Interannual variability in the gravity wave drag – vertical coupling and possible climate links

Gravity wave drag (GWD) is an important driver of the middle atmospheric dynamics. However, there are almost no observational constraints on its strength and distribution (especially horizontal). In this study we analyze orographic GWD (OGWD) output from Canadian Middle Atmosphere Model simulation with specified dynamics (CMAM-sd) to illustrate the interannual variability in the OGWD distribution at particular pressure levels in the stratosphere and its relation to major climate oscillations. We have found significant changes in the OGWD distribution and strength depending on the phase of the North Atlantic Oscillation (NAO), quasi-biennial oscillation (QBO) and El Niño–Southern Oscillation. The OGWD variability is shown to be induced by lower-tropospheric wind variations to a large extent, and there is also significant variability detected in near-surface momentum fluxes. We argue that the orographic gravity waves (OGWs) and gravity waves (GWs) in general can be a quick mediator of the tropospheric variability into the stratosphere as the modifications of the OGWD distribution can result in different impacts on the stratospheric dynamics during different phases of the studied climate oscillations.Grantová Agentura České Republiky | Ref. 16-01562JSecretaría de Estado de Investigación, Desarrollo e Innovación | Ref. CGL2015-71575-PGrantová Agentura České Republiky | Ref. 18-01625

Effect of latitudinally displaced gravity wave forcing in the lower stratosphere on the polar vortex stability

In order to investigate the impact of a locally confined gravity wave (GW) hotspot, a sensitivity study based on simulations of the middle atmosphere circulation during northern winter was performed with a nonlinear, mechanistic, general circulation model. To this end, we selected a fixed longitude range in the East Asian region (120–170∘ E) and a latitude range from 22.5 to 52.5∘ N between 18 and 30 km for the hotspot region, which was then shifted northward in steps of 5∘. For the southernmost hotspots, we observe a decreased stationary planetary wave (SPW) with wave number 1 (SPW 1) activity in the upper stratosphere and lower mesosphere, i.e., fewer SPWs 1 are propagating upwards. These GW hotspots lead to a negative refractive index, inhibiting SPW propagation at midlatitudes. The decreased SPW 1 activity is connected to an increased zonal mean zonal wind at lower latitudes. This, in turn, decreases the meridional potential vorticity gradient (qy) from midlatitudes towards the polar region. A reversed qy indicates local baroclinic instability, which generates SPWs with wave number 1 in the polar region, where we observe a strong positive Eliassen–Palm (EP) divergence. As a result, the EP flux increases towards the polar stratosphere (corresponding to enhanced SPW 1 amplitudes), where the SPWs with wave number 1 break, and the zonal mean zonal wind decreases. Thus, the local GW forcing leads to a displacement of the polar vortex towards lower latitudes. The effect of the local baroclinic instability indicated by the reversed qy also produces SPWs with wave number 1 in the lower mesosphere. The effect on the dynamics in the middle atmosphere due to GW hotspots that are located northward of 50∘ N is negligible, as the refractive index of the atmosphere is strongly negative in the polar region. Thus, any changes in the SPW activity due to the local GW forcing are quite ineffectiveDeutsche Forschungsgemeinschaft | Ref. JA836/32-1Xunta de Galicia | Ref. ED481B 2018/103Ministerio de Ciencia e Innovación | Ref. CGL2015-71575-PCzech Science Foundation (GAČR) | Ref. 16-01562JCzech Science Foundation (GAČR) | Ref. 18-01625

Revisiting internal gravity waves analysis using GPS RO density profiles: comparison with temperature profiles and application for wave field stability study

We revise selected findings regarding the utilization of Global Positioning System radio occultation (GPS RO) density profiles for the analysis of internal gravity waves (IGW), introduced by Sacha et al. (2014). Using various GPS RO datasets, we show that the differences in the IGW spectra between the dry-temperature and dry-density profiles that were described in the previous study as a general issue are in fact present in one specific data version only. The differences between perturbations in the temperature and density GPS RO profiles do not have any physical origin, and there is not the information loss of IGW activity that was suggested in Sacha et al. (2014). We investigate the previously discussed question of the temperature perturbations character when utilizing GPS RO dry-temperature profiles, derived by integration of the hydrostatic balance. Using radiosonde profiles as a proxy for GPS RO, we provide strong evidence that the differences in IGW perturbations between the real and retrieved temperature profiles (which are based on the assumption of hydrostatic balance) include a significant nonhydrostatic component that is present sporadically and might be either positive or negative. The detected differences in related spectra of IGW temperature perturbations are found to be mostly about ±10 %. The paper also presents a detailed study on the utilization of GPS RO density profiles for the characterization of the wave field stability. We have analyzed selected stability parameters derived from the density profiles together with a study of the vertical rotation of the wind direction. Regarding the Northern Hemisphere the results point to the western border of the Aleutian high, where potential IGW breaking is detected. These findings are also supported by an analysis of temperature and wind velocity profiles. Our results confirm advantages of the utilization of the density profiles for IGW analysis.Grantová Agentura České Republiky | Ref. 16-01562JMinisterstvo Školství, Mládeže a Tělovýchovy | Ref. 7AMB16AT021OeAD-GmbH | Ref. CZ 06/2016Ministerio de Ciencia e Innovación | Ref. CGL2015-71575-

