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1991
Bromirski, PD, Frazer LN, Duennebier FK.  1991.  Sediment Q from spectral ratios of converted shear reflections. Shear waves in marine sediments. ( Hovem JM, Richardson MD, Stoll RD, Eds.).:361-368., Dordrecht ; Boston: Kluwer Abstract
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1992
Bromirski, PD, Frazer LN, Duennebier FK.  1992.  Sediment shear Q from airgun OBS data. Geophysical Journal International. 110:465-485.   10.1111/j.1365-246X.1992.tb02086.x   AbstractWebsite

Direct measurement of the sediment shear-wave quality factor, Q(beta), has been hindered by the lack of an effective shear-wave source. We show that if a satisfactory horizontal component ocean bottom seismometer (OBS) is available, then sediment Q(beta) can be determined directly by using spectral ratios of converted shear-wave reflections. Spectral ratios are formed with the PS reflection from the sediment/basement interface and the PSSS multibounce sediment shear-wave reflection. As a check, we also computed Q(beta) from the peak amplitudes of PS and PSSS. We applied the spectral ratio method to airgun OBS data collected over 356 m of primarily high-porosity biosiliceous clay in 5467 m of water in the northwest Pacific at 43-degrees-55.44'N, 159-degrees-47.84'E (DSDP Site 581). An average sediment shear-wave velocity of about 0.2 km s-1 was obtained from the PS traveltime. Effective Q(beta) for the sediment column was found to be 97 +/- 11 (alpha = 0.281 +/- 0.032 dB lambda-1) in the frequency band 3-18 Hz. We tested the methods by applying them to reflectivity synthetic seismograms computed for various velocity profiles with both frequency-dependent Q and frequency-independent Q. The Q(beta) estimate obtained from synthetic seismograms was within 15 per cent of the true Q(beta) for each velocity profile. Q(beta) estimates within 25 per cent of the true Q were obtained with the addition of up to 6.5 per cent signal-generated noise, whereas the addition of only 3 per cent signal-generated noise energy makes estimates of the frequency dependence of Q unreliable using spectral ratios. We conclude that the two-octave band of the data is not wide enough to determine the frequency dependence of Q(beta). Tests on synthetic seismograms, computed from models containing alternating layers of high impedance contrast with realistic velocities, indicated that apparent attenuation due to intrabed multiples does not significantly affect the spectral ratio Q(beta) estimates, although a shift in spectral content to higher frequencies for PS and PSSS phases and a delay in the apparent arrival time of PSSS were observed. However, the alternative peak amplitude ratio method gave Q(beta) estimates more than 25 per cent lower than the true Q for multilayer sediment models. We also tested the methods on synthetic data subjected to hard and soft clipping. Spectral ratio estimates of Q(beta) from synthetic data with PS clipped up to 50 per cent, were within 25 er cent of the true Q(beta).

1993
Rudman, AJ, Mallick S, Frazer LN, Bromirski P.  1993.  Workstation computation of synthetic seismograms for vertical and horizontal profiles: A full wavefield response for a two-dimensional layered half-space. Computers & Geosciences. 19:447-474.   10.1016/0098-3004(93)90095-m   AbstractWebsite

FORTRAN code for generation of full wavefield synthetic seismograms is presented for two-dimensional horizontally layered models bounded by a free surface and a half space. Model layers are user defined by compressional and shear velocities, Q factors, densities and thicknesses. The algorithm is based on the reflectivity method and uses the propagator matrix approach. Explosion (point) and double couple (fault) sources are generated with a moment tensor representation. As evaluation of the slowness integrals involves time consuming numerical Hankel transforms, these computations are made with a generalized Filon method that saves computational time. The architecture of the program is unusual because the outermost loop is over temporal frequency and the innermost loop is over slowness. This permits the use of frequency-dependent seismic velocities, necessary for causality, while giving a factor of seven speed-up from vectorization. The codes are applicable for both vector computers and workstations. Two test cases demonstrate successful applications of the codes for both horizontal seismic profiles (receivers at one depth at successively larger offsets) and for vertical seismic profiles (receivers arranged in a vertical array at any offset). Receivers and source may be positioned within any layer. The seismograms display direct, refracted, reflected, and head-wave arrivals and their multiples. Mode converted events of compressional and shear propagation are generated and identified. The code generates seismograms for pressure, vertical and horizontal displacement sensors and for models combining acoustic and elastic layers.

