scholarly journals Tropical Wave Driving of the Annual Cycle in Tropical Tropopause Temperatures. Part II: Model Results

2006 ◽  
Vol 63 (5) ◽  
pp. 1420-1431 ◽  
Author(s):  
W. A. Norton
Keyword(s):  
Annual Cycle ◽  
Rossby Waves ◽  
Lower Stratosphere ◽  
Equation Model ◽  
Stationary Waves ◽  
Mean Flow ◽  
Flux Divergence ◽  
Tropical Tropopause ◽  
Equatorial Rossby Waves ◽  
Tropical Heating

Abstract The atmospheric response to a localized distribution of tropical heating is examined in terms of the stationary waves excited and how these impact the mean flow near the tropical tropopause. This is done by examining nonlinear simulations of the Gill model with a primitive equation model that extends from the surface up into the stratosphere. The model produces strong cooling of zonal mean temperatures near the tropical tropopause when the heating is on the equator but weaker cooling with the heating at 15°N. The model shows that equatorial Rossby waves that penetrate the lower stratosphere and changes in EP flux divergence that correspond to the observed changes between December and August. It is suggested that ascent in the upper tropical troposphere is driven by vorticity advection or equivalently potential vorticity fluxes due to these equatorial Rossby waves, particularly when the heating is close to the equator. The model results provide support to the hypothesis that the annual cycle in tropical tropopause temperatures is a result of the annual variation in latitude of tropical heating and that equatorial Rossby waves are key in producing the response in the upper troposphere and lower stratosphere.

scholarly journals Tropical Wave Driving of the Annual Cycle in Tropical Tropopause Temperatures. Part I: ECMWF Analyses

2006 ◽  
Vol 63 (5) ◽  
pp. 1410-1419 ◽  
Author(s):  
A. M. Kerr-Munslow ◽  
W. A. Norton
Keyword(s):  
Annual Cycle ◽  
Meridional Flow ◽  
Momentum Balance ◽  
Convective Heating ◽  
Stationary Waves ◽  
Flux Divergence ◽  
Wave Dissipation ◽  
Tropical Tropopause ◽  
Adiabatic Cooling ◽  
Mass Divergence

Abstract A quantitative examination of the annual cycle in the tropical tropopause temperatures, tropical ascent, momentum balance, and wave driving is performed using ECMWF analyses to determine how the annual cycle in tropical tropopause temperatures arises. Results show that the annual cycle in tropical tropopause temperatures is driven by the annual variation in ascent and consequent dynamical (adiabatic) cooling at the tropical tropopause. Mass divergence local to the tropical tropopause has the dominant contribution to ascent near the tropical tropopause. The annual cycle in mass divergence, and the associated meridional flow, near the tropical tropopause is driven by Eliassen–Palm (EP) flux divergence, that is, wave dissipation. The EP flux divergence near the tropical tropopause is dominated by stationary waves with both the horizontal and vertical components of the EP flux contributing. However, the largest annual cycle is in the divergence of the vertical EP flux and in particular from the contribution in the vertical flux of zonal momentum. These results do not match the existing theory that the annual cycle is driven by the wave dissipation in the extratropical stratosphere, that is, the stratospheric pump. It is suggested that the annual cycle is linked to equatorial Rossby waves forced by convective heating in the tropical troposphere.

A Large Annual Cycle in Ozone above the Tropical Tropopause Linked to the Brewer–Dobson Circulation

2007 ◽  
Vol 64 (12) ◽  
pp. 4479-4488 ◽  
Author(s):  
William J. Randel ◽  
Mijeong Park ◽  
Fei Wu ◽  
Nathaniel Livesey
Keyword(s):  
Annual Cycle ◽  
Seasonal Cycle ◽  
Annual Variation ◽  
Lower Stratosphere ◽  
Vertical Gradient ◽  
Relative Amplitude ◽  
Vertical Layer ◽  
Satellite Measurements ◽  
Tropical Tropopause ◽  
Vertical Gradients

