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I. Richter and C. R. Mechoso

Abstract

The impact of South American orography on subtropical stratocumulus clouds off the Peruvian coast is investigated in the context of an atmospheric general circulation model. It is found that stratocumulus incidence is significantly reduced when South American orography is removed. Key to this behavior is a decrease in lower tropospheric stability (LTS) that allows for more frequent stratocumulus destruction through the model’s cloud-top entrainment instability mechanism. The role of orography in enhancing Peruvian stratocumulus is as follows. Within the PBL, orography deflects the midlatitude westerly winds equatorward in association with cold air advection and blocking of the low-level flow from the continent. Above the PBL, the steep and high South American orography deflects a significant portion of the midlatitude westerlies equatorward. This flow sinks along the equatorward sloping isentropes, thus promoting subsidence. Both processes increase LTS over the stratocumulus region. In further AGCM experiments, the sensitivity of Peruvian stratocumulus to the use of unsmoothed orographic boundary conditions is assessed. The results show no significant differences to the control simulation, which uses smoothed orography. This suggests that, in the context of GCMs, a representation of South American orography more detailed than is generally used has little potential for improving the performance of coupled ocean–atmosphere models in the eastern tropical Pacific.

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G. L. Manney, J. D. Farrara, and C. R. Mechoso

Abstract

The evolution of the stratospheric flow during the major stratospheric sudden warming of February 1979 is studied using two primitive equation models of the stratosphere and mesosphere. The United Kingdom Meteorological Office Stratosphere-Mesosphere Model (SMM) uses log pressure as a vertical coordinate. A spectral, entropy coordinate version of the SMM (entropy coordinate model, or ECM) that has recently been developed is also used. Both models produce similar successful simulations through the peak of the warming, capturing the splitting of the vortex and the development of small-scale structures, such as narrow baroclinic zones. The ECM produces a more realistic recombination and recovery of the polar vortex in the midstratosphere after the warming, due mainly to better conservation properties for Rossby-Ertel potential vorticity in this model. Another advantage of the ECM is the automatic increase in vertical resolution near baroclinic zones. Comparison of SMM simulations with forecasts performed using the University of California, Los Angeles general circulation model confirms the previously noted sensitivity of stratospheric forecasts to tropospheric forecast and emphasizes the importance of adequate vertical resolution in modeling the stratosphere.

The ECM simulators provide a schematic description of the three-dimensional evolution of the polar vortex and the motion of air through it. During the warming, the two cyclonic vortices till westward and equatorward with height. Strong upward velocities develop in the lower stratosphere on the west (cold) side of a baroclinic zone as it forms over Europe and Asia. Strong downward velocities appear in the upper stratosphere on the east (warm) side, strengthening the temperature gradients. After the peak of the warming, vertical velocities decrease, downward velocities move into the lower stratosphere, and upward velocities move into the upper stratosphere. Transport calculations show that air with high ozone mixing ratios is advected toward the pole from low latitudes during the warming, and air with low ozone mixing ratios is transported to the midstratosphere from both higher and lower altitudes along the baroclinic zone in the polar regions. Trajectories of parcels moving around the vortex oscillate up and down as they move through regions of ascending and descending motions, with an overall increase in pressure in the polar regions. Tracer transport and trajectory calculations show enhanced diabatic descent in the region between cyclone and anticyclone during the warming, consistent with the temperature structure shown.

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G. L. Manney, J. D. Farrara, and C. R. Mechoso

Abstract

A detailed analysis is presented of the behavior of the zonal wavenumber 2 component of the flow (wave 2) for July through October in the Southern Hemisphere stratosphere. Wave 2 in the stratosphere is characterized by a broad meridional structure peaking between 55° and 65°S, and regular eastward propagation, with periods ranging from 5 to 40 days. Maximum geopotential height amplitudes for a year range from approximately 600 to 1000 m. Examination of vertical structure suggests that during episodes of largest growth, wave 2 propagates upward from the upper troposphere. Regular eastward propagation is, however, evident only within the stratosphere. There are also episodes of wave 2 growth that do not appear connected with the troposphere; in general, the wave 2 amplitude is not as large in these cases.

There are several years when wave 1 and wave 2 amplitudes are strongly anticorrelated in time during September and October. There are also years with strong positive correlation during August and September. While wave 1 is usually quasi-stationary, a number of instances where wave 1 moves eastward with wave 2 are observed, lasting from 4 to 10 days.