1995
Bromirski, PD, Frazer LN, Duennebier FK.  1995.  The Q-gram method: Q from instantaneous phase. Geophysical Journal International. 120:73-86.   10.1111/j.1365-246X.1995.tb05911.x   AbstractWebsite

The width of a seismic pulse increases monotonically with distance and with Q-1. Estimates of Q from pulse width measurements are often not robust for oscillatory arrivals or for impulsive arrivals in the presence of noise. We present a method to estimate Q from two arrivals using measurements of any signal attribute, xiBaR, that is sensitive to propagation loss. The propagation loss is defined as the change in xiBAR divided by the difference in traveltime between the arrivals. The first data arrival is used as the reference wavelet. The Q-gram method is based on propagating the reference wavelet with a plane-wave Q-propagator for various values of Q-1. The Q-propagator includes a dispersion relation and the measured difference in traveltime between the data arrivals. The plot of synthetic propagation loss between the reference and propagated wavelets, versus Q-1, is called a Q-gram. The Q-gram, together with the measured propagation loss of the data, gives the Q of the data. The averaged instantaneous frequency fBAR and the averaged instantaneous pulse width tauBAR make good signal attributes. Tests on synthetic seismograms show that the Q-gram method, using either fBAR or tauBAR for xiBAR, is applicable to both impulsive and oscillatory arrivals and is relatively robust with regard to noise, phase changes and signal clipping. We apply the Q-gram method to horizontal-component airgun ocean-bottom seismometer (OBS) data using the basement-converted shear-wave reflection, PS, as the first arrival and PSSS as the second arrival. We estimate Q(beta), the effective sediment shear-wave Q, with an fBAR-type Q-gram and a tauBAR-type Q-gram for the PS and PSSS sediment shear-wave reflections. The data indicate that Q(beta) almost-equal-to 75 +/- 15, in agreement with results from the application of the spectral-ratio method using windows that exclude interfering arrivals identified by means of the instantaneous frequency.

1999
Bromirski, PD, Flick RE, Graham N.  1999.  Ocean wave height determined from inland seismometer data: Implications for investigating wave climate changes in the NE Pacific. Journal of Geophysical Research-Oceans. 104:20753-20766.   10.1029/1999jc900156   AbstractWebsite

Knowing the wave climate along the California coast is vital from the perspectives of climatological change and planning shore protection measures. Buoy data indicate that the wave climate is very similar along much of the California coast. We show that elements of the wave climate can be accurately reconstructed using near-coastal inland broadband seismometer data. Such reconstructions are possible because swell approaching the coast generates pressure fluctuations that are locally transformed into seismic waves at the seafloor that propagate inland and are detectable by land-based seismometers. Buoy and seismometer data show that most of the microseism energy recorded inland near the coast is generated from wave events at nearby coastal locations. A site-specific, empirically derived seismic-to-wave transfer function is demonstrated to be applicable to seismic data from the same location for any year. These results suggest that ocean wave heights estimated from near-coastal broadband seismometer data are sufficiently reliable for monitoring the coastal wave height when buoy data are unavailable, provided that adequate simultaneous nearby buoy measurements are available to calibrate the seismometer data. The methodology presented here provides an important tool that allows the investigation of potential wave climate changes from reconstructions using archived seismic data collected since the 1930s.

2001
Bromirski, PD.  2001.  Vibrations from the "Perfect Storm". Geochemistry Geophysics Geosystems. 2:art.no.-2000GC000119.   10.1029/2000GC000119   AbstractWebsite

Microseismic vibrations during the famous October 1991 "Perfect Storm" were observed at seismic stations across North America. The extreme wave conditions during this storm, in conjunction with the occurrence of Hurricane Grace to the south, are ideal for studying where such vibrations originate and their inland propagation. High-amplitude primary and double-frequency (DF) microseisms were observed at broadband seismic station HRV in eastern Massachusetts. Similar spectral variation observed at seismic station ANMO at Albuquerque, New Mexico, shows transcontinental propagation of vibrations from the Perfect Storm. Cross correlation between wave spectra from widely separated buoy measurements and corresponding DF microseism spectra at HRV give high-correlation coefficients, R-2, from the New England coast to Cape Hatteras. Contours of peak R-2 scaled by the magnitude of the lag at the peak, together with similarities between wave and microseism spectral variation, imply that the dominant source area of DF microseisms during the Perfect Storm is near the southern Massachusetts coast, not in the open ocean where the highest waves occurred.

