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  • Author or Editor: C. R. Mechoso x
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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

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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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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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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