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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.

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.

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.

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.

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.

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