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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

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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Keith M. Hines
and
Carlos R. Mechoso

Abstract

Surface frontal structure during cyclogenesis, and the sensitivity of this structure to surface friction, is examined. The approach is based on the analyses of simulations using a primitive equation model, with the domain restricted to a sector of one hemisphere, and the physics reduced to surface drag, horizontal diffusion, and dry convective adjustment. The model horizontal resolution is 1.2° latitude × 1.5° longitude, and there are 21 layers in the vertical. The drag coefficient is varied in simulations with midlatitude jet streams as initial conditions. The extent to which simulations in the adiabatic framework or with highly simplified representations of physical processes succeed in producing features of cyclone evolution emphasized by recent observational analyses is evaluated.

Shallow bent-back warm fronts develop in simulations with surface drag coefficients that are zero or representative of ocean surfaces. Horizontal advection, first in strong easterly and later in strong northerly winds, is primarily responsible for the resulting bent-back structure of the warm front.

The effect of surface drag on simulated lower-tropospheric wind speeds and frontogenesis is nonuniform. Warm frontogenesis is enhanced in simulations with relatively low surface drag through a feedback process involving vorticity, deformation, convergence, and warm-air advection. Surface drag tends to inhibit warm frontogenesis by decreasing the low-level wind speed and reducing the contribution of warm advection to the feedback. Consistently, a distinct warm front does not develop in the simulation with a surface drag coefficient representative of continental surfaces. Cold frontogenesis, on the other hand, is not very sensitive to surface drag.

Further simulations with doubled horizontal resolution (0.6° latitude × 0.75° longitude), slightly higher baroclinity at lower levels in the initial conditions, and small surface drag produce bent-back fronts that spiral around the surface pressure minimum. These results suggest that there are important differences in the structure of surface fronts associated with marine and continental cyclogenesis.

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Andrew W. Robertson
and
Carlos R. Mechoso

Abstract

The characteristics of subseasonal circulation variability over the South Pacific are examined using 10-day lowpass-filtered 700-hPa geopotential height NCEP–NCAR reanalysis data. The extent to which the variability in each season is characterized by recurrent geographically fixed circulation regimes and/or oscillatory behavior is determined. Two methods of analysis (a K-means cluster analysis and a cross-validated Gaussian mixture model) both indicate three to four geographically fixed circulation regimes in austral fall, winter, and (to some extent) spring. The spatial regime structures are found to be quite similar in each season; they resemble the so-called Pacific–South American (PSA) patterns discussed in previous studies and often referred to as PSA 1 and PSA 2. Oscillatory behavior is investigated using singular spectrum analysis. This identifies a predominantly stationary wave with a period of about 40 days and a spatial structure similar to PSA 1; it is most pronounced in winter and spring and exhibits a noticeable eastward drift as it decays. The power spectrum of variability is otherwise well approximated by a red spectrum, together with enhanced broader-band 15–30-day variability.

The results presented herein indicate that low-frequency variability over the South Pacific is not dominated by a propagating wave whose quadrature phases are PSA 1 and PSA 2, as hitherto described. Rather, it is found that the variability is well described by the occurrence of three to four geographically fixed circulation regimes, with a (near) 40-day oscillation that is predominantly stationary in space. The potential subseasonal predictability implied by this duality is discussed. Only during austral spring is a strong correlation found between El Niño and the frequency of occurrence of the circulation regimes.

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Andrew W. Robertson
and
Carlos R. Mechoso

Abstract

Interannual variations of the summertime (January–March) atmospheric circulation over subtropical South America are examined during the period 1958–97 using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis data. It is found from an empirical orthogonal function analysis that an anomalous upper-tropospheric large-scale stationary eddy in the lee of the Andes tends to accompany a dipole in anomalous vertical motion. An anomalous cyclonic (anticyclonic) eddy accompanies an intensified (diffuse) South Atlantic convergence zone (SACZ), with anomalous descent (ascent) to the southwest. The cold-core equivalent barotropic vertical structure of the anomalous cyclonic eddy and the 200-hPa vorticity balance are both characteristic of a stationary Rossby wave; the tendency for the eddy to be advected downstream by the mean westerlies is compensated by meridional advection of planetary vorticity and stretching associated with vertical motion. The anomalous cyclonic flow at low levels reinforces the thermally direct circulation associated with the SACZ. A weak funneling of submonthly Rossby wave activity into this descent region is also identified.

