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Grise K.M.,University of Virginia | Polvani L.M.,Lamont Doherty Earth Observatory | Polvani L.M.,University of Applied and Environmental Sciences
Geophysical Research Letters | Year: 2014

In Coupled Model Intercomparison Project Phase 5 (CMIP5) models, the zonal-mean tropospheric circulation shifts robustly poleward in the Southern Hemisphere extratropics in response to increased atmospheric CO2 concentrations. However, in the Northern Hemisphere (NH) extratropics, the circulation response to CO2 is largely absent in the zonal mean and is instead characterized by complex regional anomalies. This study decomposes the atmospheric circulation response to CO2 forcing in CMIP5 models into two components: a direct component due to CO2 radiative forcing and an indirect component associated with sea surface temperature (SST)-mediated changes. The direct radiative forcing of CO2 drives a weak poleward jet shift in both hemispheres, whereas the indirect (SST) component of the CO2 forcing dominates the total response and drives a zonally asymmetric response in the NH. Hence, understanding the SST-mediated component of atmospheric CO2 forcing appears crucial to unlocking the mechanisms that contribute to forced extratropical circulation changes. Key Points NH circulation does not shift robustly poleward in response to increasing CO2Direct radiative forcing of CO2 causes poleward jet shift in each hemisphereSSTs dominate and cause NH asymmetric extratropical circulation response ©2014. American Geophysical Union. All Rights Reserved.

Butler A.H.,Climate Prediction Center | Polvani L.M.,University of Applied and Environmental Sciences | Polvani L.M.,Lamont Doherty Earth Observatory
Geophysical Research Letters | Year: 2011

Recent studies have suggested that El Nio-Southern Oscillation (ENSO) may have a considerable impact on Northern Hemisphere wintertime stratospheric conditions. Notably, during El Nio the stratosphere is warmer than during ENSO-neutral winters, and the polar vortex is weaker. Opposite-signed anomalies have been reported during La Nia, but are considerably smaller in amplitude than during El Nio. This has led to the perception that El Nio is able to substantially affect stratospheric conditions, but La Nia is of secondary importance. Here we revisit this issue, but focus on the extreme events that couple the troposphere to the stratosphere: major, mid-winter stratospheric sudden warmings (SSWs). We examine 53 years of reanalysis data and find, as expected, that SSWs are nearly twice as frequent during ENSO winters as during non-ENSO winters. Surprisingly, however, we also find that SSWs occur with equal probability during El Nio and La Nia winters. These findings corroborate the impact of ENSO on stratospheric variability, and highlight that both phases of ENSO are important in enhancing stratosphere-troposphere dynamical coupling via an increased frequency of SSWs. Copyright 2011 by the American Geophysical Union.

Tippett M.K.,Columbia University | Sobel A.H.,University of Applied and Environmental Sciences | Sobel A.H.,Lamont Doherty Earth Observatory | Camargo S.J.,Lamont Doherty Earth Observatory
Geophysical Research Letters | Year: 2012

Monthly U.S. tornado numbers are here related to observation-based monthly averaged atmospheric parameters. Poisson regression is used to form an index which captures the climatological spatial distribution and seasonal variation of tornado occurrence, as well as year-to-year variability, and provides a framework for extended range forecasts of tornado activity. Computing the same index with predicted atmospheric parameters from a comprehensive forecast model gives some evidence of the predictability of monthly tornado activity. Copyright 2012 by the American Geophysical Union.

Polvani L.M.,University of Applied and Environmental Sciences | Polvani L.M.,Lamont Doherty Earth Observatory | Solomon S.,Massachusetts Institute of Technology
Journal of Geophysical Research: Atmospheres | Year: 2012

The effect of ozone depletion on temperature trends in the tropical lower stratosphere is explored with an atmospheric general circulation model, and directly contrasted to the effect of increased greenhouse gases and warmer sea surface temperatures. Confirming and extending earlier studies we find that, over the second half of the 20th Century, the model's lower-stratospheric cooling caused by ozone depletion is several times larger than that induced by increasing greenhouse gases. Moreover, our model suggests that the response to different forcings is highly additive. Finally we demonstrate that when ozone depletion alone is prescribed in the model, the seasonal cycle of the resultant cooling trends in the lower stratosphere is quite similar to that recently reported in satellite and radiosonde observations: this constitutes strong, new evidence for the key role of ozone depletion on tropical lower-stratospheric temperature trends. © 2012. American Geophysical Union. All Rights Reserved.

