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Yang Zhang and Peter H. Stone

Abstract

Baroclinic eddy equilibration and the roles of different boundary layer processes in limiting the baroclinic adjustment are studied using an atmosphere–ocean thermally coupled model. Boundary layer processes not only affect the dynamical constraint of the midlatitude baroclinic eddy equilibration but also are important components in the underlying surface energy budget. The authors' study shows that baroclinic eddies, with the strong mixing of the surface air temperature, compete against the fast boundary layer thermal damping and enhance the meridional variation of surface sensible heat flux, acting to reduce the meridional gradient of the surface temperature. Nevertheless, the requirement of the surface energy balance indicates that strong surface baroclinicity is always maintained in response to the meridionally varying solar radiation. With the strong surface baroclinicity and the boundary layer processes, the homogenized potential vorticity (PV) suggested in the baroclinic adjustment are never observed near the surface or in the boundary layer.

Although different boundary layer processes affect baroclinic eddy equilibration differently with more dynamical feedbacks and time scales included in the coupled system, their influence in limiting the PV homogenization is more uniform compared with the previous uncoupled runs. The boundary layer PV structure is more determined by the strength of the boundary layer damping than the surface baroclinicity. Stronger boundary layer processes always prevent the lower-level PV homogenization more efficiently. Above the boundary layer, a relatively robust PV structure with homogenized PV around 600–800 hPa is obtained in all of the simulations. The detailed mechanisms through which different boundary layer processes affect the equilibration of the coupled system are discussed in this study.

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K. K. Tung and H. Yang

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Observational evidence for a global quasi-biennial oscillation (QBO) pattern is reviewed. In particular, the presence of an extratropical, as well as an equatorial, component of the QBO signal in column ozone is established. It is found that the ozone interannual variability is such that as one moves away from the Tropics, the frequency spectrum of the anomaly changes from one that is dominated by the equatorial QBO frequency of 1/30 mo to a two-peak spectrum around the two frequencies: 1/30 mo and 1/20 mo. Instead of treating the 1/20 mo frequency as a separate phenomenon to be filtered away in extracting the QBO in the extratropics, as was previously done, the authors argue that both peaks are integral parts of the extratropical QBO phenomenon. The 1/20 mo frequency happens to be the difference combination of the QBO frequency 1/30 mo and the annual frequency 1/12 mo. Therefore, it can represent the result of the QBO modulating an annual cycle. The authors suggest that previous methods of extracting the extratropical QBO signal severely underestimated the contribution of the QBO to the interannual variability of ozone when data are filtered to pass only the component with the period of equatorial QBO.

Further, it is argued that the transport of equatorial QBO ozone anomaly by a non-QBO circulation can at most account for 6–8 Dobson units (DU) of the observed interannual variability of column ozone in the extratropics. The remaining variability (up to 20 DU) probably cannot be produced without an anomaly in the transporting circulation in the extratropics.

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Yang Zhang and Peter H. Stone

Abstract

Baroclinic eddy equilibration under a Northern Hemisphere–like seasonal forcing is studied using a modified multilayer quasigeostrophic channel model to investigate the widely used “quick baroclinic eddy equilibration” assumption and to understand to what extent baroclinic adjustment can be applied to interpret the midlatitude climate. Under a slowly varying seasonal forcing, the eddy and mean flow seasonal behavior is characterized by four clearly divided time intervals: an eddy inactive time interval in summer, a mainly dynamically determined eddy spinup time interval starting in midfall and lasting less than one month, and a quasi-equilibrium time interval for the zonal mean flow available potential energy from late fall to late spring, with a mainly external forcing determined spindown time interval for eddy activity from late winter to late spring. The baroclinic adjustment can be clearly observed from late fall to late spring. The sensitivity study of the eddy equilibration to the time scale of the external forcing indicates that the time scale separation between the baroclinic adjustment and the external forcing in midlatitudes is only visible for external forcing cycles one year and longer.

In spite of the strong seasonality of the eddy activity, similar to the observations, a robust potential vorticity (PV) structure is still observed through all the seasons. However, it is found that baroclinic eddy is not the only candidate mechanism to maintain the robust PV structure. The role of the boundary layer thermal forcing and the moist convection in maintaining the lower-level PV structure is discussed. The adjustment and the vertical variation of the lower-level stratification play an important role in all of these mechanisms.

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R. T. Pierrehumbert and H. Yang

Abstract

The general nature of two-dimensional mixing on isentropic surfaces in the troposphere has been investigated. The daily time series of isentropic winds is obtained from a global general circulation model and is used to drive a high-resolution fully Lagrangian passive tracer model. Results are compared with the extremes of chaotic mixing by organized waves on the one hand and classical diffusion on the other and are found to lie in the middle ground. Advection by planetary-and synoptic-scale eddies generates small scales from an initially smooth tracer field exponentially fast, but given a modest degree of smoothing the tracer field evolving from a localized release rapidly attains the form of an algebraically spreading cloud. The zonal size of the cloud increases linearly with time (superdiffusively), owing to the systematic shear in the extratropical zonal jets, while the meridional spread has the square-root-of-time increase characteristic of classical diffusion. It is argued, however, that the small-scale tracer structure missing from current general circulation models and from diffusive mixing models severely compromises the fidelity with which chemical reactions and the hydrological cycle can be modeled.

