New paper in GRL uses ocean mass sensor points to measure sea level change

Figure 3. (a) Modeled (black) and measured (red) time series of OBP, and the individual contributions to OBP, from sitesN and S in the tropical Pacific. The start of each instrument deployment is marked with a red tick at the top, the first deployment being at N and all others at S. The ocean mass term is shown as modeled (black), and also as calculated from a residual of the measured OBP minus all other modeled terms, both before (pale red) and after (dark red) applying a 5-month low pass filter.

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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40 Responses to New paper in GRL uses ocean mass sensor points to measure sea level change

  1. Ed_B says:

    I fail to see why satellites are needed after the design of a longer life bottom instrument is realized. Please consider reducing government waste!

  2. Peter Melia says:

    Doesn’t sea water density change quite dramatically from place to place, varying from almost fresh to very dense?
    Presumably these differences in density will affect pressure on the seabed.
    Do these investigations allow for this?

  3. richardscourtney says:

    Anthony:

    The Abstract of the paper says

    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.

    NO!
    A model cannot measure anything except the opinions of those who constructed the model.
    Opinions are NOT reality.
    However, the output of a model can be used as an indication if the model can be shown to emulate reality with known accuracy, precision and reliability.

    The paper says

    Nonetheless, most of the Pacific shows trends below 1 mm/yr even with this unrealistically large adjustment process taking place, suggesting that the statistical uncertainty estimate of 0.28 mm/yr is probably not far from the true dynamical OBP trend. The dynamical signal is therefore small enough that a measurement of secular OBP trend at a single point could provide a valuable constraint on the secular increase in ocean mass.

    However, it is not true that
    “The dynamical signal is therefore small enough …”
    because there is no determination – only a suggestion – that
    “the statistical uncertainty estimate of 0.28 mm/yr is probably not far from the true dynamical OBP trend”.

    How far is “not far”?

    In conclusion, the paper may be right “that there is a region in the central Pacific ocean where ocean bottom pressure is a direct measure of interannual changes in ocean mass” but it only provides a “suggestion” of this being true and much more work is required to substantiate whether it is true or not.

    Richard

  4. Peter Miller says:

    Apologies for being off topic, but has anyone wondered where have all the tediously painful, pseudo-intellectual, trolls that used to post on WUWT, like R Gates, gone to?

    Answer: They seem to have emerged at Judith Curry’s Climate etc where their verbal diarrhoea is in full flow.

    As for this post, good luck with this measuring process, these guys are going to need it.

  5. Pressure changes at the bottom of a static tank might reveal surface level changes, but this is a dynamic system. Sea water weighs 64.2 lb/cuft with a linear increase with depth. A minor temperature difference in this water column could result in a perceived silly millimeter change. Terra firma weighs average 125 lb/cu ft and is floating on liquid magma. Every day the Moon lifts and drops the Earth’s crust 18 inches, known as Earthtide. This lift/fall cycle allows adjustments of under crust mantle pressure spots, meaning there is NO fixed bottom reference point. To measure the top of the ocean, from pressure sensors on the bottom, is a preposterous grant gravy fiction, approved by green blinkered, logic-free bureaucrates.

  6. David A. Evans says:

    What actually matters is what we see on our shorelines. There is no perceptible sea level rise where I live, my mothers home in Redcar is still above water. I see no need to worry.

    DaveE.

  7. Gary Pearse says:

    Has anyone noticed that most of the new, interesting work on climate related items like sea level, ice ages, paleo climate, etc., are from mainly new blood. The prolific bunch that essentially ruined scientific journals with dross and activist plonk pre-climategate (how can anyone respect Nature, Sci American and their like again) have somewhat withdrawn and taken up defensive positions, sniping in shrill hyperbole and through law suits, the same old dead stuff about CO2. This and the somewhat sad and self denigrating global rants designed by Al Gore. Al, time to sell your positions in the carbon market and retire. Kevin, its over, let the young guys do the heavy lifting now. Don’t stand on the bridge like the Mann and watch your world fall apart.