Abstract Near-equatorial ozone observations from balloon and satellite measurements reveal a large annual cycle in ozone above the tropical tropopause. The relative amplitude of the annual cycle is large in a narrow vertical layer between ∼16 and 19 km, with approximately a factor of 2 change in ozone between the minimum (during NH winter) and maximum (during NH summer). The annual cycle in ozone occurs over the same altitude region, and is approximately in phase with the well-known annual variation in tropical temperature. This study shows that the large annual variation in ozone occurs primarily because of variations in vertical transport associated with mean upwelling in the lower stratosphere (the Brewer–Dobson circulation); the maximum relative amplitude peak in the lower stratosphere is collocated with the strongest background vertical gradients in ozone. A similar large seasonal cycle is observed in carbon monoxide (CO) above the tropical tropopause, which is approximately out of phase with ozone (associated with an oppositely signed vertical gradient). The observed ozone and CO variations can be used to constrain estimates of the seasonal cycle in tropical upwelling.

scholarly journals Spectrum of Wave Forcing Associated with the Annual Cycle of Upwelling at the Tropical Tropopause

2016 ◽  
Vol 73 (2) ◽  
pp. 855-868 ◽  
Author(s):  
Joowan Kim ◽  
William J. Randel ◽  
Thomas Birner ◽  
Marta Abalos
Keyword(s):  
Annual Cycle ◽  
Lower Stratosphere ◽  
Mass Conservation ◽  
Upper Troposphere ◽  
Wave Forcing ◽  
Planetary Scale ◽  
Tropical Tropopause ◽  
Zonal Wavenumber ◽  
Wavenumber Spectrum ◽  
Transformed Eulerian Mean

Abstract The zonal wavenumber spectrum of atmospheric wave forcing in the lower stratosphere is examined to understand the annual cycle of upwelling at the tropical tropopause. Tropopause upwelling is derived based on the wave forcing computed from ERA-Interim using the momentum and mass conservation equations in the transformed Eulerian-mean framework. The calculated upwelling agrees well with other upwelling estimates and successfully captures the annual cycle, with a maximum during Northern Hemisphere (NH) winter. The spectrum of wave forcing reveals that the zonal wavenumber-3 component drives a large portion of the annual cycle in upwelling. The wave activity flux (Eliassen–Palm flux) shows that the associated waves originate from the NH extratropics and the Southern Hemisphere tropics during December–February, with both regions contributing significant wavenumber-3 fluxes. These wave fluxes are nearly absent during June–August. Wavenumbers 1 and 2 and synoptic-scale waves have a notable contribution to tropopause upwelling but have little influence on the annual cycle, except the wavenumber-4 component. The quasigeostrophic refractive index suggests that the NH extratropical wavenumber-3 component can enhance tropopause upwelling because these planetary-scale waves are largely trapped in the vertical, while being refracted toward the subtropical upper troposphere and lower stratosphere. Regression analysis based on interannual variability suggests that the wavenumber-3 waves are related to tropical convection and wave breaking along the subtropical jet in the NH extratropics.

scholarly journals Modelled thermal and dynamical responses of the middle atmosphere to EPP-induced ozone changes

2015 ◽  
Vol 15 (22) ◽  
pp. 33283-33329 ◽  
Author(s):  
K. Karami ◽  
P. Braesicke ◽  
M. Kunze ◽  
U. Langematz ◽  
M. Sinnhuber ◽  
...  
Keyword(s):  
Zonal Wind ◽  
Ozone Depletion ◽  
Rossby Waves ◽  
Lower Stratosphere ◽  
Middle Atmosphere ◽  
Wave Activity ◽  
Flux Divergence ◽  
Starting Point ◽  
Wide Range ◽  
The Impact