2002
Bromirski, PD, Duennebier FK.  2002.  The near-coastal microseism spectrum: Spatial and temporal wave climate relationships. Journal of Geophysical Research-Solid Earth. 107   10.1029/2001jb000265   AbstractWebsite

[1] Comparison of the ambient noise data recorded at near-coastal ocean bottom and inland seismic stations at the Oregon coast with both offshore and nearshore buoy data shows that the near-coastal microseism spectrum results primarily from nearshore gravity wave activity. Low double-frequency (DF), microseism energy is observed at near-coastal locations when seas nearby are calm, even when very energetic seas are present at buoys 500 km offshore. At wave periods >8 s, shore reflection is the dominant source of opposing wave components for near-coastal DF microseism generation, with the variation of DF microseism levels poorly correlated with local wind speed. Near-coastal ocean bottom DF levels are consistently similar to20 dB higher than nearby DF levels on land, suggesting that Rayleigh/Stoneley waves with much of the mode energy propagating in the water column dominate the near-coastal ocean bottom microseism spectrum. Monitoring the southward propagation of swell from an extreme storm concentrated at the Oregon coast shows that near-coastal DF microseism levels are dominated by wave activity at the shoreline closest to the seismic station. Microseism attenuation estimates between on-land near-coastal stations and seismic stations similar to150 km inland indicate a zone of higher attenuation along the California coast between San Francisco and the Oregon border.

2003
Bromirski, PD, Flick RE, Cayan DR.  2003.  Storminess variability along the California coast: 1858-2000. Journal of Climate. 16:982-993.   10.1175/1520-0442(2003)016<0982:svatcc>2.0.co;2   AbstractWebsite

The longest available hourly tide gauge record along the West Coast (U. S.) at San Francisco yields meteorologically forced nontide residuals (NTR), providing an estimate of the variation in "storminess'' from 1858 to 2000. Mean monthly positive NTR (associated with low sea level pressure) show no substantial change along the central California coast since 1858 or over the last 50 years. However, in contrast, the highest 2% of extreme winter NTR levels exhibit a significant increasing trend since about 1950. Extreme winter NTR also show pronounced quasi-periodic decadal-scale variability that is relatively consistent over the last 140 years. Atmospheric sea level pressure anomalies (associated with years having high winter NTR) take the form of a distinct, large-scale atmospheric circulation pattern, with intense storminess associated with a broad, southeasterly displaced, deep Aleutian low that directs storm tracks toward the California coast.

2005
Bromirski, PD, Duennebier FK, Stephen RA.  2005.  Mid-ocean microseisms. Geochemistry Geophysics Geosystems. 6   10.1029/2004gc000768   AbstractWebsite

The Hawaii-2 Observatory (H2O) is an excellent site for studying the source regions and propagation of microseisms since it is located far from shorelines and shallow water. During Leg 200 of the Ocean Drilling Program, the officers of the JOIDES Resolution took wind and wave measurements for comparison with double-frequency (DF) microseism data collected at nearby H2O. The DF microseism band can be divided into short-period and long-period bands, SPDF and LPDF, respectively. Comparison of the ship's weather log with the seismic data in the SPDF band from about 0.20 to 0.45 Hz shows a strong correlation of seismic amplitude with wind speed and direction, implying that the energy reaching the ocean floor is generated locally by ocean gravity waves. Nearshore land seismic stations see similar SPDF spectra, also generated locally by wind seas. At H2O, SPDF microseism amplitudes lag sustained changes in wind speed and direction by several hours, with the lag increasing with wave period. This lag may be associated with the time necessary for the development of opposing seas for DF microseism generation. Correlation of swell height above H2O with the LPDF band from 0.085 to 0.20 Hz is often poor, implying that a significant portion of this energy originates at distant locations. Correlation of the H2O seismic data with NOAA buoy data, with hindcast wave height data from the North Pacific, and with seismic data from mainland and island stations, defines likely source areas of the LPDF signals. Most of the LPDF energy at H2O appears to be generated by high-amplitude storm waves impacting long stretches of coastline nearly simultaneously, and the Hawaiian Islands appear to be a significant source of LPDF energy in the North Pacific when waves arrive from particular directions. The highest levels observed at mid-ocean site H2O occur in the SPDF band when two coincident nearby storm systems develop. Deep water, mid-ocean-generated DF microseisms are not observed at continental sites, indicating high attenuation of these signals. At near-coastal seismic stations, both SPDF and LPDF microseism levels are generally dominated by local generation at nearby shorelines.