The Climatology of Elevated Stratopause Events in the UA-ICON Model and the Contribution of Gravity Waves

The climatologies of the stratopause height and temperature in the UA-ICON model are examined by comparing them to 17-years (2005–2021) of Microwave Limb Sounder (MLS) observations. In addition, the elevated stratopause (ES) event occurrence, their main characteristics, and driving mechanisms in the UA-ICON model are examined using three 30-year time-slice experiments. While UA-ICON reasonably simulates the large-scale stratopause properties similar to MLS observations, at polar latitudes in the Southern Hemisphere the stratopause is ∼8 K warmer and ∼3 km higher than observed. A time lag of about two months also exists in the occurrence of the tropical semiannual oscillation of the stratopause compared to the observations. ES events occur in ∼20% of the boreal winters, after major sudden stratospheric warmings (SSWs). Compared to the SSWs not followed by ES events (SSW-only), the ES events are associated with the persistent tropospheric forcing and prolonged anomalies of the stratospheric jet. Our modeling results suggest that the contributions of both gravity waves (GW)s and resolved waves are important in explaining the enhanced residual circulation following ES events compared to the SSW-only events but their contributions vary through the lifetime of ES events. We emphasize the role of the resolved wave drag in the ES formation as in the sensitivity test when the non-orographic GW drag is absent, the anomalously enhanced resolved wave forcing in the mesosphere gives rise to the formation of the elevated stratopause at about 85 km

Effects of particle size and surface chemistry on the dispersion of graphite nanoplates in polypropylene composites

Carbon nanoparticles tend to form agglomerates with considerable cohesive strength, depending on particle morphology and chemistry, thus presenting different dispersion challenges. The present work studies the dispersion of three types of graphite nanoplates (GnP) with different flake sizes and bulk densities in a polypropylene melt, using a prototype extensional mixer under comparable hydrodynamic stresses. The nanoparticles were also chemically functionalized by covalent bonding polymer molecules to their surface, and the dispersion of the functionalized GnP was studied. The effects of stress relaxation on dispersion were also analyzed. Samples were removed along the mixer length, and characterized by microscopy and dielectric spectroscopy. A lower dispersion rate was observed for GnP with larger surface area and higher bulk density. Significant re-agglomeration was observed for all materials when the deformation rate was reduced. The polypropylene-functionalized GnP, characterized by increased compatibility with the polymer matrix, showed similar dispersion effects, albeit presenting slightly higher dispersion levels. All the composites exhibit dielectric behavior, however, the alternate current (AC) conductivity is systematically higher for the composites with larger flake GnP.This work was funded by National Funds through FCT-Portuguese Foundation for Science and Technology, Reference UID/CTM/50025/2013 and FEDER funds through the COMPETE 2020 Programme under the project number POCI-01-0145-FEDER-007688. We acknowledge Mrs. Uta Reuter from Leibniz-Institut für Polymerforschung Dresden e.V. for TEM specimen preparation and image acquisition for the composites.info:eu-repo/semantics/publishedVersio

Extratropical age of air trends and causative factors in climate projection simulations

Climate model simulations show an acceleration of the Brewer–Dobson circulation (BDC) in response to climate change. While the general mechanisms for the BDC strengthening are widely understood, there are still open questions concerning the influence of the details of the wave driving. Mean age of stratospheric air (AoA) is a useful transport diagnostic for assessing changes in the BDC. Analyzing AoA from a subset of Chemistry–Climate Model Initiative part 1 climate projection simulations, we find a remarkable agreement between most of the models in simulating the largest negative AoA trends in the extratropical lower to middle stratosphere of both hemispheres (approximately between 20 and 25 geopotential kilometers (gpkm) and 20–50∘ N and S). We show that the occurrence of AoA trend minima in those regions is directly related to the climatological AoA distribution, which is sensitive to an upward shift of the circulation in response to climate change. Also other factors like a reduction of aging by mixing (AbM) and residual circulation transit times (RCTTs) contribute to the AoA distribution changes by widening the AoA isolines. Furthermore, we analyze the time evolution of AbM and RCTT trends in the extratropics and examine the connection to possible drivers focusing on local residual circulation strength, net tropical upwelling and wave driving. However, after the correction for a vertical shift of pressure levels, we find only seasonally significant trends of residual circulation strength and zonal mean wave forcing (resolved and unresolved) without a clear relation between the trends of the analyzed quantities. This indicates that additional causative factors may influence the AoA, RCTT and AbM trends. In this study, we postulate that the shrinkage of the stratosphere has the potential to influence the RCTT and AbM trends and thereby cause additional AoA changes over time.Czech Science Foundation (GACˇ R) | Ref. 16- 01562JCzech Science Foundation (GACˇ R) | Ref. 18-01625SMinisterio de Ciencia e Innovación | Ref. CGL2015-71575-PXunta de Galicia | Ref. ED481B 2018/103Ministerio de Economía y Competitividad | Ref. RYC-2013-1456

