Cross-Frontal Exchange and Scotian Shelf Cross-over Workshops

10-12 October, 2000

Holiday Inn, Falmouth, Massachusetts

 


Overview

Cross-Frontal Exchange

Presentations Discussion Topics Upcoming North Sea Study (LIFECO) (St. John)

Scotian Shelf Cross-over

Presentations Discussion Questions Synthesis Topics Appendix

 

Near-bottom Dye Transport Across the Front

Secondary circulation and mixing

Robert Houghton (LDEO)

Dye injections into the bottom mixed layer were able to quantify the on-bank Lagrangian flow through the tidal front. This is illustrated in Fig. 3 which shows sections through the center of the dye patch with dye concentration, normalized to unity in each panel, superposed on the temperature field. The location of the dye injection is denoted by the crossed circle. The top panels are for the south flank experiment, with on-bank direction to the right, with sections (a) 16 hours and (b) 85 hours after injection respectively. The bottom panels are on the northeast peak with "on-bank" direction to the left for (c) 17 and (d) 55 hours after injection. The dye patch clearly moves through the tidal front in the on-bank direction. On the northeast peak (Fig. 3 d) there is evidence of isopycnal mixing or advection off-bank in the pycnocline. The more rapid dye dispersion on the northeast peak shortened the time that the dye concentrations remained at detectable levels. But since the speeds were greater is the dye actually moved further through the front.

The on-bank Lagrangian velocities relative to the front are determined by the warming of the dye patch (Fig.4) and the horizontal temperature gradient across the front. The results are presented in Table 1. The on-bank velocity is 1.9 cm/s on the south flank and 3.2 cm/s on the northeast peak. This is an extraordinary demonstration of the utility of the tracer. These velocities were determined in an environment with frontal motions as large as 100 cm/s driven by tidal forcing. Eulerian measurements would not be successful here. The diffusivities are estimated from the dispersion of the dye patch. It is the vertical heat flux that accounts for most of the warming of the dye tagged water as it moves on-bank through the front. This vertical mixing tends to make the front a barrier to lateral exchange and suggests that most of the mixing of scalers such as heat and nutrients will occur within the front before they can be transported onto the cap of the bank.

Table 1. Mixing and displacement calculations derived from the dye experiment.

 

South flank

Northeast peak

warming: T (C)

0.76

1.21

duration: t (hours)

83

43

dT/dt (x10-6 C/s)

2.5

8.0

dT/dy (x10-4 C/m)

1.3

2.4

on-bank displacement: y (km)

5.8

5.0

on-bank velocity: v (cm/s)

1.9

3.2

cross-front diffusivity: (m2/s)

17

20

along-front diffusivity: (m2/s)

33

120

vertical diff,: Kz (x10-3 m2/s)

1.4

2.5

Kz2T/z2 (x10-6 C/s)

2.8

10