scholarly journals The role of the winter residual circulation in the summer mesopause regions in WACCM

2018 ◽  
Vol 18 (6) ◽  
pp. 4217-4228 ◽  
Author(s):  
Maartje Sanne Kuilman ◽  
Bodil Karlsson
Keyword(s):  
Planetary Wave ◽  
Climate Model ◽  
Zonal Flow ◽  
Global Scale ◽  
Wave Activity ◽  
Latitudinal Gradients ◽  
Residual Circulation ◽  
Mean Flow ◽  
Vertically Propagating

Abstract. High winter planetary wave activity warms the summer polar mesopause via a link between the two hemispheres. Complex wave–mean-flow interactions take place on a global scale, involving sharpening and weakening of the summer zonal flow. Changes in the wind shear occasionally generate flow instabilities. Additionally, an altering zonal wind modifies the breaking of vertically propagating gravity waves. A crucial component for changes in the summer zonal flow is the equatorial temperature, as it modifies latitudinal gradients. Since several mechanisms drive variability in the summer zonal flow, it can be hard to distinguish which one is dominant. In the mechanism coined interhemispheric coupling, the mesospheric zonal flow is suggested to be a key player for how the summer polar mesosphere responds to planetary wave activity in the winter hemisphere. We here use the Whole Atmosphere Community Climate Model (WACCM) to investigate the role of the summer stratosphere in shaping the conditions of the summer polar mesosphere. Using composite analyses, we show that in the absence of an anomalous summer mesospheric temperature gradient between the equator and the polar region, weak planetary wave forcing in the winter would lead to a warming of the summer mesosphere region instead of a cooling, and vice versa. This is opposing the temperature signal of the interhemispheric coupling that takes place in the mesosphere, in which a cold and calm winter stratosphere goes together with a cold summer mesopause. We hereby strengthen the evidence that the variability in the summer mesopause region is mainly driven by changes in the summer mesosphere rather than in the summer stratosphere.

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 Are Sudden Stratospheric Warmings Preceded by Anomalous Tropospheric Wave Activity?

2019 ◽  
Vol 32 (21) ◽  
pp. 7173-7189 ◽  
Author(s):  
Alvaro de la Cámara ◽  
Thomas Birner ◽  
John R. Albers
Keyword(s):  
Climate Model ◽  
State Of The Art ◽  
Source Region ◽  
Lower Troposphere ◽  
Wave Activity ◽  
Mean Flow ◽  
Wave Event ◽  
Dynamical Nature ◽  
The Waves ◽  
Robust Evidence

Abstract A combination of 240 years of output from a state-of-the-art chemistry–climate model and a twentieth-century reanalysis product is used to investigate to what extent sudden stratospheric warmings are preceded by anomalous tropospheric wave activity. To this end we study the fate of lower tropospheric wave events (LTWEs) and their interaction with the stratospheric mean flow. These LTWEs are contrasted with sudden stratospheric deceleration events (SSDs), which are similar to sudden stratospheric warmings but place more emphasis on the explosive dynamical nature of such events. Reanalysis and model output provide very similar statistics: Around one-third of the identified SSDs are preceded by wave events in the lower troposphere, while two-thirds of the SSDs are not preceded by a tropospheric wave event. In addition, only 20% of all anomalous tropospheric wave events are followed by an SSD in the stratosphere. This constitutes statistically robust evidence that the anomalous amplification of wave activity in the stratosphere that drives SSDs is not necessarily due to an anomalous amplification of the waves in the source region (i.e., the lower troposphere). The results suggest that the dynamics in the lowermost stratosphere and the vortex geometry are essential, and should be carefully analyzed in the search for precursors of SSDs.

scholarly journals The Role of the Divergent Circulation for Large-Scale Eddy Momentum Transport in the Tropics. Part I: Observations

2019 ◽  
Vol 76 (4) ◽  
pp. 1125-1144 ◽  
Author(s):  
Pablo Zurita-Gotor
Keyword(s):  
Momentum Flux ◽  
Large Scale ◽  
Zonal Flow ◽  
Stationary Wave ◽  
Hadley Cell ◽  
Momentum Transport ◽  
Mean Flow ◽  
Meridional Transport ◽  
The Tropics

