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Carlos R. Mechoso

Abstract

A July integration of a GFDL spectral general circulation model is repeated after eliminating from the model the topographic elevations. Zonally averaged. mean fields, regions of frequent Cyclogenesis and cyclone tracks, and the standing waves in geopotential at 500 mb simulated for the Southern Hemisphere in both integrations are compared. It is argued that the differences in the results support the interpretation advanced by Mechoso (1980) concerning the influence of Antarctica an the general circulation of the Southern Hemisphere.

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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

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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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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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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Rafael Terra, Carlos R. Mechoso, and Akio Arakawa

Abstract

This paper examines the impact of orographically induced mesoscale heterogeneities on the macroscopic behavior of planetary boundary layer (PBL) stratiform clouds, and implements and tests a physically based parameterization of this effect in the University of California, Los Angeles (UCLA), atmospheric general circulation model (AGCM). The orographic variance and associated thermal circulations induce inhomogeneities in the cloud field that can significantly alter the PBL evolution; an effect that has been largely ignored in existing climate models. The impact of this effect on AGCM simulations is examined and the mechanisms at work are studied by analyzing a series of Cloud System Resolving Model (CSRM) simulations.

Both the CSRM and AGCM results show that, in the absence of the orographic effect, the continental PBL tends to be in one of two regimes: the solid regime characterized by a cold and overcast PBL and the broken regime characterized by a low time-mean cloud incidence and a large-amplitude diurnal cycle. Without the orographic effect, the PBL may lock in the convectively stable solid regime, with deep convection displaced to the surrounding oceans and subsidence induced over land further contributing to the persistence of the cloud deck. The inclusion of the orographic effect weakens the feedback between the cloud's albedo and the ground temperature responsible for the existence of the two regimes and, therefore, conspires against the persistence of the solid regime rendering the behavior of the PBL–ground system less bimodal. The parameterization featured in this paper also increases the amplitude of the diurnal cycle in the AGCM and reduces the excessive seasonality in PBL cloud incidence, resulting in an improved simulation of convective precipitation over regions where the solid regime was spuriously dominating.

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Jin-Yi Yu and Carlos R. Mechoso

Abstract

The hypothesis that Peruvian stratocumulus play an important role on both the annual mean and annual variations of sea surface temperature (SST) in the eastern equatorial Pacific is examined. The problem is addressed by performing sensitivity experiments using the University of California, Los Angeles, coupled atmosphere–ocean GCM with different idealized temporal variations of stratocumulus in a region along the coast of Peru.

The results obtained are consistent with the notion that Peruvian stratocumulus are a key component of the interhemispherically asymmetric features that characterize the annual mean climate of the eastern equatorial Pacific, including the cold SSTs off Peru and the absence of a southern ITCZ. The principal new finding of this study is that the annual variations (i.e., deviations from the annual mean) of Peruvian stratocumulus are linked to the differences between the amplitude, duration, and westward propagation of the warm and cold phases of the equatorial cold tongue. In the model’s context, only if the prescribed annual variations of Peruvian stratocumulus have the same phase as the observed variations are those differences successfully captured.

The impact of Peruvian stratocumulus on equatorial SST involves “dynamical” and “thermal” effects. The former develop through an enhancement of the northerly component of the surface wind from the Peruvian coast to the equator. The thermal effects develop through the special relationships between SST and surface evaporation over the equatorial cold tongue, which contributes to extend the cold phase until the end of the year. A successful portrayal of this behavior requires a realistic simulation of the annual variations of surface wind over the equatorial cold tongue.

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Andrew W. Robertson and Carlos R. Mechoso

Abstract

The time series of annual streamflow of four rivers in southeastern and south-central South America (the Negro, Paraguay, Paraná, and Uruguay Rivers) for the period 1911–93 are analyzed. Application of the multitaper method shows that the following features are significant at the 95% level: 1) a nonlinear trend, 2) a near-decadal component, and 3) interannual peaks with ENSO timescales. The trend and near-decadal components are most marked in the two more central rivers, the Paraguay and Paraná, with ENSO timescale variability most pronounced in the Negro and Uruguay rivers in the southeast. Composites of SST are made for each of the statistically significant oscillatory components of river flow, by reconstructing each component using singular spectrum analysis. These composites confirm the influence of ENSO on the streamflow variability of the Negro and Uruguay Rivers, with El Niño associated with enhanced streamflow. On the decadal timescale, high river runoff is associated with anomalously cool SSTs over the tropical North Atlantic. A very similar near-decadal oscillation in SST over this region is identified separately from a rotated empirical orthogonal function analysis of gridded annual mean SSTs. The near-decadal component of the Paraguay and Paraná Rivers is strongest in the austral summer.

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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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