Bromirski, PD, Cayan DR, Flick RE.  2005.  Wave spectral energy variability in the northeast Pacific. Journal of Geophysical Research-Oceans. 110   10.1029/2004jc002398   AbstractWebsite

The dominant characteristics of wave energy variability in the eastern North Pacific are described from NOAA National Data Buoy Center ( NDBC) buoy data collected from 1981 to 2003. Ten buoys at distributed locations were selected for comparison based on record duration and data continuity. Long- period ( LP) [ T > 12] s, intermediate- period [ 6 <= T <= 12] s, and short- period [ T < 6] s wave spectral energy components are considered separately. Empirical orthogonal function ( EOF) analyses of monthly wave energy anomalies reveal that all three wave energy components exhibit similar patterns of spatial variability. The dominant mode represents coherent heightened ( or diminished) wave energy along the West Coast from Alaska to southern California, as indicated by composites of the 700 hPa height field. The second EOF mode reveals a distinct El Nino-Southern Oscillation (ENSO)-associated spatial distribution of wave energy, which occurs when the North Pacific storm track is extended unusually far south or has receded to the north. Monthly means and principal components (PCs) of wave energy levels indicate that the 1997 - 1998 El Nino- winter had the highest basin- wide wave energy within this record, substantially higher than the 1982 - 1983 El Nino. An increasing trend in the dominant PC of LP wave energy suggests that storminess has increased in the northeast Pacific since 1980. This trend is emphasized at central eastern North Pacific locations. Patterns of storminess variability are consistent with increasing activity in the central North Pacific as well as the tendency for more extreme waves in the south during El Nino episodes and in the north during La Nina.

2006
Stephen, RA, Duennebier FK, Harris D, Jolly J, Bolmer ST, Bromirski PD.  2006.  Broadband seismic observations at the Hawaii-2 Observatory, ODP Leg 200. Proceedings of the Ocean Drilling Program, Scientific Results. 200   10.2973/odp.proc.sr.200.007.2006   AbstractWebsite

Among the groups of oceanic microfossils, only Radiolaria occur in abundances and preservation states sufficient to provide biostratigraphic control for restricted intervals within sediments recovered in Hole 1223A. The distribution of these microfossils has been divided into four major intervals, A-D. Radiolaria distribution Interval A occupies the depth range 0-3.0 meters below seafloor (mbsf), where the abundance of specimens is very low and preservation is poor. Radiolaria distribution Interval B occupies the depth range 3.02-7.1 mbsf. Radiolaria in Interval B are locally rare to abundant and well preserved, and assemblages range in age from pure early Eocene to early Eocene admixed with late Neogene taxa. Radiolaria distribution Interval C occupies the depth range 7.1-36.99 mbsf and is characterized by sediments either barren of microfossils or containing extremely rare early Eocene specimens. Radiolaria distribution Interval D occupies the depth range 36.99-38.7 mbsf (base of the recovered sedimentary section), where early Eocene Radiolaria are present in rare to common frequencies, but opal-A to opal-CT recrystallization has degraded the preservation state. The late Neogene assemblage of Radiolaria distribution Interval B is dated at 1.55-2.0 Ma, based on occurrences of Eucyrtidium matuyamai, Lamprocyclas heteroporos, and Theocorythium trachelium trachelium. The early Eocene assemblage of Radiolaria distribution Intervals B and D is somewhat problematically assigned to the Buryella clinata Zone.

