\section{The Flow Mow Experiment}

The Flow Mow experiment constituted an innovative effort to avoid both the
variability in advected plumes and the expense of acquiring measurements at
vents on the sea floor.  This dissertation presents its primary result: the
most precise measurement of the flux of heat from a hydrothermal vent field
accomplished to date.  The experimental data sets also offer new perspectives
on the plumes and currents that transport the volcanic energy into the
surrounding ocean.

\subsection{A Well-Mapped Study Site: the Main Endeavour Field}

\subsection{Measure Convective Flux in Rising Plumes}

\subsection{Use a Stable and Efficient Instrument Platform: the ABE}

\subsection{Monitor Ambient Hydrography and Currents}



\section(The Setting}

\subsection{Topography and geology}

How much does valley cross-sectional area vary along axis?  

How about the actual area surveyed on the N and S MEFCV walls?  Did the shoaling 
topography in the SW corner of the CV cause the surveyed area at the S end to be 
significantly smaller than at the N end?

\subsection{Hydrothermal activity}


In addition to Quebec, there are known sources near the MEF permiter.  There is
a cluster of high $B$ vents near the SW corner of the MEF called Cathedral (D.
Kelley, personal communication) and a lone high $B$ source $\sim$500~m NNE of
the MEF recently dubbed Raven (H.  P. Johnson, pers.\ comm.) and previously
noted as Redd Fox (V. Bhat, pers.\ comm.).


\subsection{Flow}
The Endevour segment is centered (at $48^\circ~N$) latitudinally beneath the divergence 
of the North Pacific current which shifts seasonally between $\sim45^\circ~N$ (winter) and 
$\sim50^\circ~N$ (summer) \cite{Pickard+90}.  While the surface current turns southward to 
become the California current and northward to feed the Alaska gyre, the regional flow 
within a few hundred meters of the segment topography ($\sim1800-2400$~m) is... [cite Roth 
thesis and Cannon regional papers.]

Include Marv's P record, perhaps juxtaposed with P record from mooring.  Also, Bill wants a PSD 
of the P record.  And maybe a pressure prediction from the OSU model?
Note that some meters were supplemented with $T$ and/or $P$ sensors.

Make sure you note that both moorings were deployed and recovered by Richard
Thomson of IOS/Canada.  He also kindly provided calibration and processing
according to standard IOS procedures (ref?).

\subsubsection{Mean flows}
Include table of means from current meters plus other general stats HERE (or in a different 
section (or Chapter?) on background conditions and setting)?

PSD of v and u (or PVD's?) to show that v dominates u, generally in valley?

Citations for mean + oscillatory flows in deep sea:

keller, anderson, and lavelle (1975);maybe worth overlaying where HT sources are known to be now?
Also mentioned french dives claiming 39~cm/s (~5mab?)... this is anecdotal, but similar to 
Marv's reports of Alvin being pushed around in/near MEF (get details, including sub fixes over 
time?).  Meter C is very similar to wasp -- maybe a clue that topographic constrictions really 
accentuate the mean flow that is added to tidal oscillations?  Meter C recorded a mean speed of 24.2~cm/s 
over a 50-s averaging period.  Long-term mean was 8.2~cm/s.  They also report high coherence between 
meters A and C (offset 150~m vertically and $\sim$3~km within the MAR/FAMOUS axial valley.

They cite Garner (1972) regarding similar flow in Gibbs fracture zone (cross-ridge?) 

In the vicinity of the Rainbow vent
field on the Mid-Atlantic ridge, for example, mechanical mixing at sills driven
by tidal oscillations appears to maintain a northward mean flow along an axial
valley that is open only at its south end \citep{thurnherr+02}.

\subsubsection{Power spectra}
Just refer to Rick's paper and summarize comparable articles?

The displacement that occurs during $1/2$ of a pure 
oscillation with amplitude $v_o$ and period $\tau$ is 

\begin{equation} 
$\Delta x = \int\limits_{0}^{\tau/2} v_o sin{\frac{{2\pi}{\tau}}t} dt = v_o\tau/\pi $.
\end{equation}

Assuming an extreme value for $v_o=0.19$~m/s (the maximum observed hourly mean
speed above the ridge crest) the displacement is 2.6~km for  a diurnal
($\tau=12$~hr) oscillation and 3.5~km for an inertial ($\tau=16.1$~hr)
oscillation.  During the same half-periods, the mean flow will displace fluid
1-1.5~km.  

