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

2019 ◽  
Vol 19 (11) ◽  
pp. 7627-7647 ◽  
Author(s):  
Petr Šácha ◽  
Roland Eichinger ◽  
Hella Garny ◽  
Petr Pišoft ◽  
Simone Dietmüller ◽  
...  
Keyword(s):  
Climate Change ◽  
Climate Model ◽  
Climate Projection ◽  
Residual Circulation ◽  
Wave Forcing ◽  
Open Questions ◽  
Transit Times ◽  
Causative Factors ◽  
Climate Model Simulations ◽  
Changes Over Time

Abstract. 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.

scholarly journals Extratropical Age of Air trends and causative factors in climate projection simulations

2019 ◽  
Author(s):  
Petr Šácha ◽  
Roland Eichinger ◽  
Hella Garny ◽  
Petr Pišoft ◽  
Simone Dietmüller ◽  
...  
Keyword(s):  
Climate Change ◽  
Climate Model ◽  
Climate Projection ◽  
Residual Circulation ◽  
Wave Forcing ◽  
Open Questions ◽  
Transit Times ◽  
Causative Factors ◽  
Climate Model Simulations ◽  
Changes Over Time

Abstract. Climate model simulations show a Brewer-Dobson circulation (BDC) acceleration in the course of climate change. While the mechanisms for the BDC strengthening are well understood, there are still open questions concerning its dynamical driving. Mean age of stratospheric air (AoA) is a useful transport diagnostic for accessing changes of the BDC. Analysing 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 gpkm and 25 gpkm and 20°–50°N/S). We show that the occurrence of AoA trend minima in those regions is directly related to the climatological AoA distribution being sensitive to an upward shift of the circulation in response to a climate change. But 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 like 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. Namely, we postulate a possible influence of stratospheric shrinkage on RCTT, AbM and therefore also on AoA 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.

scholarly journals Hemispheric asymmetries in recent changes of the stratospheric circulation

2022 ◽  
Author(s):  
Felix Ploeger ◽  
Hella Garny
Keyword(s):  
Hemispheric Asymmetry ◽  
Climate Models ◽  
Climate Model ◽  
Transport Model ◽  
Internal Variability ◽  
Residual Circulation ◽  
Hemispheric Asymmetries ◽  
Model Simulations ◽  
Transit Times ◽  
Climate Model Simulations

Abstract. Despite the expected opposite effects of ozone recovery, the stratospheric Brewer-Dobson circulation (BDC) has been found to weaken in the Northern hemisphere (NH) relative to the Southern hemisphere (SH) in recent decades, inducing substantial effects on chemical composition. We investigate hemispheric asymmetries in BDC changes since about 2000 in simulations with the transport model CLaMS driven with different reanalyses (ERA5, ERA-Interim, JRA-55, MERRA-2) and contrast those to a suite of free-running climate model simulations. We find that age of air increases robustly in the NH stratosphere relative to the SH in all reanalyses considered. Related nitrous oxide changes agree well between reanalysis-driven simulations and satellite measurements, providing observational evidence for the hemispheric asymmetry in BDC changes. Residual circulation metrics further show that the composition changes are caused by structural BDC changes related to an upward shift and strengthening of the deep BDC branch, resulting in longer transit times, and a downward shift and weakening shallow branch in the NH relative to the SH. All reanalyses agree on this mechanism. Although climate model simulations show that ozone recovery will lead to overall reduced circulation and age of air trends, the hemispherically asymmetric signal in circulation trends is small compared to internal variability. Therefore, the observed circulation trends over the recent past are not in contradiction to expectations from climate models. Furthermore, the hemispheric asymmetry in BDC trends imprints on the composition of the lower stratosphere and the signal might propagate into the troposphere, potentially affecting composition down to the surface.

scholarly journals Residual circulation trajectories and transit times into the extratropical lowermost stratosphere