  8. Mac the Knife says:

    The models 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.

    An hypothesis is declared (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) based on their pet models. The hypothesis needs to be tested by direct measurements at this Pacific ocean location, to determine if it is valid or false, to assess if the conditions at the location is really as the models predict. Further, samples of the bottom ooze from this location need to be retrieved and evaluated to guide the design of the sensor.

    As the hypothetical sensor will measure pressure as a proxy for ‘total ocean mass’ (Did I understand that correctly?), it must needs be incredibly accurate and stable over decades wrt to pressure and temperature while being impervious to direct penetration, permeation, corrosion, or diffusion from the ambient solution or elemental constitutients at the ocean bottom. What ambient pressures and temperature will this sensor be exposed to in service, at the bottom of the Pacific? What is the molecular and dissociated elemental make up of the primordial ooze at that depth?

    The bottom of the ocean is necessarily exposed to a continual in-fall of organic and inorganic debris from the water column above. What is the in-fall rate at this location and how will this steadily accumulating in-fall of debris effect the sensor over time?
    MtK

  9. Peter Miller says:
    November 9, 2012 at 3:26 pm

    Apologies for being off topic, but has anyone wondered where have all the tediously painful, pseudo-intellectual, trolls that used to post on WUWT, like R Gates, gone to?

    Answer: They seem to have emerged at Judith Curry’s Climate etc where their verbal diarrhoea is in full flow.

    I’ve noticed that. Her site needs a team of moderators.

  10. Peter Miller says:
    November 9, 2012 at 3:26 pm
    where have all the tediously painful, pseudo-intellectual, trolls that used to post on WUWT, like R Gates, gone to?
    There are some left here…
    e.g. the usual ones on http://wattsupwiththat.com/2012/11/06/solar-cycle-24-continues-weakly-perhaps-weakest-of-the-space-age

  11. I have not read the paper because it is behind a paywall but when scientists write a paper about pressure than one has to doubt the conclusions. Pressure is an engineering unit. OBP is a combination of a) gravity b) density of the fluid d) height of the column of fluid and e) surface pressure (ie atmospheric pressure). All of these factors may vary around the globe and some can vary over time. A standard atmosphere is 101.3 kPa but atmospheric pressure can vary from about 96kPa to 104 KPa. Standard gravity is 9.807 m2/sec but can vary in the range 9.78 to 9.83 m2/sec . By definition the density of water at 4C is 1000kg/m3.. A 10m column of pure water at 4C in a standard atmosphere will give a pressure of 98.07kPa. Density of water will vary with temperature. Pure water at 20C has a density of 998kg/m3. Sea water contains salt which changes density – the approximate salt content is 35kg/m3. The average density of a deep water column near the equator maybe 1030 kg/m3.
    I doubt if sea level can ever be measured to an accuracy of 1 mm particularly considering the variation in atmospheric pressure (work it out yourself).
    Reports on “The South Pacific Sea Level & Climate Monitoring Project” have shown variations in sea level associated with SOI (calculated from the difference in atmospheric pressure between Darwin and Tahiti)

  12. Larry Butler says:

    Dear Perfessers….
    The water ain’t gittin’ no deeper in Charleston…..
    Thank you,
    Bubba

  13. Mariss says:

    “There are some left here…
    e.g. the usual ones on http://wattsupwiththat.com/2012/11/06/solar-cycle-24-continues-weakly-perhaps-weakest-of-the-space-age

    OK; So I go there and I wonder what it is I’m expected to do? Scroll through 212 responses and try to guess which ones are trolls? A little bit of specificity would go a long way here.

  14. Mariss says:
    November 9, 2012 at 6:36 pm
    Scroll through 212 responses and try to guess which ones are trolls? A little bit of specificity would go a long way here.
    Scroll from the bottom and make up your own mind

  15. Rascal says:

    Somewhat off topic: a couple of years ago you featured a pie chart showing “What we know about climate”, “What we think we know about climate”, and “What we DON’T know about climate”.