Abstract. Energetic particles including protons, electrons and heavier ions, enter the Earth's atmosphere over the polar regions of both hemispheres, where they can greatly disturb the chemical composition of the upper and middle atmosphere and contribute to ozone depletion in the stratosphere and mesosphere. The chemistry–climate general circulation model EMAC is used to investigate the impact of changed ozone concentration due to Energetic Particle Precipitation (EPP) on temperature and wind fields. The results of our simulations show that ozone perturbation is a starting point for a chain of processes resulting in temperature and circulation changes over a wide range of latitudes and altitudes. In both hemispheres, as winter progresses the temperature and wind anomalies move downward with time from the mesosphere/upper stratosphere to the lower stratosphere. In the Northern Hemisphere (NH), once anomalies of temperature and zonal wind reach the lower stratosphere, another signal develops in mesospheric heights and moves downward. Analyses of Eliassen and Palm (EP) flux divergence show that accelerating or decelerating of the stratospheric zonal flow is in harmony with positive and negative anomalies of the EP flux divergences, respectively. This results suggest that the oscillatory mode in the downwelling signal of temperature and zonal wind in our simulations are the consequence of interaction between the resolved waves in the model and the mean stratospheric flow. Therefore, any changes in the EP flux divergence lead to anomalies in the zonal mean zonal wind which in turn feed back on the propagation of Rossby waves from the troposphere to higher altitudes. The analyses of Rossby waves refractive index show that the EPP-induced ozone anomalies are capable of altering the propagation condition of the planetary-scale Rossby waves in both hemispheres. It is also found that while ozone depletion was confined to mesospheric and stratospheric heights, but it is capable to alter Rossby wave propagation down to tropospheric heights. In response to an accelerated polar vortex in the Southern Hemisphere (SH) late wintertime, we found almost two weeks delay in the occurrence of mean dates of Stratospheric Final Warming (SFW). These results suggest that the stratosphere is not merely a passive sink of wave activity from below, but it plays an active role in determining its own budget of wave activity.

scholarly journals The Role of Extratropical Background Flow in Modulating the MJO Extratropical Response

2020 ◽  
Vol 33 (11) ◽  
pp. 4513-4536
Author(s):  
Cheng Zheng ◽  
Edmund Kar-Man Chang
Keyword(s):  
Rossby Waves ◽  
Intraseasonal Variability ◽  
Equation Model ◽  
Dominant Mode ◽  
Mean Flow ◽  
Background Flow ◽  
Heating Source ◽  
The North ◽  
Initial Location ◽  
Ocean Region

AbstractThe Madden–Julian oscillation (MJO) is the dominant mode of tropical intraseasonal variability. Many studies have found that the MJO, which acts as a tropical heating source, can excite Rossby waves that propagate into the midlatitude and modulate midlatitude circulation. The extratropical mean flow can modulate the MJO extratropical response. Rossby waves can grow or decay in different extratropical background flows, and the propagation of the Rossby waves also varies as the background flow acts as a waveguide. In this study, how extratropical mean flow modulates the MJO extratropical response is explored by using a nonlinear baroclinic primitive equation model. MJO-associated heating, as an external forcing of the model, is imposed into scenarios with different extratropical background flows. Different background flow modulates the generation and advection of the vorticity anomalies induced by the MJO, which determines the initial location and strength of the Rossby waves. The midlatitude waveguides can be different as the background flow changes. As the propagation of Rossby waves follows the waveguides, the background flow determines whether the Rossby waves are trapped in the Pacific Ocean region or can propagate to the north and to the east into North America. The experiments also show that the anomalies associated with the Rossby waves can extract energy from the midlatitude jet over the jet exit region and the southern flank of the jet. This further modulates the strength, location, and duration of the MJO extratropical response.

scholarly journals The climatology of the Brewer–Dobson circulation and the contribution of gravity waves

2019 ◽  
Vol 19 (7) ◽  
pp. 4517-4539 ◽  
Author(s):  
Kaoru Sato ◽  
Soichiro Hirano
Keyword(s):  
Gravity Waves ◽  
Rossby Waves ◽  
Lower Stratosphere ◽  
Momentum Equation ◽  
Boreal Winter ◽  
Potential Contribution ◽  
Flux Divergence ◽  
Deep Circulation ◽  
Mean Circulation ◽  
Residual Mean Circulation

