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\begin{document}


\title{The Deep Basin Experiment:\protect\( \protect \)\\
A Meeting Report}


\author{Nelson G. Hogg}

\maketitle

\section{Introduction}

From Nov 15 to 17, 1999 a meeting of Deep Basin Experiment (DBE)\cite{col96}
participants was held at the Carriage House, Woods Hole Oceanographic Institution
with 26 scientists from France, Germany, Great Britain and the United States
attending. This is a particularly critical time for the DBE: most (but not all)
of the field work has been completed. Investigators have been working singly
or in small groups on various subsets of the data. The main purposes of this
meeting were to make the group aware of what work is going on and what results
have been achieved, and to stimulate research toward the overarching questions
posed in the original DBE manifesto.

Reports summarizing data status and availability, publications completed and
in process, and further studies contemplated were presented on the first morning.
These were followed by short scientific reports grouped under the water mass
themes appropriate to the DBE: Antarctic Intermediate Water (AAIW), North Atlantic
Deep Water (NADW) and Antarctic Bottom Water (AABW). The meeting ended with
a brief discussion about the future and how we might tackle an overall synthesis.
It was agreed that the next meeting would take place in Brest, France in September
of 2000. To supplement what is summarized below the interested reader is referred
to a previously published description of the DBE given in \citet{hog96} and
a recent summary of circulation in the South Atlantic by \citet{str99}.


\section{Summary of research}

Although the meeting was structured around water mass themes it seems more appropriate
for this report to repackage this under the headings of the original DBE objectives,
namely\footnote{%
Names in brackets indicate those who spoke on the topic of the preceeding sentence(s).
}:


\subsection{To observe and quantify the deep circulation within an abyssal basin.}

The three water mass levels were instrumented with a large number of neutrally
buoyant floats. French and German scientists have used a combination of cycling
``Marvor'' floats and one-shot ``Rafos'' floats to determine the circulation
of AAIW. This has proven to be quite successful --- the large--scale, subtropical
gyre that feeds water westward at the southern boundary of the Brazil Basin
(BB) with resulting bifurcation over the Santos Plateau at the western boundary
has been well observed (O. Boebel, P. Richardson). The transports across WHP
line A17 also exhibit a ``Santos'' bifurcation and many of the features of
the circulation can be explained by a simple ventilated thermocline model (C.
Schmid). Further north the interior flow is observed less well and appears to
have smaller spatial scales. The complex system of zonal currents in the tropics
and equatorial regions that has been described by other investigators is not
apparent in the float data although there is some confirmation of the Southern
Intermediate Counter Current (SICC). Just one of the floats at this level crossed
the equator in the western boundary current but a number accomplished the task
by taking interior, nearly zonal, routes near the equator. In the neighborhood
of the Vitoria-Trindade Seamount Chain (VTSC) there is tantalizing evidence
of a recirculating zonal flow but too few floats were in this feature to make
it definitive (M. Ollitrault). 

At the NADW level a well defined Deep Western Boundary Current (DWBC) exists,
consistent with the classic Stommel-Arons \citep{sto58} picture but the interior
flow appears to be zonally banded with small meridional length scales (W. B.
Owens). A prominent eastward flow extends offshore from the VTSC between 20S
and 25S. This is the region of the Namib-Col Current but on the basis of various
computations combining direct current measures and simple box models from hydrographic
data the offshore flow seems to carry 8--20Sv at least as far east as the Mid-Atlantic
Ridge (G. Siedler, M. Vanicek, N. Hogg). What happens at the ridge is a puzzle:
does it feed a northward flowing boundary current along the ridge flanks (K.
Speer) or continue into the Angola Basin (M. Vanicek)? In addition to the 20S-25S
eastward flow, the A17 transports suggest significant escapes from the boundary
near 3 degrees on either side of the equator, at 10S, and at 30S-34S (M.Arhan).

Just north of the VTSC a moored array reveals a boundary current that carries
a surprising \( 39\pm 20 \) Sv within the NADW toward the southeast and extends
downward through the AABW layer. There is no evidence for a return recirculation
to the north (G. Weatherly).

There were fewer floats put in the bottom water layer than in those above but
the flow there seems to share the same zonal structure away from the western
boundary. In addition, although the AABW does enter the basin from the south
the flow north of the VTSC is generally southward closely resembling that within
the NADW above (W. B. Owens, G. Weatherly, M. Arhan).

A large population of small vortices has been observed in the float trajectories,
at all levels. They are of both signs of rotation with the direction of propagation
generally consistent with theoretical arguments (P. Kassis). Several have also
been found in the WOCE hydrographic surveys of the BB (G. Weatherly). Those
within the NADW have property anomalies consistent with a source at the western
boundary but they have been observed throughout the basin and generally travel
westward (P. Kassis).

The WOCE hydrographic sections from the BB have been analyzed for regional aspects
of the circulation with little, at this point, attempt to determine basin-wide
circulation patterns. Given the dominance of zonal flows in the interior the
meridional sections have been especially useful for giving support to the existence
of flow away from the western boundary (M. Arhan) and extending eastward with
CFCs playing an important role, especially near the equator (C. Andrié, M. Vanicek,
W. Smethie). 


\subsection{To distinguish between boundary and internal mixing processes.}

There are 4 major passages connecting the Brazil Basin to neighboring basins
and all were instrumented with current meter arrays during the DBE so as to
better constrain the mass and heat budgets for the bottom water relative to
that done previously \citep{hog82}. At the exits for bottom water in the north
the Romanche-Chain Fracture Zones allow \( 1.22 \) Sv (H. Mercier) and the
equatorial passage near 35W about \( 2.0 \) Sv (M. Hall) to leave. At the south
the Vema Channel and the Hunter Channel both contribute significant amounts
of bottom water to the Brazil Basin with the former estimated at 4.0 Sv and
the latter at 2.9 Sv (W. Zenk) with little net transport of AABW coming over
the Santos Plateau to the west. The Hunter array was the least well equipped
to determine the transport with any accuracy. 

