The Sensitivity of an Intermediate Model of the Midlatitude Troposphere's Equilibrium to Changes in Radiative Forcing* (original) (raw)
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Quarterly Journal of the Royal Meteorological Society, 1996
A multi-layer, quasi-geostrophic, Boussinesq model on a B-channel is used to study the sensitivity of the mid-latitude storm-track to changes in the vertical structure of the zonal mean equilibrium meridional temperature gradient. Motivation for this study is taken from the observation that the doubled CO2 climatology of generalcirculation models presents a weaker gradient in the lower troposphere and a stronger gradient in the upper troposphere than in a 'control' climatology. It is observed that the dynamics of the quasi-geostrophic model are generally more sensitive to the lower than to the upper tropospheric temperature gradient as a consequence of the importance of shallow eddies. In order to give a good representation of the eddy dynamics it is necessary to use a sufficiently high resolution, which, in the case chosen, proved to be between 10 and 20 tropospheric layers. If only three layers are used the general features of the response are still reproduced, but the model does not represent the sensitivities of the eddy meridional momentum-flux convergence adequately. It is argued that this is connected with the particular choice taken when parametrizing the surface Ekman friction. When a higher-resolution model is used, the eddy momentum-flux convergence proves to be more sensitive to changes in the upper than in the lower meridional equilibrium temperature gradient but is generally much less responsive to these changes than other quantities like the surface eddy potential-vorticity flux or eddy kinetic energy. For each variable the two sensitivities become close to each other as the baroclinicity of the equilibrium flow is increased because of the prevalence of deep eddies in the dynamics, sensitive only to the total radiative equilibrium wind vertical gradient. For very high values of the baroclinicity of the equilibrium flow the sensitivities approach a constant value in accordance with a simple scaling argument.
Experiments with a Stratospheric General Circulation Model 1. Radiative and Dynamic Aspects
1968
An 18-vertical level primitive equation general circulation model was developed from previous models of the Geophysical Fluid Dynamics Laboratory in order to study the lower stratosphere in detail. The altitude range covered was from the surface to 4 mb. (37.5 km.), the vertical resolution being optimized in the tropopause region to permit a more accurate calculation of the vertical transport terms. A polar stereographic projection was used and the model was limited to a single hemisphere.
Monthly Weather Review
The middle-latitude standing wave problem is investigated by means of a quasi-geostrophic, linear, steadJ;-state model in which the zonal current is perturbed by the lower boundary topography and by a distribution of heat sources and sinks. All the perturbations are assumed to have a single meridional wavelength and the dissipation is considered to take place in the surface boundary layer using, as a first approach, a horizontally uniform drag coefficient. After investigating some basic properties of the model atmosphere, some computations are made to determine its response to the combined forcing by topography and by diabatic heating for January 1962. The resulting perturbations are found to be in rather good agreement with the observed standing waves. The results also indicate that the standing waves forced by the topography are in about the same position as those forced by the diabatic heating and that the former have somewhat larger amplitudes than the latter. The effect of allowing the drag coefficient to have one constant value over the continents and a smaller constant value over the oceans is examined and found to be quite important when the ratio of the two values is 6, but small (yet such as t o bring the computed and observed eddies into closer agreement than in the case of a uniform drag coefficient) for a ratio of 2.
Radiative‐dynamical climatology of the first‐generation Canadian middle atmosphere model
Atmosphere-Ocean, 1997
Model of thé troposphere-stratosphere-mesosphere System, starting from thé AES/CCCma third-génération atmospheric General Circulation Model. This paper describes thé basic features of thé first-generation Canadian MAM and some aspects ofits radiative-dynamical climatology. Standard first-order mean diagnostics are presentedfor monthly means and for thé annual cycle ofzonal-mean winds and températures. The mean méridional circulation is examined, and comparison is made between thé steady diabatic, downward controlled, and residual streamfunctions. It isfound thaï downward contrat holds quite well in thé monthly mean through most of thé middle atmosphère, even during equinoctal periods. The relative rôles of différent drag processes in determining thé mean downwelling over thé wintertime polar middle stratosphère is examined, and thé vertical structure of thé drag is quantified.