2008
Bromirski, PD, Flick RE.  2008.  Storm surge in the San Francisco Bay/Delta and nearby coastal locations. Shore & Beach. 76:29-37. Abstract

California’s San Francisco Bay/Sacramento-San Joaquin Delta (bay/delta) estuary system is subject to externally forced storm surge propagating from the open ocean. In the lower reaches of the delta, storm surge dominates water level extremes and can have a significant impact on wetlands, freshwater aquifers, levees, and ecosys- tems. The magnitude and distribution of open-ocean tide generated storm surge throughout the bay/delta are described by a network of stations within the bay/delta system and along the California coast. Correlation of non-tide water levels between stations in the network indicates that peak storm surge fluctuations propagate into the bay/delta system from outside the Golden Gate. The initial peak surge propa- gates from the open ocean inland, while a trailing (smaller amplitude) secondary peak is associated with river discharge. Extreme non-tide water levels are generally associated with extreme Sacramento-San Joaquin river flows, underscoring the po- tential impact of sea level rise on the delta levees and bay/delta ecosystem.

Bromirski, PD, Kossin JP.  2008.  Increasing hurricane wave power along the US Atlantic and Gulf coasts. Journal of Geophysical Research-Oceans. 113   10.1029/2007jc004706   AbstractWebsite

Although no clear trend in tropical cyclone (TC) generated wave height is observed, a TC wave power index (WPI) increases significantly in the Atlantic during the mid-1990s, resulting largely from an increase in the frequency of middle-to-late season TCs. The WPI is related to TC strength, size, duration, and frequency and is highly correlated with the TC power dissipation index (PDI). Differences between the Atlantic and Gulf of Mexico WPIs reflect systematic changes in TC genesis regions and subsequent tracks, characterized by their relationship with the regional circulation patterns described by the Atlantic Meridional Mode. The annual wave power at near-coastal locations is closely associated with open ocean WPI. The close association of the WPI to hurricane activity implies that under rising sea level, significant coastal impacts will increase as the PDI increases, regardless of TC landfall frequency.

Cayan, DR, Bromirski PD, Hayhoe K, Tyree M, Dettinger MD, Flick RE.  2008.  Climate change projections of sea level extremes along the California coast. Climatic Change. 87:S57-S73.   10.1007/s10584-007-9376-7   AbstractWebsite

California's coastal observations and global model projections indicate that California's open coast and estuaries will experience rising sea levels over the next century. During the last several decades, the upward historical trends, quantified from a small set of California tide gages, have been approximately 20 cm/century, quite similar to that estimated for global mean sea level. In the next several decades, warming produced by climate model simulations indicates that sea level rise (SLR) could substantially exceed the rate experienced during modem human development along the California coast and estuaries. A range of future SLR is estimated from a set of climate simulations governed by lower (B1), middle-upper (A2), and higher (A1fi) GHG emission scenarios. Projecting SLR from the ocean warming in GCMs, observational evidence of SLR, and separate calculations using a simple climate model yields a range of potential sea level increases, from 11 to 72 cm, by the 2070-2099 period. The combination of predicted astronomical tides with projected weather forcing, El Nino related variability, and secular SLR, gives a series of hourly sea level projections for 2005-2100. Gradual sea level rise progressively worsens the impacts of high tides, surge and waves resulting from storms, and also freshwater floods from Sierra and coastal mountain catchments. The occurrence of extreme sea levels is pronounced when these factors coincide. The frequency and magnitude of extreme events, relative to current levels, follows a sharply escalating pattern as the magnitude of future sea level rise increases.