Abstract. The climatology of residual mean circulation – a main component of the Brewer–Dobson circulation – and the potential contribution of gravity waves (GWs) are examined for the annual mean state and each season in the whole stratosphere based on the transformed-Eulerian mean zonal momentum equation using four modern reanalysis datasets. Resolved and unresolved waves in the datasets are respectively designated as Rossby waves and GWs, although resolved waves may contain some GWs. First, the potential contribution of Rossby waves (RWs) to residual mean circulation is estimated from Eliassen–Palm flux divergence. The rest of residual mean circulation, from which the potential RW contribution and zonal mean zonal wind tendency are subtracted, is examined as the potential GW contribution, assuming that the assimilation process assures sufficient accuracy of the three components used for this estimation. The GWs contribute to drive not only the summer hemispheric part of the winter deep branch and low-latitude part of shallow branches, as indicated by previous studies, but they also cause a higher-latitude extension of the deep circulation in all seasons except for summer. This GW contribution is essential to determine the location of the turn-around latitude. The autumn circulation is stronger and wider than that of spring in the equinoctial seasons, regardless of almost symmetric RW and GW contributions around the Equator. This asymmetry is attributable to the existence of the spring-to-autumn pole circulation, corresponding to the angular momentum transport associated with seasonal variation due to the radiative process. The potential GW contribution is larger in September-to-November than in March-to-May in both hemispheres. The upward mass flux is maximized in the boreal winter in the lower stratosphere, while it exhibits semi-annual variation in the upper stratosphere. The boreal winter maximum in the lower stratosphere is attributable to stronger RW activity in both hemispheres than in the austral winter. Plausible deficiencies of current GW parameterizations are discussed by comparing the potential GW contribution and the parameterized GW forcing.

scholarly journals Wave Driving in the Tropical Lower Stratosphere as Simulated by WACCM. Part I: Annual Cycle

2009 ◽  
Vol 66 (7) ◽  
pp. 2029-2043 ◽  
Author(s):  
Masakazu Taguchi
Keyword(s):  
Annual Cycle ◽  
Lower Stratosphere ◽  
Climate Model ◽  
Forthcoming Paper ◽  
Convective Heating ◽  
Residual Circulation ◽  
Vertical Fluxes ◽  
Induced Changes ◽  
Zonal Momentum ◽  
Equatorial Rossby Waves

Abstract This study explores the climatological annual cycle of temperature, circulation, and wave driving distributions in the tropical lower stratosphere as produced in a 50-yr simulation of the Whole Atmosphere Community Climate Model (WACCM). The simulation is forced with a climatological sea surface temperature and sea ice condition. The present diagnoses verify the primary balances of the annual cycle in this region, consistent with lower temperatures, stronger residual circulation (upwelling and local meridional outflow), and nearby stronger wave driving for Northern Hemisphere (NH) winter. An in-detail analysis on the wave driving further reveals that the stronger driving, occurring mostly in the northern tropics and subtropics, is contributed by northward and upward propagation (associated with meridional and vertical fluxes of zonal momentum, respectively) of equatorial Rossby waves forced by convective heating, and also by equatorward propagation of NH extratropical planetary and synoptic waves. The results are used to discuss implications about possible factors that may affect the different observations of the wave driving. The present framework and results will be extended to investigate ENSO-induced changes in this region during NH winter in a forthcoming paper.

scholarly journals The climatology of the Brewer-Dobson circulation and the contribution of gravity waves

2020 ◽  
Author(s):  
Kaoru Sato ◽  
Soichiro Hirano
Keyword(s):  
Gravity Waves ◽  
Rossby Waves ◽  
Lower Stratosphere ◽  
Momentum Equation ◽  
Boreal Winter ◽  
Potential Contribution ◽  
Flux Divergence ◽  
Deep Circulation ◽  
Mean Circulation ◽  
Residual Mean Circulation

<p>The climatology of residual mean circulation – a main component of the Brewer–Dobson circulation – and the potential contribution of gravity waves (GWs) are examined for the annual mean state and each season in the whole stratosphere based on the transformed-Eulerian mean zonal momentum equation using four modern reanalysis datasets. Resolved and unresolved waves in the datasets are respectively designated as Rossby waves and GWs, although resolved waves may contain some GWs. First, the potential contribution of Rossby waves (RWs) to residual mean circulation is estimated from Eliassen–Palm flux divergence. The rest of residual mean circulation, from which the potential RW contribution and zonal mean zonal wind tendency are subtracted, is examined as the potential GW contribution, assuming that the assimilation process assures sufficient accuracy of the three components used for this estimation. The GWs contribute to drive not only the summer hemispheric part of the winter deep branch and low-latitude part of shallow branches, as indicated by previous studies, but they also cause a higher-latitude extension of the deep circulation in all seasons except for summer. This GW contribution is essential to determine the location of the turn-around latitude. The autumn circulation is stronger and wider than that of spring in the equinoctial seasons, regardless of almost symmetric RW and GW contributions around the Equator. This asymmetry is attributable to the existence of the spring-to-autumn pole circulation, corresponding to the angular momentum transport associated with seasonal variation due to the radiative process. The potential GW contribution is larger in September to November than in March-to-May in both hemispheres. The upward mass flux is maximized in the boreal winter in the lower stratosphere, while it exhibits semi-annual variation in the upper stratosphere. The boreal winter maximum in the lower stratosphere is attributable to stronger RW activity in both hemispheres than in the austral winter. Plausible deficiencies of current GW parameterizations are discussed by comparing the potential GW contribution and the parameterized GW forcing.</p>