Abstract This work investigates the role played by the divergent circulation for meridional eddy momentum transport in the tropical atmosphere. It is shown that the eddy momentum flux in the deep tropics arises primarily from correlations between the divergent eddy meridional velocity and the rotational eddy zonal velocity. Consistent with previous studies, this transport is dominated by the stationary wave component, associated with correlations between the zonal structure of the Hadley cell (zonal anomalies in the meridional overturning) and the climatological-mean Rossby gyres. This eddy momentum flux decomposition implies a different mechanism of eddy momentum convergence from the extratropics, associated with upper-level mass convergence (divergence) over sectors with anomalous westerlies (easterlies). By itself, this meridional transport would only increase (decrease) isentropic thickness over regions with anomalous westerly (easterly) zonal flow. The actual momentum mixing is due to vertical (cross isentropic) advection, pointing to the key role of diabatic processes for eddy–mean flow interaction in the tropics.

scholarly journals Ozone Loss and Recovery and the Preconditioning of Upward-Propagating Planetary Wave Activity

2013 ◽  
Vol 70 (12) ◽  
pp. 3977-3994 ◽  
Author(s):  
John R. Albers ◽  
Terrence R. Nathan
Keyword(s):  
Ozone Depletion ◽  
Planetary Wave ◽  
Climate Model ◽  
Stratospheric Ozone ◽  
Polar Vortex ◽  
Wave Drag ◽  
Late Winter ◽  
Residual Circulation ◽  
Wave Amplification ◽  
Ozone Loss

Abstract A mechanistic chemistry–dynamical model is used to evaluate the relative importance of radiative, photochemical, and dynamical feedbacks in communicating changes in lower-stratospheric ozone to the circulation of the stratosphere and lower mesosphere. Consistent with observations and past modeling studies of Northern Hemisphere late winter and early spring, high-latitude radiative cooling due to lower-stratospheric ozone depletion causes an increase in the modeled meridional temperature gradient, an increase in the strength of the polar vortex, and a decrease in vertical wave propagation in the lower stratosphere. Moreover, it is shown that, as planetary waves pass through the ozone loss region, dynamical feedbacks precondition the wave, causing a large increase in wave amplitude. The wave amplification causes an increase in planetary wave drag, an increase in residual circulation downwelling, and a weaker polar vortex in the upper stratosphere and lower mesosphere. The dynamical feedbacks responsible for the wave amplification are diagnosed using an ozone-modified refractive index; the results explain recent chemistry–coupled climate model simulations that suggest a link between ozone depletion and increased polar downwelling. The effects of future ozone recovery are also examined and the results provide guidance for researchers attempting to diagnose and predict how stratospheric climate will respond specifically to ozone loss and recovery versus other climate forcings including increasing greenhouse gas abundances and changing sea surface temperatures.

scholarly journals Effects of mesoscale eddies on global ocean distributions of CFC-11, CO<sub>2</sub> and Δ<sup>14</sup>C

2006 ◽  
Vol 3 (4) ◽  
pp. 1011-1063
Author(s):  
Z. Lachkar ◽  
J. C. Orr ◽  
J.-C. Dutay ◽  
P. Delecluse
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Global Scale ◽  
Global Ocean ◽  
Residual Circulation ◽  
Mean Flow ◽  
Meridional Transport ◽  
Anthropogenic Co2 ◽  
Short Time

Abstract. Global-scale tracer simulations are typically made at coarse resolution without explicitly modeling eddies. Here we ask what role do eddies play in ocean uptake, storage, and meridional transport of transient tracers. We made global anthropogenic transient-tracer simulations in non-eddying (2°cosφ×2°, ORCA2) and eddying (½°cosφ×½°, ORCA05) versions of the ocean general circulation model OPA9. We focus on the Southern Ocean where tracer air-sea fluxes are largest. Eddies have little effect on global and regional bomb Δ14C uptake and storage. Yet for anthropogenic CO2 and CFC-11, increased eddy activity reduces southern extratropical uptake by 28% and 25% respectively. There is a similar decrease in corresponding inventories, which provides better agreement with observations. With higher resolution, eddies strengthen upper ocean vertical stratification and reduce excessive ventilation of intermediate waters by 20% between 60° S and 40° S. By weakening the Residual Circulation, i.e., the sum of Eulerian mean flow and the opposed eddy-induced flow, eddies reduce the supply of tracer-impoverished deep waters to the surface near the Antarctic divergence, thus reducing the air-sea tracer flux. Consequently, inventories for both CFC-11 and anthropogenic CO2 decrease because their mixed layer concentrations in that region equilibrate with the atmosphere on relatively short time scales (15 days and 6 months, respectively); conversely, the slow air-sea equilibration of bomb Δ14C of 6 years, gives surface waters little time to exchange with the atmosphere before they are subducted.

scholarly journals Specified dynamics scheme impacts on wave-mean flow dynamics, convection, and tracer transport in CESM2 (WACCM6)