Previdi M.,Lamont Doherty Earth Observatory | Polvani L.M.,University of Applied and Environmental Sciences
Quarterly Journal of the Royal Meteorological Society | Year: 2014

We review what is presently known about the climate system response to stratospheric ozone depletion and its projected recovery, focusing on the responses of the atmosphere, ocean and cryosphere. Compared with well-mixed greenhouse gases (GHGs), the radiative forcing of climate due to observed stratospheric ozone loss is very small: in spite of this, recent trends in stratospheric ozone have caused profound changes in the Southern Hemisphere (SH) climate system, primarily by altering the tropospheric midlatitude jet, which is commonly described as a change in the Southern Annular Mode. Ozone depletion in the late twentieth century was the primary driver of the observed poleward shift of the jet during summer, which has been linked to changes in tropospheric and surface temperatures, clouds and cloud radiative effects, and precipitation at both middle and low latitudes. It is emphasized, however, that not all aspects of the SH climate response to stratospheric ozone forcing can be understood in terms of changes in the midlatitude jet. The response of the Southern Ocean and sea ice to ozone depletion is currently a matter of debate. For the former, the debate is centred on the role of ocean eddies in possibly opposing wind-driven changes in the mean circulation. For the latter, the issue is reconciling the observed expansion of Antarctic sea-ice extent during the satellite era with robust modelling evidence that the ice should melt as a result of stratospheric ozone depletion (and increases in GHGs). Despite lingering uncertainties, it has become clear that ozone depletion has been instrumental in driving SH climate change in recent decades. Similarly, ozone recovery will figure prominently in future climate change, with its impacts expected to largely cancel the impacts of increasing GHGs during the next half-century. © 2014 Royal Meteorological Society.

Bellon G.,French National Center of Weather Research | Sobel A.H.,University of Applied and Environmental Sciences
Journal of Climate | Year: 2010

A model of intermediate complexity based on quasi-equilibrium theory-a version of the Quasi-Equilibrium Tropical Circulation Model with a prognostic atmospheric boundary layer, as well as two free-tropospheric modes in momentum, and one each in moisture and temperature-is used in a zonally symmetric aquaplanet configuration to study the sensitivity of the Hadley circulation to the sea surface temperature (SST) latitudinal distribution. For equatorially symmetric SST forcing with large SST gradients in the tropics, the model simulates the classical double Hadley cell with one equatorial intertropical convergence zone (ITCZ). For small SST gradients in the tropics, the model exhibits multiple equilibria, with one equatorially symmetric equilibrium and two asymmetric equilibria (mirror images of each other) with an off-equatorial ITCZ. Further investigation of the feedbacks at play in the model shows that the assumed vertical structure of temperature variations is crucial to the existence and stability of the asymmetric equilibria. The free-tropospheric moisture-convection feedback must also be sufficiently strong to sustain asymmetric equilibria. Both results suggest that the specific physics of a given climate model condition determine the existence of multiple equilibria via the resulting sensitivity of the convection to free-tropospheric humidity and the vertical structure of adiabatic heating. The symmetry-breaking mechanism and resulting multiple equilibria have their origin in the localmultiple equilibria that can be described by a single-columnmodel using theweak temperature gradient approximation. An additional experiment using an SST latitudinal distribution with a relative minimum at the equator shows that the feedbacks controlling these multiple equilibria might be relevant to the double-ITCZ problem. © 2010 American Meteorological Society.

Shaw T.A.,University of Applied and Environmental Sciences | Pauluis O.,New York University
Journal of the Atmospheric Sciences | Year: 2012

