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Camargo S.J.,Lamont Doherty Earth Observatory
Journal of Climate | Year: 2013

Tropical cyclone (TC) activity is analyzed in 14 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). The globalTCactivity in the historical runs is comparedwith observations. The simulation of TC activity in theCMIP5 models is not as good as in higher-resolution simulations. TheCMIP5 global TCfrequency is much lower than observed, and there is significant deficiency in the geographical patterns of TC tracks and formation. Although all of the models underestimate the global frequency of TCs, the models present a wide range of globalTCfrequency. Themodelswith the highest horizontal resolution have the highest level of globalTCactivity, though resolution is not the only factor that determines model TC activity. A cold SST bias could potentially contribute to the low number of TCs in themodels. The models show no consensus regarding the difference of TC activity in two warming scenarios [representative concentration pathway 4.5 (RCP4.5) and RCP8.5] and the historical simulation. The author examined in more detail North Atlantic and eastern North Pacific TC activity in a subset ofmodels and found no robust changes acrossmodels inTCfrequency. Therefore, there is no robust signal across the CMIP5 models in global and regional TC changes in activity for future scenarios. The future changes in various large-scale environmental fields associated with TC activity were also examined globally: genesis potential index, potential intensity, vertical wind shear, and sea level pressure. The multimodel mean changes of these variables in the CMIP5 models are consistent with the changes obtained in the CMIP3 models. © 2013 American Meteorological Society.

Holtzman B.K.,Lamont Doherty Earth Observatory
Geochemistry, Geophysics, Geosystems | Year: 2016

This paper integrates current questions in rock physics on the effects and behavior of very small melt fractions (1%) in the asthenosphere. In experiment and theory, it has been shown that a very small melt fraction forming a connected network has a large effect on the diffusion creep shear viscosity, as well as in the anelastic behavior. Because small concentrations of volatiles, particularly H2O and CO2, significantly lower the peridotite solidus, a small melt fraction is expected in the asthenosphere. Even with connected networks, permeability will be low and surface tension will generate a strong force resisting complete draining of small melt fractions. The anelastic reduction of shear velocity due to melt could cause a ≥5% shear velocity contrast across the solidus, consistent with the contrast measured on features in the shallow suboceanic upper mantle that are often interpreted as the lithosphere-asthenosphere boundary. © 2015. American Geophysical Union. All Rights Reserved.

Barnes E.A.,Lamont Doherty Earth Observatory | Polvani L.,Lamont Doherty Earth Observatory
Journal of Climate | Year: 2013

This work documents how the midlatitude, eddy-driven jets respond to climate change using model output from phase 5 of the Coupled Model Intercomparison Project (CMIP5). The authors consider separately the North Atlantic, the North Pacific, and the Southern Hemisphere jets. The analysis is not limited to annualmean changes in the latitude and speed of the jets, but also explores how the variability of each jet changes with increased greenhouse gases. All jets are found to migrate poleward with climate change: the Southern Hemisphere jet shifts poleward by 2° of latitude between the historical period and the end of the twenty-first century in the representative concentration pathway 8.5 (RCP8.5) scenario, whereas both Northern Hemisphere jets shift by only 1°. In addition, the speed of the Southern Hemisphere jet is found to increase markedly (by 1.2ms-1 between 850 and 700 hPa), while the speed remains nearly constant for both jets in the Northern Hemisphere. More importantly, it is found that the patterns of jet variability are a strong function of the jet position in all three sectors of the globe, and as the jets shift poleward the patterns of variability change. Specifically, for the Southern Hemisphere and the North Atlantic jets, the variability becomes less of a north-south wobbling and more of a pulsing (i.e., variation in jet speed). In contrast, for the North Pacific jet, the variability becomes less of a pulsing and more of a north-south wobbling. These different responses can be understood in terms of Rossby wave breaking, allowing the authors to explain most of the projected jet changes within a single dynamical framework. © 2013 American Meteorological Society.

