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Papers by Peter Bromirski

Science (New York, N.Y.), Jan 22, 2009

Journal of Geophysical Research, 2008

1] Although no clear trend in tropical cyclone (TC) generated wave height is observed, a TC wave ... more 1] 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.

Continuous seismometer recordings over extended periods describe wave climate variability. Intens... more Continuous seismometer recordings over extended periods describe wave climate variability. Intense storms having steep atmospheric pressure gradients cause strong winds that generate high waves. Wave energy is coupled into ambient seismic noise by wave-wave interactions in the open ocean and along coasts, and by direct pressure fluctuations at the ocean bottom in nearshore shallow water, generating double- and single-frequency microseisms, respectively. Some of the ocean swell energy is transformed into very long period infragravity (IG) wave energy along coasts, generating the Earth's ``hum''. A portion of the coastal IG wave energy leaks off continental shelves and becomes free waves that produce seismic signals at distant locations. The spatial and temporal distribution of storms and their associated wave energy controls the locations where high amplitude ambient seismic noise is generated, but seismic levels are also dependent on near-coastal bathymetry and shoreline geometry. Changes in climate-related broad-scale atmospheric patterns that affect storm track and intensity have a corresponding affect on the spatial ambient noise distribution. Because storm activity is seasonal, winters produce the highest ambient noise levels in each hemisphere. And because ocean gravity waves propagate long distances, seismometers in both hemispheres detect signals produced by winter waves generated in the opposite hemisphere. Microseism and hum Rayleigh waves can propagate long distances, and are detected at locations far from their generation region. Although seismometers detect ambient noise signals generated simultaneously along long stretches of coastline, the nearest coastlines tend to produce the dominant signal. The integrative character of ambient noise observations provides an assessment of the amount of wave energy reaching particular coasts that is complementary to point oceanographic measurements. Increasing wave energy along coasts is of particular concern under rising sea levels, which allows more energy to reach farther shoreward.

Assessment of regional and global storm intensities is of considerable interest in the debate abo... more Assessment of regional and global storm intensities is of considerable interest in the debate about the influence of global climate change on tropical and extratropical storm frequency and intensity. Continuous digital data from the Global Seismographic Network (GSN) and nearby predecessor seismic stations now extend back nearly four decades at the longest operational sites. The incessant and prominent microseism peaks in the global seismic background noise spectrum near 7 and 14 s period arise from distinct coupling processes between the atmosphere, ocean gravity waves, and the solid Earth. Earth’s microseism record thus constitutes an independent integrative proxy for nearshore wave intensity. We tabulate and quantify extreme storm events using microseism records from globally-distributed long-running seismic stations to assess decadal-scale changes in coastal wave intensity from the longest available digital archives. An apparent strong global influence of El Nino Southern Oscillation-associated changes in microseism/wave intensity is observed. A globally widespread preponderance of upward trends in the occurrence of strong microseism events suggests that winter storm intensity has generally increased over all ocean basins since the 1970’s.

Journal of Climate, Mar 1, 2003

The longest available hourly tide gauge record along the West Coast (U.S.) at San Francisco yield... more 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.

Error-corrected hourly tide gauge data from 1858 to 1999 at San Francisco (SFO) suggests a trend ... more Error-corrected hourly tide gauge data from 1858 to 1999 at San Francisco (SFO) suggests a trend for greater coastal impacts during winter months as a result of a combination of both an increased rate in sea level rise and recently increased storminess. Changes in storminess are determined from non-tide residuals, while sea level rise impacts are estimated from tide gauge anomalies. Assuming that non-tidal forcing varies smoothly across the tide gauge spectrum, non-tide water level estimates are obtained by linearly interpolating Fourier spectral estimates across the tidal bands, with the variance of the interpolated estimates determined from spectral level variation on both sides of the band. Tide gauge anomalies are determined from the difference between the raw tide gauge data and mean of the monthly means. Thus, tide gauge anomalies can include significant contributions resulting from El Niño related thermal expansion along the West Coast as well as long term sea level rise, while non-tide residuals exclude water level variation at time-scales greater than 90 days and are more closely associated with storminess. Tide gauge anomalies show an increasing trend in both the number of hours and occurrence of extreme water levels (above the 98th percentile) beginning about 1930, inferred to be primarily the result of an increase in the rate of sea level rise. Five-year moving variance analyses show quasi-periodic decadal-scale variability from 1858 onward for both non-tide residuals and tide gauge anomalies, with variance peaks generally centered near extreme ENSO episodes. The range in non-tide variation suggests that storminess during ENSO episodes has not increased substantially since 1858 (except for perhaps the last 10 yrs). Measures of non-tide variability indicate that storminess comparable to or exceeding the great El Niño's of 1982-83 and 1997-98 occurred during decadal-scale periods centered near 1878 and 1916, suggesting that any climate change that may have occurred has not caused significant changes in storm trends during the last 140 years that are visible in the sea level record at SFO. The California Department of Boating and Waterways supported this research.

