Peter D. Bromirski

Storminess Variability along the California Coast: 1858–2000

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.

Ocean wave height determined from inland seismometer data: Implications for investigating wave climate changes in the NE Pacific

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.

Monitoring and Understanding Changes in Extremes: Extratropical Storms, Winds, and Waves

This scientific assessment examines changes in three climate extremes—extratropical storms, winds, and waves—with an emphasis on U.S. coastal regions during the cold season. There is moderate evidence of an increase in both extratropical storm frequency and intensity during the cold season in the Northern Hemisphere since 1950, with suggestive evidence of geographic shifts resulting in slight upward trends in offshore/coastal regions. There is also suggestive evidence of an increase in extreme winds (at least annually) over parts of the ocean since the early to mid-1980s, but the evidence over the U.S. land surface is inconclusive. Finally, there is moderate evidence of an increase in extreme waves in winter along the Pacific coast since the 1950s, but along other U.S. shorelines any tendencies are of modest magnitude compared with historical variability. The data for extratropical cyclones are considered to be of relatively high quality for trend detection, whereas the data for extreme winds and waves ar...

Dominant source regions of the Earth's “hum” are coastal

[1] 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.

Response of the Ross Ice Shelf, Antarctica, to ocean gravity-wave forcing

Abstract 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.

Geophysics. Earth vibrations.

Seismic “noise” can be used to monitor climate change effects, locate storms, and elucidate the structure of Earths crust. Intense cyclonic storm systems generate strong ocean-surface winds that transfer atmospheric energy into ocean gravity waves. Some of the ocean wave energy couples to the solid earth, causing what seismologists have long considered as ambient “noise,” because it interferes with the study of earthquake signals measured by seismometers. However, rising ambient noise levels imply increasing oceanic storminess (1), which is linked to climate change. In this context, the roles are reversed, with earthquakes being the noise that needs to be excluded from the climate-related signals. Studies of long-term seismic records suggest that wave-generated ambient noise is increasing globally (2).

Wave power variability and trends across the North Atlantic influenced by decadal climate patterns

Climate variations influence North Atlantic winter storm intensity and resultant variations in wave energy levels. A 60 year hindcast allows investigation of the influence of decadal climate variability on long-term trends of North Atlantic wave power, PW, spanning the 1948–2008 epoch. PW variations over much of the eastern North Atlantic are strongly influenced by the fluctuating North Atlantic Oscillation (NAO) atmospheric circulation pattern, consistent with previous studies of significant wave height, Hs. Wave activity in the western Atlantic also responds to fluctuations in Pacific climate modes, including the Pacific North American (PNA) pattern and the El Nino/Southern Oscillation. The magnitude of upward long-term trends during winter over the northeast Atlantic is strongly influenced by heightened storm activity under the extreme positive phase of winter NAO in the early 1990s. In contrast, PW along the United States East Coast shows no increasing trend, with wave activity there most closely associated with the PNA. Strong wave power “events” exhibit significant upward trends along the Atlantic coasts of Iceland and Europe during winter months. Importantly, in opposition to the long-term increase of PW, a recent general decrease in PW across the North Atlantic from 2000 to 2008 occurred. The 2000–2008 decrease was associated with a general shift of winter NAO to its negative phase, underscoring the control exerted by fluctuating North Atlantic atmospheric circulation on PW trends.

Ross ice shelf vibrations

Broadband seismic stations were deployed across the Ross Ice Shelf (RIS) in November 2014 to study ocean gravity wave-induced vibrations. Initial data from three stations 100 km from the RIS front and within 10 km of each other show both dispersed infragravity (IG) wave and ocean swell-generated signals resulting from waves that originate in the North Pacific. Spectral levels from 0.001 to 10 Hz have the highest accelerations in the IG band (0.0025–0.03 Hz). Polarization analyses indicate complex frequency-dependent particle motions, with energy in several frequency bands having distinctly different propagation characteristics. The dominant IG band signals exhibit predominantly horizontal propagation from the north. Particle motion analyses indicate retrograde elliptical particle motions in the IG band, consistent with these signals propagating as Rayleigh-Lamb (flexural) waves in the ice shelf/water cavity system that are excited by ocean wave interactions nearer the shelf front.

Shallow-water seismoacoustic noise generated by tropical storms Ernesto and Florence

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.

Propagation of microseisms from the deep ocean to land

Ocean-generated microseisms are faint Earth vibrations that result from pressure fluctuations at the sea floor generated by the interaction between ocean surface gravity waves, and are continuously recorded as low frequency seismic noise. Here we investigate microseism propagation away from deep-ocean source regions using the spectral element method for an oceanic model that contains realistic northeast Atlantic Ocean irregular-layered structure composed of water, sediment, and upper crust. It also includes structural heterogeneities and continental slope and shelf bathymetry. Numerical simulations of coupled acoustic and elastic wave propagation in both simplified models and the full realistic model show that most microseism energy is confined to sediment and water column waveguides. We also show that a significant portion of microseism energy is reflected back to the deep ocean by the continental slope, while only a small fraction of deep-ocean-generated microseism energy reaches land. We conclude that terrestrially observed microseisms are largely generated in shallow water on continental shelves.