Several different analyses, including a study of the spatial distribution of finite-time Lyapunov exponents, indicate the presence of a partial barrier to mixing between the tropics and extratropics. In contrast with previous studies using simpler advecting flow fields, the extratropical mixing regions appear to be zonally homogeneous, and there are no impediments to zonal homogeneization. Our calculations indicate that a zonal wave 3 tracer pattern would be mixed away in 10–15 days. If the results can be taken as indicative of potential vorticity mixing, the implication is that under normal circumstances mixing due to synoptic eddies exerts a damping effect on planetary-scale waves. Implications of the results for the general circulation of the troposphere are discussed.

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H. Yang and R. T. Pierrehumbert

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The authors have explored the factors governing upper-tropospheric relative humidity with a simple model based on isentropic mixing and condensation. Our analysis has focused on the Northern Hemisphere winter season and on the 315-K (dry) isentropic surface.

The advection–condensation model yields the following results. In the absence of moisture resupply, about half of the mass of water is lost from the isentropic surface after only 10 days, with the main brake on drying being the weak mixing between Tropics and extratropics. The moist plumes escaping from the Tropics take the form of filamentary structures, which are more numerous and space filling in the summer/Southern Hemisphere than in the winter/Northern Hemisphere. These moist plumes are accompanied by substantial importation of extratropical dry air into the Tropics. The probability distributions of midlatitude relative humidity are bimodal, with a prominent dry peak having a lognormal tail and a spike representing saturated air; the summer hemisphere has generally higher relative humidity than the winter hemisphere. When moisture is maintained by periodically resaturating the Tropics, the resulting cloud and moisture fields exhibit a fractal character, with a tendency to become less space filling with distance from the Tropics.

Some tentative comparisons with data are made, which tend to confirm the advective control of relative humidity patterns outside the Tropics. There are indications, however, that the advection-condensation model with moisture resupply only from the Tropics yields an upper troposphere that is far too dry. The authors suggest that the missing moisture is supplied from the 295-K isentropic surface via diabatic mixing arising in ascending, convecting saturated trajectories near that surface.

As found in earlier passive tracer studies, the permeable mixing barrier between the Tropics and extratropics has the potential to exert a controlling influence on the global climate.

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K. K. Tung and H. Yang

Abstract

Although the phenomenon of equatorial quasi-biennial oscillation is relatively well understood, the problem of how the equatorially confined QBO wave forcing can induce a signal in the extratroics of comparable or larger magnitude remains unsolved. A simple mechanistic model is constructed to provide a quantitative test of the hypothesis that the phenomenon of extratropical QBO is mainly caused by an anomalous seasonal circulation induced by an anomalous Eliassen-Palm flux divergence. The anomaly in E–P flux divergence may be caused in turn by the relative poleward and downward shift of the region of irreversible mixing (breaking) of the extratropical planetary waves during the easterly phase of the equatorial QBO as compared to its westerly phase. The hemispheric nature of the anomaly wave forcing in solstice seasons (viz., no wave breaking in the summer hemisphere) induces a global circulation anomaly that projects predominantly into the first few zonal Hough modes of Plumb. Such a global QBO circulation pattern, although difficult to measure directly, is reflected in the distribution of stratospheric tracers transported by it. Our model produces a global pattern of QBO anomaly in column ozone that appears to account for much of the unfiltered interannual variability in the column ozone observed by the TOMS instrument aboard the Nimbus satellite. Furthermore, the model produces the characteristic spectrum of the observation with peaks at periods of 20 and 30 months.

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Yang Yang, Robert H. Weisberg, Yonggang Liu, and X. San Liang

Abstract

A recently developed tool, the multiscale window transform, along with the theory of canonical energy transfer is used to investigate the roles of multiscale interactions and instabilities in the Gulf of Mexico Loop Current (LC) eddy shedding. A three-scale energetics framework is employed, in which the LC system is reconstructed onto a background flow window, a mesoscale eddy window, and a high-frequency eddy window. The canonical energy transfer between the background flow and the mesoscale windows plays an important role in LC eddy shedding. Barotropic instability contributes to the generation/intensification of the mesoscale eddies over the eastern continental slope of the Campeche Bank. Baroclinic instability favors the growth of the mesoscale eddies that propagate downstream to the northeastern portion of the well-extended LC, eventually causing the shedding by cutting through the neck of the LC. These upper-layer mesoscale eddies lose their kinetic energy back to the background LC through inverse cascade processes in the neck region. The deep eddies obtain energy primarily from the upper layer through vertical pressure work and secondarily from baroclinic instability in the deep layer. In contrast, the canonical energy transfer between the mesoscale and the high-frequency frontal eddy windows accounts for only a small fraction in the mesoscale eddy energy balance, and this generally acts as a damping mechanism for the mesoscale eddies. A budget analysis reveals that the mesoscale eddy energy gained through the instabilities is balanced by horizontal advection, pressure work, and dissipation.