\subsubsection{Coherence and phase}
These meters show less powerful, but significant spectral density peaks at the 
diurnal and inertial frequencies (Figure \ref{cohphase}b).  The N mooring has
less diurnal power and more inertial power than the S mooring (ref Mihaly
and/or others here?).   There is significant N/S coherence at each of these
frequencies, but the diurnal oscillation is $\sim$180{$^\circ$} out of phase,
while the inertial oscillation has a +20-120{$^\circ$} phase difference.

\begin{figure}[htbp]
\begin{center}
\includegraphics[width=0.9\textwidth]{figs/power_coh_phase_NS.eps2}
\end{center}
\caption{(a) Power spectral density of the along-axis component of flow
measured at the N and S moorings.  (b) Coherence and phase between the 2
moorings.  The level of 95\% confidence is indicated by the horizontal line.
The three significant peaks (excluding the low frequency shoulder) correspond
with the diurnal, inertial, and semidiurnal periods.  Positive phase means that
S leads N.} \label{cohphase} \end{figure}


\subsection{Hydrography}

General NE Pacific $\theta-S$ plot and interpretation with respect to water masses.  
Pretty good overview in Pickard+Emery.  Juxtapose station Papa with Levitus?  Cite Reid and Si-maximum.  

\cite{cannon+93}: Good context on NE Pacific $\theta$--$S$ and flow relative to the JdFR.   
``Common water'' ($\sigma_{\theta}=27.76$, ) and ``Intermediate water'' ($\sigma_{\theta}=27.28, 
z=$800-900~m).  Ref to Munk, Abyssal recipes, 1966.  Also to Roemmich and McCallister (1989) on 
large-scale flow in the North Pacific.  Maybe good to combine his mean vectors with 
those from Franks in 1 figure to give overview?



Now compare all available ''background'' casts.
Need a map of the locations: MZ stns, REVEL stns?, FM stns, Hautala stns.
Then pressure-binned overlain profile plots for full water column and plume depths

The Flow Mow CTD stations located near the axis of the segment show, on average, positive 
$\theta$ (Figure \ref{{axial_mean_sig-th}) and $S$ (Figure \ref{{axial_mean_sig-S}) 
anomalies relative to the background trends that are linear with respect 
to density ($\sigma-\theta$ and $\sigma-\theta$).   This deviation from a linear 
background was first observed by Lupton et al. and can be used to define density-referenced 
(hereafter \emph{isopycnal}) anomalies in hydrothermal environments.   Over the Endeavour axis, 
typical isopycnal temperature anomalies ($\Delta_{\rho}\theta$) are $+\sim0.03\circ~C$ 
and typical isopycnal salinity anomalies ($\Delta_{\rho}S$) are $+\sim0.003psu$.  Thus, 
the overall influence of the hydrothermal system on the ocean appears to be a shift from 
the background to saltier, warmer conditions at all depths.  

\begin{figure}
\begin{center} 
\includegraphics*[width=.45\textwidth]{figs/setting/sgth_T2.mean.vs.bkgrd.eps2}\hfill
\includegraphics*[width=.45\textwidth]{figs/setting/sgth_S2.mean.vs.bkgrd.eps2}
%\includegraphics*[width=.75\textwidth]{figs/setting/sgth_T2.mean.vs.bkgrd.eps2}
%\includegraphics*[width=.75\textwidth]{figs/setting/sgth_S2.mean.vs.bkgrd.eps2}
\caption[Background and axial CTD stations: $\sigma_\theta$ versus $\theta_0$ and $S$]
{$\sigma_\theta$ versus mean $\theta_0$ and $S$ for Flow Mow background and axial CTD 
stations.  The solid line is the mean background (both stations and both pairs of sensors), 
while the open circles represent 10~m depth-binned averages from stations located along the 
ridge axis.  }
\label{axial_mean_sig-th+s}
%\label{axial_mean_sig-S}
\end{center}
\end{figure}

Individual axial casts reveal intermittent returns to the background trend  
at variable depths, but almost exclusively above the ridge crests.  The lower fluid, 
enclosed within the axial valley topography, almost always has positive isopycnal anomalies of
$\theta$ and $S$, even within 5~m of the sea floor.  The only exceptions are found within 
buoyant plumes.  Further from the axis, however, 
deep casts make it clear that downstream of the volcano the positive
isopycnal anomalies have been embedded within the linear background; descending through 
the plume depths, the positive anomalies decay back to the background trend near the depths 
of the axial valley floor.  This downstream return to background conditions below plume 
equilibration depths has been used to justify the definition of an anomaly by extrapolating 
a slope from above the plume through intermediate depths.