Science of Tsunami Hazards

Spectral characteristics of sea level fluctuations during the May 1960 Chilean Earthquake tsunami... more Spectral characteristics of sea level fluctuations during the May 1960 Chilean Earthquake tsunami are investigated using digitized strip chart recordings from two docks within Crescent City Harbor. Peaks in sea level spectra at the two docks near 10 -3 Hz and near 2.1 x10 -3 Hz correspond to the two lowest frequency harbor modes, occurring above the frequency band most strongly excited by the tsunami. Tidal modulation of harbor spectral structure at very short periods is observed. Theoretical estimates of shelf edge wave resonant modes fall within the frequency band strongly excited by the tsunami, in contrast to modeled edge waves from a seismic event near Cape Mendocino that show no evidence of the reflection necessary for a strong shelf resonance. This suggests that heightened susceptibility of sea level (but not necessarily currents) at Crescent City to tsunami is not due primarily to either harbor or shelf resonances.

Analysis of long time series, broadband (0.001-60Hz) seismometer and hydrophone data from the Haw... more Analysis of long time series, broadband (0.001-60Hz) seismometer and hydrophone data from the Hawaii-2 Observatory reveals many time-independent characteristics in power spectral density and coherence that persist regardless of the type or location of the noise sources. These characteristics can be attributed to the water depth, sediment thickness, igneous crustal structure, and other geological features local to the observatory. It is important to recognize these characteristics as due to local structure so that they do not confuse the interpretation of noise generated by storms and earthquakes in terms of other physical processes such as infra-gravity wave excitation and propagation, wave-wave interaction, breaking waves, Rayleigh/Stoneley/Scholte wave effects, and propagation and leakage from the ocean wave guide. Some examples of local, physical mechanisms include: 1) shear wave resonances (modes) in sediments [24], 2) water multiples (organ pipe modes) in the ocean [31], and 3) secondary scattering of Scholte waves from local seafloor heterogeneities [23].

The Hawaii-2 Observatory (H2O), located about half way between Hawaii and California, is an excel... more The Hawaii-2 Observatory (H2O), located about half way between Hawaii and California, is an excellent site for studying the origin and propagation of microseisms since it is located in the mid-ocean far from shorelines and shallow water. During the period between Dec 26, 2001 and January 24, 2002 on Leg 200 of the Ocean Drilling Program, the bridge crew of the JOIDES Resolution took environmental measurements (wind speed and direction, wave height, etc.) for comparison with the data collected by the H2O seismic system nearby. Comparison of the ship's weather log with the seismic data at frequencies from about 0.2 to 0.5 Hz shows a strong correlation of seismic amplitude with wind speed and direction, implying that the energy reaching the ocean floor (4977 m below) is generated locally by ocean gravity waves at the sea surface. These signals result from the well-known double-frequency (DF) microseism wave-wave interaction mechanism. Near-shore seismic stations on the U.S. West Co...

We show that the sea-level pressure North Pacific High (NPH) winter-time anomaly provides a super... more We show that the sea-level pressure North Pacific High (NPH) winter-time anomaly provides a superior predictor of inter-annual precipitation variability in California, including seasonal precipitation totals, drought, and extreme precipitation intensity, compared to traditional ENSO indices such as SOI, MEI, NINO3.4, and others. Large-scale climatic variables have been used as predictors of precipitation totals and extremes in many studies, and are used operationally in weather forecasts to circumvent the difficulty in obtaining robust dynamical simulations of precipitation – which is among the most complex of all climate variables in its mathematical representation in dynamical models. Here we show that the NPH anomaly more closely reflects the influence of the Pacific basin conditions over California. We employ statistical models that show the effectiveness of the NPH winter (Oct.-Mar.) anomaly as a predictor of total winter precipitation and the likelihood of daily precipitation ...