Kim W.-Y.,Lamont Doherty Earth Observatory
Journal of Geophysical Research: Solid Earth | Year: 2013

Over 109 small earthquakes (Mw 0.4-3.9) were detected during January 2011 to February 2012 in the Youngstown, Ohio area, where there were no known earthquakes in the past. These shocks were close to a deep fluid injection well. The 14 month seismicity included six felt earthquakes and culminated with a Mw 3.9 shock on 31 December 2011. Among the 109 shocks, 12 events greater than Mw 1.8 were detected by regional network and accurately relocated, whereas 97 small earthquakes (0.4 < Mw < 1.8) were detected by the waveform correlation detector. Accurately located earthquakes were along a subsurface fault trending ENE-WSW - consistent with the focal mechanism of the main shock and occurred at depths 3.5-4.0 km in the Precambrian basement. We conclude that the recent earthquakes in Youngstown, Ohio were induced by the fluid injection at a deep injection well due to increased pore pressure along the preexisting subsurface faults located close to the wellbore. We found that the seismicity initiated at the eastern end of the subsurface fault - close to the injection point, and migrated toward the west - away from the wellbore, indicating that the expanding high fluid pressure front increased the pore pressure along its path and progressively triggered the earthquakes. We observe that several periods of quiescence of seismicity follow the minima in injection volumes and pressure, which may indicate that the earthquakes were directly caused by the pressure buildup and stopped when pressure dropped. ©2013. American Geophysical Union. All Rights Reserved.

DeMenocal P.B.,Lamont Doherty Earth Observatory
Science | Year: 2011

Climate change and its effects on African ecosystems may have played a key role in human evolution.

Malinverno A.,Lamont Doherty Earth Observatory
Earth and Planetary Science Letters | Year: 2010

At Site U1325 (IODP Exp. 311, Cascadia margin), gas hydrates occupy 20-60% of pore space in thin sand layers (<5 cm) surrounded by fine-grained intervals (2.5. m thick on average) that contain little or no hydrate. This is a common occurrence in gas hydrate-bearing marine sequences, and it has been related to the inhibition of hydrate formation in the small pores of fine-grained sediments. This paper applies a mass balance model to gas hydrate formation in a stack of alternating fine- and coarse-grained sediment layers. The only source of methane considered is in situ microbial conversion of a small amount of organic carbon (<0.5% dry weight fraction). The results show that in a sequence such as that at Site U1325 the methane concentration never reaches the supersaturation needed to form gas hydrates in the fine-grained layers. Methane generated in these layers is transported by diffusion into the coarse-grained layers where it forms concentrated gas hydrate deposits. The vertical distribution and amount of gas hydrate observed at Site U1325 can be explained by in situ microbial methane generation, and a deep methane source is not necessary. © 2010 Elsevier B.V.

Ekstrom G.,Lamont Doherty Earth Observatory
Geophysical Journal International | Year: 2011

A large number of fundamental-mode Love and Rayleigh wave dispersion curves were determined from seismograms for 3330 earthquakes recorded on 258 globally distributed seismographic stations. The dispersion curves were sampled at periods between 25 and 250 s to determine propagation-phase anomalies with respect to a reference earth model. The data set of phase anomalies was first used to construct global isotropic phase-velocity maps at specific frequencies using spherical-spline basis functions with a nominal uniform resolution of 650km. Azimuthal anisotropy was then included in the parametrization, and its importance for explaining the data explored. Only the addition of 2ζ azimuthal variations for Rayleigh waves was found to be resolved by the data. In the final stage of the analysis, the entire phase-anomaly data set was inverted to determine a global dispersion model for Love and Rayleigh waves parametrized horizontally using a spherical-spline basis, and with a set of B-splines to describe the slowness variations with respect to frequency. The new dispersion model, GDM52, can be used to calculate internally consistent global maps of phase and group velocity, as well as local and path-specific dispersion curves, between 25 and 250 s. © 2011 The Author Geophysical Journal International © 2011 RAS.