Plotting these same $\theta$ and $S$ data against each other (rather than density) also 
reveals positive temperature anomalies (Figure \ref{axial_mean_th-S}), though in this case 
they are salinity referenced (hereafter \emph{isohaline}).   On average, the isohaline temperature 
anomaly ($\Delta_S\theta) is $\sim0.05\circ~C} roughly $2\times$ the isopycnal anomaly (ref sxn on 
correction and conversion factors).  As with the isopycnal anomalies, 
individual axial casts show highly variable isohaline anomalies above the ridge crests, and relatively 
uniform conditions within the confines of the axial valley.  Stations as close as a kilometer from the 
axis can reveal positive isohaline anomalies near the plume equilibration depths bounded above and 
below by background fluid.  

\begin{figure}
\begin{center} 
\includegraphics*[width=.75\textwidth]{T2-S2.mean.vs.bkgrd.eps2}
\caption[$\theta - S$ data from Flow Mow background and axial CTD stations]
{The depth-binned average $\theta$ and $S$ from background (solid line) and axial (open 
circle) stations, over the depth range common to all stations.  }
\label{axial_mean_th-S}
\end{center}
\end{figure}

A number of mixing processes could be responsible for the elevated $\theta-S$ 
conditions at the axis.  The relatively rough topography of the ridge may cause enhanced 
vertical mixing through turbulence or boundary layers, but in the absence of a warm, salty water 
mass, this process can only redistribute the $\theta-S$ data along the background trend.  The 
elevated values could be due to fluid from the west side of the ridge (opposite the Flow Mow 
background stations), deeper (saltier) background water warmed geothermally, shallower (warmer) 
background water to which salt has been added, or an end member that is both warmer and saltier 
than the background.  These alternatives are illustrated schematically in figure \ref{th-s_diag}.  

Interleaving of distinct water masses across the Mid-Atlantic ridge has been observed near crest 
depths \cite{Thurnherr00}.  The Northeast Pacific, however, has a much more uniform hydrography.  
Stations from multiple years and both sides of the Endeavour segment suggest that the 
regional hydrography is remarkably stable (Figure \ref{th-s_regional_bkg_comp.eps2}.  [Cite 
Cannon/Reid here?]

A final and illuminating way to visualize the density, temperature, and salinity fields over the 
Endeavour segment is in space.  A depth profile of the mean density observed at background and axial 
stations (Figure \ref{}) immediately highlights the dynamic interaction of the hydrothermal plumes and the 
deep ocean.  The difference between background and axial data at a particular depth defines a depth-referenced, 
or level-to-level, anomaly (hereafter denoted $\Delta_z$).  
On average, the fluid above 2000~m is denser than the surrounding ocean, while the 
deeper axial water is less dense than the isobathic background.   

Show individual $\theta-S$ scatter plots juxtaposed with density profiles: stn10 
tn10_th-s-sig-z<F3><F4>stn10_th-s-sig-z
Figure \ref{stn10_th-s-sig-z} for buoyant plumes 
and stns 28/33 for temporal averages (explain the linear tails!)

\begin{figure}
\begin{center} 
\includegraphics*[width=.75\textwidth]{stn10.th.s.sig1.z.ps}
\caption[]{
}
\label{stn10_th-s-sig-z}
\end{center}
\end{figure}

Maybe this is the place to show the $\theta-S$ trajectories from ABE flythroughs, as well?

Geographic grouping of the vertical profiles provides further insight into how hydrothermal 
activity causes the transition from background to axial conditions.  Figure 
\ref{profile_comp_sigt-th-s} emphasizes how the isobathic anomalies increase as one 
progresses from the background stations to the southern axial stations and northward 
up into the axial valley.  Both near the bottom and at upper levels, the absolute 
maxima of $\Delta_z\sigma_{theta}$, $\Delta_z\theta$, and $\Delta_zS$ are 
measured at the SoMEF and NoMEF stations, in the vicinity of the MEF.   

\begin{figure}
\begin{center} 
\includegraphics*[width=.3\textwidth]{S-N.mean_sgt2_z.eps2}\hfill
\includegraphics*[width=.3\textwidth]{S-N.meanT2_z.eps2}\hfill
\includegraphics*[width=.3\textwidth]{S-N.meanS2_z.eps2}
% Andreas used a macro with:
%       \epsfig{figure=#2.eps,width=#1\unitwidth}\hfill
%       \epsfig{figure=#3.eps,width=#1\unitwidth}
\caption[Depth profiles of mean $\sigma_{\theta}$, $\theta$, and $S$: geographic transition]
{Depth profiles of mean $\sigma_{\theta}$, $\theta$, and $S$ from Flow Mow stations grouped geographically.  
Emphasis is on the transitions between background and axial stations.  Axial stations are:
``Deep S," the southernmost stations, from the SE flank to the Mothra vicinity; ``SoMEF," stations 
4, 6, and 33, $\sim500~m$ S of MEF; ``NoMEF," stations 3, 5, 14, and 28, $\sim500~m$ N of MEF; 
``Deep N," stations just N and S of the saddle.  }
\label{profile_comp_sigt-th-s}
\end{center}
\end{figure}

