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Analysis of the 1999 Georges Bank Tidal Mixing Front Moored Array Data

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July 1, 2004 through June 30, 2005

Dr. Robert C. Beardsley and Dr. James Lerczak
Physical Oceanography Department
Woods Hole Oceanographic Institution

Program Manager: Dr. Lisa Dilling, NOAA – Office of Global Programs

As part of this NOAA/WHOI CICOR funded project, the PIs provided support for the processing and analysis of data from a moored array of instruments and other data deployed across the tidal mixing front (TMF) on the southeastern flank of Georges Bank in 1999 by Dr. Ronald Schlitz (NOAA/NMFS) as part of the U.S. GLOBEC Northwest Atlantic/Georges Bank Phase III field program. The focus of this analysis has been on determining the dynamics which drive the tidal and sub-tidal circulation in the vicinity of the TMF and quantifying the structure of the cross-frontal and along-frontal sub-tidal circulation and temperature field as they varied over the 160 day deployment period from early spring, when the water column was homogeneous, to summer, when the water column was highly-stratified offshore of the bank.

The tidal mixing front (TMF), which separates well-mixed water on the crest of Georges Bank from stratified water offshore, is a prominent feature along the perimeter of the bank from late spring through fall of each year. The exchange of physical and biological properties across the TMF influences the supply of nutrients for primary production on the bank, the retention of larval fish and their prey on the bank, and interactions between fish species, their prey and their predators. The advective heat flux driven by the circulation in the vicinity of the TMF may also regulate the heat budget on George Bank. The size and direction of biological exchange and heat fluxes are sensitive to the details of the circulation and thermal structure at the TMF, which varies on seasonal, spring/neap and weather-band time scales.

In this NOAA/WHOI CICOR funded project, the PIs used current, temperature, salinity and bottom pressure data from an array of moorings deployed on the southeastern flank of Georges Bank as well as wind stress data from an NDBC buoy (Fig. 1) to determine the dynamics which drive the tidal and sub-tidal circulation in the vicinity of the TMF and quantify the structure of the cross-frontal and along-frontal sub-tidal circulation as it varied over the deployment period. The array of seven moorings was deployed for a 160 day period during the spring and summer of 1999, spanning approximately 20 km in the cross-bank direction and centered on the 60 m isobath, the typical location of the TMF on the southern flank.

The tight spacing betw een moorings (2 to 3 km), allowed the PIs to determine the dominant balances in the sub-tidal, cross-front momentum equations and the detailed structure of the tidal and sub-tidal cross-front and along-front circulation during early spring, when the water column was well-mixed and the TMF was not present, and during the summer, when a strong thermal TMF was observed. Notably, the cross-frontal tidal stress terms associated with tidal rectification were directly calculated.

The work involved data processing, data quality control, as well as analysis and dynamical interpretation of the data. This work was principally undertaken by Dr. James Lerczak (WHOI).

Most of the processing of the acoustic Doppler current profiler (ADCP), temperature, salinity, and pressure data from the moored arrays was completed by the end of 2003. Inconsistencies in the orientation of the ADCP currents were identified, and corrected current time series were generated. Time series of processed currents, temperature, salinity, and pressure interpolated onto a common time base, and stored as MATLAB files were provided to Dr. Schlitz, and are available for use by other investigators, in particular other scientists involved in the Georges Bank/GLOBEC program.