Well supplied in life, set aside in death: A multi‐isotope study of Justinian plague victims from Saint‐Doulchard (France, 7th–8th centuries AD )

Objectives: Justinian plague and its subsequent outbreaks were major events influencing Early Medieval Europe. One of the affected communities was the population of Saint‐Doulchard in France, where plague victim burials were concentrated in a cemetery enclosure ditch. This study aimed to obtain more information about their life‐histories using the tools of isotope analysis. Materials and Methods: Dietary analysis using carbon and nitrogen isotopes was conducted on 97 individuals buried at Le Pressoir in Saint‐Doulchard, with 36 of those originating from the enclosure ditch. This sample set includes all individuals analyzed for plague DNA in a previous study. Mobility analysis using strontium isotope analysis supplements the dietary study, with 47 analyzed humans. The results are supported by a reference sample set of 31 animal specimens for dietary analysis and 9 for mobility analysis. Results: The dietary analysis results showed significantly different dietary behavior in individuals from the ditch burials, with better access to higher quality foods richer in animal protein. 87Sr/86Sr ratios are similar for both studied groups and indicate a shared or similar area of origin. Discussion: The results suggest that the ditch burials contain an urban population from the nearby city of Bourges, which overall had a better diet than the rural population from Saint‐Doulchard. It is implied that city's population might have been subjected to high mortality rates during the plague outbreak(s), which led to their interment in nearby rural cemeteries

Stratospheric contraction caused by increasing greenhouse gases

Rising emissions of anthropogenic greenhouse gases (GHG) have led to tropospheric warming and stratospheric cooling over recent decades. As a thermodynamic consequence, the troposphere has expanded and the rise of the tropopause, the boundary between the troposphere and stratosphere, has been suggested as one of the most robust fingerprints of anthropogenic climate change. Conversely, at altitudes above ∼55 km (in the mesosphere and thermosphere) observational and modeling evidence indicates a downward shift of the height of pressure levels or decreasing density at fixed altitudes. The layer in between, the stratosphere, has not been studied extensively with respect to changes of its global structure. Here we show that this atmospheric layer has contracted substantially over the last decades, and that the main driver for this are increasing concentrations of GHG. Using data from coupled chemistry-climate models we show that this trend will continue and the mean climatological thickness of the stratosphere will decrease by 1.3 km following representative concentration pathway 6.0 by 2080. We also demonstrate that the stratospheric contraction is not only a response to cooling, as changes in both tropopause and stratopause pressure contribute. Moreover, its short emergence time (less than 15 years) makes it a novel and independent indicator of GHG induced climate change

Observed and Modeled Mountain Waves from the Surface to the Mesosphere Near the Drake Passage

Four state-of-the-science numerical weather prediction (NWP) models were used to perform mountain wave- (MW) resolving hind-casts over the Drake Passage of a 10-day period in 2010 with numerous observed MW cases. The Integrated Forecast System (IFS) and the Icosahedral Nonhydrostatic (ICON) model were run at Δx ≈ 9 and 13 km globally. TheWeather Research and Forecasting (WRF) model and the Met Office Unified Model (UM) were both configured with a Δx = 3 km regional domain. All domains had tops near 1 Pa (z ≈ 80 km). These deep domains allowed quantitative validation against Atmospheric InfraRed Sounder (AIRS) observations, accounting for observation time, viewing geometry, and radiative transfer. All models reproduced observed middle-atmosphere MWs with remarkable skill. Increased horizontal resolution improved validations. Still, all models underrepresented observed MW amplitudes, even after accounting for model effective resolution and instrument noise, suggesting even at Δx ≈ 3 km resolution, small-scale MWs are under-resolved and/or over-diffused. MWdrag parameterizations are still necessary in NWP models at current operational resolutions of Δx ≈ 10 km. Upper GW sponge layers in the operationally configured models significantly, artificially reduced MW amplitudes in the upper stratosphere and mesosphere. In the IFS, parameterized GW drags partly compensated this deficiency, but still, total drags were ≈ 6 time smaller than that resolved at Δx ≈ 3 km. Meridionally propagating MWs significantly enhance zonal drag over the Drake Passage. Interestingly, drag associated with meridional fluxes of zonal momentum (i.e. u'v') were important; not accounting for these terms results in a drag in the wrong direction at and below the polar night jet