Aster, RC, McNamara DE, Bromirski PD.  2008.  Multidecadal climate-induced variability in microseisms. Seismological Research Letters. 79:194-202.   10.1785/gssrl.79.2.194   AbstractWebsite

Microseisms are the most ubiquitous continuous seismic signals on Earth at periods between approximately 5 and 25 s (Peterson 1993; Kedar and Webb 2005). They arise from atmospheric energy converted to (primarily) Rayleigh waves via the intermediary of wind-driven oceanic swell and occupy a period band that is uninfluenced by common anthropogenic and wind-coupled noise processes on land (Wilson et al. 2002; de la Torre et al. 2005). “Primary” microseisms (near 8-s period) are generated in shallow water by breaking waves near the shore and/or the nonlinear interaction of the ocean wave pressure signal with the sloping sea floor (Hasselmann 1963). Secondary microseisms occur at half of the primary period and are especially strongly radiated in source regions where opposing wave components interfere (Longuett-Higgins 1950; Tanimoto 2007), which principally occurs due to the interaction of incident swell and reflected/scattered wave energy from coasts (Bromirski and Duennebier 2002; Bromirski, Duennebier, and Stephen 2005). Coastal regions having a narrow shelf with irregular and rocky coastlines are known to be especially efficient at radiating secondary microseisms (Bromirski, Duennebier, and Stephen 2005; Shulte-Pelkum et al. 2004). The secondary microseism is globally dominant, and its amplitudes proportional to the square of the standing wave height (Longuett-Higgins 1950), which amplifies its sensitivity to large swell events (Astiz and Creager 1994; Webb 2006).

Traer, J, Gerstoft P, Bromirski PD, Hodgkiss WS, Brooks LA.  2008.  Shallow-water seismoacoustic noise generated by tropical storms Ernesto and Florence. Journal of the Acoustical Society of America. 124:EL170-EL176.   10.1121/1.2968296   AbstractWebsite

Land-based seismic observations of double frequency (DF) microseisms generated during tropical storms Ernesto and Florence are dominated by signals in the 0.15-0.5 Hz band. In contrast, data from sea floor hydrophones in shallow water (70 m depth, 130 km off the New Jersey coast) show dominant signals in the ocean gravity-wave frequency band, 0.02-0.18 Hz, and low amplitudes from 0.18 to 0.3 Hz, suggesting significant opposing wave components necessary for DF microseism generation were negligible at the site. Florence produced large waves. over deep water while Ernesto only generated waves in coastal regions, yet both storms produced similar spectra. This suggests near-coastal shallow water as the dominant region for observed microseism generation. (C) 2008 Acoustical Society of America.

2009
Bromirski, PD, Gerstoft P.  2009.  Dominant source regions of the Earth's "hum'' are coastal. Geophysical Research Letters. 36   10.1029/2009gl038903   AbstractWebsite

Hum beam power observations using the USArray EarthScope transportable array, combined with infragravity wave observations, show that the dominant source area of the Earth's hum over the 120-400 s period band during winter months is the Pacific coast of North America, with the western coast of Europe a secondary source region. Correlation of hum with model ocean wave heights indicates that the Pacific coast of Central America is an important hum source region when impacted by austral storm waves. Hum is excited by relatively local infragravity wave forcing as ocean swell propagates along coasts, with no indication of significant deep-ocean hum generation. Citation: Bromirski, P. D., and P. Gerstoft (2009), Dominant source regions of the Earth's "hum'' are coastal, Geophys. Res. Lett., 36, L13303, doi: 10.1029/2009GL038903.

Bromirski, PD.  2009.  Earth Vibrations. Science. 324:1026-1027.   10.1126/science.1171839   AbstractWebsite
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2010
Zhang, JA, Gerstoft P, Bromirski PD.  2010.  Pelagic and coastal sources of P-wave microseisms: Generation under tropical cyclones. Geophysical Research Letters. 37   10.1029/2010gl044288   AbstractWebsite

Nonlinear wave-wave interactions generate double-frequency (DF) microseisms, which include both surface waves (mainly Rayleigh-type) and compressional (P) waves. Although it is unclear whether DF surface waves generated in deep oceans are observed on land, we show that beamforming of land-based seismic array data allows detection of DF P waves generated by ocean waves from Super Typhoon Ioke in both pelagic and coastal regions. Two distinct spectral bands associated with different P-wave source locations are observed. The short-period DF band (0.16-0.35 Hz) is dominated by P waves generated in the deep ocean by local wind seas under the storm. In contrast, P waves in the long-period DF band (0.1-0.15 Hz) are weaker and generated closer to the coast of Japan from swell interactions. The accurate identification of DF P-wave microseism source areas is useful to monitor ocean wave-wave interactions due to tropical cyclones and to image Earth structure using ambient seismic noise. Citation: Zhang, J., P. Gerstoft, and P. D. Bromirski (2010), Pelagic and coastal sources of P-wave microseisms: Generation under tropical cyclones, Geophys. Res. Lett., 37, L15301, doi: 10.1029/2010GL044288.

