Jeffrey Park

Global‐scale modes of surface temperature variability on interannual to century timescales

Using 100 years of global temperature anomaly data, we have performed a singular value decomposition of temperature variations in narrow frequency bands to isolate coherent spatio-temporal “modes” of global climate variability. Statistical significance is determined from confidence limits obtained by Monte Carlo simulations. Secular variance is dominated by a globally coherent trend, with nearly all grid points warming in phase at varying amplitude. A smaller, but significant, share of the secular variance corresponds to a pattern dominated by warming and subsequent cooling in the high latitude North Atlantic with a roughly centennial timescale. Spatial patterns associated with significant peaks in variance within a broad period range from 2.8 to 5.7 years exhibit characteristic El Nino-Southern Oscillation (ENSO) patterns. A recent transition to a regime of higher ENSO frequency is suggested by our analysis. An interdecadal mode in the 15-to-18 years period range appears to represent long-term ENSO variability. This mode has a sizeable projection onto global-average temperature, and accounts for much of the anomalous global warmth of the 1980s. A quasi-biennial mode centered near 2.2-years period and a mode centered at 7-to-8 years period both exhibit predominantly a North Atlantic Oscillation (NAO) temperature pattern. A potentially significant “decadal” mode centered on 11-to-12 years period also exhibits an NAO temperature pattern and may be modulated by the century-scale North Atlantic variability.

Receiver Functions from Multiple-Taper Spectral Correlation Estimates

Teleseismic P waves are followed by a series of scattered waves, particularly P -to- S converted phases, that form a coda. The sequence of scattered waves on the horizontal components can be represented by the receiver function (RF) for the station and may vary with the approach angle and azimuth of the incoming P wave. We have developed a frequency-domain RF inversion algorithm using multiple-taper correlation (MTC) estimates, instead of spectral division, using the pre-event noise spectrum for frequency-dependent damping. The multitaper spectrum estimates are leakage resistant, so low-amplitude portions of the P -wave spectrum can contribute usefully to the RF estimate. The coherence between vertical and horizontal components can be used to obtain a frequency-dependent uncertainty for the RF. We compare the MTC method with two popular methods for RF estimation, time-domain deconvolution (TDD), and spectral division (SPD), both with damping to avoid numerical instabilities. Deconvolution operators are often biased toward the frequencies where signal is strongest. Spectral-division schemes with constant water-level damping can suffer from the same problem in the presence of strong signal-generated noise. Estimates of uncertainty are scarce for TDD and SPD, which impedes developing a weighted average of RF estimates from multiple events. Multiple-taper correlation RFs are more resistant to signal-generated noise in test cases, though a “coherent” scattering effect, like a strong near-surface organ-pipe resonance in soft sediments, will overprint the Ps conversions from deeper interfaces. The MTC RF analysis confirms the broad features of an earlier RF study for the Urals foredeep by Levin and Park (1997a) using station ARU of the Global Seismographic Network (GSN), but adds considerable detail, resolving P -to- S converted energy up to f = 4.0 Hz.

joint Spatiotemporal Modes of Surface Temperature and Sea Level Pressure Variability in the Northern Hemisphere during the Last Century

Abstract Coherent spatiotemporal modes of climatic variability are isolated based on a multivariate frequency domain singular value decomposition (SVD) of nearly a century of monthly Northern Hemisphere sea level pressure (SLP) and surface temperature data. Insight into the underlying physical processes associated with potential climatic signals is obtained by examining the relationship between surface temperature and inferred atmospheric circulation patterns as they evolve over the a typical cycle, taking potential seasonal distinctions into account. Our analysis provides evidence for two significant independent secular variations describing a secular warming trend (and accompanying changes in circulation patterns) and a century timescale “oscillation” marked by high-amplitude variations in temperature and SLP in the North Atlantic that are similar to those observed in recent model simulations. Quasi-oscillatory interdecadal (16–18 yr timescale) variability also displays a pattern similar to those predic...

Mantle flow at a slab edge: Seismic anisotropy in the Kamchatka region

The junction of the Aleutian Island and the Kamchatka peninsula defines a sharp turn in the boundary of the Pacific and North American plates, terminating the subduction zones of the northwest Pacific. The regional pattern of shear-wave birefringence near the junction indicates that trench-parallel strain follows the seismogenic Benioff zone, but rotates to trench-normal beyond the slab edge. Asthenospheric mantle is inferred to flow around and beneath the disrupted slab edge, and may influence the shallowing dip of the Benioff zone at the Aleutian junction.

