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Search SpringerLink Search. Abstract Barotropic-Baroclinic instability of horizontally and vertically shearing mean monsoon flow during July is investigated numerically by using a layer quasi-geostrophic model. References Charney, J. Google Scholar Colton, D.
Google Scholar Klein, W. Google Scholar Krishnamurti, T. Google Scholar Shukla, J. Understanding the extratropical atmospheric response to thermal and mechanical forcing is central to a range of current problems in climate dynamics. In all cases, the mechanisms that drive the tropospheric response are not fully understood. The problem lies not in the balanced response of the extratropical atmosphere to external forcing.
The geostrophically and hydrostatically balanced response to thermal and mechanical forcing is both well understood and straightforward to estimate Haynes and Shepherd ; Haynes et al. Rather, the problem lies in understanding and predicting the subsequent changes in the extratropical eddy fluxes of heat and momentum. The eddy-driven jet is collocated with large eddy fluxes of heat in the lower troposphere and convergence of the eddy-momentum flux at the tropopause level.
Thus, understanding and predicting the response of the jet to external forcing can be accomplished only through understanding and predicting the response of its attendant wave fluxes of heat and momentum.
The wave fluxes of momentum are particularly important, as they determine the barotropic component of the flow, project strongly onto the annular modes and their attendant climate impacts, and influence the lower-tropospheric baroclinicity. The response of the wave fluxes of momentum to a given forcing can arise through two sets of processes:. The goal of this study is to present a methodology to investigate the relative importance of barotropic and baroclinic eddy feedbacks in determining the structure and amplitude of the extratropical circulation response to mechanical forcing.
The study is modeled on the experiments performed in Ring and Plumb , hereafter RP07 , in which the dynamical core of a general circulation model is subject to mechanical torques placed over a range of extratropical latitudes. Here we perform similar experiments, but apply a wider range of mechanical forcings to a hierarchy of numerical models with varying representations of extratropical wave—mean flow interactions.
As such, the results provide insight into 1 the relationships between the forcing and response latitudes, 2 the relationships between the forcing latitude and climatological-mean jet position, and 3 the physical feedbacks that play a key role in determining the amplitude and structure of the atmospheric response to mechanical forcing. The experiments are described in section 2 , results are given in sections 3 — 5 , and discussion and conclusions are given in section 6.
We conduct a series of experiments similar to those run in RP07 , in which the extratropical atmosphere is subjected to a series of mechanical torques centered at a range of latitudes. In all experiments the torque is applied as a tendency in the zonal-mean zonal wind. The relative importance of barotropic and baroclinic processes in determining the circulation response to the imposed mechanical torques is assessed using the following hierarchy of numerical experiments.
Citation: Journal of the Atmospheric Sciences 71, 1; In this study, we distinguish barotropic feedbacks as those simulated by a barotropic model with fixed stirring constant eddy source. This definition of barotropic feedbacks thus includes the interaction of the background flow with the wave propagation and dissipation.
We note, however, that the barotropic model also includes the influence of the background vorticity gradient on the pseudomomentum source, which can also modulate the eddy fluxes see Barnes and Garfinkel for discussion of this feedback. Baroclinic feedbacks are defined as changes in the position and strength of the eddy source due to changes in the low-level baroclinicity.
We note, however, that other distinctions between barotropic and baroclinic feedbacks are also possible. For example, baroclinic processes may modulate wave characteristics such as phase speed and wavenumber rather than just the strength and position of the wave generation. We will not be directly simulating these feedbacks in the barotropic model experiments.
In the GCM experiments we apply the zonal torques to the spectral dry dynamical core used in Held and Suarez The model parameters are identical to those in Held and Suarez unless otherwise mentioned.
The model is integrated at T42 resolution, with 20 evenly spaced sigma levels and a time step of s. The model forcing is zonally and hemispherically symmetric. The applied torques are identical at all model pressure levels. The GCM is run under three different control climatologies. The zonal-mean zonal-wind field is evaluated in the lower troposphere hPa , since that is where friction acting on the wind field balances the vertically integrated eddy-momentum flux convergence.
The eddy-momentum flux convergence is pressure-weighted averaged from hPa to the top of the atmosphere, where the fluxes are first calculated at each pressure level before the vertical average is applied.
