Pedro Leite da Silva Dias

Simulations of the Summer Circulation over the South American Region with an Eta Coordinate Model

Abstract A multilevel limited-area primitive equations model in eta coordinate is used to simulate the effects of the Amazonian latent heat source and the Andean topography on the summer (December, January, February) circulation over the South American region. The observed circulation features, such as the Bolivian high and the trough near the coast of Northeast Brazil in the upper troposphere, the elongated South Atlantic convergence zone (SACZ) oriented northwest to southeast from the southern Amazon region into the Atlantic Ocean, and the subtropical anticyclones in the South Pacific and South Atlantic Oceans in the lower levels are well simulated. Although heating alone produces the SACZ, the mountain and the basic flow put it in the observed position. It is found that regular daily pulses of heating are necessary for the maintenance of the SACZ. The position and intensity of the SACZ change rather substantially from the no-mountain case to the mountain case, thus showing the importance of the effect ...

Large-Scale Response of the Tropical Atmosphere to Transient Convection

Abstract We consider the problem of the linear response of a stratified, equatorial, β-plane model atmosphere to specified transient sources of heat and momentum. The method of solution involves transforms in all three spatial coordinates. A finite Sturm-Liouville transform is used in z, a Fourier transform in x, and a generalized Hermite transform in y. The resulting spectral equations can then be solved analytically for a specified forcing. Of particular interest is the case of a Gaussian-shaped heat source centered at latitude yo and with e-folding radius a. The heat source is transient and has time scale 1/α. Using the Parceval relation we compute how the forced energy is partitioned between Kelvin, mixed Rossby-gravity, Rossby and gravity modes as a function of a, yo, α. Model results using a heat source centered at 11°S with an e-folding radius of 750 km and a time scale of about a day indicate that many aspects of the summertime upper tropospheric circulation over South America can be explained by ...

Analysis of Tropical–Extratropical Interactions with Influence Functions of a Barotropic Model

Abstract Influence functions (IFs) of a linear barotropic vorticity equation model are computed in order to determine the regions in which anomalous divergence at upper levels, related to tropical heating anomalies, has the largest impact on certain prominent low-frequency anomalies. The present computation differs from that of Branstator in two aspects: (a) the model includes the effects of the basic-flow divergence and the advection by anomalous divergent wind and (b) the influence functions directly assess the influence of upper-level divergence anomalies rather than sources of vorticity. The IFs are applied to the study of low-frequency tropical–extratropical interactions at the interannual (ENSO) and intraseasonal (30–60-day oscillation) timescales. The origin of well-known teleconnection patterns is explored through the identification of comma influence regions in the Tropics and subtropics for their main action centers. The subtropical west and central-east Pacific, north of the equator, is an impo...

Geostrophic Adjustment in an Axisymmetric Vortex

Abstract A linearized system of equations for the atmospheres first internal mode in the vertical is derived. The system governs small-amplitude, forced, axisymmetric perturbations on a basic-state tangential flow which is independent of height. When the basic flow is at rest, solutions for the transient and final adjusted state are found by the method of Hankel transforms. Two examples are considered, one with an initial top hat potential vorticity and one with an initial Gaussian-type potential vorticity. These two examples, which extend the work of Fischer (1963) and Obukhov (1949), indicate that the energetical efficiency of cloud-cluster-scale heating in producing balanced vortex flow is very low, on the order of a few percent. The vast majority of the energy is simply partitioned to gravity-inertia waves. In contrast the efficiency of cloud-cluster-scale vorticity transport is very high. When the basic state possesses positive relative vorticity in an inner region, the energy partition can be subst...

