitoring sites for dissolved oxygen (DO) measurements. Experiments were carried out using the Ensemble Kalman Filter (EnKF) for data assimilation with focus on initial and boundary perturbations. Running a well-validated water quality model with a perturbed initial field of DO but “true” boundary forcing conditions, the model is capable of restoring DO back to the true state without data assimilation over a recovery time scale of about a month. Since DO in Massachusetts Bay has a bay-wide correlation scale, placing a monitoring site of DO near the northern boundary or at a location that has maximum correlation to the entire domain can shorten the restoring time to a week. Running the model with perturbed boundary forcing without data assimilation, the results show that the errors propagate into Massachusetts Bay following the inflow from the northern boundary and spread southward to Cape Cod Bay over a time scale of about a month. Using a DO monitoring site located near the northern entrance, the data assimilation can efficiently control the error propagation and prevent the model field from deviating from the true state. The model shows that the inflow from the northern entrance, which is connected to the upstream Western Maine Coastal Current, plays an important role in controlling the DO variation in Massachusetts Bay, and the residence time of the bay controlled by this flow is about one month. Understanding the upstream boundary-control nature of this system is critical for optimal design of sampling strategies of water quality variables in this region.
Wetland-estuarine-shelf interactions in the Plum Island Sound and Merrimack River in the Massachusetts coast

(American Geophysical Union, 2010-10-16) Zhao, Liuzhi ; Chen, Changsheng ; Vallino, Joseph J. ; Hopkinson, Charles S. ; Beardsley, Robert C. ; Lin, Huichan ; Lerczak, James A.

Wetland-estuarine-shelf interaction processes in the Plum Island Sound and Merrimack River system in the Massachusetts coast are examined using the high-resolution unstructured grid, finite volume, primitive equations, coastal ocean model. The computational domain covers the estuarine and entire intertidal area with a horizontal resolution of 10–200 m. Driven by five tidal constituents forcing at the open boundary on the inner shelf of the eastern coast of the Gulf of Maine, the model has successfully simulated the 3-D flooding/drying process, temporal variability, and spatial distribution of salinity as well as the water exchange flux through the water passage between the Plum Island Sound and Merrimack River. The model predicts a complex recirculation loop around the Merrimack River, shelf, and Plum Island Sound. During the ebb tide, salt water in the Plum Island Sound is injected into the Merrimack River, while during flood tide, a significant amount of the freshwater in the Merrimack River is forced into Plum Island Sound. This water exchange varies with the magnitude of freshwater discharge and wind conditions, with a maximum contribution of ∼30%–40% variability in salinity over tidal cycles in the mouth of the Merrimack River. Nonlinear tidal rectification results in a complex clockwise residual recirculation loop around the Merrimack River, shelf, and Plum Island Sound. The net water flux from Plum Island Sound to the Merrimack River varies with the interaction between tide, river discharge, and wind forcing. This interaction, in turn, affects the salt transport from this system to the shelf. Since the resulting water transport into the shelf significantly varies with the variability of the wind, models that fail to resolve this complex estuarine and shelf system could either overestimate or underestimate the salt content over the shelf.
Complexity of the flooding/drying process in an estuarine tidal-creek salt-marsh system : an application of FVCOM

(American Geophysical Union, 2008-07-30) Chen, Changsheng ; Qi, Jianhua ; Li, Chunyan ; Beardsley, Robert C. ; Lin, Huichan ; Walker, Randy ; Gates, Keith

The tidal flooding/drying process in the Satilla River Estuary was examined using an unstructured-grid finite-volume coastal ocean model (FVCOM). Driven by tidal forcing at the open boundary and river discharge at the upstream end, FVCOM produced realistic tidal flushing in this estuarine tidal-creek intertidal salt-marsh complex, amplitudes and phases of the tidal wave, and salinity observed at mooring sites and along hydrographic transects. The model-predicted residual flow field is characterized by multiscale eddies in the main channel, which are verified by ship-towed ADCP measurements. To examine the impact of complex coastal geometry on water exchange in an estuarine tidal-creek salt-marsh system, FVCOM was compared with our previous structured-grid finite difference Satilla River Estuary model (ECOM-si). The results suggest that by failing to resolve the complex coastal geometry of tidal creeks, barriers and islands, a model can generate unrealistic flow and water exchange and thus predict the wrong dynamics for this estuary. A mass-conservative unstructured-grid model is required to accurately and efficiently simulate tidal flow and flushing in a complex geometrically controlled estuarine dynamical system.
Studies of the Canadian Arctic Archipelago water transport and its relationship to basin-local forcings : results from AO-FVCOM