Bromirski, PD, Sergienko OV, MacAyeal DR.  2010.  Transoceanic infragravity waves impacting Antarctic ice shelves. Geophysical Research Letters. 37   10.1029/2009gl041488   AbstractWebsite

Long-period oceanic infragravity (IG) waves (ca. [250, 50] s period) are generated along continental coastlines by nonlinear wave interactions of storm-forced shoreward propagating swell. Seismic observations on the Ross Ice Shelf show that free IG waves generated along the Pacific coast of North America propagate transoceanically to Antarctica, where they induce a much higher amplitude shelf response than ocean swell (ca. [30, 12] s period). Additionally, unlike ocean swell, IG waves are not significantly damped by sea ice, and thus impact the ice shelf throughout the year. The response of the Ross Ice Shelf to IG-wave induced flexural stresses is more than 60 dB greater than concurrent ground motions measured at nearby Scott Base. This strong coupling suggests that IG-wave forcing may produce ice-shelf fractures that enable abrupt disintegration of ice shelves that are also affected by strong surface melting. Bolstering this hypothesis, each of the 2008 breakup events of the Wilkins Ice Shelf coincides with wave-model-estimated arrival of IG-wave energy from the Patagonian coast. Citation: Bromirski, P. D., O. V. Sergienko, and D. R. MacAyeal (2010), Transoceanic infragravity waves impacting Antarctic ice shelves, Geophys. Res. Lett., 37, L02502, doi:10.1029/2009GL041488.

Aster, RC, McNamara DE, Bromirski PD.  2010.  Global trends in extremal microseism intensity. Geophysical Research Letters. 37   10.1029/2010gl043472   AbstractWebsite

Globally ubiquitous seismic background noise peaks near 7 and 14 s period are generated via distinct mechanisms that transfer storm-generated gravity wave energy to the seismic wave field. We utilize continuous digital ground motion data recorded by the Global Seismographic Network and precursor instrumentation to chronicle microseism power extreme events for 1972-2009. Because most land-observed microseism surface-wave energy is generated at or near coasts, microseism metrics are particularly relevant to assessing changes in coastal ocean wave energy. Extreme microseism winter storm season event counts reveal the widespread influence of the El Nino Southern Oscillation (ENSO). Individual station and ensemble slopes trend positive for this study period for Northern Hemisphere stations. The double-frequency microseism is particularly volatile, suggesting that the weaker single-frequency microseism directly generated by ocean swell at coasts is likely a more representative seismic proxy for broad-scale ocean wave energy estimation. Citation: Aster, R. C., D. E. McNamara, and P. D. Bromirski (2010), Global trends in extremal microseism intensity, Geophys. Res. Lett., 37, L14303, doi: 10.1029/2010GL043472.

2011
Bromirski, PD, Miller AJ, Flick RE, Auad G.  2011.  Dynamical suppression of sea level rise along the Pacific coast of North America: Indications for imminent acceleration. Journal of Geophysical Research-Oceans. 116   10.1029/2010jc006759   AbstractWebsite

Long-term changes in global mean sea level (MSL) rise have important practical implications for shoreline and beach erosion, coastal wetlands inundation, storm surge flooding, and coastal development. Altimetry since 1993 indicates that global MSL rise has increased about 50% above the 20th century rise rate, from 2 to 3 mm yr(-1). At the same time, both tide gauge measurements and altimetry indicate virtually no increase along the Pacific coast of North America during the satellite epoch. Here we show that the dynamical steric response of North Pacific eastern boundary ocean circulation to a dramatic change in wind stress curl, tau(xy), which occurred after the mid-1970s regime shift, can account for the suppression of regional sea level rise along this coast since 1980. Alarmingly, mean tau(xy) over the North Pacific recently reached levels not observed since before the mid-1970s regime shift. This change in wind stress patterns may be foreshadowing a Pacific Decadal Oscillation regime shift, causing an associated persistent change in basin-scale tau(xy) that may result in a concomitant resumption of sea level rise along the U.S. West Coast to global or even higher rates.

