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  • Author or Editor: Carlos R. Mechoso x
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Carlos R. Mechoso

Abstract

Available observations of the atmospheric circulation over the coast of Antarctica indicate the presence of a core of westerly winds in the upper troposphere. The linear stability of these westerlies is studied by using a semi-spectral numerical model with which the linearized, shallow, anelastic hydrostatic equations are integrated. The influence on the stability of the westerlies of both the slope and amplitude of the topography representative of East Antarctica is analyzed. The results obtained for several basic flows taken as idealizations of possible mean states indicate that although the topography exerts a somewhat stabilizing influence, the doubling times for the unstable perturbations are less than two days in all cases.

It is shown by using a three-level primitive equation model that the combined action of finite-amplitude baroclinic waves migrating from middle latitudes, the topography of Antarctica, and the meridional temperature gradients around the continent can generate westerlies with jetlike structure over the topographic slopes. Furthermore, none of those mechanisms acting separately can generate such a jet.

The results suggest that the region around Antarctica, far from being a place where all baroclinic processes are damped out by topographic slopes, is baroclinically very active with a complicated energy cascade, and that the distinctive topographic characteristics of Antarctica are fundamental to the permanence of low temperatures in its overlying atmosphere.

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Carlos R. Mechoso

Abstract

In the two-layer quasi-geostrophic model with boundaries sloping perpendicular to the basic flow, the ratios of the slopes of the bottom and the top to that of the interface between the fluid layers in the basic state are important parameters in the expression of the growth rate of unstable waves. When Eady's (1949) model is extended to include sloping bottom and top boundaries, the growth rates of unstable waves depend on the ratios of the slopes of the bottom and the top to that of the isentropes of the basic state. For the Eady model with sloping bottom, an important parameter characterizing the instability is the ratio between the vertical and horizontal heat transports by the wave divided by the slope of the isentropes of the basic state. An interpretation of these ratios and their relations clarifies the stabilization of the system for large slopes, the variation of the wavelength of the most unstable wave with the bottom slope, and the destabilization of some short waves for negative bottom slopes. It is found that the most unstable wave of the system has zero vertical energy flux convergence at the sloping bottom.

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Heng Xiao
and
Carlos R. Mechoso

Abstract

The present paper examines ways in which the seasonal cycle influences the evolution of El Niño in the tropical Pacific. The following hypotheses and associated physical mechanisms are investigated: (i) Hypothesis 1 (H1)—the seasonal warming of the cold tongue early in the calendar year (January–April) favors the initial growth of an event; (ii) hypothesis 2 (H2)—during an event, the warm surface waters migrating in the western basin from the Southern to the Northern Hemisphere during the northern spring (April–May) trigger enhanced convection along the equator, which contributes to reinforce the event; and (iii) hypothesis 3 (H3)—the warm surface waters returning in the western basin from the Northern to the Southern Hemisphere toward the end of the calendar year (November–January) favor the demise of ongoing events.

Hypothesis-validation experiments are performed with a coupled atmosphere–ocean general circulation model (CGCM)—the tropical Pacific version of the University of California, Los Angeles (UCLA) CGCM. The anomaly-coupling technique is applied, in which the simulated seasonal cycle and interannual variability can be separated and artificially modified to highlight the aspect targeted for examination, thus allowing for comparisons of simulations in which seasonal conditions in the CGCM’s atmospheric component are either fixed or time varying. The results obtained in the experiments are supportive of hypotheses H1 and H3. No supportive evidence is found for the validity of hypothesis H2.

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Heng Xiao
and
Carlos R. Mechoso

Abstract

This study examines whether shifts between the correlative evolutions of ENSO and the seasonal cycle in the tropical Pacific Ocean can produce effects that are large enough to alter the evolution of the coupled atmosphere–ocean system. The approach is based on experiments with an ocean general circulation model (OGCM) of the Pacific basin, in which the seasonal and nonseasonal (interannually varying) components of the surface forcing are prescribed with different shifts in time. The shift would make no difference in terms of ENSO variability if the system were linear. The surface fluxes of heat and momentum used to force the ocean are taken from 1) simulations in which the OGCM coupled to an atmospheric GCM produces realistic ENSO variability and 2) NCEP reanalysis data corrected by Comprehensive Ocean–Atmosphere Data Set climatology for the 20-yr period 1980–99. It is found that the response to the shifts in terms of eastern basin heat content can be 20%–40% of the maximum interannual anomaly in the first experiment, whereas it is 10%–20% in the second experiment. In addition, the response to the shift is event dependent. A response of this magnitude can potentially generate coupled atmosphere–ocean interactions that alter subsequent event evolution. Analysis of a selected event shows that the major contribution to the response is provided by the anomalous zonal advection of seasonal mean temperature in the equatorial band. Additional OGCM experiments suggest that both directly forced and delayed signals provide comparable contributions to the response. An interpretation of the results based on the “delayed oscillator” paradigm and on equatorial wave–mean flow interaction is given. It is argued that the same oceanic ENSO anomalies in different times of the oceanic seasonal cycle can result in different ENSO evolutions because of nonlinear interactions between equatorially trapped waves at work during ENSO and the seasonally varying upper-ocean currents and thermocline structure.

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Douglas M. Sinton
and
Carlos R. Mechoso

Abstract

A two-layer, shallow-water frontal model on an f-plane is used to study the nonlinear evolution of frontal waves. The fluid is confined to a periodic channel with parallel vertical walls. It is found that, at an advanced stage in the evolution of frontal waves, small-scale disturbances develop along the cold front while the warm front evolves in a smooth fashion. It is shown that the motion field associated with the primary low advects kinetic energy and low potential vorticity into the cold-frontal region. That kinetic energy is transferred by barotropic processes to the secondary disturbances at locations along the cold front where advection of low potential vorticity results in an enhancement of the horizontal shears. On the other hand, kinetic energy is removed from the warm-frontal region, which remains undisturbed.

