Publications

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2015
Mikhalevsky, PN, Sagen H, Worcester PF, Baggeroer AB, Orcutt J, Moore SE, Lee CM, Vigness-Raposa KJ, Freitag L, Arrott M, Atakan K, Beszczynska-Moeller A, Duda TF, Dushaw BD, Gascard JC, Gavrilov AN, Keers H, Morozov AK, Munk WH, Rixen M, Sandven S, Skarsoulis E, Stafford KM, Vernon F, Yuen MY.  2015.  Multipurpose Acoustic Networks in the Integrated Arctic Ocean Observing System. Arctic. 68:11-27. AbstractWebsite

The dramatic reduction of sea ice in the Arctic Ocean will increase human activities in the coming years. This activity will be driven by increased demand for energy and the marine resources of an Arctic Ocean accessible to ships. Oil and gas exploration, fisheries, mineral extraction, marine transportation, research and development, tourism, and search and rescue will increase the pressure on the vulnerable Arctic environment. Technologies that allow synoptic in situ observations year-round are needed to monitor and forecast changes in the Arctic atmosphere-ice-ocean system at daily, seasonal, annual, and decadal scales. These data can inform and enable both sustainable development and enforcement of international Arctic agreements and treaties, while protecting this critical environment. In this paper, we discuss multipurpose acoustic networks, including subsea cable components, in the Arctic. These networks provide communication, power, underwater, and under-ice navigation, passive monitoring of ambient sound (ice, seismic, biologic, and anthropogenic), and acoustic remote sensing (tomography and thermometry), supporting and complementing data collection from platforms, moorings, and vehicles. We support the development and implementation of regional to basin-wide acoustic networks as an integral component of a multidisciplinary in situ Arctic Ocean observatory.

2013
Farrell, WE, Munk W.  2013.  Surface gravity waves and their acoustic signatures, 1-30 Hz, on the mid-Pacific sea floor. Journal of the Acoustical Society of America. 134:3134-3143.   10.1121/1.4818780   AbstractWebsite

In 1999, Duennebier et al. deployed a hydrophone and geophone below the conjugate depth in the abyssal Pacific, midway between Hawaii and California. Real time data were transmitted for 3 yr over an abandoned ATT cable. These data have been analyzed in the frequency band 1 to 30 Hz. Between 1 and 6 Hz, the bottom data are interpreted as acoustic radiation from surface gravity waves, an extension to higher frequencies of a non-linear mechanism proposed by Longuet-Higgins in 1950 to explain microseisms. The inferred surface wave spectrum for wave lengths between 6 m and 17 cm is saturated (wind-independent) and roughly consistent with the traditional Phillips kappa(-4) wave number spectrum. Shorter ocean waves have a strong wind dependence and a less steep wave number dependence. Similar features are found in the bottom record between 6 and 30 Hz. But this leads to an enigma: The derived surface spectrum inferred from the Longuet-Higgins mechanism with conventional assumptions for the dispersion relation is associated with mean square slopes that greatly exceed those derived from glitter. Regardless of the generation mechanism, the measured bottom intensities between 10 and 30 Hz are well below minimum noise standards reported in the literature. (C) 2013 Acoustical Society of America.

2011
Munk, W.  2011.  The sound of climate change. Tellus Series a-Dynamic Meteorology and Oceanography. 63:190-197.   10.1111/j.1600-0870.2010.00494.x   AbstractWebsite

It came as a great shock in the 1960s that the oceans, like the atmosphere, have an active weather (i.e. ocean storms are called eddies). The traditional expedition mode of individual research vessels making independent measurements was no longer adequate. Ocean Acoustic Tomography was developed in direct response to the 'eddie revolution'. Sound travels faster in warmer water; acoustic waves transmitted from ship to ship give information about the temperature and currents in the intervening waters. The transmission scale has widened over the years from 100 to 1000 to 10 000 km, approaching the antipodal scale of Ewing's 1960 transmission from Perth (Australia) to Bermuda. In 1991 we successfully transmitted from a source ship on Heard Island in the Indian Ocean to receiver ships in the north and south Atlantic and Pacific Oceans. Brian Dushaw is planning to repeat Ewing's experiment; he expects a reduction in travel time of approximately 10 s as confirmation of global ocean warming over the last 50 yr. Sea level rose 15 cm in the 20th century. The rate has since doubled; values up to 2 m by 2100 are now being quoted. To make accurate predictions we must understand the melting processes of continental ice sheets. Floating ice sheets from Antarctic and Greenland glaciers cover huge ocean caverns that have not to date been accessible to observation. We propose probing these caverns with sound waves to study the ocean dynamics at the underside of the floating ice sheets: a daring venture.