2003
Midwinter storm track response to zonal variations in midlatitude sea surface temperatures (SSTs) has been investigated using an atmospheric general circulation model under aquaplanet and perpetual-January conditions. Zonal wavenumber-1 SST variations with a meridionally confined structure are placed at various latitudes. Having these SST variations centered at 30ЊN leads to a zonally localized storm track, while the storm track becomes nearly zonally uniform when the same SST forcing is moved farther north at 40Њ and 50ЊN. Large (small) baroclinic energy conversion north of the warm (cold) SST anomaly near the axis of the storm track (near 40ЊN) is responsible for the large (small) storm growth. The equatorward transfer of eddy kinetic energy by the ageostrophic motion and the mechanical damping are important to diminish the storm track activity in the zonal direction. Significant stationary eddies form in the upper troposphere, with a ridge (trough) northeast of the warm (cold) SST anomaly at 30ЊN. Heat and vorticity budget analyses indicate that zonally localized condensational heating in the storm track is the major cause for these stationary eddies, which in turn exert a positive feedback to maintain the localized storm track by strengthening the vertical shear near the surface. These results indicate an active role of synoptic eddies in inducing deep, tropospheric-scale response to midlatitude SST variations. Finally, the application of the model results to the real atmosphere is discussed.
Quarterly Journal of The Royal Meteorological Society, 2002
The effect of spatial and temporal variations in the radiative damping rate on the response to an imposed forcing or diabatic heating is examined in a zonal-mean model of the middle atmosphere. Attention is restricted to the extratropics, where a linear approach is viable. It is found that regions with weak radiative damping rates are more sensitive in terms of temperature to the remote influence of the diabatic circulation. The delay in the response in such regions can mean that ‘downward’ control is not achieved on seasonal time-scales. A seasonal variation in the radiative damping rate modulates the evolution of the response and leaves a transient-like signature in the annual mean temperature field.Several idealized examples are considered, motivated by topical questions. It is found that wave drag outside the polar vortex can significantly affect the temperatures in its interior, so that high-latitude, high-altitude gravity-wave drag is not the only mechanism for warming the southern hemisphere polar vortex. Diabatic mass transport through the 100 hPa surface is found to lag the seasonal evolution of the wave drag that drives the transport, and thus cannot be considered to be in the downward control regime. On the other hand, the seasonal variation of the radiative damping rate is found to make only a weak contribution to the annual mean temperature increase that has been observed above the ozone hole. Copyright © 2002 Royal Meteorological Society.
Journal of the Meteorological Society of Japan. Ser. II, 1991
Evidence of large temporal and spatial variability in the eddy fluxes of sensible heat in the lower troposphere (100-50 kPa layer) in January, from 1946 to 1987, is presented. The January climatological (1946-87) spatial distribution of the meridional standing eddy heat flux is characterized by four main features, or "centers of action" : (1) a region north of Korea (extreme eastern Siberia), (2) northeastern Atlantic Ocean, (3) the Gulf of Alaska, and (4) a region over midwestern Canada. Even though the center just north of Korea is the most intense, much of the interannual variability of the meridional January standing eddy sensible heat transport is associated with the two centers over the northeastern Atlantic and the Gulf of Alaska. Spatial and temporal variability of these centers are correlated with the intensities of the Icelandic Low and the Aleutian Low, respectively. The standing eddy heat transport in the Greenland Sea is well correlated with the ice margin there during years of large abnormal heat transport by the standing eddy component. The long term variability of the heat transport over the period 1946-87 shows a clear interdecadal signal in the Gulf of Alaska. Over the North Atlantic, the variability is instead dominated by a large perturbation in the early 1970's. The January climatological spatial distribution of the meridional transient eddy heat flux is dominated by a center over the Aleutian Peninsula, and an elongated cell stretching from eastern United States to North Sea, with a center off Newfoundland and another over Iceland. The spatial pattern follows closely the climatological tracks of baroclinic disturbances, i. e., synoptic storms. The geographical distribution of the spatial and temporal variability of the January meridional transient eddy heat flux is less organized than the pattern associated with the standing eddy heat flux. Much of the variability is confined to the western hemisphere, from the mid-North Atlantic Ocean, across North America, to the eastern half of the North Pacific. The centers of variability are located at (1) Gulf of Alaska, (2) western and eastern Canada, and (3) southeastern United States.
Breaking down the tropospheric circulation response by forcing
This study describes simulated changes in the general circulation during the twentieth and twenty-first centuries due to a number of individual direct radiative forcings and warming sea surface temperatures, by examining very long time-slice simulations created with an enhanced version of the Geophysical Fluid Dynamics Laboratories Atmospheric Model AM 2.1. We examine the effects of changing stratospheric ozone, greenhouse gas concentrations, and sea surface temperatures individually and in combination over both hemispheres. Data reveal robust poleward shifts in zonal mean circulation features in present-day simulations compared to a pre-industrial control, and in future simulations compared to present-day. We document the seasonality and significance of these shifts, and find that the combined response is well approximated by the sum of the individual responses. Our results suggest that warming sea surface temperatures are the main driver of circulation change over both hemispheres, and we project that the southern hemisphere jet will continue to shift poleward, albeit more slowly during the summer due to expected ozone recovery in the stratosphere.