Biasutti M.,Lamont Doherty Earth Observatory
Journal of Geophysical Research: Atmospheres | Year: 2013

The simulations of the fifth Coupled Models Intercomparison Project (CMIP5) strengthen previous assessments of a substantial role of anthropogenic emissions in driving precipitation changes in the Sahel, the semiarid region at the southern edge of the Sahara. Historical simulations can capture the magnitude of the centennial Sahel drying over the span of the 20th century and confirm that anthropogenic forcings have contributed substantially to it. Yet, the models do not reproduce the amplitude of observed oscillations at multidecadal timescales, suggesting that either oscillations in the forcing or the strength of natural variability are underestimated. Projections for Sahel rainfall are less robust than the 20th century hindcast and outlier projections persist, but overall the CMIP5 models confirm the CMIP3 results in many details and reaffirm the prediction of a rainy season that is more feeble at its start, especially in West Africa, and more abundant at its core across the entire Sahel. Out of 20 models, four buck this consensus. Idealized simulations from a subset of the CMIP5 ensemble - simulations designed to separate the fast land-atmosphere response to increased greenhouse gases (GHGs) from the slow response mediated through changes in sea surface temperature (SST) - confirm that the direct effect of CO2 is to enhance the monsoon, while warmer SST induce drying over the Sahel. At the same time, these simulations suggest that the seasonal evolution of the rainfall trends in the scenario simulations, spring drying and fall wetting, is an inherently coupled response, not captured by the linear superposition of the fast and slow response to CO2. Key PointsAnthropogenic forcing contributed to past drought in the Sahel.Projections are converging on a delay and intensification of the rainy interactions are important for the response © 2013. American Geophysical Union. All Rights Reserved.

Smerdon J.E.,Lamont Doherty Earth Observatory
Wiley Interdisciplinary Reviews: Climate Change | Year: 2012

Millennium-length, forced transient simulations with fully coupled general circulation models have become important new tools for addressing uncertainties in global and hemispheric temperature reconstructions targeting the Common Era (the last two millennia). These model simulations are used as test beds on which to evaluate the performance of paleoclimate reconstruction methods using controlled and systematic investigations known as pseudoproxy experiments (PPEs). Such experiments are motivated by the fact that any given real-world reconstruction is the product of multiple uncontrolled factors, making it difficult to isolate the impact of one factor in reconstruction assessments and comparisons. PPEs have established a common experimental framework that can be systematically altered and evaluated, and thus test reconstruction methods and their dependencies. Although the translation of PPE results into real-world implications must be done cautiously, their experimental design attributes allow researchers to test reconstruction techniques beyond what was previously possible with real-world data alone. This review summarizes the development of PPEs and their findings over the last decade. The state of the science and its implications for global and hemispheric temperature reconstructions is also reviewed, as well as near-term design improvements that will expand the utility of PPEs. © 2011 John Wiley & Sons, Ltd.

Seager R.,Lamont Doherty Earth Observatory | Vecchi G.A.,National Oceanic and Atmospheric Administration
Proceedings of the National Academy of Sciences of the United States of America | Year: 2010

Climate models robustly predict that the climate of southwestern North America, defined as the area from the western Great Plains to the Pacific Ocean and from the Oregon border to southern Mexico, will dry throughout the current century as a consequence of rising greenhouse gases. This regional drying is part of a general drying of the subtropics and poleward expansion of the subtropical dry zones. Through an analysis of 15 coupled climate models it is shown here that the drying is driven by a reduction of winter season precipitation associated with increased moisture divergence by the mean flow and reduced moisture convergence by transient eddies. Due to the presence of large amplitude decadal variations of presumed natural origin, observations to date cannot confirm that this transition to a drier climate is already underway, but it is anticipated that the anthropogenic drying will reach the amplitude of natural decadal variability by midcentury. In addition to this drop in total precipitation, warming is already causing a decline in mountain snow mass and an advance in the timing of spring snow melt disrupting the natural water storage systems that are part of the region's water supply system. Uncertainties inhowradiative forcing will impact the tropical Pacific climate system create uncertainties in the amplitude of drying in southwest North America with a La Niña-like response creating a worst case scenario of greater drying.

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