2010
Farrell, WE, Munk W.  2010.  Booms and busts in the deep. Journal of Physical Oceanography. 40:2159-2169.   10.1175/2010jpo4440.1   AbstractWebsite

Deep sea (5 km) pressure and velocity at the Hawaii-2 Observatory (H2O), midway between Hawaii and California, exhibit a number of remarkable features that are interpreted using the Longuet-Higgins theory of acoustic radiation from oppositely directed surface waves. A change in the slope of the bottom spectra near 5 Hz can be associated with a transition near 2.5 Hz (25-cm wavelength) of the surface wave spectrum from the classical kappa(-4) saturated (wind independent) Phillips spectrum to a distinct band of ultragravity waves. Bottom spectra are remarkably stable. Occasional 15-dB busts in the gravities and booms in the ultragravities are prominent features in the bottom records and can be associated with calms and storms at the sea surface. For strong winds, two broad lobes in the directional spectrum of the gravity waves are nearly perpendicular to the wind; as the wind drops, the lobes become narrower and more nearly aligned with the wind, leading to busts.

Gleick, PH, Adams RM, Amasino RM, Anders E, Anderson DJ, Anderson WW, Anselin LE, Arroyo MK, Asfaw B, Ayala FJ et al..  2010.  Climate change and the integrity of science. Science. 328:689-690.   10.1126/science.328.5979.689  
2009
Dushaw, BD, Worcester PF, Munk WH, Spindel RC, Mercer JA, Howe BM, Metzger K, Birdsall TG, Andrew RK, Dzieciuch MA, Cornuelle BD, Menemenlis D.  2009.  A decade of acoustic thermometry in the North Pacific Ocean. Journal of Geophysical Research-Oceans. 114   10.1029/2008jc005124   AbstractWebsite

Over the decade 1996-2006, acoustic sources located off central California (1996 1999) and north of Kauai (1997-1999, 2002-2006) transmitted to receivers distributed throughout the northeast and north central Pacific. The acoustic travel times are inherently spatially integrating, which suppresses mesoscale variability and provides a precise measure of ray-averaged temperature. Daily average travel times at 4-day intervals provide excellent temporal resolution of the large-scale thermal field. The interannual, seasonal, and shorter-period variability is large, with substantial changes sometimes occurring in only a few weeks. Linear trends estimated over the decade are small compared to the interannual variability and inconsistent from path to path, with some acoustic paths warming slightly and others cooling slightly. The measured travel times are compared with travel times derived from four independent estimates of the North Pacific: (1) climatology, as represented by the World Ocean Atlas 2005 (WOA05); (2) objective analysis of the upper-ocean temperature field derived from satellite altimetry and in situ profiles; (3) an analysis provided by the Estimating the Circulation and Climate of the Ocean project, as implemented at the Jet Propulsion Laboratory (JPL-ECCO); and (4) simulation results from a high-resolution configuration of the Parallel Ocean Program (POP) model. The acoustic data show that WOA05 is a better estimate of the time mean hydrography than either the JPL-ECCO or the POP estimates, both of which proved incapable of reproducing the observed acoustic arrival patterns. The comparisons of time series provide a stringent test of the large-scale temperature variability in the models. The differences are sometimes substantial, indicating that acoustic thermometry data can provide significant additional constraints for numerical ocean models.