Calculations show that the zonal mean state of the Southern Hemisphere stratosphere frequently satisfies conditions for instability. It is suggested that instability of zonally symmetric and asymmetric states, and nonlinear interactions between wave 1 and wave 2 both play a role in determining the behavior of wave 2 in the Southern Hemisphere winter and spring stratosphere.

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Y. Gu, J. Farrara, K. N. Liou, and C. R. Mechoso

Abstract

A contemporary radiation parameterization scheme has been implemented in the University of California, Los Angeles (UCLA), atmospheric GCM (AGCM). This scheme is a combination of the delta-four-stream method for solar flux transfer and the delta-two-and-four-stream method for thermal infrared flux transfer. Both methods have been demonstrated to be computationally efficient and at the same time highly accurate in comparison with exact radiative transfer computations. The correlated-k distribution method for radiative transfer has been used to represent gaseous absorption in multiple-scattering atmospheres. The single-scattering properties for ice and water clouds are parameterized in terms of ice/liquid water content and mean effective size/radius. In conjunction with the preceding radiative scheme, parameterizations for fractional cloud cover and cloud vertical overlap have also been devised in the model in which the cloud amount is determined from the total cloud water mixing ratio. For radiation calculation purposes, the model clouds are vertically grouped in terms of low, middle, and high types. Maximum overlap is first used for each cloud type, followed by random overlap among the three cloud types. The preceding radiation and cloud parameterizations are incorporated into the UCLA AGCM, and it is shown that the simulated cloud cover and outgoing longwave radiation fields without any special tuning are comparable with those of International Satellite Cloud Climatology Project (ISCCP) dataset and derived from radiation budget experiments. The use of the new radiation and cloud schemes enhances the radiative warming in the mid- to upper tropical troposphere and alleviates the cold bias that is common to many AGCMs. Sensitivity studies show that ice crystal size and cloud inhomogeneity significantly affect the radiation budget at the top of the atmosphere and the earth’s surface.

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A. W. Robertson, C-C. Ma, M. Ghil, and C. R. Mechoso

Abstract

Two multiyear simulations with a coupled ocean-atmosphere general circulation model (GCM)-totaling 45 years-are used to investigate interannual variability at the equator. The model consists of the UCLA global atmospheric GCM coupled to the GFDL oceanic GCM, dynamically active over the tropical Pacific. Multichannel singular spectrum analysis along the equator identifies ENSO-like quasi-biennial (QB) and quasi-quadrennial (QQ) modes. Both consist of predominantly standing oscillations in sea surface temperature and zonal wind stress that peak in the central or east Pacific, accompanied by an oscillation in equatorial thermocline depth that is characterized by a phase shift of about 90° across the basin, with west leading east. Simulated interannual variability is weaker than observed in both simulations. One of these is dominated by the QB, the other by the QQ mode, although the two differ only in details of the surface-layer parameterizations.

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A. W. Robertson, C-C. Ma, C. R. Mechoso, and M. Ghil

Abstract

A multiyear simulation with a coupled ocean-atmosphere general circulation model (GCM) is presented. The model consists of the UCLA global atmospheric GCM coupled to the GFDL oceanic GCM; the latter is dynamically active over the tropical Pacific, while climatological time-varying sea surface temperatures (SST) are prescribed elsewhere. The model successfully simulates the main climatological features associated with the seasonal cycle, including the east-west gradient in SST across the equatorial Pacific. The most apparent deficiencies include a systematic cold bias (∼2 K) across most of the tropical Pacific and underestimated wind stress magnitudes in the equatorial band. Multichannel singular spectrum analysis is used to describe the multivariate structure of the seasonal cycle at the equator in both the model and observed data. The annual harmonic in equatorial SST is primarily wind driven, while air-sea interaction strongly affects the semiannual harmonic.