2010 ◽  
Vol 10 (7) ◽  
pp. 16837-16860 ◽  
Author(s):  
T. Birner ◽  
H. Bönisch
Keyword(s):  
Time Scale ◽  
Climate Model ◽  
Global Scale ◽  
Reanalysis Data ◽  
Residual Circulation ◽  
Deep Circulation ◽  
Deep Branch ◽  
Tropical Tropopause ◽  
Transit Times ◽  
Part Time

Abstract. Transport into the extratropical lowermost stratosphere (LMS) can be divided into a slow part (time-scale of several months to years) associated with the global-scale stratospheric residual circulation and a fast part (time-scale of days to a few months) associated with (mostly quasi-horizontal) mixing (i.e. two-way irreversible transport, including stratosphere-troposphere exchange). The stratospheric residual circulation can be considered to consist of two branches: a deep branch more strongly associated with planetary waves breaking in the middle to upper stratosphere, and a shallow branch more strongly associated with synoptic-scale waves breaking in the subtropical lower stratosphere. In this study the contribution due to the stratospheric residual circulation alone to transport into the LMS is quantified using residual circulation trajectories, i.e. trajectories driven by the (time-dependent) residual mean meridional and vertical velocities. This contribution represents the advective part of the overall transport into the LMS and can be viewed as providing a background onto which the effect of mixing has to be added. Residual mean velocities are obtained from a comprehensive chemistry-climate model as well as from reanalysis data. Transit times of air traveling from the tropical tropopause to the LMS along the residual circulation streamfunction are evaluated and compared to recent mean age of air estimates. A clear time-scale separation with much smaller transit times into the mid-latitudinal LMS than into polar LMS is found that is indicative of a clear separation of the shallow from the deep branch of the residual circulation. This separation between the shallow and the deep circulation branch is further manifested in a clear distinction in the aspect ratio of the vertical to meridional extent of the trajectories as well as the integrated mass flux along the residual circulation trajectories. The residual transit time distribution reproduces qualitatively the observed seasonal cycle of youngest air in the extratropical LMS in fall and oldest air in spring.

scholarly journals How robust are stratospheric age of air trends from different reanalyses?

2019 ◽  
Author(s):  
Felix Ploeger ◽  
Bernard Legras ◽  
Edward Charlesworth ◽  
Xiaolu Yan ◽  
Mohamadou Diallo ◽  
...  
Keyword(s):  
Climate Model ◽  
Decadal Variability ◽  
Reanalysis Data ◽  
Forced Response ◽  
Lagrangian Model ◽  
Current Generation ◽  
Transit Times ◽  
Age Variations ◽  
Dipole Pattern ◽  
Climate Model Simulations

Abstract. An accelerating Brewer-Dobson circulation (BDC) is a robust signal of climate change in model predictions but has been questioned by trace gas observations. We analyze stratospheric mean age of air and the full age spectrum as measures for the BDC and its trend. Age of air is calculated with the Chemical Lagrangian Model of the Stratosphere (CLaMS) driven by ERA-Interim, JRA-55 and MERRA-2 reanalysis data to assess the robustness of the representation of the BDC in current generation meteorological reanalyses. We find that climatological mean age significantly depends on the reanalysis, with JRA-55 showing the youngest and MERRA-2 the oldest mean age. Consideration of the age spectrum indicates that the older age for MERRA-2 is related to a stronger spectrum tail, likely related to weaker tropical upwelling and stronger recirculation. Seasonality of stratospheric transport is robustly represented in reanalyses, with similar mean age variations and age spectrum peaks. Long-term changes over 1989–2015 turn out to be similar for the reanalyses with mainly decreasing mean age accompanied by a shift of the age spectrum peak towards shorter transit times, resembling the forced response in climate model simulations to increasing greenhouse gas concentrations. For the shorter periods 1989–2001 and 2002–2015 age of air changes are less robust. Only ERA-Interim shows the hemispheric dipole pattern in age changes during 2002–2015 as viewed by recent satellite observations. Consequently, the representation of decadal variability of the BDC in current generation reanalyses appears less robust and a major uncertainty of modelling the BDC.

