**Discussion**
There are several aspects of these observations which at first appear contradictory. In particular,
onshore and westward flow in the BBL is not consistent with Ekman dynamics. This apparent contradiction
is resolved in non-linear model calculations by Chapman and Lentz (1994) where they consider
frontogenesis on the shelf driven by a coastal buoyancy source. The offshore migration of the front is
arrested when in the BBL offshore flow of buoyant shelf water converges with an onshore flow underneath
the frontal boundary. The cross-shelf convergence zone in the BBL is at the shoreward side of the frontal
boundary. The thermal wind shear in response to the increased cross-shelf density gradient reverses the
alongshore flow at the foot of the front. However, when an alongshore pressure gradient is added to
simulate the observed mean southwestward flow along the northeast continental margin there is no reversal
in the alongshore flow in the BBL under the front (see their Fig. 13). Thus our observations substantiate
several important features of their frontogenesis model.
Since we are following the dye tagged water in a Lagrangian sense the observed cooling, dT/dt
~ 4x10^{-6o}C/s, must be the result of diffusive mixing. From the cross-shelf spreading of the dye patch we
estimate a cross-shelf diffusivity of K_{x} ~ 10 m^{2}/s. The cross-shelf variance of the dye patch increases
approximately as time squared indicating a major contribution from shear dispersion. From measured
cross-shelf temperature gradients an upper bound for T_{xx} is ~ 0.1x10-6^{o}C/m^{2} so that the cross-shelf heat
flux K_{x}T_{xx} ~ 1x10-6^{o}C/s. Since this is only one forth of dT/dt there must be significant vertical heat flux
through the top of the BBL to achieve a local heat balance. An estimate of the vertical diffusivity K_{z} across
the highly stratified top of the BBL is problematical but a value of K_{z} ~ 10^{-5} m^{2}/s yields K_{z}T_{zz} ~ 4x10^{-6 o}C/s which is approximately the magnitude required to account for the cooling of the dye patch. Using the
same diffusivity results in a loss of approximately 1-3 l of dye which when subtracted from the 16 l
initially injected into the BBL is consistent with the final dye inventory of 11.5 l. There is no evidence that
double-diffusive mixing is a factor here. First, the density ratio,
is approximately 0.4 and
not near unity where double diffusive processes are more active. Second, the T-S values of the water
parcel evolves along the mean mixing curve between Shelf and Slope Water properties with no evidence of
a counterclockwise rotation as modeled by Schmidt (1981). When warrented by better dye patch sampling
in future experiments these flux calculations will be repeated by properly integrating over the entire patch
rather than using mean patch values. However, even these crude estimates impose an upper bound on the
diffusive flux through the highly stratified boundary of the BBL near the foot of the front.
If to some extent the structure depicted in
Fig. 4
represents a steady-state condition in the coastal
regime with no alongshore variation then both mass and heat balance must apply. Mass balance is probably
achieved by an offshore flow confined to the stratified layer at the top of the BBL. Houghton et al. (1982)
estimate that during the summer the 'cold pool', shelf water beneath the warm surface mixed layer, warms
at approximately 1^{o}C /month. Approximating the cold pool as a wedge 100 km wide and 60m thick at the
shelfbreak front this warming is equivalent to a heat flux of 4.9x106 W per meter of alongshelf distance.
When mixed to 6^{o}C the onshore flow of 9^{o}C water in the BBL 6m thick at the measured speed of 0.015 m/s
represents a flux of 1.1x106 W. Thus the heat flux associated with the onshore flow in the BBL at the foot
of the front could contribute significantly to the Shelf Water heat balance and to exchange of Shelf and
Slope Water properties via diapycnal mixing across the top of the BBL.

Designed by:
D. Jarvis Belinne

Lamont-Doherty Earth Observatory of Columbia University