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Carlos R. Mechoso
and
Douglas M. Sinton

Abstract

The energy analysis of the two-layer frontal model of Kotchin (1932) and Orlanski (1968) is reformulated. The new formulation is based on separating the contributions to the eddy kinetic energy of the unstable waves by the changes in 1) the difference in relative momentum between the layers (multiplied by the shear), and in 2) the available potential energy. Such a separation results in a clear characterization of the instabilities, particularly near the Rayleigh, Helmholtz and baroclinic instability limits. The mean meridional circulation induced by the unstable waves is analyzed.

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Koji Yamazaki
and
Carlos R. Mechoso

Abstract

The evolution of the flow in the Southern Hemisphere during the period 31 August-10 November 1979 is examined. The final stratospheric warming of 1979 and the associated reversal of the flow above 10 mb occurred during this period. It is found that this warming processs was newly monotonic but modulated by a series of events with enhanced eddy activity.

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Carlos R. Mechoso
and
Dennis L. Hartmann

Abstract

Southern Hemisphere analyses from the surface to 2 mb and from 20 to 80°S for the period May-September 1979 have been used to study the structure of traveling planetary waves. Space-time cross- spectral analysis of the height field has been employed to define the amplitude and phase for both the eastward and westward moving components of particular combinations of zonal wavenumber and frequency band. Latitude-height contour plots of power, phase and coherence squared show that westward moving waves have structure characteristic of barotropic external modes and are coherent across a broad range of latitudes and from the surface to 2 mb, the highest level analyzed. Eastward-moving waves, on the other hand, have more rapid phase variations, especially in the troposphere, and appear more baroclinic. The tropospheric structure of the moving components of wavenumbers 1–4 is as one would expect for baroclinically unstable modes of the Charney type. Wavenumbers 1 and 2 both have double amplitude maxima in the troposphere, separated by ∼20° of latitude. These amplitude maxima are coherent with each other and are about 180° out of phase. The variances of the eastward components of wavenumbers 1 and 2 increase rapidly with altitude in the stratosphere, but the variance in the upper stratosphere is not coherent with that in the troposphere. To explain these observations it is suggested that two linearly independent eastward moving modes are present simultaneously in the Southern Hemisphere, and that these modes are manifestations of the baroclinic instability of the zonal mean flow. One of the modes dominates the variance in the troposphere (Charney mode) and the other dominates the variance in the stratosphere (Green mode).

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Annarita Mariotti
,
Carlos R. Mechoso
,
Bernard Legras
, and
Vincent Daniel

Abstract

The ozone evolution in the lower stratosphere of the Southern Hemisphere during the period 5–10 August 1994 is analyzed. The analysis focuses on the ozone “collar” (the band of maximum values in ozone mixing ratio around the Antarctic ozone “hole” at these altitudes) and the development of “collar filaments.” Ozone mixing ratios provided by the Microwave Limb Sounder (MLS) on board the Upper Atmosphere Research Satellite and by an ER-2 aircraft participating in the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft campaign are compared with values at corresponding locations in high-resolution isentropic maps obtained by using the numerical scheme of “contour advection with surgery” (CAS).

The CAS reconstructed ozone maps provide a view of the way in which air masses are exported from the outskirts of the collar to form the “tongues” of higher mixing ratios observed at lower latitudes on MLS synoptic maps. There is an overall consistency between the datasets insofar as the collar location is concerned. This location seems to be primarily defined by the local properties of the flow. Nevertheless the CAS reconstructed collar tends to become weaker than that depicted by MLS data. By means of radiative calculation estimates, it is argued that diabatic descent may be responsible for maintaining the ozone concentration approximately constant in the collar while filaments isentropically disperse collarlike mixing ratios from this region toward lower latitudes.

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Carlos R. Mechoso
,
J. David Neelin
, and
Jin-Yi Yu

Abstract

The realistic simulation of El Niño–Southern Oscillation (ENSO) by the University of California, Los Angeles (UCLA), coupled atmosphere–ocean general circulation model (CGCM) is used to test two simple theoretical models of the phenomenon: the recharge oscillator model of Jin and the delayed oscillator model of Schopf, Suarez, Battisti, and Hirst (SSBH). The target for the simple models is provided by the CGCM results prefiltered with singular spectrum analysis to extract the leading oscillatory mode. In its simplest form, the Jin model can be reduced to two first ordinary differential equations. If the parameters of the model are fit in this reduced form, it appears to capture the period of the CGCM oscillatory mode. If the Jin model is instead fit using the individual physical balances that are used to derive it, substantial misfits to the CGCM are encountered. The SSBH model can likewise be expressed either in a condensed form or a larger set of individual physical balances with highly analogous results.

It is shown that the misfits in both simple models can be greatly reduced by introducing a spinup timescale for wind stress relative to eastern equatorial Pacific SST. In the CGCM, this spinup time appears to be associated with a combination of atmospheric and ocean mixed layer processes in a way consistent with the “mixed mode” regime discussed by Syu and Neelin, which is not included in the Jin and SSBH models. These appear indistinguishable in this analysis, although the latter is more sensitive to fitting.

This paper provides a bridge between work on ENSO by theoreticians and numerical modelers. The CGCM results validate the conceptual framework of the simple models by demonstrating that they can provide a plausible representation of ENSO with realistic sets of parameters. The results also suggest that, in terms of realistic ENSO variability, the framework of the simple models can be made substantially more complete by including the adjustment time between wind stress and eastern Pacific SST required by the coupled spinup of the atmosphere and the ocean mixed layer processes outside this region.

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