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L. J. Gelinas, R. L. Walterscheid, C. R. Mechoso, and G. Schubert

Abstract

Spectral analyses of time series of zonal winds derived from locations of balloons drifting in the Southern Hemisphere polar vortex during the Vorcore campaign of the Stratéole program reveal a peak with a frequency near 0.10 h−1, more than 25% higher than the inertial frequency at locations along the trajectories. Using balloon data and values of relative vorticity evaluated from the Modern Era Retrospective-Analyses for Research and Applications (MERRA), the authors find that the spectral peak near 0.10 h−1 can be interpreted as being due to inertial waves propagating inside the Antarctic polar vortex. In support of this claim, the authors examine the way in which the low-frequency part of the gravity wave spectrum sampled by the balloons is shifted because of effects of the background flow vorticity. Locally, the background flow can be expressed as the sum of solid-body rotation and shear. This study demonstrates that while pure solid-body rotation gives an effective inertial frequency equal to the absolute vorticity, the latter gives an effective inertial frequency that varies, depending on the direction of wave propagation, between limits defined by the absolute vorticity plus or minus half of the background relative vorticity.

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G. L. Manney, L. S. Elson, C. R. Mechoso, and J. D. Farrara

Abstract

Eastward-traveling waves 2 and 3 are frequently observed to grow in the Southern Hemisphere stratosphere during late winter and early spring. Observations show times when wave 2 growth appears to be confined to the stratosphere. This suggests that instability is one in situ mechanism that should be considered. The stability of stratospheric flows derived from data is examined during some of these times, using several linear models of quasigeostrophic instability.

Unstable modes of both wave 2 and wave 3 have periods and spatial structures similar to observations. Wave 2 and wave 3 momentum fluxes are similar in observations and model results and are consistent with the transfer of kinetic energy from the zonal-mean flow to the wave. When a barotropic model with a zonally symmetric basic flow is used, wave 3 is usually most unstable. Including a stationary wave 1 in the basic flow destabilizes both wave 2 and wave 3, but has little effect on their periods or spatial structures. When a zonally symmetric flow with realistic meridional and vertical structure is used, resulting unstable modes have shorter periods and slower growth rates than for barotropic flows. Wave 2 is usually more unstable than wave 3 when realistic vertical structure is included.

The similarity between observed fields and model results in a number of cases when wave 2 appears to grow within the stratosphere suggests that in situ instabilities play a role in the evolution of the eastward-traveling wave 2 characteristic of the Southern Hemisphere winter and early spring stratosphere.

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H.-Y. Ma, X. Ji, J. D. Neelin, and C. R. Mechoso

Abstract

The present study examines the mechanisms for the connection between the precipitation variability in eastern Brazil and the South Atlantic convergence zone (SACZ) convective margin (eastern Brazil/SACZ convective margin) and the variability of low-level inflow on interannual time scales during austral summer. The authors' methodology is based on the analysis of observational datasets and simulations by the University of California, Los Angeles (UCLA) atmospheric general circulation model (AGCM) coupled to the Simplified Simple Biosphere Model.

It is demonstrated that the inflow variability is associated with the leading mode of wind variability over subtropical South America, and the connection is established through the mechanism of an analytic prototype for convective margin shifts proposed in previous studies. Over the eastern Brazil/SACZ convective margin, the weaker (stronger) convection tends to occur together with stronger (weaker) low-level inflows in reference to the mean easterly trades. By changing the “ventilation” effect, stronger (weaker) inflows with low moist static energy from the Atlantic Ocean suppress (promote) convection. The causal relationship is verified by AGCM mechanism-testing experiments performed in perpetual-February mode, in which low-level, nondivergent wind perturbations are imposed in a region overlapping eastern Brazil and the western Atlantic Ocean. With solely the imposed-wind perturbations acting on the moisture advection in the model equation, the AGCM can reproduce the precipitation variability in the eastern Brazil/SACZ convective margin. The capability of the AGCM in capturing such precipitation sensitivity to the low-level inflow variability also suggests that the mechanism can be applied to other regions of convective margins or to other time scales.

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John D. Farrara, Michael Ghil, Carlos R. Mechoso, and Kingtse C. Mo

Abstract

We present a new empirical orthogonal function (EOF) analysis of winter 500 mb geopotential height anomalies in the Southern Hemisphere. An earlier EOF analysis by two of the present authors prefiltered the anomalies to exclude wavenumbers 5 and higher; we do not. The different preprocessing of data affects the results. All three distinct planetary flow regimes identified in the winter circulation of the Southern Hemisphere by a pattern correlation method are captured by the new set of EOFs; only two of those regimes were captured by the earlier set. The new results, therefore, lend further support to the idea that EOFs point to distinct planetary

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