ABSTRACT:
Combining ocean and earth models, we show that there is a region in the central Pacific ocean where ocean bottom pressure is a direct measure of interannual changes in ocean
mass, with a noise level for annual means below 3 mm water equivalent, and a trend error below 1 mm/yr. We demonstrate this concept using existing ocean bottom pressure
measurements from the region, from which we extract the annual cycle of ocean mass (amplitude 8.5 mm, peaking in late September), which is in agreement with previous
determinations based on complex combinations of global data sets. This method sidesteps a number of limitations in satellite gravity-based calculations, but its direct implementation is currently limited by the precision of pressure sensors, which suffer from significant drift. Development of a low-drift method to measure ocean bottom pressure at a few sites could provide an important geodetic constraint on the earth system.
Citation: Hughes, C. W., M. E. Tamisiea, R. J. Bingham, and J. Williams (2012), Weighing the ocean: Using a single mooring to measure changes in the mass of the ocean, Geophys. Res. Lett., 39, L17602, doi:10.1029/2012GL052935.
Introduction:
The GRACE satellite gravity mission has revolutionized our ability to monitor regional mass redistribution in the earth system, and hence monitor changes in ocean mass and the source of those changes. However, GRACE does not monitor the degree 1 terms in mass movement, associated with geocenter motion, and is weak for the C2,0 harmonic [Chen et al., 2006; Swenson et al., 2008; Leuliette and Miller, 2009]. It also suffers from limited spatial resolution, making it hard to distinguish the much larger land signals from ocean signals near the ocean boundaries [Chambers et al., 2007], and secular trends include a contribution from glacial isostatic adjustment (GIA), the solid earth’s ongoing response to the change in load since the last glaciations [Tamisiea, 2011]. Together, these difficulties lead to an uncertainty approaching 1 mm/yr in the measured mass component of global sea level trend.
If sea level changes were spatially uniform, then variation of the volume of the ocean could be monitored using a single tide gauge. Similarly, spatially uniform changes in ocean bottom pressure (OBP) would mean ocean mass changes could be monitored with a single OBP Recorder. However, spatial variations mean that sea level measurements must be made over the entire ocean (by satellite altimetry), or statistical extrapolation must be used to mitigate the sampling problems of tide gauge data [Hughes and Williams, 2010; Church and White, 2006; Jevrejeva et al., 2006]. Fortunately, as we will show, OBP observations in one specific region do allow us to measure ocean mass changes with a single station.
h/t to Dr. Leif Svalgaard who has the full paper here
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@Rob L
We have another circular problem with the GPS system.
The accuracy of a GPS position depends upon knowing the orbital accuracy of each satellite.
Each satellite is monitored as it orbits over monitoring stations on terra firma, and new position data is uploaded to each satellite, for rebroadcast to users.
Satellite orbit accuracy depends upon the accuracy of the ground stations position. If the ground station moves up or down by a few millimetres then all users using the system will have their positions ‘adjusted’ automatically.
Geologists etc use GPS to monitor the movement of plates over the earths surface.
This link shows that Seattle is rising and falling by +/-20mm per year with an indicated sinking trend.
http://pbo.unavco.org/station/overview/SEAT
If GPS requires a stable earth surface to remain in useful calibration and the earth’s surface is moving quite a lot, I think it will be difficult to use a pressure sensor on the ocean floor to accurately measure water height.
markx says:
November 9, 2012 at 9:48 pm
I may be a troll in your eyes but I think it is relevant that my mothers home in Redcar is not under water. It’s less than 3 feet above high water and has been so for the best part of a century. Spring tide gets to within a few inches of the front step, it’s done that for the best part of a century too.
I’ve lived in this area for over 50 years so no discernible rise is also relevant.
Oh, I drive by more than once. 😉
DaveE.
“NOAA has officially ended their “El Nino Watch”, meaning they no longer believe it’s coming and instead, say the Pacific Ocean waters will be near normal — or neutral — conditions.”
http://climaterealists.com/?id=10554
Ever hear of ‘water displacement’?
So you’re telling me that all the stuff we built in the ocean,all the ships in the sea,all the erosion,all the rocks i skipped.Doesn’t cause unmeasureable(well it may be estimateable but its constantly changing)changes to the sea level?…………………………WUWT?
Thanks for the interesting articles and comments
As Richard Cortney points out, using OBP looks like a good idea (cheaper than a fleet of satellites!) but this paper is based on nothing but using model output as “data” so shows nothing.
“[5] For a running annual mean of OBP, this spectrum
translates into a standard deviation of less than 3 mm of
water in the tropical Pacific ”
OMG , here we go again. The LAST thing you do with your data before doing a frequency analysis is distort its frequency characteristics with crap filter like a runny mean.
Good idea on OBP but these guys just failed on the basic maths and signal processing modules. Give the job to someone else.
Rascal
The pie chart of climate knowledge devised by Willis Eschenbach and overlaid on Turner’s “Rain, Steam, and Speed – The Great Western Railway” is here:
http://wattsupwiththat.com/2010/05/08/climate-actually-changes-film-at-1100/
FWIW, the above curve of total OBP (ocean bottom pressure) seems flat over the last 10 years. No evidence of SLR.
The “equilibrium tide” looks to be model. Low resolution with a visual decline in amplitude and bottom value. The scales look crude, not 1 mm/yr.