2022 ◽  
Vol 22 (1) ◽  
pp. 197-214
Author(s):  
Nicholas A. Davis ◽  
Patrick Callaghan ◽  
Isla R. Simpson ◽  
Simone Tilmes
Keyword(s):  
Climate Model ◽  
Tracer Transport ◽  
Residual Circulation ◽  
Mean Flow ◽  
Time Step ◽  
Modeling Tools ◽  
Transport Pathways ◽  
Systematic Assessment ◽  
Model Version ◽  
Wide Range

Abstract. Specified dynamics schemes are ubiquitous modeling tools for isolating the roles of dynamics and transport on chemical weather and climate. They typically constrain the circulation of a chemistry–climate model to the circulation in a reanalysis product through linear relaxation. However, recent studies suggest that these schemes create a divergence in chemical climate and the meridional circulation between models and do not accurately reproduce trends in the circulation. In this study we perform a systematic assessment of the specified dynamics scheme in the Community Earth System Model version 2, Whole Atmosphere Community Climate Model version 6 (CESM2 (WACCM6)), which proactively nudges the circulation toward the reference meteorology. Specified dynamics experiments are performed over a wide range of nudging timescales and reference meteorology frequencies, with the model's circulation nudged to its own free-running output – a clean test of the specified dynamics scheme. Errors in the circulation scale robustly and inversely with meteorology frequency and have little dependence on the nudging timescale. However, the circulation strength and errors in tracers, tracer transport, and convective mass flux scale robustly and inversely with the nudging timescale. A 12 to 24 h nudging timescale at the highest possible reference meteorology frequency minimizes errors in tracers, clouds, and the circulation, even up to the practical limit of one reference meteorology update every time step. The residual circulation and eddy mixing integrate tracer errors and accumulate them at the end of their characteristic transport pathways, leading to elevated error in the upper troposphere and lower stratosphere and in the polar stratosphere. Even in the most ideal case, there are non-negligible errors in tracers introduced by the nudging scheme. Future development of more sophisticated nudging schemes may be necessary for further progress.

scholarly journals On the possible causes of recent increases in northern hemispheric total ozone from a statistical analysis of satellite data from 1979 to 2003

2006 ◽  
Vol 6 (5) ◽  
pp. 1165-1180 ◽  
Author(s):  
S. Dhomse ◽  
M. Weber ◽  
I. Wohltmann ◽  
M. Rex ◽  
J. P. Burrows
Keyword(s):  
Total Ozone ◽  
Planetary Wave ◽  
Linear Trend ◽  
Wave Activity ◽  
High Latitudes ◽  
Residual Circulation ◽  
Ozone Loss ◽  
Eddy Heat Flux ◽  
Northern Hemispheric ◽  
Polar Ozone

Abstract. Global total ozone measurements from various satellite instruments such as SBUV, TOMS, and GOME show an increase in zonal mean total ozone at northern hemispheric (NH) mid to high latitudes since the mid-nineties. This increase could be expected from the peaking and start of decline in the effective stratospheric halogen loading, but the rather rapid increase observed in NH zonal mean total ozone suggests that another physical mechanism such as winter planetary wave activity has increased which has led to higher stratospheric Arctic temperatures. This has enhanced ozone transport into higher latitudes in recent years as part of the residual circulation and at the same time reduced the frequency of cold Arctic winters with enhanced polar ozone loss. Results from various multi-variate linear regression analyses using SBUV V8 total ozone with explanatory variables such as a linear trend or, alternatively, EESC (equivalent effective stratospheric chlorine) and on the other hand planetary wave driving (eddy heat flux) or, alternatively, polar ozone loss (PSC volume) in addition to proxies for stratospheric aerosol loading, QBO, and solar cycle, all considered to be main drivers for ozone variability, are presented. It is shown that the main contribution to the recent increase in NH total ozone is from the combined effect of rising tropospheric driven planetary wave activity associated with reduced polar ozone loss at high latitudes as well as increasing solar activity. This conclusion can be drawn regardless of the use of linear trend or EESC terms in our statistical model. It is also clear that more years of data will be needed to further improve our estimates of the relative contributions of the individual processes to decadal ozone variability. The question remains if the observed increase in planetary wave driving is part of natural decadal atmospheric variability or will persist. If the latter is the case, it could be interpreted as a possible signature of climate change.