2012
Bromirski, PD, Stephen RA.  2012.  Response of the Ross Ice Shelf, Antarctica, to ocean gravity-wave forcing. Annals of Glaciology. 53:163-172.   10.3189/2012AoG60A058   AbstractWebsite

Comparison of the Ross Ice Shelf (RIS, Antarctica) response at near-front seismic station RIS2 with seismometer data collected on tabular iceberg B15A and with land-based seismic stations at Scott Base on Ross Island (SBA) and near Lake Vanda in the Dry Valleys (VNDA) allows identification of RIS-specific signals resulting from gravity-wave forcing that includes meteorologically driven wind waves and swell, infragravity (IG) waves and tsunami waves. The vibration response of the RIS varies with season and with the frequency and amplitude of the gravity-wave forcing. The response of the RIS to IG wave and swell impacts is much greater than that observed at SBA and VNDA. A spectral peak at near-ice-front seismic station RIS2 centered near 0.5 Hz, which persists during April when swell is damped by sea ice, may be a dominant resonance or eigenfrequency of the RIS. High-amplitude swell events excite relatively broadband signals that are likely fracture events (icequakes). Changes in coherence between the vertical and horizontal sensors in the 8-12 Hz band from February to April, combined with the appearance of a spectral peak near 10 Hz in April when sea ice damps swell, suggest that lower (higher) temperatures during austral winter (summer) months affect signal propagation characteristics and hence mechanical properties of the RIS.

Traer, J, Gerstoft P, Bromirski PD, Shearer PM.  2012.  Microseisms and hum from ocean surface gravity waves. Journal of Geophysical Research-Solid Earth. 117   10.1029/2012jb009550   AbstractWebsite

Ocean waves incident on coasts generate seismic surface waves in three frequency bands via three pathways: direct pressure on the seafloor (primary microseisms, PM), standing waves from interaction of incident and reflected waves (double-frequency microseisms, DF), and swell-transformed infragravity wave interactions (the Earth's seismic hum). Beamforming of USArray seismic data shows that the source azimuths of the generation regions of hum, PM and DF microseisms vary seasonally, consistent with hemispheric storm patterns. The correlation of beam power with wave height over all azimuths is highest in near-coastal waters. Seismic signals generated by waves from Hurricane Irene and from a storm in the Southern Ocean have good spatial and temporal correlation with nearshore wave height and peak period for all three wave-induced seismic signals, suggesting that ocean waves in shallow water commonly excite hum (via infragravity waves), PM, and DF microseisms concurrently.

2013
Moon, JH, Song YT, Bromirski PD, Miller AJ.  2013.  Multidecadal regional sea level shifts in the Pacific over 1958-2008. Journal of Geophysical Research-Oceans. 118:7024-7035.   10.1002/2013jc009297   AbstractWebsite

Altimeter data have significantly improved our understanding of regional sea level variability and trends, but their relatively short records do not allow either evaluation of the ocean state prior to 1993 or multidecadal low-frequency signals in the ocean. Here we characterize and quantify the multidecadal regional sea level rise (rSLR) and related ocean heat content in the Pacific from a non-Boussinesq ocean circulation model in comparison with data sets from altimeters, two sea level reconstructions, and in situ ocean profiles from 1958 to 2008. We show that the rSLR trends have undergone two shifts, during the mid-1970s and in the early 1990s, with an east-west dipole pattern in the tropical Pacific. In each of these phases, rSLR accelerated on one side of the Pacific, but decelerated on the other side. The multidecadal sea level shifts can be explained by the dynamical (steric) upper-ocean responses to the surface wind forcing associated with the Pacific Decadal Oscillation (PDO), with negligible contributions from internal (depth-integrated) ocean mass changes. Additional model experimentation further confirms that the Pacific wind stress trend over the recent two decades has played an important role in strengthening the rSLR in the western Pacific while suppressing the rSLR in the eastern Pacific. The climate-forced large-scale rSLR variability is likely to impose a long-term and uneven impact on coastal communities.