Munk, W.  2009.  An inconvenient sea truth: spread, steepness, and skewness of surface slopes. Annual Review of Marine Science. 1:377-415., Palo Alto: Annual Reviews   10.1146/annurev.marine.010908.163940   Abstract

Breon and Henriot (BH) have collected eight million globally distributed satellite images of sunglitter, which yield a few simple, robust rules about the statistics of surface slopes: 1) constant angular spread, 2) linear steepness, and 3) sigmoid (near stepwise) skewness (all with respect to wind speed). Yet die information is sparse because it says nothing about time and space scales. The BH rules are an inconvenient sea truth, too fundamental to be ignored, too incomplete to be understood. With regard to BH rule 1 (BH:1), I suggest that the constant spread is associated with a wake-like geometry of the short gravities. Steepness linearity (BH:2) remains an enigma. Skewness (BH:3) appears to be correlated with a rather sudden onset of breaking for winds above 4 in s(-1) do not think that skewness comes from parasitic capillaries. These are tentative conclusions; I look forward to intensive sea-going experiments over the next few years demolishing the proposed interpretations.

2008
Munk, W, Day D.  2008.  Glimpses of oceanography in the postwar period. Oceanography. 21:14-21.   10.5670/oceanog.2008.30   AbstractWebsite

Several recent histories give a critical review of American oceanography in the postwar period. Ronald Rainger (2000a,b, 2001), who has written extensively about the history of oceanography, reviews the Navy-oceanography partnership, and Jacob Darwin Hamblin’s monograph, Oceanographers and the Cold War, examines the history of Soviet-US oceanographic relations at a time of increasing Navy sponsorship of US oceanography (Hamblin, 2005). Written by professional historians, these books give a long view of oceanography. Here, I share some personal glimpses of this era, which differ from the historians’ accounts, especially in regard to relationships between oceanographers and the Navy. Oceanography is again in a state of great flux—the reviews by Rainger and Hamblin may offer guidance concerning future developments.

Farrell, WE, Munk W.  2008.  What do deep sea pressure fluctuations tell about short surface waves? Geophysical Research Letters. 35   10.1029/2008gl035008   AbstractWebsite

Short waves centered at the gravity to capillary transition (13.5 Hz) dominate the slope statistics of the sea surface and are responsible for most of the air-sea momentum transfer (wind stress). Little is known about short "gravities.'' In contrast, long gravities, with their characteristic k(-4) spectrum, have extensive observational support. We propose a fundamental distinction between long gravities, with their saturated (wind-independent) spectrum, and short gravities, with their wind-dependent spectrum. Evidence comes (surprisingly) from sea-floor pressure fluctuations associated with non-linear interactions between oppositely traveling surface waves of half their frequency. The bottom pressure spectrum shows a transition at about 6 Hz (3 Hz surface wave frequency) from an f(-7) to an f(-3) dependence that we associate with the long to short surface gravity wave transition. Further, the requirement of oppositely traveling energy places an integral restraint on the directional spread of the surface waves.

2007
Munk, W, Bills B.  2007.  Tides and the climate: Some speculations. Journal of Physical Oceanography. 37:135-147.   10.1175/jpo3002.1   AbstractWebsite

The important role of tides in the mixing of the pelagic oceans has been established by recent experiments and analyses. The tide potential is modulated by long-period orbital modulations. Previously, Loder and Garrett found evidence for the 18.6-yr lunar nodal cycle in the sea surface temperatures of shallow seas. In this paper, the possible role of the 41 000-yr variation of the obliquity of the ecliptic is considered. The obliquity modulation of tidal mixing by a few percent and the associated modulation in the meridional overturning circulation ( MOC) may play a role comparable to the obliquity modulation of the incoming solar radiation ( insolation), a cornerstone of the Milankovic theory of ice ages. This speculation involves even more than the usual number of uncertainties found in climate speculations.

2006
Rudnick, DL, Munk W.  2006.  Scattering from the mixed layer base into the sound shadow. Journal of the Acoustical Society of America. 120:2580-2594.   10.1121/1.2338813   AbstractWebsite

Long-range sound transmissions in the ocean are largely controlled by the SOFAR wave guide. The agreement between the measured and predicted arrival patterns is generally excellent, except for an observed extension downward by many hundreds of meters from the lower caustics into the deep sound shadow. The deepening of early arrivals is proposed to be associated with the sharp transition zone marking the lower boundary of the surface mixed layer. This mixed layer base (MLB) is distorted by internal waves. Multiple collisions of the ray-like acoustic transmission with the wavy MLB lead to a mean deepening of the lower caustics of 2 y(a)(-1) 1 sigma(2) approximate to 35 in per collision, where y(a) = (1/C)dC/dz = 1.13 x 10(-2) km(-1) is the (fractional) abyssal adiabatic sound speed gradient and sigma(2) = 2 x 10(-4) is the variance in MLB slope. There are typically 20 such collisions in a 1000 kin transmission. Monte Carlo numerical experiments yield statistics of ray inclination, range, travel time, and lower turning point. The resulting time front includes a deepening by several hundred meters. The acoustic signatures provide the possibility for monitoring upper ocean processes with abyssal. acoustic arrays. (c) 2006 Acoustical Society of America.