scholarly journals Formation and Maintenance of the Tropical Cold-Point Tropopause in a Dry Dynamic-Core GCM

2015 ◽  
Vol 72 (8) ◽  
pp. 3097-3115 ◽  
Author(s):  
Joowan Kim ◽  
Seok-Woo Son
Keyword(s):  
Lower Stratosphere ◽  
Model Simulation ◽  
Equilibrium Temperature ◽  
Equation Model ◽  
Synoptic Scale ◽  
Mean Flow ◽  
Model Driven ◽  
Planetary Scale ◽  
Radiative Processes ◽  
Cold Point

Abstract The formation of the tropical cold-point tropopause (CPT) is examined using a dry primitive equation model driven by the Held–Suarez forcing. Without moist and realistic radiative processes, the dry model successfully reproduces the zonal-mean structure of the CPT. The modeled CPT is appreciably colder (~10 K) than the prescribed equilibrium temperature, and it is maintained by upwelling in the tropical upper troposphere and lower stratosphere (UTLS). A transient simulation starting from an axisymmetric steady state without the CPT shows that the evolution and maintenance of the CPT are closely related to the zonal-mean-flow response to wave driving in the stratosphere. The transformed Eulerian-mean analysis indicates that the wave driving is mostly due to convergence of synoptic-scale waves originating from the midlatitude troposphere and propagating into the subtropical UTLS in this model simulation. The modeled CPT also shows a large sensitivity to increased baroclinicity in the equilibrium temperature. Although planetary-scale waves are not considered in this simulation, the result confirms that wave-driven upwelling in the tropical UTLS is a crucial process for the formation and maintenance of the CPT. In addition, it also implies that synoptic-scale waves may play a nonnegligible role in this mechanism, particularly in the seasons when planetary-scale wave activity in the lower stratosphere is weak.

scholarly journals Influence of the Barotropic Mean Flow on the Width and the Structure of the Atlantic Equatorial Deep Jets

2014 ◽  
Vol 44 (9) ◽  
pp. 2485-2497 ◽  
Author(s):  
Martin Claus ◽  
Richard J. Greatbatch ◽  
Peter Brandt
Keyword(s):  
Rossby Waves ◽  
Current System ◽  
Low Frequency ◽  
Water Model ◽  
Kelvin Wave ◽  
Mean Flow ◽  
The North ◽  
Basin Mode ◽  
The Mean ◽  
Equatorial Rossby Waves

Abstract A representation of an equatorial basin mode excited in a shallow-water model for a single high-order baroclinic vertical normal mode is used as a simple model for the equatorial deep jets. The model is linearized about both a state of rest and a barotropic mean flow corresponding to the observed Atlantic Equatorial Intermediate Current System. It was found that the eastward mean flow associated with the North and South Intermediate Counter Currents (NICC and SICC, respectively) effectively shields the equator from off-equatorial Rossby waves. The westward propagation of these waves is blocked, and focusing on the equator due to beta dispersion is prevented. This leads to less energetic jets along the equator. On the other hand, the westward barotropic mean flow along the equator reduces the gradient of absolute vorticity and hence widens the cross-equatorial structure of the basin mode. Increasing lateral viscosity predominantly affects the width of the basin modes’ Kelvin wave component in the presence of the mean flow, while the Rossby wave is confined by the flanking NICC and SICC. Independent of the presence of the mean flow, the application of sufficient lateral mixing also hinders the focusing of off-equatorial Rossby waves, which is hence an unlikely feature of a low-frequency basin mode in the real ocean.

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