III. Vertical Heat Flux
1
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 School of Oceanography, University of Washington, Box 357940, Seattle, WA, 98195-7940 | [email protected] | www2.ocean.washington.edu/~scottv/ III. Vertical Heat Flux CTD IV. Heat Budget Implications H highB + H lowB + H top + H sides = 0 H lowB = H top + H sides - H highB ~ 520 +100 -340* = 280 MW therefore, H lowB ~ H highB ~ 300 MW * ~mean H highB from Bemis et al (1993) and Ginster et al (1994) Sea floor Ridge crest 250 50 150 200 100 mab 2200 2100 Depth H sides H top H lowB H highB I. Background II. Lateral Heat Flux <> S =0.047 o C <> N =0.051 o C The Flow Mow Experiment: 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. Methodology: Because southward tidal displacement rarely exceeds 50m, the south side of the control volume is usually colder than the north side. The mean from any side can be multiplied by the orthogonal component of velocity (v), as well as density (), heat capacity (C p ), and area (A), to estimate the heat flux (H) through that surface: H = C p A v Pooling and streaming cause high variance : Histograms of mean potential temperature anomaly ( ) 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 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. Magnitude and variance: ABE 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... Observed magnitude and modeled variance: Proximity of observations to MEF: near ABE50 distant all < > N < > S [oC] Velocity data source duration <v> (cm/s) 0.025 0.02 0.002 0.004 South mooring 2168 74 days 1.6 27 22 2.2 4.4 South, survey only 17 days 1.1 24 19 1.9 3.8 North WASP 2161 142 days 4.1 91 73 7.3 15 North, survey only 17 days 5.1 114 91 9.1 18.2 North, ABE dive 50 only ~1 day 6.1 109 South, ABE dive 50 only ~1 day 3.9 70 Notes: C p ~3800; A=300m x 75m [MW] Method of calculation: H top = C p A (1/N) [( > S ) w] [Watts] Area (A) is the survey coverage (300m x 720m). > 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 and C p of each top is used. Summation is over all points of ABE’s trackline. o C Each heat flux value below is estimated by multiplying a mean velocity by the mean difference between N and S sides of the field (< > N < > S ). Modeled variance for each current meter record is given at right. Recently discovered fields HIGH BUOYANCY FLUX FIELDS: SAS = Sasquatch SDF = Salty Dawg HRF = High Rise MEF = Main Endeavour MF = Mothra LOW BUOYANCY FLUX FIELDS: C = Cirque D = Dune CB = Clam Bed Q = Quebec Bathymetric contour interval: 100m SAS N 2000 MEF ‘95 S 2000 Current Meters ABE survey Ridge crests South of MEF: North of MEF: North side South side ABE dive 50 MEF perimeter : 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 and v (below). 51766 51768 51770 51772 51774 51776 51778 0 100 200 300 400 500 600 700 800 900 1000 Tim e (m od.julian days) H eatflux (M W) M E F heatflux from 300 x 720m plane (box-top)@ 2100m depth ABE 51 ABE 50 ABE 49 ABE 48 ABE 46 ABE 45 ABE 44 MEAN 524 M W ABE surveyed the top of the MEF control volume 15 times; 12 of these “tops” led to robust estimates of H top . 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. Sout h Nort h Variance of observations: σ = 236.3 MW Variance of mean: [σ / (12) 1/2 ] = 65.3 MW Simulated MEF H sides Normalized horizontal heat flux
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250. 200. CTD. 150. ABE. 100. 50. Bathymetric contour interval: 100m. SAS. N 2000. MEF ‘95. S 2000. Current Meters. LOW BUOYANCY FLUX FIELDS: C = Cirque D = Dune CB = Clam Bed Q = Quebec. HIGH BUOYANCY FLUX FIELDS: SAS = Sasquatch SDF = Salty Dawg HRF = High Rise - PowerPoint PPT Presentation
Transcript of III. Vertical Heat Flux
Measurements and Models of Heat Flux Magnitude and Variance from the Main Endeavour Hydrothermal Vent FieldScott R. Veirs, Fritz R. Stahr, Russell E. McDuff, Richard E. Thomson, Dana R. Yoerger, and Albert M. Bradley
School of Oceanography, University of Washington, Box 357940, Seattle, WA, 98195-7940 | [email protected] | www2.ocean.washington.edu/~scottv/
III. Vertical Heat Flux
CTD
IV. Heat Budget Implications
HhighB + HlowB + Htop + Hsides= 0
HlowB = Htop + Hsides - HhighB
~ 520 +100 -340* = 280 MW
therefore, HlowB ~ HhighB ~ 300 MW
* ~mean HhighB from Bemis et al (1993) and Ginster et al (1994)
Sea floor
Ridge crest
250
50
150
200
100
mab 2200
2100
Depth
Hsides
Htop
HlowBHhighB
I. Background II. Lateral Heat Flux
<>S=0.047 oC <>N=0.051 oC
The Flow Mow Experiment: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.
Methodology:Because southward tidal displacement rarely exceeds 50m, the south side of the control volume is usually colder than the north side. The mean from any side can be multiplied by the orthogonal component of velocity (v), as well as density (), heat capacity (Cp), and area (A), to estimate the heat flux (H) through that surface: H = Cp A
v
Pooling and streaming cause high variance :
Histograms of mean potential temperature anomaly () 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 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.
Magnitude and variance:
ABE
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...
Observed magnitude and modeled variance:
Proximity of observations to MEF: near ABE50 distant all <>N<>S [oC]
Velocity data source duration <v> (cm/s) 0.025 0.02 0.002 0.004
South mooring 2168 74 days 1.6 27 22 2.2 4.4
South, survey only 17 days 1.1 24 19 1.9 3.8
North WASP 2161 142 days 4.1 91 73 7.3 15
North, survey only 17 days 5.1 114 91 9.1 18.2
North, ABE dive 50 only ~1 day 6.1 109
South, ABE dive 50 only ~1 day 3.9 70
Notes:Cp~3800; A=300m x 75m [MW]
Method of calculation:Htop = Cp A (1/N) [(>S ) w] [Watts]
Area (A) is the survey coverage (300m x 720m). >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 and Cp
of each top is used. Summation is over all points of ABE’s trackline.
oC
Each heat flux value below is estimated by multiplying a mean velocity by the mean difference between N and S sides of the field (<>N<>S ). Modeled variance for each current meter record is given at right.
Recently discovered fields
HIGH BUOYANCY FLUX FIELDS:
SAS = Sasquatch
SDF = Salty Dawg
HRF = High Rise
MEF = Main Endeavour
MF = Mothra
LOW BUOYANCY FLUX FIELDS:
C = Cirque
D = Dune
CB = Clam Bed
Q = Quebec
Bathymetric contour
interval: 100m
SAS
N 2000MEF ‘95
S 2000
Current Meters
ABE surveyRidge crests
South of MEF: North of MEF:
North side
South side
ABE dive 50
MEF perimeter :
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 and v (below).
51766 51768 51770 51772 51774 51776 51778 0
100
200
300
400
500
600
700
800
900
1000
Time (mod. julian days)
Hea
tflu
x (M
W)
MEF heatflux from 300 x 720m plane (box-top) @ 2100m depth
ABE 51
ABE 50
ABE 49
ABE 48
ABE 46
ABE 45
ABE 44
MEAN524 MW
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.
South North
Variance of observations: σ = 236.3 MW
Variance of mean: [σ / (12)1/2] = 65.3 MW
Simulated MEF Hsides
Normalized horizontal heat flux
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