The spectrum of meridional latent heat transport in the tropics and subtropics by disturbances to the zonal mean during all seasons is analyzed. The transport is divided into stationary and transient planetary- and subplanetary-scale eddy contributions. The stationary transport is largest in the subtropical lower troposphere and dominates the overall transport during summer. It is of planetary scale and the zonal scale of the transport corresponds to the number of subtropical anticyclones. The transient transport is large from the surface up to the midtroposphere and from the tropics to subpolar latitudes. It is dominated by the subplanetary-scale contribution during all seasons. Westward (eastward)- propagating waves dominate the transport in the tropics (subtropics and midlatitudes). The analysis reveals that, while the total eddy meridional latent heat transport is seamless from the deep tropics to the pole, it represents the sum of transport by distinct dynamical features. The role of the eddy meridional latent heat transport in the moist isentropic circulation is assessed using the statistical transformed Eulerian mean formulation, which converts the eddy transports into streamfunctions. The addition of the eddy latent heat streamfunction to the Eulerian mean plus eddy sensible heat streamfunction increases the mass transport by a factor of 2-3 in the subtropics and midlatitudes. The eddy transport is found to dominate the transport across the subtropical boundary. During Northern Hemisphere summer there is virtually no circulation in the absence of eddy latent heat transport. The results highlight the important role of latent heat transport by subtropical anticyclones and tropical and baroclinic waves in the general circulation. © 2012 American Meteorological Society.

Kang S.M.,Columbia University | Polvani L.M.,University of Applied and Environmental Sciences
Journal of Climate | Year: 2011

Astrong correlation between the latitudes of the eddy-driven jet and of the Hadley cell edge, on interannual time scales, is found to exist during austral summer, in both the NCEP-NCAR reanalysis and the models participating in the Coupled Model Intercomparison Project, phase 3 (CMIP3). In addition, a universal ratio close to 1:2 characterizes the robust connection between these two latitudes on a year-to-year basis: for a 2° shift of the eddy-driven jet, the edge of the Hadley cell shifts by 1°. This 1:2 interannual ratio remains the same in response to climate change, even though the values of the two latitudes increase. The corresponding trends are also highly correlated; in the CMIP3 scenario integrations, however, no universal ratio appears to exist connecting these long-term trends. In austral winter and in the Northern Hemisphere, no strong interannual correlations are found. © 2011 American Meteorological Society.

Wang S.,Columbia University | Gerber E.P.,Courant Institute of Mathematical Sciences | Polvani L.M.,University of Applied and Environmental Sciences
Journal of Climate | Year: 2012

The circulation response of the atmosphere to climate change-like thermal forcing is explored with a relatively simple, stratosphere-resolving general circulation model. The model is forced with highly idealized physics, but integrates the primitive equations at resolution comparable to comprehensive climate models. An imposed forcing mimics the warming induced by greenhouse gasses in the low-latitude upper troposphere. The forcing amplitude is progressively increased over a range comparable in magnitude to the warming projected by Intergovernmental Panel on Climate Change coupled climate model scenarios. For weak to moderate warming, the circulation response is remarkably similar to that found in comprehensive models: the Hadley cell widens and weakens, the tropospheric midlatitude jets shift poleward, and the Brewer-Dobson circulation (BDC) increases. However, when the warming of the tropical upper troposphere exceeds a critical threshold,;5K, an abrupt change of the atmospheric circulation is observed. In the troposphere the extratropical eddy-driven jet jumps poleward nearly 108. In the stratosphere the polar vortex intensifies and the BDC weakens as the intraseasonal coupling between the troposphere and the stratosphere shuts down. The key result of this study is that an abrupt climate transition can be effected by changes in atmospheric dynamics alone, without need for the strong nonlinearities typically associated with physical parameterizations. It is verified that the abrupt climate shift reported here is not an artifact of the model's resolution or numerics. © 2012 American Meteorological Society.

Tuakuila J.,University of Applied and Environmental Sciences
International Journal of Hygiene and Environmental Health | Year: 2013

Background and objectives: Data on human exposure to chemicals in Africa are scarce. A biomonitoring study was conducted in a representative sample of the population in Kinshasa (Democratic Republic of Congo) to document exposure to benzene. Methods: S-phenylmercapturic acid (S-PMA) was measured by LC-MS/MS in spot urine samples from 220 individuals (50.5% women), aged 6-70 years living in the urban area and from 50 additional subjects from the sub-rural area of Kinshasa. Data were compiled as arithmetic means, geometric means, percentile 95th and range expressed in μg/L. Results: Overall, living in urban Kinshasa was associated with increased levels of S-PMA in urine as compared to a population living in the sub-rural area. Increased levels were also found by comparison with some date from literature. Conclusions: This study reveals the high benzene exposure of the Kinshasa population requiring the determination of benzene concentrations in ambient air of Kinshasa and limit values for the protection of human health. © 2013 Elsevier GmbH.

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