Experiment

Both injection and detection of the dye were accomplished using a sled (0.5m high, 2 m long, weighing ~150 kg and ballast to remain horizontal) towed at speeds ranging from 1-4 kts. The sled was fitted with a downward looking altimeter, a Sea Cat SBE-19 CTD, and two Chelsea MK II Aquatracka fluorometers with optical filters suited to detect Chlorophyll a and Rhodamine-WT dye. The chlorophyll measurement was used to remove spurious background signal in the dye channel due to in situ chlorophyll. Thirty-five kilograms of Rhodamine-WT dye in a 20% water solution mixed with isopropyl alcohol to achieve in situ density was pumped into the BBL producing an initial streak 1 km in length parallel to the 100 m isobath in 8.9°C water just on the shoreward side of the center of the front. The vertical temperature gradient at the top of the BBL at the foot of the front was much greater than depicted in Fig. 1 . Above a mixed layer 4-7 m thick the transition to cold, fresh water above was a 2.4°C decrease in 3 m corresponding to a 0.4 kg/m ³ density decrease and a Brunt-Vèisèlè period of 2.9 minutes. Because of the fluctuations in sled depth and the unexpected sharpness of the top of the BBL only approximately half of the dye was actually injected into the BBL. Within 24 hours all evidence of streakiness in the dye patch disappeared and dye injected above the BBL was advected out of the study area by the vertical velocity shear.

The subsequent dispersal of the dye patch in the BBL was mapped for the next 3 days (Fig. 2). Measurable dye concentrations, level of detection was a concentration of 10-11 by volume, were confined to the BBL. The sled altitude was maintained between 2-5 m above bottom with brief vertical excursions to measure the BBL thickness and stratification by adjusting the towing speed and length of cable paidout. The westward drift of the patch (~0.06 m/s) roughly parallel to the local bathymetry was expected, the sudden onshore displacement of approximately 12 km in 24 hours (~0.14 m/s) was not. There was also a displacement of the frontal boundary inferred from the cross-shelf position of the 8°C isotherm. Since during this event cross-shelf temperature gradients within the BBL did not change it is inferred that at least the lower portion of the frontal boundary was displaced onshore and not just stretched.

As the dye patch dispersed its mean temperature decreased. The salinity also decreased with T-S values of the dye patch evolving along the mean T-S curve for the BBL water. This cooling is illustrated by representative sections across the dye patch (Fig. 3) measured during successive surveys of the dye patch. The dye inventory derived from the final survey indicates that 11.5 l or approximately 70% of the original dye injected into the BBL was still in the dye patch. Therefore the dye patch is truly a Lagrangian follower and this temperature change represents cooling of the water parcel tagged by the dye. Using the temperature at the peak dye concentration to represent the temperature of the center of mass of the patch and constructing a mean cross-shelf front BBL temperature profile from a composite of all the cross front sections the dye patch position relative to the front was constructed (Table I). Over the 3 day period of the survey the dye patch and hence the tagged water parcel in the BBL moved onshore by 3.5 km with a mean speed of 0.015 m/s. Thus the water moved in a direction opposite to that expected.

The situation is clarified by considering the spatial variations of other parameters in the BBL structure (Table I). Although the standard deviation associated with individual measurements is approximately 20-50% of the mean the trends are well defined. Moving onshore to colder temperatures the cross-shelf temperature gradient Tx increases, while the vertical temperature gradient Tz at the top of the BBL decreases, and the thickness of the BBL, ho, decreases. These trends indicate that the dye patch had been injected on the seaward side of the convergence zone of the frontal boundary instead of the shoreward side as intended.

These observations are used to construct a schematic diagram (Fig. 4) illustrating the evolution of the dye patch shown in the BBL. Although the diagram is two-dimensional it represents the cross-shelf motion of a water parcel that is flowing nearly 20 km alongshore to the west. Thus, there is an implicit three-dimensionality to the circulation. From its injection point near 9°C the center of mass of the dye patch moves onshore towards the frontal convergence point as it broadens. Within the BBL the dye was well mixed vertically. Traces of dye appeared to penetrate into the stratified layer above the BBL as far as the 7°C isotherm. Here the decrease in dye concentration was abrupt suggesting a convergence in the center of the stratified layer above the BBL as indicated by the arrows in Fig. 4. The thickness of the BBL defined by the height of the maximum gradient region diminished onshore so the dye patch was squeezed as it moved onshore. The arrows pointing onshore indicate water flow defined by the dye patch motion while the other arrows are a more speculative representation of inferred flow in the BBL at the foot of the frontal boundary.

Lamont-Doherty Earth Observatory of Columbia University