Figure 2a shows the hPa zonal-wind profile for the GCM45 control integration solid black line. The near-surface westerlies are maintained against drag by the eddies and as evidenced in Fig. The EMFC maximizes in midlatitudes and exhibits the largest divergence on the jet flanks where breaking Rossby waves produce an easterly torque.
The goal of the study is to identify the relative roles of barotropic and baroclinic feedbacks in the extratropical atmospheric response to mechanical forcing. To help identify the role of barotropic feedbacks, we analyze output from a stirred barotropic model on the sphere. In the model, stirring of the vorticity parameterizes the wave source. The distribution of the stirring i. Details of the model are given in Barnes and Garfinkel and Vallis et al. The barotropic model is spectral and nondivergent.
Stirring is applied as an additional term in the vorticity tendency equation and is scale specific; stirring occurs over total wavenumbers 8—12, which requires that the zonal wavenumber be greater than 3 in order to emphasize synoptic-scale eddies. We will be comparing output from the barotropic and general circulation models to test the relative importance of different eddy feedbacks in the response to identical forcings.
For this reason, we wish to limit as much as possible the differences between the model climatologies. To do this, we set the damping time scale, amplitude, and location of the stirring so that two aspects of the climatology in the barotropic model match as closely as possible those from full GCM: 1 the latitude and strength of the maximum zonal-mean zonal wind and 2 the magnitude of the eddy-momentum flux convergence see Table 1.
Summary of mean states in the control simulations. The GCM values are calculated using the hPa winds. Values have been rounded to the nearest 0. The crosses in Fig. Here we have used a frictional time scale of 6. The latitude and strength of the maximum zonal-mean zonal winds agree well with that of the GCM by construction Fig.
However, the wind profiles themselves are determined purely by the eddy fluxes in each model i. The agreement between the climatological-mean zonal flow of the GCM and barotropic model attest to the utility of the barotropic model for simulating that part of the GCM zonal wind that is driven by eddy-momentum fluxes.
The feedbacks are introduced on day of the control BARO experiment to allow the jet and eddies to come into equilibrium without the baroclinic feedback present and then spun up an additional days before the day integration. Both the GCM and the barotropic model show distributions of jet latitude that are narrower than the distributions of the eddy-momentum flux convergence; highlighting that the maximum eddy forcing on daily time scales does not always align with the zonal jet.
This is possible when the zonal-wind acceleration due to the shifted eddy forcing is not enough to shift the zonal-wind maximum. Note, however, that the eddy-momentum flux convergence and the surface winds must balance in steady state.
Careful comparison of Figs. The histograms are shown in Figs. This propensity for the eddies and jet to migrate poleward is likely due to the mechanism first explored Feldstein and Lee , where the preference for waves to propagate and break on the equatorward flank of the jet causes the jet and eddies to shift poleward over time.
This value compares reasonably well with the observed e -folding time scale of the tropospheric southern annular mode Gerber et al. This bias in the GCM toward long time scales is well documented and appears to be sensitive to model resolution, topography, and mean state Gerber and Vallis ; Wang et al. Annular-mode time scales for the BARO and FDBK runs with varying feedback strengths are given in Table 3 , and the persistence of the annular mode increases with increasing feedback strength.
The findings for these additional experiments are presented in appendix A. The magnitude of the response changes as the feedback changes, but the results are otherwise qualitatively similar.
We will first discuss the circulation response in the GCM45 experiments. By construction, the response includes the full suite of dry baroclinic and barotropic feedbacks. We will then compare the full GCM responses to those derived from the barotropic model experiments with different representations of the eddy feedbacks. Figure 4 shows the zonal-mean near-surface zonal-wind response in the GCM45 experiments. The format used to construct Fig. The abscissa denotes the latitude at which the forcing is centered, the ordinate is used to denote the latitude of the response, and the dotted black line denotes the one-to-one line i.
The thick solid lines denote the position of the control jet and the dashed lines denote the centers of action of the model annular mode in the zonal-mean zonal wind. In the GCM45 simulations, the control jet lies at The results in Fig. Figure 5d shows the corresponding changes in the eddy-momentum flux convergence. The most robust aspect of the GCM eddy response is that the imposed torque leads to changes in the eddy fluxes of momentum, regardless of the latitude of the forcing.
Beyond this, the response can be divided into two regimes:. That the eddy response lies poleward of the forcing latitude is consistent with the nature of meridionally propagating waves. In regions where the flow already permits a range of phase speeds, increases in the flow have little effect on the range of phase speeds that are permitted there. In contrast, in regions where the flow is relatively weak, incremental changes in the zonal flow have a much larger effect on the range of phase speeds permitted there.
