1. Introduction

Process-oriented experiments with FVCOM were conducted to simulate the fluorescent dye trajectory that were injected and tracked on the southern flank of GB in late May 1999. The objective of this study is 1) to validate the accuracy of FVCOM to resolve the tracer movement under small-scale fluctuation flow field and 2) to examine physical processes controlling the cross-frontal water transport on the southern flank of GB. This project is funded by NSF-US GLOBEC/GB Phase IV Program.

The comparison was made with the concentration and trajectories of observed fluorescent dyes. Model results show that the dye movement depends on 1) vertical stratification and 2) temporal and spatial variability of the location and intensity of the tidal mixing front. Onset of vertical stratification tends to trap the dye within the bottom mixed layer by reducing vertical mixing. The small-scale meander of the tidal mixing front caused the dye to move onbank. Horizontal resolution of the model plays an essential role in the spatial distribution, movement and horizontal diffusion of the dye. A 500-m horizontal resolution seems to be a minimum requirement to resolve the spatial size of the dye and capture right physics of horizontal diffusion. Good agreements in locations and shapes of model-predicted and observed dyes after including the assimilation of the temperature and salinity data suggest that the cross-frontal water exchange in the southern flank of GB is mainly controlled by the buoyancy-induced secondary subtidal current rather than tidal residual flow. This study also raises a fundamental question on the requirement of the high-resolution measurement of the hydrographic data to resolve the cross-frontal water transport on the southern flank of GB.

The model results are being written up into a paper by C. Chen at SMAST/UMASSD, R. C. Beardsley at WHOI, R. Houghton at Lamont, and Q. Xu at SMAST/UMASSD. A PPT format poster is available at the MEDM Laboratory for people who are interested in learning more details about the model-dye comparison. Please contact C. Chen if one wants to receive the poster.

2. Design of numerical experiments

The model-dye experiments were conducted under the 1999 May flow field predicted by the FVCOM-based GoM/GB circulation model. The model was driven by the wind stress and heat flux re-produced by the meso-scale meteorological model (MM5) and realistic tidal forcings, with an initial field of T and S specified using the 1999 US GLOBEC/GB broad scale survey data on GB plus the April climatologic hydrographic field in adjacent computational regions. The 2 case studies were carried out: 1) simulated and assimilated. In the assimilated case, the hydrographic data during scanfish surveys were used to merge the model-predicted temperature and salinity fields to the observation. The model-data comparison was made with a focus on the case with a horizontal resolution of 0.5 km. Experiments were also made for different horizontal resolutions of 4, 2 and 0.25 km in the dye tracking area.

Fig. 1: Bathymetry of the GoM/GB region and unstructured grids of the FVCOM-based GoM/GB circulation model. Upper: an overview of the entire computational domain; middle: bathymetry and triangular grids with a horizontal resolution of 2 km; lower: bathymetry and triangular grids with a horizontal resolution of 0.5 km. The model dye was injected in control volumes closest to the bottom at 16: 33 GMT , 22 May, 1999 . Sections 1-3 were Scanfish transects made at 15:18 , 19: 27 , and 21:38 GMT , 23 May, 1999 , about one day after the dye was injected.

3. Model-data comparison of dye distributions

There are some issues that must be addressed on the model-data comparison of dye distributions. Firstly, the center of the fluorescent dye was determined with adjusted location using the instantaneous ship-board ADCP current. Since each dye survey usually took about 4-7 hours to cover the possible area of the dye concentration after the dye released and there are significant variations in tidal current, the center location of the dye concentration determined by “constant” instantaneous ADCP current was probably not very accurate. Secondly, there is some argument if the hydrographic data could be adjusted only based on the tidal current without consideration of local mixing. This problem would directly affect the accuracy of the determination for the center location of the dye concentration.

Therefore, comparing the model results with the empirically-determined center location of the fluorescent dye would cause misleading. This is the reason why we compared our model results directly with the center of the dye concentration measured on the Scanfish transects.

To make a fair comparison, we output the model-computed water temperature, salinity and dye concentration following all Scanfish tracks. Comparison was made for two cases: 1) simulation and 2) assimilation (with hydrographic data). Examples of these comparison results are shown in Fig. 2 where the white dashed line is the Scanfish tracks. The results used for the comparison are output from the case with a horizontal resolution of 500 m.

These comparison results clearly show that once the model is capable to reproduce the small-scale temperature and salinity structure, it can provide a very accurate simulation of the dye in both vertical and horizontal distributions. It also implies that the cross-isobath distribution of the dye concentration depends on 1) vertical stratification, and 2) location and intensity of the tidal mixing front.

We also compared the model-predicted locations of the center of the dye on the center of the dye measured on each Scanfish transect that were taken after the dye released. It should be noticed here that the location shown in Fig. 3 is not the location of the center of the dye in a 3-D view. It is the location of the center of the dye on individual Scanfish transect taken during different phases of the tide. The results demonstrate that under the condition of the accurate simulation of the small-scale variation of water temperature and salinity, FVCOM is capable to reproduce accurately the trajectory of the dye movement.

Fig. 3: Comparison between observed and model-predicted centers of the dye concentration at selected Scanfish track transect during the May 1999 dye experiment for simulated and assimilated cases. Note: the dye center defined here is the center of the vertically averaged dye concentration on the Scanfish track transect.

We have examined the physical mechanism responsible for the cross-isobath movement of the dye. In the simulation case, the model started with the setup of the initial field of the April climatologic water temperature and salinity. Because the climatologic field does not resolve the small-scale variation of temperature and salinity, the model-predicted center of the dye moves mainly along the local isobath. In the assimilation case, the model does predict the cross-isobath movement of the dye along with the small-scale meander of the tidal mixing front. In this case, the dye is still within the frontal zone, no cross-event occurs! If this model result is correct, it is questionable if the experiment with fluorescent dyes (which is usually valid for double of days) is sufficient enough to resolve the cross-frontal transport of the tracer. On the other hand, to simulate accurately the cross-isobath secondary circulation on the southern flank of GB, the model must resolve the small-scale field of water temperature and salinity. It is easy for configuring high resolution grids of FVCOM on GB. However, since meteorological forcing used to drive the model has a resolution of 10 km, it is difficult to resolve such small-scale variability of temperature and salinity in this domain. It is a challenging issue for the field measurement, too.

Fig. 4: Trajectories of the vertically-averaged dye concentration overlapping the field of the water temperature for simulated and assimilated cases. Contours: the vertically-averaged temperature; red dots: the locations of the center of the vertically-averaged dye concentration at every M 2 tidal cycle.