The key scientific results from these analyses are:
• Vertically-averaged, subtidal flow is predominantly in the along-bank direction, clockwise around Georges Bank, with an amplitude as large as 25 cm/s (Figs. 2a and 2b). When stratification is weak (before day 120; Fig. 2c), the current is intermittent and varies significantly in amplitude. During the summer, when stratification is strong (Fig. 2c), the along-isobath flow is less intermittent and has an average amplitude of about 10 cm/s.
• Vertical shear in the subtidal, along-isobath currents increases significantly from spring to summer with the increase in stratification (Figure 2d). This shear is predominantly in thermalwind balance.
• Cross-bank, tidal Reynolds stress (tidal average of the nonlinear terms vuy and vvy) are dominant terms in the sub-tidal momentum budgets. The vertical structure and amplitude of the tidal Reynolds stress varies with stratification. For example, the vvy term is vertically uniform when stratification is weak and has a mode two structure when stratification is strong. This term may be important in driving the subtidal, cross-bank flow in the vicinity of the TMF.
• The TMF is apparent in the structure of the density field in the vicinity of the array, but not all the time. During the summer (after day 170), the stratification is stronger off-bank than onbank (Figure 3a), indicative of the presence of the TMF. This cross-bank variation of stratification is intermittent, at times during the summer being large (pink shaded region in Figure 3) while at other times during the summer being small (e.g., day 202). The presence of the front is also apparent the vertical structure of the cross-bank density gradient (Figure 3b). At the surface water tends to be denser on-bank than off-bank. However, near the bottom, offbank water tends to be denser than on-bank. The vertically-averaged density tends to be higher off-bank than off bank. This is consistent with the structure of a TMF, for which mixing of the water column is more efficient on-bank of the front and less so off-bank of the front. We believe the short period (~5 to 10 days) variations in stratification and cross-bank density gradient are due to the cross-bank advection of the TMF in the vicinity of the mooring array.
• The different dynamical balances between spring and summer conditions lead to differences in the cross-frontal and along-frontal circulation (Fig. 4). During the spring, the along-bank, sub-with an amplitude of about 8 cm/s. The cross-front circulation is mode-one in structure, with onbank flow near the bottom and offbank flow near the surface, and with an amplitude of about 3 cm/s (Fig. 4a). During the summer, a strong subsurface along-bank jet with an amplitude of 30 cm/s was  observed at the TMF. The sub-tidal, cross-front circulation is stronger and more complicated than during the spring. At the location of the jet, the cross-front flow is on-bank in the middle of the water column, with an amplitude of 5 cm/s. The flow is offbank near the surface and bottom of the water column (Fig. 4b).

Two manuscripts are in preparation. The first describes the tidal and sub-tidal barotropic (vertically-averaged) momentum budgets and determines the relative importance of tidal rectification, geostrophy and wind stress in driving the vertically-averaged sub-tidal circulation. This manuscript is anticipated to be completed by the end of spring, 2005. The second describes the vertically-varying sub-tidal circulation and temperature field at the mooring array. The different dynamics driving the cross-frontal circulation during homogeneous conditions and periods when the TMF is present are quantified. This manuscript is anticipated to be completed by the end of the summer, 2005.

The PIs also plan to collaborate with Dr. C. Chen (UMass Dartmouth) and Dr. Schlitz to make detailed comparisons of the structure of the TMF circulation, stratification and fluxes as determined from the mooring array data and those produced by the Finite Volume Coastal Ocean Model (FVCOM).

Lerczak, J., R. Schlitz, S. Lentz, and R. Beardsley. 2003. Sub-Tidal Circulation at the Tidal Mixing Front: Analysis of the Moored Instrument Array. Presentation at the U.S. Globec Georges Bank Science Meeting. 18-20 November 2003, Rhode Island.

Lerczak, J. A., R. J. Schlitz, R. C. Beardsley. 2003. The seasonally-varying, sub-tidal structure of the tidal mixing front on the southern flank of Georges Bank. Eos Trans. AGU, 84(52), Ocean Sci. Meet. Suppl., Abstract OS51J-09.

Lerczak, J. A., R. C. Beardsley, and R. J. Schlitz. 2004. The dynamics and cross-frontal circulation at a tidal mixing front under well-mixed and stratified conditions. Eos Trans. AGU, 85(47), Fall Meet. Suppl., Abstract OS33D-04.

The tidal mixing front mooring array was collected by Dr. Ronald Schlitz (NOAA/NMFS/NEFSC). Data processing, analysis and interpretation were conducted in collaboration with Dr. Schlitz. In addition, this work was presented at the NOAA sponsored U.S. Globec Georges Bank Science Meeting held in November 2003 at the Whispering Pines Conference Center, Rhode Island.

This project provided partial support for Dr. James Lerczak in 2002, 2003 and 2004.

Last updated: August 19, 2008

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