One significant error in models throws out the conclusion here, does it not?
David A. Evans says: November 10, 2012 at 5:45 am
“…I may be a troll in your eyes but I think it is relevant that my mothers home in Redcar is not under water. It’s less than 3 feet above high water and has been so for the best part of a century. Spring tide gets to within a few inches of the front step, it’s done that for the best part of a century too. I’ve lived in this area for over 50 years so no discernible rise is also relevant………DaveE….”
That is indeed not trolling.
Apologies from me David. I seem to have misinterpreted your intent.
Rob L says:
November 10, 2012 at 3:40 am
“…We have great GPS based capacity to measure altitude with sub millimeter accuracy nowadays…”
Not so Rob, as Anthony [showed] recently with the article on the proposed GRASP satellite. changes are needed because the current system (including GPS satellite data cobbled into an array of data from other satellites) still is not accurate to within the 1 mm required.
As Steve Richards points out apparently there are problems with the TRF (Terrestrial Reference Frame):
http://ilrs.gsfc.nasa.gov/docs/GRASP_COSPAR_paper.pdf
What on the Earth’s surface (your choice of sea surface or sea bottoms) could be considered stable? And stable with reference to what? Satellites orbit the Earth’s CoG, but modified by intervening masscons, the Diurnal Bulge, etc. The waters of the oceans and lakes slosh and flow about in somewhat, but not sufficiently, periodic and predictable ways, modified by significant pulls from the Moon and Sun on a daily basis. Land surfaces wander slowly about the surface of the semi-liquid mantle, bobbing, flexing, cracking, and jerking as they do so. The Atlantic expands, the Pacific shrinks. Islands grow and erode.
Where, anywhere in all that, is a reference point which is not purely arbitrary? And fraught with consequences once chosen?
In order to remove confusing annual cyclic noise from the plots presented, I would recommend that all such data be plotted as one year moving averages.
I am not sure if this study measures and accounts for slow changes in the volume of the ocean basin due to tectonic activity or if it has been determined that such changes are negligible.
Well, I am always intrigued by measurement concepts that “seem” to defy common sense, and I do mean seem to, as sometimes they actually do work.
On a trip to my alma mater a few years ago, one of the Physics profs described some ocean temperature measurement work they were doing. As I recall, it had something to do with the velocity of sound as a function of Temperature, so the propagation time from A to B depended only on the “average” temperature (near surface) between A & B. Supposedly, they could determine the average temperature between NZ (Auckland) and Hawaii. Didn’t know you can even hear that far. Well maybe I got the factors wrong; a problem of old age, but it was one of those “can you really do that ?” things. So I’m not going to pre-judge this OBP busines till I read some more.
But ANY time I hear about this kind of data gathering and filtering and statisticating, the very first thing that comes to my mind, is: This is a sampled data system, so prove to me that it satisfies the Nyquist samplin theorem; because no amount of filtering o statistical machinations ca buy you a reprieve if you violate Nyquist.
So what did they say the density of water; excuse me that’s sea water, well no it’s actually sea food fish soup; actually is, and how stable with time is that ?
I would expect that ocean biomass changes seasonally and geographically, to make any ocean mass computations somewhat suspect.
But as I said; stranger things have happened. The primary astigmatism of a thin lens with the aperture stop in contact depends only on the focal length, and is quite independent of the lens material, and of the object and image conjugate distances (magnification), and also independent of any surface curvatures, or asphericity in the lens prescription; and no process short of adding other elements (lenses) can alter it, let alone correct it, and the amount of it is huge; yes it is quite independent of the lens speed ( f-number).
So I am ready to learn how they know this ocean weighing really works.
RE: george e. smith:
November 13, 2012 at 7:37 pm
“… Physics profs described some ocean temperature measurement work they were doing. As I recall, it had something to do with the velocity of sound as a function of Temperature …”
The velocity of sound is affected by temperature, salinity, and pressure to the extent that they change the density or bulk elasticity of the water. Also the sound path will curve due to progressive refraction as the sound velocity changes with depth (or any other spatial dimension). This is the cause of acoustic shadows, as sound ray curvature bends emitted sound away from the shadow zone. Ref: Principles of Underwater Sound, Robert J. Urick, 3rd Ed, 1996.
RE: george e. smith:
November 13, 2012 at 7:37 pm
So I am ready to learn how they know this ocean weighing really works.
Based on the statement in the abstract, It would appear that they are using the pressure at the bottom of the ocean to determine the total mass of water per unit surface area above that point. That is what this pressure indicates when the surface pressure is subtracted out. To measure the mass of the ocean as a whole, one needs to know the total volume of the ocean bed and also know how that volume changes as the level of the sea changes. If one knows the vertical temperature, salinity, and pressure profile at a given location, it is possible to make a reasonably accurate measure of the ocean bottom depth by acoustic echo sounding.
I suspect they are assuming that the volume and shape of the ocean basin is not changing and that is the reason that they can rely on a single measurement point. The average slope of the shore impacts total mass increase with increasing average sea surface rise with respect to the center of the Earth. Multiple measurements, all over the world, would be required to estimate the total rate of ocean basin volume change from slow tectonic effects.
Of course, if all they really want to measure is level changes, then they do not need to know the true mass of the ocean.