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

A Regime Perspective on the North Atlantic Eddy-Driven Jet Response to Sudden Stratospheric Warmings

Changes to the preferred states, or regime behavior, of the North Atlantic eddy-driven jet (EDJ) following a major sudden stratospheric warming (SSW) is examined using a large ensemble experiment from the Canadian Middle Atmosphere Model in which the stratosphere is nudged toward an SSW. In the 3 months following the SSW (January–March), the North Atlantic EDJ shifts equatorward by ~3°, on average; this arises from an increased occurrence of the EDJ’s south regime and reductions in its north and central regimes. Qualitatively similar behavior is shown in a reanalysis dataset. We show that under SSW conditions the south regime becomes more persistent and that this can explain the overall increase in the EDJ latitude decorrelation time scale. A cluster analysis reveals that, following the SSW, the south EDJ regime is characterized by weaker low-level baroclinicity and eddy heat fluxes in the North Atlantic Ocean. We hypothesize, therefore, that the increased persistence of the south regime is related to the weaker baroclinicity leading to slower growth rates of the unstable modes and hence a slower buildup of eddy heat flux, which has been shown to precede EDJ transitions. In the North Atlantic sector, the surface response to the SSW projects onto a negative North Atlantic Oscillation (NAO) pattern, with almost no change in the east Atlantic (EA) pattern. This behavior appears to be distinct from the modeled intrinsic variability in the EDJ, where the jet latitude index captures variations in both the NAO and EA patterns. The results offer new insight into the mechanisms for stratosphere–troposphere coupling following SSWs

The Topology of Parabolic Character Varieties of Free Groups

Let G be a complex affine algebraic reductive group, and let K be a maximal compact subgroup of G. Fix elements h_1,...,h_m in K. For n greater than or equal to 0, let X (respectively, Y) be the space of equivalence classes of representations of the free group of m+n generators in G (respectively, K) such that for each i between 1 and m, the image of the i-th free generator is conjugate to h_i. These spaces are parabolic analogues of character varieties of free groups. We prove that Y is a strong deformation retraction of X. In particular, X and Y are homotopy equivalent. We also describe explicit examples relating X to relative character varieties.Comment: 16 pages, version 2 includes minor revisions and some modified proofs, accepted for publication in Geometriae Dedicat

Living with and without an intestinal stoma: Factors that promote psychological well-being and self-care: A cross sectional study.:Psychological well-being and stoma

Abstract Aims This study compared those living with and without an intestinal stoma in relation to physical and psychological health, stress and coping, quality of life and resilience. Also, identifying factors that could be used to promote better self‐care in stoma patients in the future. Design A cross‐sectional and comparative study design was employed. Methods Participants were recruited via email and social media (Facebook and Twitter) between August 2018 and March 2019, to complete an online survey. The data were analysed using analysis of variance to examine group difference and a series of hierarchical linear regression analyses determining predictors of psychological well‐being. Results Of 278 participants aged 18–68 years who completed the survey, 129 (46%) had a stoma and reported significantly poorer physical health. Approximately one‐fifth experienced problems with stoma management. Psychological well‐being was mediated by the duration of living with a stoma (under 3 years) and frequency of leaks (weekly and monthly)

Canonical Causal Diagrams to Guide the Treatment of Missing Data in Epidemiological Studies

With incomplete data, the missing at random (MAR) assumption is widely understood to enable unbiased estimation with appropriate methods. The need to assess the plausibility of MAR and to perform sensitivity analyses considering missing not at random (MNAR) scenarios have been emphasized, but the practical difficulty of these tasks is rarely acknowledged. What MAR means with multivariable missingness is difficult to grasp, while in many MNAR scenarios unbiased estimation is possible using methods commonly associated with MAR. Directed acyclic graphs (DAGs) have been proposed as an alternative framework for specifying practically accessible assumptions beyond the MAR-MNAR dichotomy. However, there is currently no general algorithm for deciding how to handle the missing data given a specific DAG. We construct "canonical" DAGs capturing typical missingness mechanisms in epidemiological studies with incomplete exposure, outcome and confounders. For each DAG, we determine whether common target parameters are "recoverable", meaning that they can be expressed as functions of the observed data distribution and thus estimated consistently, or if sensitivity analyses are necessary. We investigate the performance of available case and multiple imputation procedures. Using the Longitudinal Study of Australian Children, we illustrate how our findings can guide the treatment of missing data in point-exposure studies