Colosi, JA, Munk W.  2006.  Tales of the venerable Honolulu tide gauge. Journal of Physical Oceanography. 36:967-996.   10.1175/jpo2876.1   AbstractWebsite

Surface expressions of internal tides constitute a significant component of the total recorded tide. The internal component is strongly modulated by the time-variable density structure, and the resulting perturbation of the recorded tide gives a welcome look at twentieth-century interannual and secular variability. Time series of mean sea level h(SL)(t) and total recorded M-2 vector a(TT)(t) are extracted from the Honolulu 1905-2000 and Hilo 1947-2000 (Hawaii) tide records. Internal tide parameters are derived from the intertidal continuum surrounding the M-2 frequency line and from a Cartesian display of a(TT)(t), yielding a(ST) = 16.6 and 22.1 cm, a(IT) = 1.8 and 1.0 cm for surface and internal tides at Honolulu and Hilo, respectively. The proposed model a(TT)(t) = a(ST) + a(IT) cos theta(IT)(t) is of a phase-modulated internal tide generated by the surface tide at some remote point and traveling to the tide gauge with velocity modulated by the underlying variable density structure. Mean sea level h(SL)(t) [ a surrogate for the density structure and hence for theta(IT)(t)] is coherent with a(IT)(t) within the decadal band 0.2-0.5 cycles per year. For both the decadal band and the century drift the recorded M-2 amplitude is high when sea level is high, according to Omega a(TT) = O(0.1 delta h(SL)). The authors attribute the recorded secular increase in the Honolulu M-2 amplitude from a(TT) = 16.1 to 16.9 cm between 1915 and 2000 to a 28 rotation of the internal tide vector in response to ocean warming.

2005
Colosi, JA, Baggeroer AB, Cornuelle BD, Dzieciuch MA, Munk WH, Worcester PF, Dushaw BD, Howe BM, Mercer JA, Spindel RC, Birdsall TG, Metzger K, Forbes AMG.  2005.  Analysis of multipath acoustic, field variability and coherence in the finale of broadband basin-scale transmissions in the North Pacific Ocean. Journal of the Acoustical Society of America. 117:1538-1564.   10.1121/1.1854615   AbstractWebsite

The statistics of low-frequency, long-range acoustic transmissions in the North Pacific Ocean are presented. Broadband signals at center frequencies of 28, 75, and 84 Hz are analyzed at propagation ranges of 3252 to 5171 km, and transmissions were received on 700 and 1400 in long vertical receiver arrays with 35 in hydrophone spacing. In the analysis we focus on the energetic "finale" region of the broadband time front arrival pattern, where a multipath interference pattern exists. A Fourier analysis of 1 s regions in the finale provide narrowband data for examination as well. Two-dimensional (depth and time) phase unwrapping is employed to study separately the complex field phase and intensity. Because data sampling occured in 20 or 40 min intervals followed by long gaps, the acoustic fields are analyzed. in terms of these 20 and 40 min and multiday observation times. An analysis of phase, intensity, and complex envelope variability as a function of depth and time is presented in terms of mean fields, variances, probability density functions (PDFs), covariance, spectra, and coherence. Observations are compared to a random multipath model of frequency and vertical wave number spectra for phase and log intensity, and the observations are compared to a broadband multipath model of scintillation index and coherence. 2005 Acoustical Society of America.