The changes in the wave forcing should thus peak on the flanks of the jet, where the flow is relatively weak, thus shifting the jet poleward or equatorward, in the case of a low-latitude torque.
For example, consider Fig. The red curves denote the total eddy-momentum flux convergence profiles for each integration. Figures 5a,d demonstrate that the eddies induce a dipolar response in the winds for forcing on the flanks of the control jet. When the torque is applied at the latitude of the control jet, the zonal-wind response is weak since the eddies oppose the torque there; that is, there is anomalous divergence at the torque latitude.
Similar conclusions were reached in RP07 , but our inclusion of forcings across a wider range of latitudes yields the following additional insights into the GCM response to mechanical forcing:. Since part of the motivation for this work is to extend the results of RP07 , appendix B presents additional GCM simulations using parameters similar to those used in RP The response of the GCM to mechanical forcing includes both dry barotropic and baroclinic eddy feedbacks.
The middle and right columns of Fig. The wind responses in both barotropic model configurations are dominated by accelerated winds along the torque axis, with the weakest responses found when the forcing is near the control jet latitude as is true for the GCM. Both experiments also exhibit dipolar responses in the winds when the forcing is placed on the flanks of the jet.
In all cases, the wind responses are weaker in the runs without the baroclinic eddy feedback. For the most part, it appears that barotropic dynamics may play a key role in setting the structure of the response in the GCM, while baroclinic feedbacks set the amplitude.
Figure 7 quantifies the similarities and differences between 1 the GCM45 response and 2 the responses of the two barotropic model configurations. The response fields are first interpolated to a 0. This is done to account for differences in the mean states of the various model configurations. The covariance rather than correlation is chosen so as to take into account both the pattern and magnitude of the responses, and the values are scaled so that the largest agreement is equal to 1.
Figure 7a reveals that the addition of a baroclinic-like feedback to the barotropic model acts to noticeably improve the zonal-wind response similarities with the full GCM response. The improvement is evident for all forcing latitudes.
Figure 7b shows the associated spatial covariances of the eddy responses Figs. And again, the agreement with the GCM is lowest just south of the control jet latitude for both experiments. Thus, the FDBK results suggest that a key to simulating the GCM response for forcing away from the jet is allowing the stirring region, and thus the baroclinic zone, to move with the circulation.
Finally, we quantify the magnitude of the wind response due solely to the eddies in the barotropic experiments. In the barotropic model, the control winds are purely eddy driven, allowing the direct response of the zonal winds to the torque to be computed.
As shown in Eq. We have performed such integrations, and find that the maximum wind response is approximately 6. Equation 9 predicts a maximum of 6.
By subtracting Fig. Note that since the torque is zonally symmetric and thus applied only to the zonal-mean budget, the eddy response is brought about solely by changes in the zonal-mean winds and thus signifies either a barotropic or baroclinic-like eddy-mean flow feedback. As expected, eddy feedbacks explain all of the wind response away from the torque latitude. For forcing near the jet center, the eddies generally oppose the torque.
In this section, we investigate the role of the mean state on the response of the circulation to an external torque. We perform this analysis based upon the recent results of Garfinkel et al. Consistent with those studies, Barnes and Hartmann and Barnes and Polvani demonstrate that the meridional shifts in the flow associated with the annular mode varies across a range of models as a function of the mean jet latitude, with higher-latitude jets experiencing smaller shifts in the flow, and vice versa.
By modifying the equilibrium temperature gradient to move the tropospheric jet refer to section 2 , we can investigate to what degree the response magnitude to the same mechanical torque is a function of the mean jet latitude. We will show that the latitude of the jet appears to play a role in modulating the response and that this effect is present in the barotropic model runs.
Figure 9 displays results for the three GCM configurations outlined in section 2 , with the GCM45 experiment repeated for comparison. The jet latitude and jet speed for each run are summarized in Table 1.
The vertical structure of the zonal-mean zonal winds is shown in the top rows of Fig. The second and third rows of Fig. What is of interest to us are the differences in the responses between the three simulations. Comparison of the responses in Fig. The maximum eddy response does align remarkably well with the EOFs of the eddy-momentum flux convergence, as shown by the dashed lines in Fig.