The interannual time series of the eddy is significantly correlated with north–south dipolar sea surface temperature (SST) anomalies over the southwest Atlantic; one standard deviation 200-hPa wind speed anomalies of up to 5 m s−1 are accompanied by SST anomalies of up to 0.3°C. A near-cyclic 15-yr component is identified, which the authors corroborate from independent analyses of southwest Atlantic SSTs and river flows; both are found to exhibit very similar oscillatory components. When the SACZ is intensified, the Paraná and Paraguay rivers in southern Brazil tend to swell, while the Uruguay and Negro rivers to the south tend to ebb; this north–south contrast in streamflow anomalies is most marked on the interdecadal timescale.

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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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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
Steven W. Lyons

Abstract

The impact of sea surface temperature (SST) anomalies observed during the Northern Hemisphere spring of 1984, which include the growing phase of an intense Atlantic warm event on the atmospheric circulation over the tropical Atlantic and Pacific is investigated using the nine-layer, low resolution version of the UCLA general circulation model. This impact is contrasted with that for the same period during 1983, when SST anomalies include the decaying phase of the strongest Pacific El Niño on record. Results obtained in control and anomaly simulations, consisting, respectively, of extended integrations with and without the observed SST anomalies, are analyzed.

It is found that simulated anomalies in the atmospheric circulation corresponding to 1984 include low-level westerlies over the equatorial Atlantic and easterlies over the equatorial Pacific. There are centers of anomalous low-level convergence and divergence off the northeast coast of Brazil and equatorial Brazil, respectively, which are associated with positive and negative precipitation anomalies. Differences between results corresponding to 1984 and 1983 show the impact of El Niño over the Pacific. Further, positive precipitation anomalies over the equatorial Atlantic shift from generally north of the equator in 1983 to south of the equator in 1994 (dry and wet years for northeast Brazil, respectively).

These simulated anomalies and interannual differences in the atmospheric circulation are in good general agreement with those observed. This agreement strongly suggests that the atmospheric anomalies observed during the northern springs of 1984 and 1983 over the tropical Atlantic and Pacific were primarily due to the SST anomalies.

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John D. Horel
and
Carlos R. Mechoso

Abstract

The persistence of the planetary-scale simulation over the North Pacific Ocean is investigated during 18 Northern Hemisphere winters from 1965/66 to 1982/83. Quasi-stationary flow patterns dominate 20 periods during the 6 El Niño winters. In contrast, 29 such periods are observed during the remaining 12 winters. Nearly all of the quasi-stationary episodes during El Niño winters exhibit negative 500 mb geopotential height anomalies in the Gulf of Alaska-Aleutian Island region. During the other 12 winters, episodes characterized by positive height anomalies in that region occur as frequently as those exhibiting negative height anomalies.

The observed persistence of the planetary circulation is contrasted to that simulated by the UCLA general circulation model. Ten winters of model output are analyzed: during five winters, sea surface temperatures (SSTs) are prescribed to evolve through their climatological seasonal cycle while during the other five winters, SST anomalies corresponding to idealized or observed El Niño conditions are added to the climatological field. The model atmosphere has less intraseasonal variability, and quasi-stationary events are less frequent than observed. However, the model is successful in simulating the observed preponderance of quasi-stationary regimes which exhibit below-normal 500 mb geopotential height anomalies in the Gulf of Alaska during winters with positive SST anomalies in the equatorial Pacific. The evolution of the model's quasi-stationary events suggests that they result directly from dynamical processes in midlatitudes, but their characteristics are apparently affected by SST conditions.

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