Climate sensitivity constrained by CO2 concentrations over the past 420 million years.

A firm understanding of the relationship between atmospheric carbon dioxide concentration and temperature is critical for interpreting past climate change and for predicting future climate change. A recent synthesis suggests that the increase in global-mean surface temperature in response to a doubling of the atmospheric carbon dioxide concentration, termed ‘climate sensitivity’, is between 1.5 and 6.2 °C (5–95 per cent likelihood range), but some evidence is inconsistent with this range. Moreover, most estimates of climate sensitivity are based on records of climate change over the past few decades to thousands of years, when carbon dioxide concentrations and global temperatures were similar to or lower than today, so such calculations tend to underestimate the magnitude of large climate-change events and may not be applicable to climate change under warmer conditions in the future. Here we estimate long-term equilibrium climate sensitivity by modelling carbon dioxide concentrations over the past 420 million years and comparing our calculations with a proxy record. Our estimates are broadly consistent with estimates based on short-term climate records, and indicate that a weak radiative forcing by carbon dioxide is highly unlikely on multi-million-year timescales. We conclude that a climate sensitivity greater than 1.5 °C has probably been a robust feature of the Earth’s climate system over the past 420 million years, regardless of temporal scaling.

Shear wave splitting in the Appalachians and the Urals: A case for multilayered anisotropy

Observations of shear wave splitting in the northeastern U.S. Appalachians and in the foredeep of the Urals vary significantly with the back azimuth and incidence angle of the incoming phase. These variations suggest two or more layers within the upper mantle with different anisotropic properties. Synthetic seismograms for simple multilayered anisotropic structures show that shear wave splitting parameters tend to vary substantially with the direction of approach. Relying on a subset of back-azimuth and incidence angle may strongly bias the model inferred, especially if the observations are averaged. On the other hand, the azimuthal splitting pattern provides additional constraints on vertical or lateral variation of anisotropic properties in the Earth. Using a new error estimation technique for splitting, we find that individual measurements from broadband data have errors of the order of δ = 3°-7°for the fast direction and 0.1 - 0.2 s for the delay of split shear waves. The azimuthal variation of splitting parameters is broadly consistent throughout the Appalachian terranes in the northeast United States, especially for two long-running stations in the northeast United States, HRV (Harvard, Massachusetts) and PAL (Palisades, New York). Observations can be separated into two distinct populations, with mean fast-axis azimuths of N60°E±4°and N119°E±2°, Delay values within each population range from near zero to ∼1 s. Azimuthal splitting variation for ARU (Arti, Russia) in the foredeep of Uralian mountains is characterized by sharp transitions between different groups of observations. Using synthetic seismograms in simple structures, we develop one-dimensional anisotropic models under stations HRV and ARU. The model for HRV includes two layers of anisotropic material under an isotropic crust, with fast-axis azimuths N53°E and N115°E for the bottom and the top layers, respectively. The model for the upper mantle under ARU includes a layer with a fast-axis at N50°E atop a layer with fast axis azimuth N90°E. Our modeling confirms the need for a layer of strong anisotropy with a slow axis of symmetry in the lower crust under ARU, reported by Levin and Park [1997a]. Our results suggest that both Urals and Appalachians possess a relict anisotropy in the tectosphere, associated with past continental collision and accretion, underlain by anisotropy with orientation similar to the local absolute plate motion, suggesting an asthenospheric component to the signal.

Oscillatory spatiotemporal signal detection in climate studies : A multiple-taper spectral domain approach

Publisher Summary This chapter introduces a methodology for signal detection and reconstruction of irregular spatiotemporal oscillatory signals—the multiple-taper spectrum estimation method (MTM)–singular-value decomposition (SVD) methodology. This methodology is offered as an alternative technique which avoids most of the problems encountered in traditional techniques and provides an efficient exploratory method for climate signal detection. The associated signal-detection parameter—the local fractional variance spectrum (LFV) spectrum—yields the correct null distribution for a very general class of spatiotemporal climate noise processes and the correct inferences when signals are present. The methodology allows for a faithful reconstruction of the arbitrary spatiotemporal patterns of narrowband signals immersed in spatially correlated noise. The results of the MTM–SVD approach are robust to the temporal and spatial sampling inhomogeneities that are common in actual climate data. Applied to observational climate data, the MTM–SVD analysis yields insight into secular trends, low-frequency, and high-frequency quasi-oscillatory variations in the climate system. The dominant mode of secular variation has been a long-term global warming trend associated with some anomalous atmospheric circulation patterns that show similarity to the modeled response of the climate to increased greenhouse gases.