Influence of energetic particle precipitation on polar vortex mediated by planetary wave activity

2021 ◽  
Author(s):  
Timo Asikainen ◽  
Antti Salminen ◽  
Ville Maliniemi ◽  
Kalevi Mursula
Keyword(s):  
Planetary Wave ◽  
Energetic Electron ◽  
Polar Vortex ◽  
Analysis Data ◽  
Wave Activity ◽  
Necessary Condition ◽  
Atmospheric Conditions ◽  
Mean Flow ◽  
Ozone Loss ◽  
Near Earth Space

&lt;p&gt;The northern polar vortex experiences considerable inter-annual variability, which is also reflected to tropospheric weather. Recent research has established a link between polar vortex variations and energetic electron precipitation (EEP) from the near-Earth space into the polar atmosphere, which is mediated by EEP-induced chemical changes causing ozone loss in the mesosphere and stratosphere. However, the most dramatic changes in the polar vortex are due to strong enhancements of planetary wave activity, which typically result in a sudden stratospheric warming (SSW), a momentary breakdown of the polar vortex. Here we use the SSWs as an indicator of high planetary wave activity and consider their influence of SSWs on the atmospheric response to EEP in 1957-2017 using combined ERA-40 and ERA-Interim re-analysis data and geomagnetic activity as a proxy of EEP. We find that the EEP-related enhancement of the polar vortex and other associated dynamical responses are seen only during winters when a SSW occurs, and that the EEP-related changes take place slightly before the SSW onset. We show that the atmospheric conditions preceding SSWs favor enhanced wave-mean-flow interaction, which can dynamically amplify the initial polar vortex enhancement caused by ozone loss. These results highlight the importance of considering SSWs and sufficient level of planetary wave activity as a necessary condition for observing the effects of EEP on the polar vortex dynamics.&lt;/p&gt;

scholarly journals The role of the winter residual circulation in the summer mesopause regions in WACCM

2017 ◽  
Author(s):  
Maartje Sanne Kuilman ◽  
Bodil Karlsson
Keyword(s):  
Gravity Waves ◽  
General Circulation ◽  
Climate Model ◽  
Temperature Response ◽  
Circulation Model ◽  
Coupling Mechanism ◽  
Residual Circulation ◽  
Mesospheric Temperature ◽  
Temperature Signal

Abstract. High winter planetary wave activity warms the summer polar mesopause via a link between the two hemispheres. In a recent study carried out with the Kühlungsborn Mechanistic general Circulation Model (KMCM), it was shown that the net effect of this interhemispheric coupling mechanism is a cooling of the summer polar mesospheres and that this temperature response is tied to the strength of the gravity wave-driven winter mesospheric flow. We here reconfirm the hypothesis that the summer polar mesosphere would be substantially warmer without the circulation in the winter mesosphere, using the widely-used Whole Atmosphere Community Climate Model (WACCM). In addition, the role of the stratosphere in shaping the conditions of the summer polar mesosphere is investigated. Using composite analysis, we show that if winter gravity waves are absent, a weak stratospheric Brewer-Dobson circulation would lead to a warming of the summer mesosphere region instead of a cooling, and vice versa. This is opposing the temperature signal of the interhemispheric coupling in the mesosphere, in which a cold winter stratosphere goes together with a cold summer mesopause. We hereby strengthen the evidence that the equatorial mesospheric temperature response, driven by the winter gravity waves, is a crucial step in the interhemispheric coupling mechanism.

scholarly journals Zonal Flow Regime Changes in a GCM and in a Simple Quasigeostrophic Model: The Role of Stratospheric Dynamics

2009 ◽  
Vol 66 (5) ◽  
pp. 1366-1383 ◽  
Author(s):  
Isabella Bordi ◽  
Klaus Fraedrich ◽  
Michael Ghil ◽  
Alfonso Sutera
Keyword(s):  
General Circulation ◽  
Baroclinic Instability ◽  
Three Dimensional ◽  
Vertical Wind Shear ◽  
Circulation Model ◽  
Zonal Flow ◽  
Heat Fluxes ◽  
Mean Flow ◽  
Single Wave

Abstract The atmospheric general circulation is characterized by both single- and double-jet patterns. The double-jet structure of the zonal mean zonal wind is analyzed in Southern Hemisphere observations for the two calendar months of November and April. The observed features are studied further in an idealized quasigeostrophic and a simplified general circulation model (GCM). Results suggest that capturing the bimodality of the zonal mean flow requires the parameterization of momentum and heat fluxes associated with baroclinic instability of the three-dimensional fields. The role of eddy heat fluxes in generating the observed double-jet pattern is ascertained by using an analytical Eady model with stratospheric easterlies, in which a single wave disturbance interacts with the mean flow. In this model, the dual jets are generated by the zonal mean flow correction. Sensitivity of the results to the tropospheric vertical wind shear (or, equivalently, the meridional temperature gradient in the basic state’s troposphere) is also studied in the Eady model and compared to related experiments using the simplified GCM.

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