Measurements and Models of Heat Flux Magnitude and Variance from the Main Endeavour Hydrothermal Vent Field
Scott R. Veirs, Fritz R. Stahr, Russell E. McDuff, Richard E. Thomson, Dana R. Yoerger, and Albert M. Bradley
ABE
CTD
School of Oceanography, University of Washington, Box 357940, Seattle, WA, 98195-7940 | [email protected] | www2.ocean.washington.edu/~scottv/
I. Background
III. Vertical Heat Flux
II. Lateral Heat Flux
Bathymetric contour
interval: 100m
The Flow Mow
Experiment:
SAS
N 2000
MEF ‘95
S 2000
Methodology:
MEF perimeter Dq :
Method of calculation:
North side
Htop = r Cp A (1/N) [(Dq - <Dq>S ) w] [Watts]
Because southward tidal displacement
rarely exceeds 50m, the south side of the
control volume is usually colder than the
north side. The mean Dq from any side
can be multiplied by the orthogonal
South side
component of velocity (v), as well as
density (r), heat capacity (Cp), and area
(A), to estimate the heat flux (H) through
that surface:
oC
H = r Cp Dq A v
Area (A) is the survey coverage (300m x 720m).
<Dq>S characterizes entrained fluid that was
advected into the field from the south. Vertical
velocity (w) derives from MAVS minus ABE or
from a dynamic model of ABE’s response to
vertical advection within plumes. Mean r and Cp
of each top is used. Summation is over all points
of ABE’s trackline.
Current Meters
Magnitude and variance:
Pooling and streaming cause high variance :
LOW BUOYANCY FLUX FIELDS:
HIGH BUOYANCY FLUX FIELDS:
C = Cirque
SAS = Sasquatch
D = Dune
SDF = Salty Dawg
CB = Clam Bed
HRF = High Rise
Q = Quebec
MEF = Main Endeavour
Recently discovered fields
MF = Mothra
Local currents and hydrography:
Within the Endeavour segment’s axial valley (>2100m
depth), currents are dominated by semidiurnal oscillations
superimposed on a mean northward ~2-4cm/s flow.
Please refer to the gentleman shown at left (Rick
Thomson) for further details...
Periods when currents stream steadily through the control volume are times when
heat flux can be estimated accurately by differencing the flux through opposite
sides. But warmed fluid pools within the MEF during almost every tidal cycle.
We use a numerical “puff” model to generate time series of flux through any side
of the MEF and understand the high variance in the observed Dq and v (below).
South
North
MEF heatflux from 300 x 720m plane (box-top) @ 2100m depth
1000
900
ABE 46
800
ABE 44
700
Heatflux (MW)
In August 2000, we measured the
flux of heat through a control
volume enclosing the Main
Endeavour hydrothermal vent field
(MEF). Vertical flux was
monitored ~75m above the vents
with a CTD and acoustic velocity
sensor (MAVS) mounted on the
Autonomous Benthic Explorer
(ABE). Lateral heat flux was
estimated by combining ABE data,
CTD observations, and current
meter records acquired near the
MEF and close to the seafloor.
ABE dive 50
600
400
ABE 49
300
ABE 50
200
100
ABE 45
North of MEF:
<Dq>N=0.051 oC
<Dq>S=0.047 oC
Observed magnitude and modeled variance:
Ridge crests
Each heat flux value below is estimated by multiplying a mean velocity by the
mean Dq difference between N and S sides of the field (<Dq >N- <Dq >S ).
Modeled variance for each current meter record is given at right.
ABE survey
Proximity of Dq observations to MEF:
Velocity data source
Histograms of mean potential temperature anomaly (Dq ) within different
depth bins reveal that fluid confined by the axial valley is warmer north of
the MEF than to the south. The variance of Dq at these depths also
increases to the north. A simple explanation for this near-bottom
hydrography is that the mean flow transports MEF heat northward within
the valley while tidal oscillations enhance variability.
MEAN
524 MW
500
0
51766
South of MEF:
ABE 51
ABE 48
South mooring 2168
South, survey only
North WASP 2161
North, survey only
duration
74 days
17 days
142 days
17 days
<v> (cm/s)
1.6
1.1
4.1
5.1
51770
51772
Time (mod. julian days)
51774
51776
51778
Variance of observations: σ = 236.3 MW
Variance of mean: [σ / (12)1/2] = 65.3 MW
IV. Heat Budget Implications
Simulated MEF Hsides
near
ABE50 distant all
<Dq >N- <Dq >S [oC]
0.025
27
24
91
114
0.02
22
19
73
91
North, ABE dive 50 only ~1 day
6.1
109
South, ABE dive 50 only ~1 day
3.9
70
Notes: r~1037.5; Cp~3800; A=300m x 75m
51768
ABE surveyed the top of the
MEF control volume 15 times;
12 of these “tops” led to
robust estimates of Htop. The
magnitude of each is shown
here versus the mid-point of
dive time. (Note that dives 44,
50 and 51 included multiple
tops.) The black line is the
mean for the experiment:
524 MW.
0.002
2.2
1.9
7.3
9.1
250
HhighB + HlowB + Htop + Hsides= 0
0.004
Htop
200
150
4.4
HlowB = Htop + Hsides - HhighB
3.8
Hsides
100
Ridge crest
50
15
~ 520 +100 -340* = 280 MW
18.2
2100
mab
Sea floor
2200
Depth
therefore, HlowB ~ HhighB ~ 300 MW
[MW]
HhighB
HlowB
* ~mean HhighB from Bemis et al (1993) and Ginster et al (1994)
Normalized horizontal heat flux