    I found it to be very enlightening (and humbling) when discussing various theories and observations, but have lost it.

    Could you re-post it?

  16. markx says:

    David A. Evans says: November 9, 2012 at 4:44 pm “…. no perceptible sea level rise where I live, my mothers home in Redcar is still above water. I see no need to worry….”

    Larry Butler says: November 9, 2012 at 6:20 pm “…The water ain’t gittin’ no deeper in Charleston…..”

    Mariss says: November 9, 2012 at 6:36 pm “…. I wonder what it is I’m expected to do? Scroll through 212 responses and try to guess which ones are trolls? A little bit of specificity would go a long way here…..”

    I dunno, still seem to be plenty of trolls here, though they are the ‘drive by’ sort. Some people are even too lazy to do their own reading.

  17. John F. Hultquist says:

    Beginning about 18,000 years ago as the glacial ended and rapid sea level rise followed, these types of high-tech activities seem most suitable — large changes in a short time hardly confused by other factors. Currently, with small changes, there are confounding issues, such as water mining, increased sedimentation, spreading ridges, crustal uplift and sinking, and perhaps a few others. While pressure measurements may be useful for some purpose, David A. Evans @ 4:44 says “What actually matters is what we see on our shorelines.” And how does pressure tell us that?

  18. markx says:

    Mariss says: November 9, 2012 at 6:36 pm

    “……I wonder what it is I’m expected to do? Scroll through 212 responses and try to guess which ones are trolls? A little bit of specificity would go a long way here….”

    Mariss, thinking further, and having just visited the site, I wonder what you really are wanting to discover? I find when I go there, I read through the comments in order to understand the discussion (and they are nicely threaded), and even follow the links and read discussed papers!!!!?

    Can you enlighten me as to your methods of research, opinion forming, and entering into discussion?

    It may save me an awful lot of time.

  19. markx says:

    mods … sorry ….looks like an open HTML tag in my previous post…

    Reply: No problem. Fixed. -ModE

  20. Mark and two Cats says:

    markx said:
    November 9, 2012 at 9:48 pm
    [clipped examples]
    I dunno, still seem to be plenty of trolls here…
    ——————————————————————
    Those are trolls? Hah! He must not read many forums.

  21. E.M.Smith says:

    @MarkX:

    Here near San Francisco, the old Alviso Harbor is now a reed bed and the docks have been removed. The channel is flanked by salt marshes, and the place where, during World War II, they built ships for the war effort is effectively land locked. The bay is significantly smaller than it was when ‘discovered’ a few hundred years ago. There isn’t any water rise, but the land does seem to be rising.

    Then again, this IS California and we kind of expect our land to be moving around a lot ;-)

    That kind of observation isn’t ‘trolling’, it’s data.

    The Trolls tend to toss out snark for the purpose of instigating a food fight.

    IMHO, they have faded (as they typically do) due to the Election and will be back soon enough. They also fade about the time some government funded party is being held at the pork barrel ( i.e. such things as the Ipcc and related free rider events). But they come back when the new semester starts and they have to be back at “work”…

    @Marris:

    Who is a Troll is somewhat (highly?) subjective. It’s not really possible for me to tell you ‘who is a Troll’, only my opinion of who might be. So look for dogged repeating of “talking points”. Look for “talking past” the other person and not addressing points they make / raise. Look for emotional loaded language and ‘attack the messenger’ (and avoidance of an actual ‘searching for truth’ behavior). And look especially for tossing red meat insults, invective, and innuendo intended to start arguments / fights rather than reach answers. Then look for that same moniker doing those same things over and over and over…

    IMHO, though, that particular thread / link doesn’t give a lot of good examples. Mostly it has examples of snark and sniping between the “Sun can’t do it” side represented by Leif, and the “Sun does it” crowd (such as Geoff) in their usual “does so – does not” pebble tossing (doesn’t really rise to mud tossing or rock throwing ;-)