The Role of Mesoscale Eddies in the Rectification of the Southern Ocean Response to Climate Change

2010 ◽  
Vol 40 (7) ◽  
pp. 1539-1557 ◽  
Author(s):  
Riccardo Farneti ◽  
Thomas L. Delworth ◽  
Anthony J. Rosati ◽  
Stephen M. Griffies ◽  
Fanrong Zeng
Keyword(s):  
Climate Change ◽  
Southern Ocean ◽  
Climate Model ◽  
Coupled Model ◽  
Ekman Transport ◽  
Dynamical Response ◽  
Geophysical Fluid Dynamics Laboratory ◽  
Climate Change Scenarios ◽  
Residual Circulation ◽  
Atmospheric Changes

Abstract Simulations from a fine-resolution global coupled model, the Geophysical Fluid Dynamics Laboratory Climate Model, version 2.4 (CM2.4), are presented, and the results are compared with a coarse version of the same coupled model, CM2.1, under idealized climate change scenarios. A particular focus is given to the dynamical response of the Southern Ocean and the role played by the eddies—parameterized or permitted—in setting the residual circulation and meridional density structure. Compared to the case in which eddies are parameterized and consistent with recent observational and idealized modeling studies, the eddy-permitting integrations of CM2.4 show that eddy activity is greatly energized with increasing mechanical and buoyancy forcings, buffering the ocean to atmospheric changes, and the magnitude of the residual oceanic circulation response is thus greatly reduced. Although compensation is far from being perfect, changes in poleward eddy fluxes partially compensate for the enhanced equatorward Ekman transport, leading to weak modifications in local isopycnal slopes, transport by the Antarctic Circumpolar Current, and overturning circulation. Since the presence of active ocean eddy dynamics buffers the oceanic response to atmospheric changes, the associated atmospheric response to those reduced ocean changes is also weakened. Further, it is hypothesized that present numerical approaches for the parameterization of eddy-induced transports could be too restrictive and prevent coarse-resolution models from faithfully representing the eddy response to variability and change in the forcing fields.

scholarly journals Predicting the effects of climate change on bluefin tuna (Thunnus thynnus) spawning habitat in the Gulf of Mexico

2011 ◽  
Vol 68 (6) ◽  
pp. 1051-1062 ◽  
Author(s):  
Barbara A. Muhling ◽  
Sang-Ki Lee ◽  
John T. Lamkin ◽  
Yanyun Liu
Keyword(s):  
Climate Change ◽  
Gulf Of Mexico ◽  
Climate Model ◽  
Bluefin Tuna ◽  
Spawning Ground ◽  
Thunnus Thynnus ◽  
Spawning Habitat ◽  
Western Atlantic ◽  
Model Simulations ◽  
Climate Model Simulations

Abstract Muhling, B. A., Lee, S-K., Lamkin, J. T., and Liu, Y. 2011. Predicting the effects of climate change on bluefin tuna (Thunnus thynnus) spawning habitat in the Gulf of Mexico. – ICES Journal of Marine Science, 68: 1051–1062. Atlantic bluefin tuna (BFT) is a highly migratory species that feeds in cold waters in the North Atlantic, but migrates to tropical seas to spawn. Global climate-model simulations forced by future greenhouse warming project that upper-ocean temperatures in the main western Atlantic spawning ground, the Gulf of Mexico (GOM), will increase substantially, potentially altering the temporal and spatial extent of BFT spawning activity. In this study, an ensemble of 20 climate model simulations used in the Intergovernmental Panel for Climate Change fourth Assessment Report (IPCC-AR4) predicted mean temperature changes within the GOM under scenario A1B through to 2100. Associations between adult and larval BFT in the GOM and sea temperatures were defined using 20th century observations, and potential effects of warming on the suitability of the GOM as a spawning ground were quantified. Areas in the GOM with high probabilities of larval occurrence decreased in late spring by 39–61% by 2050 and 93–96% by the end of the 21st century. Conversely, early spring may become more suitable for spawning. BFT are therefore likely to be vulnerable to climate change, and there is potential for significant impacts on spawning and migration behaviours.

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