2004
Dzieciuch, M, Munk W, Rudnick DL.  2004.  Propagation of sound through a spicy ocean, the SOFAR overture. Journal of the Acoustical Society of America. 116:1447-1462.   10.1121/1.1772397   AbstractWebsite

Using a closely sampled 1000-km hydrographic section in the eastern North Pacific, the sound-speed finestructure is separated into two component fields: (i) isopyncal tilt dominated by internal waves (the traditional view) and (ii) "spicy" (cold-fresh to hot-salty) millifronts associated with upper ocean stirring. Numerical transmission experiments show significant scatter within the mixed layer from the spicy fronts. This scattered energy arrives near the start of the SOFAR sequence, and is superimposed on a triplication of the channel dispersion at the transition from reflected to upper ocean refracted energy. (This SOFAR overture is totally different from the finale which has been prominent for over 50 years.) The critical dependence of the overture on mixed layer processes suggests a scheme for acoustically monitoring the upper oceans at surface-conjugate depths (3 to 5 km), offering some advantages over in situ monitoring. (C) 2004 Acoustical Society of America.

Wadhams, P, Munk W.  2004.  Ocean freshening, sea level rising, sea ice melting. Geophysical Research Letters. 31   10.1029/2004gl020039   AbstractWebsite

Estimates of 20th Century sea level rise are typically 1.5 to 2 mm/y, with a steric contribution of (0.5 +/- 0.2) mm/y. Estimates of the eustatic contribution vary widely between -1.1 and + 1.3 mm/y. We attempt an independent estimate of eustatic sea level rise based on the measured freshening of the global ocean, and with attention to the contribution from melting of sea ice (which affects freshening but not sea level). Our estimate is based on a secular decrease in global average salinity estimated by Antonov et al. [2002] which, if assumed due entirely to run-off, would produce a eustatic rise of (1.8 +/- 0.7) mm/y, and would correspond to a run-off volume of 650 cu km/y. Measurements with upward looking sonars mounted on submarines have suggested a historical thinning of the arctic ice sheet equivalent to 525 +/- 105 cu km/y. Allowing for some growth in Antarctic sea ice, a reduced figure of (430 +/- 130) cu km/y is obtained, allowing about 220 cu km/y of run-off from land sources such as glaciers. This would produce a eustatic rise of only 0.6 mm/y, for a total of 1.1 mm/y, somewhat less than IPCC estimates. This also has implications for our understanding of glacial retreat for a total of 1.1 mm/y.

2003
Worcester, P, Munk W.  2003.  The experience with ocean acoustic tomography. Marine Technology Society Journal. 37:78-82. AbstractWebsite
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Munk, W.  2003.  Ocean freshening, sea level rising. Science. 300:2041-2043.   10.1126/science.1085534   AbstractWebsite
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2002
Munk, W.  2002.  Twentieth century sea level: An enigma. Proceedings of the National Academy of Sciences of the United States of America. 99:6550-6555.   10.1073/pnas.092704599   AbstractWebsite

Changes in sea level (relative to the moving crust) are associated with changes in ocean volume (mostly thermal expansion) and in ocean mass (melting and continental storage): zeta(t) = zeta(steric)(t) + zeta(eustatic)(t). Recent compilations of global ocean temperatures by Levitus and coworkers are in accord with coupled ocean/atmosphere modeling of greenhouse warming; they yield an increase in 20th century ocean heat content by 2 x 10(23) J (compared to 0.1 X 10(23) J of atmospheric storage), which corresponds to zeta(greenhouse)(2000) = 3 cm. The greenhouse-related rate is accelerating, with a present value zeta(greenhouse)(2000) approximate to 6 cm/century. Tide records going back to the 19th century show no measurable acceleration throughout the late 19th and first half of the 20th century; we take ,zeta(historic) = 18 cm/century. The Intergovernmental Panel on Climate Change attributes about 6 cm/century to melting and other eustatic processes, leaving a residual of 12 cm of 20th century rise to be accounted for, The Levitus compilation has virtually foreclosed the attribution of the residual rise to ocean warming (notwithstanding our ignorance of the abyssal and Southern Oceans): the historic rise started too early, has too linear a trend, and is too large. Melting of polar ice sheets at the upper limit of the Intergovernmental Panel on Climate Change estimates could close the gap, but severe limits are imposed by the observed perturbations in Earth rotation. Among possible resolutions of the enigma are: a substantial reduction from traditional estimates (including ours) of 1.5-2 mm/y global sea level rise; a substantial increase in the estimates of 20th century ocean heat storage; and a substantial change in the interpretation of the astronomic record.