Science (New York, N.Y.), Jan 22, 2009

Journal of Geophysical Research, 2008

1] Although no clear trend in tropical cyclone (TC) generated wave height is observed, a TC wave ... more 1] 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.

Continuous seismometer recordings over extended periods describe wave climate variability. Intens... more Continuous seismometer recordings over extended periods describe wave climate variability. Intense storms having steep atmospheric pressure gradients cause strong winds that generate high waves. Wave energy is coupled into ambient seismic noise by wave-wave interactions in the open ocean and along coasts, and by direct pressure fluctuations at the ocean bottom in nearshore shallow water, generating double- and single-frequency microseisms, respectively. Some of the ocean swell energy is transformed into very long period infragravity (IG) wave energy along coasts, generating the Earth's ``hum''. A portion of the coastal IG wave energy leaks off continental shelves and becomes free waves that produce seismic signals at distant locations. The spatial and temporal distribution of storms and their associated wave energy controls the locations where high amplitude ambient seismic noise is generated, but seismic levels are also dependent on near-coastal bathymetry and shoreline geometry. Changes in climate-related broad-scale atmospheric patterns that affect storm track and intensity have a corresponding affect on the spatial ambient noise distribution. Because storm activity is seasonal, winters produce the highest ambient noise levels in each hemisphere. And because ocean gravity waves propagate long distances, seismometers in both hemispheres detect signals produced by winter waves generated in the opposite hemisphere. Microseism and hum Rayleigh waves can propagate long distances, and are detected at locations far from their generation region. Although seismometers detect ambient noise signals generated simultaneously along long stretches of coastline, the nearest coastlines tend to produce the dominant signal. The integrative character of ambient noise observations provides an assessment of the amount of wave energy reaching particular coasts that is complementary to point oceanographic measurements. Increasing wave energy along coasts is of particular concern under rising sea levels, which allows more energy to reach farther shoreward.

Assessment of regional and global storm intensities is of considerable interest in the debate abo... more Assessment of regional and global storm intensities is of considerable interest in the debate about the influence of global climate change on tropical and extratropical storm frequency and intensity. Continuous digital data from the Global Seismographic Network (GSN) and nearby predecessor seismic stations now extend back nearly four decades at the longest operational sites. The incessant and prominent microseism peaks in the global seismic background noise spectrum near 7 and 14 s period arise from distinct coupling processes between the atmosphere, ocean gravity waves, and the solid Earth. Earth’s microseism record thus constitutes an independent integrative proxy for nearshore wave intensity. We tabulate and quantify extreme storm events using microseism records from globally-distributed long-running seismic stations to assess decadal-scale changes in coastal wave intensity from the longest available digital archives. An apparent strong global influence of El Nino Southern Oscillation-associated changes in microseism/wave intensity is observed. A globally widespread preponderance of upward trends in the occurrence of strong microseism events suggests that winter storm intensity has generally increased over all ocean basins since the 1970’s.

Journal of Climate, Mar 1, 2003

The longest available hourly tide gauge record along the West Coast (U.S.) at San Francisco yield... more 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.

Error-corrected hourly tide gauge data from 1858 to 1999 at San Francisco (SFO) suggests a trend ... more Error-corrected hourly tide gauge data from 1858 to 1999 at San Francisco (SFO) suggests a trend for greater coastal impacts during winter months as a result of a combination of both an increased rate in sea level rise and recently increased storminess. Changes in storminess are determined from non-tide residuals, while sea level rise impacts are estimated from tide gauge anomalies. Assuming that non-tidal forcing varies smoothly across the tide gauge spectrum, non-tide water level estimates are obtained by linearly interpolating Fourier spectral estimates across the tidal bands, with the variance of the interpolated estimates determined from spectral level variation on both sides of the band. Tide gauge anomalies are determined from the difference between the raw tide gauge data and mean of the monthly means. Thus, tide gauge anomalies can include significant contributions resulting from El Niño related thermal expansion along the West Coast as well as long term sea level rise, while non-tide residuals exclude water level variation at time-scales greater than 90 days and are more closely associated with storminess. Tide gauge anomalies show an increasing trend in both the number of hours and occurrence of extreme water levels (above the 98th percentile) beginning about 1930, inferred to be primarily the result of an increase in the rate of sea level rise. Five-year moving variance analyses show quasi-periodic decadal-scale variability from 1858 onward for both non-tide residuals and tide gauge anomalies, with variance peaks generally centered near extreme ENSO episodes. The range in non-tide variation suggests that storminess during ENSO episodes has not increased substantially since 1858 (except for perhaps the last 10 yrs). Measures of non-tide variability indicate that storminess comparable to or exceeding the great El Niño's of 1982-83 and 1997-98 occurred during decadal-scale periods centered near 1878 and 1916, suggesting that any climate change that may have occurred has not caused significant changes in storm trends during the last 140 years that are visible in the sea level record at SFO. The California Department of Boating and Waterways supported this research.