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H. Yang, E. Olaguer, and K. K. Tung

Abstract

This paper documents our two-dimensional model which incorporates comprehensive radiative transfer and chemistry modules coupled with self-consistent dynamical transports.

A simultaneous simulation of a range of chemical trace gas species with different photochemical time scales, latitudinal and vertical gradients, and with tropospheric or stratospheric sources is attempted and the result compared with available satellite and in situ observations.

The 2-D model utilizes all zonally averaged physical equations of momentum, energy and mass, and self-consistently determines both its advective and diffusive transport parameters from the observed temperature specific to the period of observation. A major assumption in the formulation is that diffusive mixing is caused by large-scale planetary waves which act predominantly along isentropic surfaces. It is also assumed that it is planetary waves that drive the stratophere away from radiative equilibrium, resulting in diabatic vertical and meridional advective transport. It is in this way that energy, momentum and tracer budgets are interconnected.

Family approximation is used and the transported species include Ox, NOy, N2O, Cly, CH4, CO, CFCs and HF. Partition within a family is calculated assuming photochemical equilibrium. Diurnal variation of nitrogen species is obtained by solving an ordinary differential equation analytically.

The comparison of the model result with observations is very favorable. Some previously known common model deficiencies have largely been overcome. Simulation of climatological ozone, including the details of seasonal, latitudinal and vertical distributions, is especially successful using the present coupled model. The problem of NOy deficit in the equatorial lower stratosphere also appears to have been resolved, and a correct latitudinal profile for nitric acid column is obtained.

We give physical reasons for the improvements in the model results and discuss possible explanations for the remaining systematic deficiencies, now occurring mostly in the model upper stratosphere and mesosphere, where breaking gravity waves may become an important transport process.

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William H. Klein, Amir Shabbar, and Runhua Yang

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Specification equations for monthly mean air temperature anomalies at 68 surface stations in Canada and Alaska are derived by applying a forward selection screening procedure to simultaneous monthly mean 500 mb height anomalies at 110 grid points in the Northern Hemisphere and the previous month's 1ocal temperature anomaly. For the annual average, these equations explain about 72% of the temperature variance (on dependent data) by means of only 4.3 variables, but with marked regional and seasonal differences.

The average properties of the specification equations closely resemble those derived previously from the field of 700 mb heights. The 500 mb equation however, explain about 2% more of the temperature variance by means of 0.7 fewer terms in the equations. This small but consistent superiority is evident during each month of the year and for equations based on heights only, as well as those including previous local temperature. The superiority is produced by higher correlations of temperature with local heights and suggests that the 500 mb level is more equivalent barotropic than 700 mb.

The specification equations are tested by means of several verification statistics computed for 5 years of independent data at 51 Canadian stations. The results show that equations based on concurrent 500 mb heights and previous local temperature perform slightly better than those based on heights only and much better than climatology or month-to-month persistence.

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Chunzai Wang, Robert H. Weisberg, and Huijun Yang

Abstract

The thermodynamical process of latent heat flux is added to an analogical delayed oscillator model of the El Niño–Southern Oscillation (ENSO) that mainly considers equatorial ocean dynamics and produces regular, non–phase-locked oscillations. Latent heat flux affects the model sea surface temperature (SST) variations by a positive feedback between the surface wind speed and SST operating through evaporation, which is called the wind speed–evaporation–SST feedback. The wind speed–evaporation–SST feedback in which the atmosphere interacts thermodynamically with the ocean through surface heat flux differs from the conventional zonal wind stress–SST feedback in which the atmsophere interacts dynamically with the ocean through momentum flux.

The combination of equatorial ocean dynamics and thermodynamics produces relatively more realistic model oscillations. When the annual cycle amplitude of the zonal wind in the wind speed–evaporation–SST feedback is gradually increased, the model solution undergoes a transition from periodic to chaotic and then to periodic oscillations for some ranges of the parameters, whereas for other ranges of the parameters the transition goes from periodic to quasiperiodic and then to periodic oscillations. The route to chaos is the intermittency route. Along with such irregularity, the nonlinear interactions between the annual and interannual cycles operating through the wind speed–evaporation–SST feedback also produce a phase-locking of ENSO to the seasonal cycle. The model ENSO onset and peak occur in the boreal winter and spring, respectively, consistent with the observed phase-locking of ENSO in the far eastern Pacific. It is shown that ENSO decadal or interdecadal variability may result from the nonlinear interactions between the annual and interannual cycles in the Tropics.

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