Munk, W, Dzieciuch M, Jayne S.  2002.  Millennial climate variability: Is there a tidal connection? Journal of Climate. 15:370-385.   10.1175/1520-0442(2002)015<0370:mcvita>2.0.co;2   AbstractWebsite

Orbital forcing has long been the subject of two quite separate communities: the tide community is concerned with the relatively rapid gravitational forces (periods up to 18.6 yr) and the climate community with the long-period Milankovitch insolation terms (exceeding 20 000 yr). The wide gap notwithstanding, the two subjects have much in common. Keeling and Whorf have proposed that the millennial climate variability is associated with high-frequency tidal forcing extending into the 10-octave gap by some nonlinear process. Here, the authors distinguish between two quite distinct processes for generating low frequencies: (i) the "traditional'' analogy with eclipse cycles associated with near coincidence of the appropriate orbital alignment of the Sun, the Moon, and Earth, and (ii) sum and differences of tidal frequencies and their harmonics producing low beat frequencies. The first process is associated with long time intervals between extreme tides, but the events are of short duration and only marginally higher than conventional high tides. With proper nonlinearities, (ii) can lead to low-frequency tidal forcing. A few candidate frequencies in the centurial and millennial band are found, which prominently include the Keeling and Whorf forcing at 1795 yr. This is confirmed by a numerical experiment with a computer-generated tidal time series of 275 000 yr. Tidal forcing is very weak and an unlikely candidate for millennial variability; the Keeling and Whorf proposal is considered as the most likely among unlikely candidates.

2001
Munk, W.  2001.  Spirals on the sea. Scientia Marina. 65:193-198. AbstractWebsite

Spiral eddies were first seen in the sun glitter on the Apollo Mission 30 years ago; they have since been recorded on SAR missions and in the infrared. The spirals are globally distributed, 10-25 km in size and overwhelmingly cyclonic. They have not been explained. Under light winds favorable to visualization, linear surface features with high surfactant density and low surface roughness are of common occurrence. We have proposed that frontal formations concentrate the ambient shear and prevailing surfactants. Horizontal shear instabilities ensue when the shear becomes comparable to the coriolis frequency. The resulting vortices wind the liner features into spirals. The hypothesis needs to be tested by prolonged measurements and surface truth. Spiral eddies are a manifestation of a sub-mesoscale oceanography associated with upper ocean stirring; dimensional considerations suggest a horizontal diffusivity of order 103 m(2)s(-1).

Townes, C, Munk W.  2001.  William Aaron Nierenberg - Obituaries. Physics Today. 54:74-75.   10.1063/1.1387603   Website
Dzieciuch, M, Worcester P, Munk W.  2001.  Turning point filters: Analysis of sound propagation on a gyre-scale. Journal of the Acoustical Society of America. 110:135-149.   10.1121/1.1377869   AbstractWebsite

Acoustic transmissions from Pioneer Seamount off California to a vertical array near Hawaii are analyzed using a technique which we call a turning-point filter. The observables, travel time and axial inclination, are interpreted in terms of the ocean sound-speed field. The method permits a uniform treatment of the arrival pattern, from the early ray-like arrivals to the late mode-like arrivals, including peak arrivals which cannot be identified as either rays or modes. An adiabatic range dependence is assumed, and Wentzel-Kramers-Brillouin-Jeffreys formalism is applied. (C) 2001 Acoustical Society of America.

2000
Munk, W, Armi L, Fischer K, Zachariasen F.  2000.  Spirals on the sea. Proceedings of the Royal Society of London Series a-Mathematical Physical and Engineering Sciences. 456:1217-1280. AbstractWebsite

Spiral eddies were first seen in the sunglitter on the Apollo Mission 30 years ago; they have since been recorded on synthetic aperture radar (SAR) images and in the infrared. We present a small sample of images. The spirals are broadly distributed over the world's oceans, 10-25 km in size and overwhelmingly cyclonic. Under light winds favourable to visualization, linear surface features with high surfactant density and low surface roughness are of common occurrence. The linear features are wound into spirals in vortices associated with horizontal shear instability, modified by rotation, in regions where the shear is comparable with the Coriolis frequency. Two models for concentrating shear are presented: a softened version of the classical sharp Margules front, and the time-dependent Lagrangian model of Hoskins & Bretherton. Horizontal shear instabilities and both frontal models favour cyclonic shear and cyclonic spirals, but for different reasons.