Seismic evidence for catastrophic slab loss beneath Kamchatka

In the northwest Pacific Ocean, a sharp corner in the boundary between the Pacific plate and the North American plate joins a subduction zone running along the southern half of the Kamchatka peninsula with a region of transcurrent motion along the western Aleutian arc. Here we present images of the seismic structure beneath the Aleutian–Kamchatka junction and the surrounding region, indicating that: the subducting Pacific lithosphere terminates at the Aleutian–Kamchatka junction; no relict slab underlies the extinct northern Kamchatka volcanic arc; and the upper mantle beneath northern Kamchatka has unusually slow shear wavespeeds. From the tectonic and volcanic evolution of Kamchatka over the past 10 Myr (refs 3, 4–5) we infer that at least two episodes of catastrophic slab loss have occurred. About 5 to 10 Myr ago, catastrophic slab loss shut down island-arc volcanic activity north of the Aleutian–Kamchatka junction. A later episode of slab loss, since about 2 Myr ago, seems to be related to the activity of the worlds most productive island-arc volcano, Klyuchevskoy. Removal of lithospheric mantle is commonly discussed in the context of a continental collision, but our findings imply that episodes of slab detachment and loss are also important agents in the evolution of oceanic convergent margins.

The global seismographic network surpasses its design goal

This year, the Global Seismographic Network (GSN) surpassed its 128-station design goal for uniform worldwide coverage of the Earth. A total of 136 GSN stations are now sited from the South Pole to Siberia, and from the Amazon Basin to the sea floor of the northeast Pacific Ocean—in cooperation with over 100 host organizations and seismic networks in 59 countries worldwide (Figure 1). Established in 1986 by the Incorporated Research Institutions for Seismology (IRIS) to replace the obsolete, analog Worldwide Standardized Seismograph Network (WWSSN),the GSN continues a tradition in global seismology that dates back more than a century to the network of Milne seismographs that initially spanned the globe. The GSN is a permanent network of state-of-the-art seismological and geophysical sensors connected by available telecommunications to serve as a multi-use scientific facility and societal resource for scientific research, environmental monitoring, and education for our national and international community.

Crust and upper mantle of Kamchatka from teleseismic receiver functions

Teleseismic receiver functions (RFs) from a yearlong broadband seismological experiment in Kamchatka reveal regional variations in the Moho, anisotropy in the supra-slab mantle wedge, and, along the eastern coast, Ps converted phases from the steeply dipping slab. We analyze both radial- and transverse-component RFs in bin-averaged epicentral and backazimuthal sweeps, in order to detect Ps moveout and polarity variations diagnostic of interface depth, interface dip, and anisotropic fabric within the shallow mantle and crust. At some stations, the radial RF is overprinted by near-surface resonances, but anisotropic structure can be inferred from the transverse RF. Using forward modeling to match the observed RFs, we find Moho depth to range between 30 and 40 km across the peninsula, with a gradational crust–mantle transition beneath some stations along the eastern coast. Anisotropy beneath the Moho is required to fit the transverse RFs at most stations. Anisotropy in the lower crust is required at a minority of stations. Modeling the amplitude and backazimuthal variation of the Ps waveform suggests that an inclined axis of symmetry and 5–10% anisotropy are typical for the crust and the shallow mantle. The apparent symmetry axes of the anisotropic layers are typically trench-normal, but trench-parallel symmetry axes are found for stations APA and ESS, both at the fringes of the central Kamchatka depression. Transverse RFs from east-coast stations KRO, TUM, ZUP and PETare fit well by two anisotropic mantle layers with trench-normal symmetry axes and opposing tilts. Strong anisotropy in the supraslab mantle wedge suggests that the mantle ‘‘lithosphere’’ beneath the Kamchatka volcanic arc is actively deforming, strained either by wedge corner flow at depth or by trenchward suction of crust as the Pacific slab retreats. D 2002 Elsevier Science B.V. All rights reserved.