(John Wiley & Sons, 2016-06-25) Zhang, Yu ; Chen, Changsheng ; Beardsley, Robert C. ; Gao, Guoping ; Lai, Zhigang ; Curry, Beth ; Lee, Craig M. ; Lin, Huichan ; Qi, Jianhua ; Xu, Qichun

A high-resolution (up to 2 km), unstructured-grid, fully coupled Arctic sea ice-ocean Finite-Volume Community Ocean Model (AO-FVCOM) was employed to simulate the flow and transport through the Canadian Arctic Archipelago (CAA) over the period 1978–2013. The model-simulated CAA outflow flux was in reasonable agreement with the flux estimated based on measurements across Davis Strait, Nares Strait, Lancaster Sound, and Jones Sounds. The model was capable of reproducing the observed interannual variability in Davis Strait and Lancaster Sound. The simulated CAA outflow transport was highly correlated with the along-strait and cross-strait sea surface height (SSH) difference. Compared with the wind forcing, the sea level pressure (SLP) played a dominant role in establishing the SSH difference and the correlation of the CAA outflow with the cross-strait SSH difference can be explained by a simple geostrophic balance. The change in the simulated CAA outflow transport through Davis Strait showed a negative correlation with the net flux through Fram Strait. This correlation was related to the variation of the spatial distribution and intensity of the slope current over the Beaufort Sea and Greenland shelves. The different basin-scale surface forcings can increase the model uncertainty in the CAA outflow flux up to 15%. The daily adjustment of the model elevation to the satellite-derived SSH in the North Atlantic region outside Fram Strait could produce a larger North Atlantic inflow through west Svalbard and weaken the outflow from the Arctic Ocean through east Greenland.
Influence of ocean freshening on shelf phytoplankton dynamics

(American Geophysical Union, 2007-12-28) Ji, Rubao ; Davis, Cabell S. ; Chen, Changsheng ; Townsend, David W. ; Mountain, David G. ; Beardsley, Robert C.

Climate change-induced freshening of the ocean can enhance vertical stratification and alter circulation patterns in ways that influence phytoplankton dynamics. We examined the timing of spring phytoplankton blooms and the magnitude of net primary productivity in the Nova Scotian Shelf (NSS) - Gulf of Maine (GoM) region with respect to seasonal and interannual changes in surface water freshening from 1998 to 2006. The general pattern of temporal westward progression of the phytoplankton bloom corresponds with the gradient of increasing sea surface salinity from the NSS in the east to the western GoM. Increased freshening enhances the spatial gradients in bloom timing by stimulating earlier blooms upstream (NSS), but it has less impact downstream (the western GoM). Strong spatial gradients (increasing westward) of mean chlorophyll concentration and net primary productivity during post-bloom months (May–June) indicate that lower sea surface salinity upstream can likely impede nutrient fluxes from deep water and therefore affect overall productivity.
Impacts of oceanic mixed layer on hurricanes: a simulation experiment with Hurricane Sandy

(American Geophysical Union, 2020-10-07) Li, Siqi ; Chen, Changsheng ; Wu, Zhongxiang ; Beardsley, Robert C. ; Li, Ming

Influences of the ocean mixed layer (OML) dynamics on intensity, pathway, and landfall of October 2012 Hurricane Sandy were examined through an experiment using the Weather Research and Forecasting (WRF) model. The WRF model was run for two cases with or without coupling to the OML. The OML in the WRF was calculated by an oceanic mixed layer submodel. The initial conditions of the depth and mean water temperature of the OML were specified using Global‐FVCOM and Global‐HYCOM fields. The comparison results between these two cases clearly show that including the OML dynamics enhanced the contribution of vertical mixing to the air‐sea heat flux. When the hurricane moved toward the coast, the local OML rapidly deepened with an increase of storm wind. Intense vertical mixing brought cold water in the deep ocean toward the surface to produce a cold wake underneath the storm, with the lowest sea temperature at the maximum wind zone. This process led to a significant latent heat loss from the ocean within the storm and hence rapid drops of the air temperature and vapor mixing ratio above the sea surface. As a result, the storm was intensified as the central sea level pressure dropped. Improving air pressure simulation with OML tended to reduce the storm size and strengthened the storm intensity and hence provided a better simulation of hurricane pathway and landfall.
Wind-induced interannual variability of sea level slope, along-shelf flow, and surface salinity on the Northwest Atlantic shelf

(John Wiley & Sons, 2014-04-16) Li, Yun ; Ji, Rubao ; Fratantoni, Paula S. ; Chen, Changsheng ; Hare, Jonathan A. ; Davis, Cabell S. ; Beardsley, Robert C.