    FWIW, in looking through a couple of postings / comments just now for a ‘good example’, I couldn’t find one… So now I’m wondering where the AGW True believers have all run off to as well…. Perhaps they were instructed to not rile the ‘skeptics’ before the election, lest they go vote… or they are still hungover from the party… ;-)

    Per the subject of the posting:

    Again with the mythical precision via magical math…. fixing up that chaotic ocean surface and the thousand and one ‘moving targets’ that change the ocean… Heck, just the way the moon has a constantly changing tidal influence with various cycles from a month to 1200 years (and more?) means a larger error term for any one spot on the ocean floor (not to mention changes in circumpolar currents at the S. Pole and…

  22. David L. Hagen says:

    cementafriend

    Please learn about the international measurement system SI and how the density of water varies with temperature. The density of water is NOT “By definition “1000kg/m3″ “at 4C” “by definition”.
    See ISPWA

    4.3 Density Maximum
    Liquid water at atmospheric pressure (0.101 325 MPa) has a maximum density at a temperature slightly below 4 °C. This point is often recommended as a reference standard for precise density measurement. The CCM (Consultative Committee for Mass and Related Quantities) of the CIPM (Comité International des Poids et Mesures) has adopted a value for this maximum density (for the isotopic composition of VSMOW) as a part of its standard table for temperatures from 0 °C to 40 °C [16]. This value is (999.974 95 ± 0.000 84) kg/m3.

    Guideline on the Use of Fundamental Physical Constants and Basic Constants of Water, The International Association for the Properties of Water and Steam 2001

  23. Chris Schoneveld says:

    Even the bottom of the ocean is not tectonically stable hence the sensor cannot be considered a reliable reference point.

  24. The sea floor is covered with debris, dead plankton and other micro animal skeleta, which rains down all the time. Since these new detectors are so sensitive this debris will affect readings. I do not expect sea floor movement due to plate tectonics to have much effect since this is a few cm per year.

  25. Rob L says:

    We have great GPS based capacity to measure altitude with sub millimetric accuracy nowadays, this means that we can measure the rate of land rise or fall in harbours where we measure sea level, and as a result can correct for land fall and rise. From this and historical measures of sea level we can accurately determine the rate of sea level rise at about 2mm/year over most of the globe. As a result I am suspicious of results that don’t agree with this figure.

    As far as sea level rise as a result of temperature change (stearic) goes; cooling can also cause sea level rise! Water is a very anomalous fluid in that it gets less dense below 4°C so water getting colder than 4°C can lead to sea level rise too – It also means that due to relative buoyancy effects the only place where cold water can penetrate below the point where temperature hits 4°C is at the poles where higher salinity water is created by salt formation.

    So could higher rates of ice formation every winter (due to larger seasonal variation) actually be causing global cooling of the deep ocean due to the creation of greater volumes of higher salinity cold water, and thereby raising sea levels? Not the kind of question or hypothesis that is likely to be asked by the cult of AGW.

  26. Steve Richards says:

    @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.

  27. David A. Evans says:

    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.

  28. mwhite says:

    “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

  29. mbur says:

    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

  30. P. Solar says:

    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.

  31. marchesarosa says:

    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/

  32. phlogiston says:

    FWIW, the above curve of total OBP (ocean bottom pressure) seems flat over the last 10 years. No evidence of SLR.

  33. Doug Proctor says:

    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?

  34. markx says:

    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.

  35. markx says:

    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):

    For example, Morel and Willis [2005] looked at the errors in mean sea level arising from errors in the geocenter or scale determinations of the ITRF. They found that a 10 mm error in the Z component of the reference frame can lead to an error of –1.2 mm in the determination of mean sea level, with a strong regional systematic error signal at the high latitudes.

    http://ilrs.gsfc.nasa.gov/docs/GRASP_COSPAR_paper.pdf

  36. Brian H says:

    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?

  37. Spector says:

    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.

  38. george e. smith says:

    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.

  39. Spector says:

    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.

  40. Spector says:

    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.

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