Science of Tsunami Hazards

Spectral characteristics of sea level fluctuations during the May 1960 Chilean Earthquake tsunami... more Spectral characteristics of sea level fluctuations during the May 1960 Chilean Earthquake tsunami are investigated using digitized strip chart recordings from two docks within Crescent City Harbor. Peaks in sea level spectra at the two docks near 10 -3 Hz and near 2.1 x10 -3 Hz correspond to the two lowest frequency harbor modes, occurring above the frequency band most strongly excited by the tsunami. Tidal modulation of harbor spectral structure at very short periods is observed. Theoretical estimates of shelf edge wave resonant modes fall within the frequency band strongly excited by the tsunami, in contrast to modeled edge waves from a seismic event near Cape Mendocino that show no evidence of the reflection necessary for a strong shelf resonance. This suggests that heightened susceptibility of sea level (but not necessarily currents) at Crescent City to tsunami is not due primarily to either harbor or shelf resonances.

Analysis of long time series, broadband (0.001-60Hz) seismometer and hydrophone data from the Haw... more Analysis of long time series, broadband (0.001-60Hz) seismometer and hydrophone data from the Hawaii-2 Observatory reveals many time-independent characteristics in power spectral density and coherence that persist regardless of the type or location of the noise sources. These characteristics can be attributed to the water depth, sediment thickness, igneous crustal structure, and other geological features local to the observatory. It is important to recognize these characteristics as due to local structure so that they do not confuse the interpretation of noise generated by storms and earthquakes in terms of other physical processes such as infra-gravity wave excitation and propagation, wave-wave interaction, breaking waves, Rayleigh/Stoneley/Scholte wave effects, and propagation and leakage from the ocean wave guide. Some examples of local, physical mechanisms include: 1) shear wave resonances (modes) in sediments [24], 2) water multiples (organ pipe modes) in the ocean [31], and 3) secondary scattering of Scholte waves from local seafloor heterogeneities [23].

The Hawaii-2 Observatory (H2O), located about half way between Hawaii and California, is an excel... more The Hawaii-2 Observatory (H2O), located about half way between Hawaii and California, is an excellent site for studying the origin and propagation of microseisms since it is located in the mid-ocean far from shorelines and shallow water. During the period between Dec 26, 2001 and January 24, 2002 on Leg 200 of the Ocean Drilling Program, the bridge crew of the JOIDES Resolution took environmental measurements (wind speed and direction, wave height, etc.) for comparison with the data collected by the H2O seismic system nearby. Comparison of the ship's weather log with the seismic data at frequencies from about 0.2 to 0.5 Hz shows a strong correlation of seismic amplitude with wind speed and direction, implying that the energy reaching the ocean floor (4977 m below) is generated locally by ocean gravity waves at the sea surface. These signals result from the well-known double-frequency (DF) microseism wave-wave interaction mechanism. Near-shore seismic stations on the U.S. West Co...

We show that the sea-level pressure North Pacific High (NPH) winter-time anomaly provides a super... more We show that the sea-level pressure North Pacific High (NPH) winter-time anomaly provides a superior predictor of inter-annual precipitation variability in California, including seasonal precipitation totals, drought, and extreme precipitation intensity, compared to traditional ENSO indices such as SOI, MEI, NINO3.4, and others. Large-scale climatic variables have been used as predictors of precipitation totals and extremes in many studies, and are used operationally in weather forecasts to circumvent the difficulty in obtaining robust dynamical simulations of precipitation – which is among the most complex of all climate variables in its mathematical representation in dynamical models. Here we show that the NPH anomaly more closely reflects the influence of the Pacific basin conditions over California. We employ statistical models that show the effectiveness of the NPH winter (Oct.-Mar.) anomaly as a predictor of total winter precipitation and the likelihood of daily precipitation ...