An Evaluation of the Large‐Scale Atmospheric Circulation and Its Variability in CESM2 and Other CMIP Models

The Community Earth System Model 2 (CESM2) is the latest Earth System Model developed by the National Center for Atmospheric Research in collaboration with the university community and is significantly advanced in most components compared to its predecessor (CESM1). Here, CESM2's representation of the large‐scale atmospheric circulation and its variability is assessed. Further context is providedthrough comparison to the CESM1 large ensemble and other models from the Coupled Model Intercomparison Project (CMIP5 and CMIP6). This includes an assessment of the representation of jet streams and storm tracks, stationary waves, the global divergent circulation, the annular modes, the North Atlantic Oscillation, and blocking. Compared to CESM1, CESM2 is substantially improved in the representation of the storm tracks, Northern Hemisphere (NH) stationary waves, NH winter blocking and the global divergent circulation. It ranks within the top 10% of CMIP class models in many of these features. Some features of the Southern Hemisphere (SH) circulation have degraded, such as the SH jet strength, stationary waves, and blocking, although the SH jet stream is placed at approximately the correct location. This analysis also highlights systematic deficiencies in these features across the new CMIP6 archive, such as the continued tendency for the SH jet stream to be placed too far equatorward, the North Atlantic westerlies to be too strong over Europe, the storm tracks as measured by low‐level meridional wind variance to be too weak and a lack of blocking in the North Atlantic sector

The Whole Atmosphere Community Climate Model Version 6 (WACCM6)

The Whole Atmosphere Community Climate Model version 6 (WACCM6) is a major update of the whole atmosphere modeling capability in the Community Earth System Model (CESM), featuring enhanced physical, chemical and aerosol parameterizations. This work describes WACCM6 and some of the important features of the model. WACCM6 can reproduce many modes of variability and trends in the middle atmosphere, including the Quasi‐Biennial Oscillation, Stratospheric Sudden Warmings and the evolution of Southern Hemisphere springtime ozone depletion over the 20th century. WACCM6 can also reproduce the climate and temperature trends of the 20th century throughout the atmospheric column. The representation of the climate has improved in WACCM6, relative to WACCM4. In addition, there are improvements in high latitude climate variability at the surface and sea ice extent in WACCM6 over the lower top version of the model (CAM6) that come from the extended vertical domain and expanded aerosol chemistry in WACCM6, highlighting the importance of the stratosphere and tropospheric chemistry for high latitude climate variability

Revisiting the Relationship among Metrics of Tropical Expansion

There is mounting evidence that the width of the tropics has increased over the last few decades, but there are large differences in reported expansion rates. This is, likely, in part due to the wide variety of metrics that have been used to define the tropical width. Here we perform a systematic investigation into the relationship among nine metrics of the zonal-mean tropical width using preindustrial control and abrupt quadrupling of CO2 simulations from a suite of coupled climate models. It is shown that the latitudes of the edge of the Hadley cell, the midlatitude eddy-driven jet, the edge of the subtropical dry zones, and the Southern Hemisphere subtropical high covary interannually and exhibit similar long-term responses to a quadrupling of CO2. However, metrics based on the outgoing longwave radiation, the position of the subtropical jet, the break in the tropopause, and the Northern Hemisphere subtropical high have very weak covariations with the above metrics and/or respond differently to increases in CO2 and thus are not good indicators of the expansion of the Hadley cell or subtropical dry zone. The differing variability and responses to increases in CO2 among metrics highlights that care is needed when choosing metrics for studies of the width of the tropics and that it is important to make sure the metric used is appropriate for the specific phenomena and impacts being examined