With this in mind, one would not necessarily expect the wind response to align with the zonal-wind EOF, as the wind response is a function of both the eddy response and the direct forcing by the torque. Hence, the pattern of variability in the EMFC may be a better indicator of the structure of the circulation response to external forcing, at least on the poleward flank of the jet. In the rest of this section we will focus on the weakening of the wind and eddy responses to the torque in Fig.
A dependence on latitude of the tropospheric response to stratospheric perturbations was found by Garfinkel et al. A weakening of the eddy response can be brought about in two ways or a combination of the two : 1 a decrease in the difference between the magnitude of the forced and control EMFC while the structure of the EMFC remains fixed or 2 a decrease in the shift of the EMFC while the magnitude of the EMFC remains fixed.
We do, however, find evidence of the latter—that is, that the eddy fluxes shift less for higher-latitude jets. This is evident in Fig. The amount of shift is the distance between the peak EMFC and the zero line. Going from the lowest-latitude jet to the highest blue curve, black curve, red curve , the amount that the eddy fluxes shift with the forcing decreases.
The differences in eddy responses among the three GCM experiments feed back on the mean flow, and Fig. These results are consistent with those of Garfinkel et al. Thank you! Published by Mario Elman Modified over 6 years ago. The atmosphere heated at the equator is then cooled at the poles, forming a kind of thermal convection. Looking at satellite pictures, the clouds around the equator do seem to be moving in a convection-like fashion. However, the cloud movement at mid-latitudes doesn't look like a convective current at all.
By looking at global movement of clouds, we can see a prominent east-west tendency in the wind, i. The east and west winds that circle the globe are know as the trade winds and the westerlies. Hadley theorized that the atmosphere moved in one large thermal convection current from the equator to the poles. The movement of the atmosphere isn't quite that simple. Mostly due to the rotation of the earth.
Let's make a simple experiment. We put water into a donut-shaped container, which will be the earth's atmosphere. We'll heat the outer edge of the container, and cool the inner edge, so the inner edge will be the poles, and the outer edge will be the equator.
Because warm water is light, and cold water is heavy, this difference makes a pressure gradient between the inner and outer edges. If the earth wasn't rotating, this pressure gradient pushes the cold water under the lighter warm water, causing a convective current. As the pressure difference begins to move water along the pressure gradient, the Coriolis force changes the direction of the flow.
It ends up with the flow in which the pressure gradient balances with the Coriolis force resulting what is called the geostrophic flow. In this model of the atmosphere, if the distribution of temperature is concentric, then the resulting geostrophic flow will also be concentric. In this situation, the flow of water in the upper layers is in the same direction as the rotating table, like the westerlies which we see in the earth's atmosphere. In this photo, the flow of water is made visible by liquid crystal capsules, and temperature is represented by different colors.
A convection current is meant to carry heat from hot areas to cold areas, but with a concentric flow of water, this obviously doesn't happen. In other words, a concentric flow of water makes for an extremely inefficient thermal convection. If you speed up the rotation of the model, this concentric flow becomes unstable, and begins to meander.
This is called baroclinic instability. When water meanders, it moves back and forth from hot to cold areas, carrying the heat as it goes.
Compared to a concentric flow, this meandering flow transfers heat far more efficiently. This circulation is called a Ferrel cell. The flow of the atmosphere at the mid-latitudes is characterized by the baroclinic instability, which causes high and low pressure systems.
On the other hand, the circulation of air at low latitudes is close to the model that Hadley proposed long ago, so these areas are called Hadley cells. Wind clockwise, sinking air and as sinks gets warmer and therefore no rain. In centre air rising therefore low pressure. Air moves in an anticlockwise direction. That means, moving in any direction away from the "High" will result in a decrease in pressure.
A high pressure center also represents the center of an anticyclone and is indicated on a weather map by a blue "H". Winds flow clockwise around a high pressure center in the northern hemisphere, while in the southern hemisphere, winds flow counterclockwise around a high.
This is why fair weather is commonly associated with an area of high pressure. That means, moving in any horizontal direction away from the "Low" will result in an increase in pressure. Low pressure centers also represent the centers of cyclones.
A low pressure center is indicated on a weather map by a red "L" and winds flow counterclockwise around a low in the northern hemisphere. The opposite is true in the southern hemisphere, where winds flow clockwise around an area of low pressure. Rossby Waves are like rivers of air in the upper troposphere and they gradually meander.
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