CMO Dye Studies - Aug '97 Cruise

Introduction

The third cruise of the CMO dye studies was performed aboard the R/V Oceanus (leg 309) between July 30-August 13 1997. Data were collected from CTD casts, the dye injection and sampling systems, shipboard ADCP, shipboard meteorological instruments, and drogues. Again, two dye experiments were performed, the first using rhodamine and the second using fluorescein. The rhodamine experiment is presented at length here. However, since the 1997 fluorescein experiment was performed within the bottom boundary layer rather than higher in the water column like the other experiments, data from this experiment were not analyzed at length by Sundermeyer (1998). Thus many of the graphics shown here for the other four experiments were not available for the latter experiment.
A cruise report for this leg, written by J. Ledwell (chief scientist) is also available.

Hydrography

Hydrographic transects were made at the beginning and end of the 1997 cruise. The first transect consisted of eighteen CTD stations extending from the 50 m to the 120 m isobath and was occupied over a 14 hr period between July 30-31 (shown to right). The second transect (a tow-yo transect, not shown) was made during the return trip to Woods Hole on August 12, and consisted of forty-one profiles between the 50 and 90 m isobaths.
The August 1997 transect shows the shelf-slope front intersecting the bottom close to the 90 m isobath, approximately 10 km inshore of its climatological mean position. As in the hydrography from previous years, the cold pool was on the inshore edge of the shelf-slope front and was the dominant feature in the temperature signal. However, unlike the 1995 and 1996 hydrography, in 1997 the cold pool extended inshore to the 60 m isobath.

Experiment 1, 1997

The first dye experiment of the 1997 cruise was conducted between September 1-6. Rhodamine dye was released in a single streak and subsequently sampled over the course of five days during three surveys. The target density surface was sigma_theta = 24.6 kg m^-3, which corresponded to a mean depth of approximately 20 m.
Again 100 kg of dye were released over a period of 1 hour, giving an injection streak about 2 km in length. The standard deviation of density during the injection was 0.05 kg m^-3 which corresponded to a standard deviation in pressure of 0.23 dbar.
Time series of the CTD data show that the target density surface gradually shoaled from 22 m to 18 m over the course of the injection. This trend was roughly consistent with the isopycnal tilting associated with the inshore edge of the cold pool as seen in the hydrographic sections. Also, temperature and salinity show slight cooling (approximately 0.3^oC) and freshening (approximately 0.05 PSU) as the injection proceeded offshore.
As in previous experiments, stratification remained constant over the course of the experiment, with buoyancy frequencies ranging from 10 to 20 cph, and typical values of 13-18 cph at the level of the dye injection.
Six holey sock drogues were also released during the injection.

Survey 1

The first survey of the 1997 rhodamine dye experiment was performed between 3 and 15 hours after injection and consisted of 16 east/west transects through the dye patch. The patch boundary was well-delimited by this survey, with a mass budget calculation indicating that 81% of the dye was found.
A number of interesting features were seen in this survey. First, the patch was elongated in the meridional direction (see stick plot). In addition, during the first half of the survey, transects across the northern portion of the patch were tilted in the east/west direction (see transects below). Over the course of the survey, this tilting gradually reversed direction so that by the end of the survey, the deeper half of the patch was tilted in the opposite sense as when it started.

The boomerang-shaped dye distributions seen in some transects further indicate that a significant portion of the vertical shear was higher than mode one, consistent with the ADCP EOF analysis described by Sundermeyer (1998). Also noteworthy is that in many of these transects, the patch was rather disjointed, exhibiting three or four well-defined tracer concentration maxima within a single transect. As indicated by Sundermeyer and Ledwell (2001), and discussed at length by Sundermeyer (1998), the latter suggests the presence of some stirring mechanism acting on the tracer patch.

Survey 2

The second survey for the 1997 rhodamine dye experiment was performed between 44 and 59 hours after injection and consisted of twelve zonal transects through the dye patch. Again the patch boundary was well-delimited by this survey, with a mass budget calculation indicating that all of the dye was found.

Individual transects from this survey again show some patchiness, although the dye was more homogeneous than in the previous survey. Again horizontal elongation in the meridional direction is apparent, and a slight vertical tilting of the patch in the zonal direction was seen during the first few transects.

Survey 3

The final survey of the 1997 rhodamine dye experiment was performed between 92 and 119 hours after injection and consisted of thirty-three zonal transects through the dye patch. Once again the patch was well delimited by the survey, with a mass budget calculation indicating that all the dye was found.

Patchiness is again evident in numerous transects of this survey. In the zonal direction, no tilting was apparent. However, as the survey progressed southward, the depth of peak dye concentration increased, indicating a meridional tilt of the dye patch.

Vertical Diffusivity

For the first survey, the mean vertical variance was sigma_z = 1.25 m², which corresponds to a mean vertical extent of 4*sigma_z = 4.5 m. By the final survey, the variance had grown to sigma_z 3.9 m², or a vertical extent of 4*sigma_z = 7.9 m. The vertical diffusivity from these data based on a Fickian diffusion model was K_z = 0.1-0.4 x 10^-5 m² s^-1.

Horizontal Diffusivity

Horizontal diffusivities were again estimated in two ways. First, the overall dispersion K_tot was estimated based on the vertically-integrated tracer. Second, the irreversible component of diffusivity, K_irrev, was estimated based on the tracer along the target density surface.
Based on these two measures, the horizontal diffusivities inferred from the above two surveys were K_tot=5.4 (1.7 to 30.0) m² s^-1 and K_irrev=0.3 (0.1 to 0.6) m² s^-1.

-- Analysis for Experiment 2 Still in Progress --

Experiment 2, 1996

The second dye experiment of the 1996 cruise was conducted between August 7-12. 100 kg of fluorescein dye was released in a single streak, and subsequently sampled during three surveys. The total inject time was about 1 hour spread over a 4 hour time period, and was at a depth of approximately 65 m, 5 meters off the bottom. Again the initial streak was about 2 km long.
The stratification at the level of the release was about 15 cph just above the release surface, and more than 20 cph in the 2 or 3 meters below it. If there was a well-mixed bottom boundary layer at the time of the release, it was confined to the bottom meter.

Sampling Surveys and Vertical Diffusivity Estimates

The patch was sampled in three surveys over a period of 5 days, with microstructure profiling to the bottom interspersed by Neil Oakey of Bedford Institute of Oceanography. As in the previous experiment, the vertical diffusivity, K_z, again appeared to be less than 0.1 x 10^-5 m² s^-1.
Inshore, the dye was spread into a smooth patch, 5 km wide along shore by 10 km long across shore, that ultimately contacted the bottom on the inshore end. Offshore, the dye extended into a 1 km wide streak, stretched 30 km to the east (along shore) at the end of 5 days. Unlike the previous experiments, the mean advection of the patch was weakly eastward, compared to the sometimes strong westward flow seen previously.