In this study, we examine the importance of regional wind forcing in modulating advective processes and hydrographic properties along the Northwest Atlantic shelf, with a focus on the Nova Scotian Shelf (NSS)-Gulf of Maine (GoM) region. Long-term observational data of alongshore wind stress, sea level slope, and along-shelf flow are analyzed to quantify the relationship between wind forcing and hydrodynamic responses on interannual time scales. Additionally, a simplified momentum balance model is used to examine the underlying mechanisms. Our results show significant correlation among the observed interannual variability of sea level slope, along-shelf flow, and alongshore wind stress in the NSS-GoM region. A mechanism is suggested to elucidate the role of wind in modulating the sea level slope and along-shelf flow: stronger southwesterly (northeastward) winds tend to weaken the prevailing southwestward flow over the shelf, building sea level in the upstream Newfoundland Shelf region, whereas weaker southwesterly winds allow stronger southwestward flow to develop, raising sea level in the GoM region. The wind-induced flow variability can influence the transport of low-salinity water from the Gulf of St. Lawrence to the GoM, explaining interannual variations in surface salinity distributions within the region. Hence, our results offer a viable mechanism, besides the freshening of remote upstream sources, to explain interannual patterns of freshening in the GoM.
Impact of high-frequency nonlinear internal waves on plankton dynamics in Massachusetts Bay

(Sears Foundation for Marine Research, 2010-03-01) Lai, Zhigang ; Chen, Changsheng ; Beardsley, Robert C. ; Rothschild, Brian ; Tian, Rucheng

A simple Nutrient-Phytoplankton-Zooplankton (NPZ) model was coupled with the non-hydrostatic Finite-Volume Coastal Ocean Model (FVCOM-NH) to study the impact of high-frequency nonlinear internal waves on plankton dynamics in Massachusetts Bay (MB) during the stratified summer season. The temporal and spatial variability of phytoplankton concentration follows the vertical isopycnal displacement to the lowest order as the waves are generated by the semidiurnal tidal flow over Stellwagen Bank (SB) and propagate westward across MB. The tidally-averaged distribution of phytoplankton is characterized by three distinct zones of low subsurface concentration: (I) the western flank of Stellwagen Bank; (II) the center of Stellwagen Basin; and (III) the upper western flank of Stellwagen Basin. The result of a model dye experiment suggests that these zones are created by the following physical processes which are dominant in each zone: (I) hydraulic jump; (II) strong internal wave-tidal current nonlinear interaction; and (III) energetic internal wave dissipation and subsequent mixing processes. The nonlinear interaction of the internal waves and offshore tidal currents significantly enhances the vertical velocity, and increases wave dissipation, thus causing an onshore transport of phytoplankton in zone II. Although the phytoplankton patchy structure can be produced using the hydrostatic FVCOM, the resulting phytoplankton concentration is overestimated due to the unrealistic intensification of vertical velocity and thus vertical nutrient flux from the deep water. It suggests that non-hydrostatic dynamics should be considered for certain small-scale biological processes that are driven primarily by the physics.
Surface circulation in Block Island Sound and adjacent coastal and shelf regions : a FVCOM-CODAR comparison

( 2016-02-29) Sun, Yunfang ; Chen, Changsheng ; Beardsley, Robert C. ; Ullman, Dave ; Butman, Bradford ; Lin, Huichan

