Sistema Regional de Información
en línea para Revistas Científicas de América Latina,
el Caribe, España y Portugal

The interanual, seasonal fluctuations of precipitation in the S. Paulo State were analyzed. The meteorological data were collected by Departamento de Agua e Energia Eletrica (DAEE). These data comprise the years 1948 to 1988. The seasonal scale was studied to classify the precipitation and to understand the winter (six months) and summer (six months) variabilities. We have used different statistical parameters, such as mean, correlation, standard deviation. The interanual fluctuations were studied through the relation of the precipitation in S. Paulo State with South Oscillation (SO). Virtually all significant variations were associated with extreme phases of the SO. It has also been verified that the most important caracteristic of the SO effect is the persistence of weak precipitation anomalies for several months. Although statistically significant SO signal can be associated with either above or below normal precipitation.

A structure of the matrix resulting from the normal mode stability of tonal non-divergent flows on a sphere is analyzed. The analysis is based on using the recurrent formula derived for the nonlinear triad interaction coefficients. As an application, it is shown that a zonal flow of the form of a Legendre polynomial of degree j is exponentially and algebraically stable to all the small-scale perturbations whose zonal wave number is greater than j.

Different types of South American annual precipitation series are analyzed for stations located south of the parallel 15ºS. It has been found that the most typical pattern of change in the long-term averages for 30-years consecutive series, corresponding to meteorological stations eastward of the Andes Cordillera, is represented by a "jump" or discontinuity which, for these stations, is always positive. The maximum signal is spotted between the 1950's and the 1960's, showing the largest values on the continental zone of Argentina's territory. On the other hand, for stations on the windward side of the Andes, in Chile, the change is characterized by a decreasing tendency during the years, being steeper over the Northern dessert. Evidences of impacts produced by these changes are shown and possible physical causes of these long-term changes are inferred.

The zonally-averaged steady state circulation in Phillips' general circulation model is determined using the same physical parameters as in the original experiment. The approximate zonally-symmetric state is determined both by spectral procedures and by solving the differential equations directly. The results are compared with a similar determination of the same circulation carried out by Charney resulting in very large wind velocities. It is concluded that the new results are more realistic than the extremely intensive circulation determined by Charney, but also that the zonal steady state has zonally averaged velocities of a few hundred meters per second. Employing a more realistic determination of the atmospheric averaged heat budget resulting in a Newtonian form of the heating when the Boltzman terms are linearized gives finally acceptable results in general agreement with Chameys later results.

For a subcritical value of the Rayleigh number the Lorenz attractor has one unstable and two stable stationary states. The asymptotic state of a long time integration may one of the two steady states. A stable steady state will be the asymptotic state if the starting position is in the immediate neighbourhood of the steady state. However, in the region midway between the two stable states the asymptotic state can be found only by a numerical integration. Small changes in the initial state may cause a change in the asymptotic state from one stable steady state to the other. The final asymptotic state is sensitive not only to the small changes in the initial state, but also to small changes in the value of the Rayleigh number. This behavior indicates that at least the border region may be of a fractal nature. In view of the behavior of the time integrations a stochastic model, including only second order statistics, was formulated. This model can be used to investigate the asymptotic states as a function of the magnitude of the uncertainty in the initial state. Several examples will be given. The stochastic model is compared with an exact solution for the steady states given the initial position and the initial variances.

A theoretical model has been constructed to explain the formation of traveling convection vortices. This model proposes that traveling convection vortices correspond to kink instabilities generated in magnetospheric regions where the current is strong enough to create an azimuthal field Bθ of the same size as the field aligned component Bz in the equatorial plane. It is assumed that in these regions (around 4Re), this current produces an azimuthal magnetic field which creates a kind of instability, known as the kink instability. The results of model calculations based on typical values observed on spacecraft have been compared with the patterns of traveling convection vortices derived from ground based magnetometers. The multiple current filaments which are often observed in these patterns are proposed to be explained by the occurrence of kink instabilities in the magnetosphere.