Given the positive $\Delta_{\rho}\theta$ and $\Delta_{\rho}S$ in figure \ref{axial_mean_sig-th+s} at all 
densities, a critical question is how the fluid below $\sim1975~m$ in figure \ref{profile_comp_sigt-th-s} 
has negative $\Delta_z\sigma_{\theta}$.  The same fluid has positive $\Delta_z\theta$ and negative 
$\Delta_zS$.  Taken all together, these observations indicate that average isopycnal, isohaline, and isotherm 
surfaces all dip over the axis.  Isopycnal surfaces are level near 1975~m and bowed upward between 1975~m 
and $\sim1800~m$.  Isohaline surfaces mimic the isopycnal ones, although they are level deeper,  
near 2075~m, and must slope more steeply than the isopycnals to generate positive 
$\Delta_{\rho}S$ at all depths.  Isotherms dip downward at all depths (also more steeply than do isopycnals) 
below 1900~m and may bow slightly upwards at shallower depths.  There is some indication that at the N end 
of the segment the isotherms begin to dip only below $\sim1975~m$.  

bkg-S2.transition_profiles.mean.eps2
bkg-T2.transition_profiles.mean.eps2

depth_bin_dth_hist_stn34.10m.gt2100.eps2
depth_bin_dth_hist_stn24.10m.gt2100.eps2
depth_bin_dth_hist_stn20.10m.gt2100.eps2



ALTERNATIVE PHRASING:
Plotted against density, both $\theta$
and $S$ have positive anomalies at all depths.  Depth profiles of $\theta$ and
$S$, however, suggest that the deep layer is isobathically fresh and warm.
This means that isopycnals must be sloped differently at different depths (as
predicted by plume equilibration theory) --- plunging into topography below the
mean equilibration depth ($z^*$) and bowing upward above.  These profiles also
imply that isohalines dip downward less than isopycnals below 2100~m, and bend
upward more than isopycnals above $\sim$2100~m.

[include Figure of background and NoSoMEF P-binned means and raw Stn 10
$\theta$-$S$, and z vs $\sigma_2$?  add ref or $S$ and $\theta$ profiles akin
to Lavelle technical report?]

Axial profiles of $\sigma_2$ are often nearly vertical in the same depth range,
evidencing a well mixed boundary layer that is $\sim$50~m thick.  The fluid
below $\sim$2150m is much clearer ($1/5$ backscatter intensity) than in
overlying equilibrated plumes.  Taken together, these observations suggest that
the increase in $\Delta\theta$ within the valley is caused by an addition of
heat to fluid as it flows into the axial valley.  This could occur through sea
floor geothermal heating of relatively fresh water, injection of
warm, fresh vent fluid, or downward mixing of overlying plume fluid.
Subsequent mechanical mixing may help to homogenize the added heat, resulting
in a reduction of $\Delta\theta$ variance with increasing time and/or distance
from a source.
END ALTERNATIVE PHRASING



The implication for vertical calculation is that south is consistently cooler than north.
Can we add evidence that the near-bottom layer (especially below 2150m) always is
clear and has a T-profile toe at 1.70$^\circ{C}$ , whereas the toe on north side (and E side?)
has toe at 1.72?  What is the toe, anyways? 


\subsection{Other measurements} 

analyze cm (T,U) time series 


The near-bottom current meters deployed in 1995 and 2000 were equipped with
high-precision thermisters.  These sensors provide long and continuous
records of water temperature variability within the axial valley near the
MEF.

Table summarizing expanded temperature data from current meter moorings

A) 1000 m N of MEF, WASP mooring Jun?-Sep? 2000
B) 200 m NE of MEF, RCM5 mooring Jun?-July? 1995
C) 1200 m S of MEF, RCM5 mooring Jun-Sep? 2000

Elevation       Depth (m)       Mean(Te)        max(Te) min(Te) std(Te)
A) 15 mab       2168
B) 25 mab       2175
C) 50 mab       2168

The observed variance in expanded temperature could be due to
1) Vertical displacement of isotherms by non-hydrothermal forcing
2) Advection of a temperature field that is heterogeneous due to
non-uniform (in X or t) input of hydrothermal heat
3) Advection of a temperature field that is homogeneous at the depth
of the thermisters, perhaps due to mechanical, near-bottom mixing