CODAR-derived surface currents in Block Island Sound over the period of June 2000 through September 2008 were compared to currents computed using the Northeast Coastal Ocean Forecast System (NECOFS). The measurement uncertainty of CODAR-derived currents, estimated using statistics of a screened nine-year time series of hourly-averaged flow field, ranged from 3-7 cm/s in speed and 4°-14° in direction. The CODAR-derived and model-computed kinetic energy spectrum densities were in good agreement at subtidal frequencies, but the NECOFS-derived currents were larger by about 28% at semi-diurnal and diurnal tidal frequencies. The short-term (hourly to daily) current variability was dominated by the semidiurnal tides (predominantly the M2 tide), which on average accounted for ~87% of the total kinetic energy. The diurnal tidal and subtidal variability accounted for ~4% and ~9% of the total kinetic energy, respectively. The monthly-averaged difference between the CODAR-derived and model-computed velocities over the study area was 6 cm/s or less in speed and 28° or less in direction over the study period. An EOF analysis for the low-frequency vertically-averaged model current field showed that the water transport in the Block Island Sound region was dominated by modes 1 and 2, which accounted for 89% and 7% of the total variance, respectively. Mode 1 represented a relatively stationary spatial and temporal flow pattern with a magnitude that varied with season. Mode 2 was characterized mainly by a secondary cross-shelf flow and a relatively strong along-shelf flow. Process-oriented model experiments indicated that the relatively stationary flow pattern found in mode 1 was a result of tidal rectification and its magnitude changed with seasonal stratification. Correlation analysis between the flow and wind stress suggested that the cross-shelf water transport and its temporal variability in mode 2 were highly correlated to the surface wind forcing. The mode 2 derived onshore and offshore water transport, and was consistent with wind-driven Ekman theory. The along-shelf water transport over the outer shelf, where a large portion of the water flowed from upstream Nantucket Shoals, was not highly correlated to the surface wind stress.
Variability of currents in Great South Channel and over Georges Bank : observation and modeling

(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1992-06) Chen, Changsheng

This thesis consists of two parts: (I) variability of currents and water properties in late spring in the northern Great South Channel and (II) numerical study of stratified tidal rectification over Georges Bank. In part I, the combined analysis of CTD, ADCP, satellite-track drifters, and zooplankton distributions in the northern Great South Channel clearly shows (1) the seasonal evolution of the surface low salinity plume, (2) the threedimensional structure of residual flow, and (3) a coherent relationship between the surface low salinity plume and high concentration of zooplankton. In part II, the numerical model of Georges Bank shows that as the fluid becomes stratified , tidal mixing and rectification intensify both along- and cross-bank residual currents and modify the vertical structure of the flow. Along- and cross-bank residual currents increase as either stratification increases or the depth of the bank decreases. Model results over Georges Bank are in good agreement with observation, particularly in the position of the tidal mixing front and residual currents on the northern flank of the Bank.
Wind-induced, cross-frontal exchange on Georges Bank : a mechanism for early summer on-bank biological particle transport

(American Geophysical Union, 2003-11-29) Chen, Changsheng ; Schlitz, Ronald J. ; Lough, R. Gregory ; Smith, Keston W. ; Beardsley, Robert C. ; Manning, James P.

Water exchange across the tidal-mixing front on the southern flank of Georges Bank (GB) is examined using a two-dimensional (2D) primitive equation ocean model. The model domain features a cross-frontal transect including a June 1999 hydrographic (CTD)/ADCP study made as part of the U.S. GLOBEC Northwest Atlantic/Georges Bank program. The model was initialized with temperature and salinity fields taken on the 15 June 1999 CTD section and run prognostically with tidal forcing, measured winds, and representative surface heat flux. The results show that fluctuations of wind plus tidal mixing can play the following essential role in the short-term transport of water and particles from the stratified region to the mixed region on GB in early summer, when stratification is just developing with a weak thermocline at a depth of about 10 m. First, a passing weather front drives a wind-induced on-bank Ekman transport of the upper part of the water column at the tidal-mixing front and associated particles in the surface mixed layer. Then, when the wind relaxes or changes direction, the water in the on-bank extension of the front (above the thermocline) mixes quickly through enhanced tidal motion in shallower depths of water. As a result, particles that are advected along the extended front stay in the previously well-mixed region of the bank. Surface heating tends to increase the strength of the thermocline and reduce the thickness of the surface mixed layer. This in turn accelerates the on-bank movement of the front under an easterly wind favorable for Ekman transport and thus enhances the on-bank, cross-frontal transport of particles. Since the wind-induced, cross-frontal on-bank transport of water can occur episodically during passages of meteorological fronts, these could produce a larger net cross-frontal flux than that produced by just tidal forcing on equivalent timescales. Therefore wind-induced processes can be important in the on-bank cross-frontal flux of copepods and other zooplankton species that exhibit shallow maxima in their vertical distributions over the southern flank of GB in early summer.

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