This study evaluates precipitation variability in Baja California in relation to the El Niño/Southern Oscillation (ENSO) using the Southern Oscillation Index (SOI). To evaluate precipitation climatology, data for 102 weather stations were analyzed. Data were directly averaged for stations with records longer than 30 years, but normalized for stations with shorter records. To test for uniformity of precipitation departures, the SOI was compared with average precipitation departures for long-term stations (established before 1960), in eight subregions of Baja California. The results revealed that, unlike California, the interannual variability of both annual and monthly precipitation is strongly linked to SOI. During El Niño events, above-normal precipitation occurs largely in February and March; but precipitation amounts are subnormal during La Niña events, and mostly limited to December and January. Gradients of precipitation departure tend to be uniform across Baja California during individual years. The variability of precipitation is attributed to the interannual dislocation by ENSO of the polar-front jet stream along the Pacific coast, as described in other studies. In El Niño events, the circulation acquires a positive-phase Pacific-North America (PNA) pattern, with an enhanced Tropical Northern Hemisphere (TNH) teleconnection mode expressed in a southern-branch jet stream across Baja California. There is strong on-shore advection of moist surface westerlies on the West coast of North America. In La Niña events, the composite circulation acquires a negative-phase PNA pattern, and the jet stream is mostly poleward of Baja California and California. This results in weak on-shore advection by moist surface westerlies. Seasonal shifts in precipitation anomalies may be related to geographic shifts in diabatic forcing from the equatorial warm pool, and consequent teleconnections into the extra-tropical wave-train in the Pacific-North American region. The annual uniformity of precipitation departures across the eight subregions of Baja California suggests that frontal storms do not produce anomalous orographic precipitation gradients during El Niños. Timely, long-term precipitation forecasts could help accommodate the region's landuse to its cyclical pattern of drought and flood.  

Several studies had shown that the anomalies of the sea surface temperature of the Tropical Atlantic and Pacific Oceans, are related to variations in the duration and timing of the rainy season in Central America. Cluster analysis was used to identify common patterns of 72 rain gauge stations of the region, with their anomaly time series as grouping variables, five clusters where identified through this process. A Vector Auto Regressive-Moving Average (VARMA) model was fitted to the data to quantify the ocean-atmosphere interaction between the oceanic indices of the Tropical North and South Atlantic, the Tropical Eastern Pacific and the first EOF's of the regional rainfall clusters. This model shows that the Tropical North Atlantica has the largest influence over the region when compared with the influence of the other indices, having positive correlation with the rainfall EOF’s. The Niño 3, instead, was found to have lower correlation with the rainfall of the region, influencing only the Pacific related clusters. This work shows that the variability of Tropical North Atlantic sea surface temperature anomaly (SSTA) presents stronger associations with the Central America rainfall, than the Tropical Eastern Pacific SSTA. The Tropical North Atlantic SSTA is maynly related to the degree of development of the Tropical Upper Tropospheric Through (TUTT) and SSTA of the Niño 3 region with meridian position of the Intertropical Convergence Zone (ITCZ).

We used the Laboratorie de Metereologie Dynamique General Circulation Model (LMDZ GCM) to study the sensitivity of the fields to prescribed changes in the extratropical sea surface temperature. It was observed the strong sensitivity of the dynamic fields to the anomalous forcing. Particularly, we noticed the northward displacement of the polar jet, due to the weakening of both, the baroclinicity in low levels and the transient momentum fluxes.

A numerical spectral model of the barotropic atmosphere is used to simulate well-known exact solutions of the vorticity equation for an ideal incompressible fluid on a rotating sphere. Primary emphasis is received to the behavior of the relative error between the exact and numerical solutions as well as to preserving the total kinetic energy, integral enstrophy, and geometric structure of the solutions (zonal flows, Rossby-Haurwitz waves, Wu-Verkley solutions, and Verkley's dipole modons). The 10-day integrations carried out with the model show that the classical exact solutions (RH waves) can be calculated to a good approximation. However, the instability of some exact generalized solutions with respect to initial errors and the errors associated with nonzero numerical model forcing can be a serious obstacle in simulating long-time behavior of such solutions. If it is the case then even highly truncated model with very small time step fails to resolve the problem, and the paths of the numerical and exact solutions diverge from each other with time. Nevertheless, the total energy and integral enstrophy of all the numerical solutions are conserved with a high degree of precision at least during first 10 days.