The downward influence of stratospheric sudden warmings

Abstract The coupling between the stratosphere and the troposphere following two major stratospheric sudden warmings is studied in the Canadian Middle Atmosphere Model using a nudging technique by which the zonal-mean evolution of the reference sudden warmings are artificially induced in an ~100-member ensemble spun off from a control simulation. Both reference warmings are taken from a freely running integration of the model. One event is a displacement, the other is a split, and both are followed by extended recoveries in the lower stratosphere. The methodology permits a statistically robust study of their influence on the troposphere below. The nudged ensembles exhibit a tropospheric annular mode response closely analogous to that seen in observations, confirming the downward influence of sudden warmings on the troposphere in a comprehensive model. This tropospheric response coincides more closely with the lower-stratospheric annular mode anomalies than with the midstratospheric wind reversal. In addition to the expected synoptic-scale eddy feedback, the planetary-scale eddies also reinforce the tropospheric wind changes, apparently responding directly to the stratospheric anomalies. Furthermore, despite the zonal symmetry of the stratospheric perturbation, a highly zonally asymmetric near-surface response is produced, corresponding to a strongly negative phase of the North Atlantic Oscillation with a much weaker response over the Pacific basin that matches composites of sudden warmings from the Interim ECMWF Re-Analysis (ERA-Interim). Phase 5 of the Coupled Model Intercomparison Project models exhibit a similar response, though in most models the response’s magnitude is underrepresented.PH acknowledges support from ERC project no 267760 - ACCI and an NSERC Postdoctoral Fellowship. IRS was supported by a Lamont-Doherty Earth Observatory postdoctoral fellowship and NSF award AGS-1317469. TThis is the author accepted manuscript. The final version is available from AMS at http://journals.ametsoc.org/doi/abs/10.1175/JAS-D-14-0012.1

Zonally symmetric adjustment in the presence of artificial relaxation

Abstract Numerical experiments, presented in a companion paper, have been performed in which the zonal-mean state of the stratosphere in a comprehensive, stratosphere-resolving, general circulation model is strongly relaxed (or “nudged”) toward the evolution of a reference sudden warming event in order to investigate its influence on the freely evolving troposphere below. Similar approaches have been used in a number of other studies. This raises the question of whether such an artificial relaxation induces the adiabatic and diabatic adjustments expected below the region of nudging, even in the absence of the stratospheric wave driving responsible for the reference event. Motivated by this question, the zonally symmetric quasigeostrophic diabatic response to zonal forces (representing wave driving) in a system nudged to a time-dependent reference state is studied. In the presence of wave driving in the nudging region that differs from the reference state, the meridional mass circulation of the reference state is reproduced only in the region below the nudging up to a correction that is inversely proportional to the strength of the nudging. The anomalous circulation is confined because of an effective boundary condition at the interface of the nudging layer. The nudging also produces an artificial “sponge-layer feedback” immediately below the region of the nudging in response to differences in the tropospheric wave driving. The strength of this artificial feedback is closely related to the strength of the effective boundary condition; however, the time scale required for the sponge-layer feedback to be established is typically much longer than that required for the confinement.This is the final version. It first appeared at http://journals.ametsoc.org/doi/abs/10.1175/JAS-D-14-0013.1

The response of the lower stratosphere to zonally symmetric thermal and mechanical forcing

Abstract The response of the atmosphere to zonally symmetric applied heating and mechanical forcing is considered, allowing for the fact that the response may include a change in the wave force (or “wave drag”). A scaling argument shows that an applied zonally symmetric heating is effective in driving a steady meridional circulation provided that the wave force (required to satisfy angular momentum constraints) is sufficiently sensitive to changes in the mean flow in the sense that the ratio is large, where K is a measure of the sensitivity of the wave force; α, N, and f are the radiative damping rate, buoyancy frequency, and Coriolis parameter, respectively; and and are the horizontal and vertical length scales of the heating, respectively. Furthermore, in the “narrow heating” regime where this ratio is large, the structure of the meridional circulation response is only weakly dependent on the details of the wave force. The scaling arguments are verified by experiments in a dry dynamical circulation model. Consistent with the scaling prediction, the regime does not apply when the width of the imposed heating is increased. The narrow-heating regime is demonstrated to be relevant to the double peak in tropical lower-stratospheric upwelling considered in a companion paper, supporting the hypothesis that this feature is radiatively driven. Similar arguments are applied to show that a narrow zonally symmetric applied mechanical forcing is primarily balanced by a change in wave force. This provides an explanation for the recently identified compensation between resolved and parameterized waves in driving modeled trends in the Brewer–Dobson circulation.The authors thank Amanda Maycock for help with the radiation code and for helpful discussions. AM and PHi acknowledge funding support from the European Research Council through the ACCI project (grant number 267760) lead by John Pyle. PHi also acknowledges support from an NSERC postdoctoral fellowship. The authors are grateful to Stephan Fueglistaler and Tom Flannaghan for conversations that stimulated some of this work. We also thank Robin Hogan, Alessio Bozzo and Irina Sandu from ECMWF for their help with reproducing the longwave heating rates as well as Ulrike Langematz and Markus Kunze for the ozone climatology. We received detailed and helpful comments from three anonymous reviewers that improved this manuscript.This is the final version of the article. It first appeared from the American Meteorological Society via http://dx.doi.org/10.1175/JAS-D-15-0294.