These refined passage transports have been used to recompute the basin averaged
diapycnal diffusivity of between 3 and 5 \( cm^{2}s^{-1} \), not significantly
different from the earlier values (M. Hall). The real question for the DBE is
how uniform this mixing process actually is. The original concept was to coarsely
subdivide the basin with hydrographic surveys and make use of direct velocity
measurements to constrain box inversion so as to be able to distinguish between
regions that were over smooth (i.e. central basin) and rough (i.e. western boundary,
Mid-Atlantic Ridge) areas. The work based on inverse techniques is still in
its infancy but the Brazil Basin Tracer Release Experiment (BBTRE) has somewhat
finessed the outcome. The release of a small quantity of sulphur hexafluoride
close to the 1.6C (potential temperature) surface and subsequent sampling at
periods of 12 and 22 months has permitted the direct estimation of diapycnal
diffusivity. Rates are between 2 and 10 \( cm^{2}s^{-1} \) depending on how
close to the (rough) bottom one is (J. Ledwell). A reasonable candidate for
the source of the mixing is the conversion of barotropic tidal energy into internal
tides by interaction with small scale bathymetry and subsequent breaking through
intensified shears (K. Polzin). Horizontal spreading is consistent with a lateral
diffusivity of order 100 \( cm^{2}s^{-1} \) (J. Ledwell). The combination of
direct estimates of mixing from microstructure observations with regional distribution
of water properties has permitted estimation of the local 3-dimensional flow
regime (L. St. Laurent). This inverse model predicts upwelling where density
surfaces intersect the western flanks of the ridge (within canyons formed by
the fracture zones) whose strength is sufficient to account for virtually all
that demanded by the basin averaged budgets (if extrapolated along the length
of the ridge). Such mixing will drive flow away from the ridge that is very
different from that of the old Stommel--Arons scheme (M. Spall).


\subsection{To understand how passages affect the water flowing through them.}

As well as being important tactical locations to measure the mass and heat flux
of bottom water passages are likely places for water modification through mixing
in the accelerated flows. Two modeling studies are underway, one centered on
the Romanche Fracture Zone at the equator (B. Ferron) and the other on the Vema
Channel (T. Müller). By way of contrast a density current flows downhill some
hundreds of meters at the exit of the Romanche and results in intense mixing
(\( >300cm^{2}s^{-1} \)) while the Vema Channel and its northward extension
has a much gentler exit which, along with the geostrophic tendency to force
flow to follow depth contours, results in little mixing. A concentration of
hydrographic sections near the Vema Channel has permitted a special focus on
this region (E. McDonagh). Its outflow has been traced to about 13S with little
change but thereafter seems to split into 2 cores, one goes to deeper depth
contours toward the Romanche and the other to the 35W equatorial exit to the
western North Atlantic (G. Weatherly).


\subsection{To study the means by which deep water flows across the equator.}

This topic has been mostly dealt with by other groups outside the DBE and was,
by no means, fully covered at the meeting although some examples of floats within
the AAIW traversing the equator were given (M. Ollitrault) and the zonally banded
structure implied by the CFCs was presented (C. Andrié). Once again the reader
is referred to the recent summary of \citet{str99}.


\section{The future}

Much of the research effort is now turning toward analysis of the collected
data and synthesis of the various data sets so that the regional circulation
will be better defined and the above objectives more completely addressed. Overlapping
groups of investigators will pursue a variety of subjects during this WOCE AIMS
phase. Virtually all of the DBE data sets will be made public by the end of
this year. Exceptions are the A14 meridional line in the Angola Basin, promised
for summer of 2000, and the last half of the Marvor float data from the AAIW
layer for which tracking has not been completed. The preliminary results have
already provoked new measurement programs aimed at quantifying the long period
changes in the Vema Channel (T. Müller) and the 35W equatorial passage (R. Limburner)
and following up on questions related to enhanced mixing over rough topography
(J. Toole). The possibility of a zonal connection between the NADW DWBC near
the Vitoria-Trindade Seamounts and the Angola Basin stimulated the interest
of a number of participants and seems likely to lead to future field programs.

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\bibliography{/home/ngh/docs/dbe.bib}



\section*{Participants}

\begin{itemize}
\item Chantal Andrié (LODYC - IRD)
\item Michel Arhan (IFREMER)
\item Olaf Boebel (URI)
\item Mindy Hall (WHOI)
\item Nelson Hogg (WHOI)
\item Patty Kassis (MIT/WHOI)
\item Jim Ledwell (WHOI)
\item Dick Limburner (WHOI)
\item Herlé Mercier (CNRS)
\item Mike McCartney (WHOI)
\item Elaine McDonagh (U. East Anglia)
\item Tom Müller (IfM-Kiel)
\item Michel Ollitrault (IFREMER, WHOI)
\item Breck Owens (WHOI)
\item Kurt Polzin (WHOI)
\item Phil Richardson (WHOI)
\item Claudi Schmid (Miami)
\item Gerold Siedler (IfM-Kiel)
\item Bill Smethie (LDEO)
\item Mike Spall (WHOI)
\item Kevin Speer (FSU)
\item John Toole (WHOI)
\item Lou St. Laurent (WHOI)
\item Michal Vanicek (WHOI)
\item Jack Whitehead (WHOI)
\item Walter Zenk (IfM-Kiel)
\end{itemize}
\end{document}