The double peak in upwelling and heating in the tropical lower stratosphere

Abstract The processes responsible for double-peak latitudinal structures in the time-averaged tropical lower-stratospheric upwelling, centered near 70 hPa and 20°N/S, previously noted in ERA-Interim and other reanalysis and model datasets, are considered. It is demonstrated that the structure of the wave force resolved by ERA-Interim consistently balances the angular momentum transport associated with the double peak. Analysis of the corresponding structures in diabatic heating rates from ERA-Interim indicates that the peaks arise predominantly from the meridional structure in ozone concentrations and the associated absorption of both shortwave and longwave radiation. Additional smaller contributions arise from local absorption of longwave radiation emitted from the relatively warm layers above and below, as well as from cloud-related radiative effects and nonradiative diabatic heating. The temperature at 70 hPa is slightly higher near 20°N/S than at the equator, opposite of what would be expected if the latitudinal structure in radiative heating were associated with local relaxation. It is proposed on the basis of this analysis that the primary cause of the peaks in upwelling is the externally imposed (i.e., nonrelaxational) part of the radiative heating field. The dynamical plausibility of this hypothesis is investigated in a companion paper.AM and PHi acknowledge funding support from the European Research Council through the ACCI project (grant number 267760) lead by John Pyle. PHi also acknowledges support from an NSERC postdoctoral fellowship.This is the final version of the article. It first appeared from the American Meteorological Society via http://dx.doi.org/10.1175/JAS-D-15-0293.

Stratospheric control of planetary waves

The effects of imposing at various altitudes in the stratosphere zonally symmetric circulation anomalies associated with a stratospheric sudden warming are investigated in a mechanistic circulation model. A shift of the tropospheric jet is found even when the anomalies are imposed only above 2�hPa. Their influence is communicated downward through the planetary wave field via three distinct mechanisms. First, a significant fraction of the amplification of the upward fluxes of wave activity prior to the central date of the warming is due to the coupled evolution of the stratospheric zonal mean state and the wave field throughout the column. Second, a downward propagating region of localized wave, mean-flow interaction is active around the central date but does not penetrate the tropopause. Third, there is deep, vertically synchronous suppression of upward fluxes following the central date. The magnitude of this suppression correlates with that of the tropospheric jet shift.European Research Council ACCI grant project 267760

Dual Geometric Worm Algorithm for Two-Dimensional Discrete Classical Lattice Models

We present a dual geometrical worm algorithm for two-dimensional Ising models. The existence of such dual algorithms was first pointed out by Prokof'ev and Svistunov \cite{ProkofevClassical}. The algorithm is defined on the dual lattice and is formulated in terms of bond-variables and can therefore be generalized to other two-dimensional models that can be formulated in terms of bond-variables. We also discuss two related algorithms formulated on the direct lattice, applicable in any dimension. These latter algorithms turn out to be less efficient but of considerable intrinsic interest. We show how such algorithms quite generally can be "directed" by minimizing the probability for the worms to erase themselves. Explicit proofs of detailed balance are given for all the algorithms. In terms of computational efficiency the dual geometrical worm algorithm is comparable to well known cluster algorithms such as the Swendsen-Wang and Wolff algorithms, however, it is quite different in structure and allows for a very simple and efficient implementation. The dual algorithm also allows for a very elegant way of calculating the domain wall free energy.Comment: 12 pages, 6 figures, Revtex

Automatic speech recognition research at NASA-Ames Research Center

A trainable acoustic pattern recognizer manufactured by Scope Electronics is presented. The voice command system VCS encodes speech by sampling 16 bandpass filters with center frequencies in the range from 200 to 5000 Hz. Variations in speaking rate are compensated for by a compression algorithm that subdivides each utterance into eight subintervals in such a way that the amount of spectral change within each subinterval is the same. The recorded filter values within each subinterval are then reduced to a 15-bit representation, giving a 120-bit encoding for each utterance. The VCS incorporates a simple recognition algorithm that utilizes five training samples of each word in a vocabulary of up to 24 words. The recognition rate of approximately 85 percent correct for untrained speakers and 94 percent correct for trained speakers was not considered adequate for flight systems use. Therefore, the built-in recognition algorithm was disabled, and the VCS was modified to transmit 120-bit encodings to an external computer for recognition

Phase Coherence and Superfluid-Insulator Transition in a Disordered Bose-Einstein Condensate

We have studied the effects of a disordered optical potential on the transport and phase coherence of a Bose-Einstein condensate (BEC) of 7Li atoms. At moderate disorder strengths (V_D), we observe inhibited transport and damping of dipole excitations, while in time-of-flight images, random but reproducible interference patterns are observed. In-situ images reveal that the appearance of interference is correlated with density modulation, without complete fragmentation. At higher V_D, the interference contrast diminishes as the BEC fragments into multiple pieces with little phase coherence.Comment: 4 pages, 5 figures, distortions in figures 1 and 4 have been fixed in version 3. This paper has been accepted to PR

Past and future conditions for polar stratospheric cloud formation simulated by the Canadian Middle Atmosphere Model

We analyze here the polar stratospheric temperatures in an ensemble of three 150-year integrations of the Canadian Middle Atmosphere Model (CMAM), an interactive chemistry-climate model which simulates ozone depletion and recovery, as well as climate change. A key motivation is to understand possible mechanisms for the observed trend in the extent of conditions favourable for polar stratospheric cloud (PSC) formation in the Arctic winter lower stratosphere. We find that in the Antarctic winter lower stratosphere, the low temperature extremes required for PSC formation increase in the model as ozone is depleted, but remain steady through the twenty-first century as the warming from ozone recovery roughly balances the cooling from climate change. Thus, ozone depletion itself plays a major role in the Antarctic trends in low temperature extremes. The model trend in low temperature extremes in the Arctic through the latter half of the twentieth century is weaker and less statistically robust than the observed trend. It is not projected to continue into the future. Ozone depletion in the Arctic is weaker in the CMAM than in observations, which may account for the weak past trend in low temperature extremes. In the future, radiative cooling in the Arctic winter due to climate change is more than compensated by an increase in dynamically driven downwelling over the pole