Diurnally Forced Tropical Tropospheric Circulation over South America

Abstract The impact of diurnally varying thermal forcing on the tropical circulation is studied with a linearized shallow water model on an equatorial beta plane. The mass and flow fields are interpreted as an internal mode with equivalent depth of the order of 250 m. This is the preferred vertical mode excited by typical cumulus heating profiles in the tropics. The diurnal forcing with the horizontal scale of the Amazon source partitions approximately 40% of the input energy to slow modes. The fast modes are trapped by the critical latitudes contributing significantly to oscillations in the divergence field, which emanate out of the source region. The divergence field at the source is approximately in phase with the forcing; the vorticity field lags the forcing by approximately 10 hours. A comparison of the model results with the 500 mb level vertical motion, derived from the National Meteorological Center analysis, shows a similar pattern of propagating oscillations. It is suggested that the diurnally f...

Resonant Wave Interactions in the Presence of a Diurnally Varying Heat Source

Abstract Resonant interactions among equatorial waves in the presence of a diurnally varying heat source are studied in the context of the diabatic version of the equatorial β-plane primitive equations for a motionless, hydrostatic, horizontally homogeneous and stably stratified background atmosphere. The heat source is assumed to be periodic in time and of small amplitude [i.e., O(e)] and is prescribed to roughly represent the typical heating associated with deep convection in the tropical atmosphere. In this context, using the asymptotic method of multiple time scales, the free linear Rossby, Kelvin, mixed Rossby–gravity, and inertio-gravity waves, as well as their vertical structures, are obtained as leading-order solutions. These waves are shown to interact resonantly in a triad configuration at the O(e) approximation, and the dynamics of these interactions have been studied in the presence of the forcing. It is shown that for the planetary-scale wave resonant triads composed of two first baroclinic e...

Excitation Mechanism of Mixed Rossby–Gravity Waves in the Equatorial Atmosphere: Role of the Nonlinear Interactions among Equatorial Waves

Abstract One possible explanation for the relatively high signal of the mixed Rossby–gravity waves observed in the tropical atmosphere is explored in this paper. This explanation is based on the nonlinear interactions among equatorial waves, and is made by adopting the nonlinear shallow water equations on the equatorial β plane. These equations are solved by a spectral method that uses the eigensolutions of the linear problem as the expansion basis. Numerical simulations are performed with a specified stationary mass source representative of the tropospheric heating associated with the typical convective activity over the Amazon Basin during the austral summer period. The numerical results show that the mixed Rossby–gravity waves are excited by a nonlinear mechanism in which the slow modes excited by the thermal forcing generate a quasigeostrophic basic state that supplies energy especially to the mixed Rossby–gravity waves with zonal wavenumbers 4 and 5, which have periods of the order of 4 days. The pha...

A Spectral Cumulus Parameterization for Use in Numerical Models of the Tropical Atmosphere

Abstract The spectral cumulus parameterization theory of Arakawa and Schubert is presented in the convective flux form as opposed to the original detrainment form. This flux form is more convenient for use in numerical prediction models. The equations are grouped into one of three categories that are members of a control flow diagram: feedback, static control, and dynamic control. The dynamic control, which determines the cloud base mass flux distribution, is formulated as an optimization problem. This allows quasi-equilibrium to be satisfied as closely as possible while maintaining the necessary nonnegativity constraint on the cloud base mass flux. Results of two applications of the parameterization are shown. The first illustrates the dependence of the predicted cloud mass flux distribution on the vertical profile of the large-scale vertical motion field. According to the assumption of quasi-equilibrium of the cloud work function, the mass flux associated with deep clouds is controlled by large-scale ve...

Asymptotic approach for the nonlinear equatorial long wave interactions

In the present work we use an asymptotic approach to obtain the long wave equations. The shallow water equation is put as a function of an external parameter that is a measure of both the spatial scales anisotropy and the fast to slow time ratio. The values given to the external parameters are consistent with those computed using typical values of the perturbations in tropical dynamics. Asymptotically, the model converge toward the long wave model. Thus, it is possible to go toward the long wave approximation through intermediate realizable states. With this approach, the resonant nonlinear wave interactions are studied. To simplify, the reduced dynamics of a single resonant triad is used for some selected equatorial trios. It was verified by both theoretical and numerical results that the nonlinear energy exchange period increases smoothly as we move toward the long wave approach. The magnitude of the energy exchanges is also modified, but in this case depends on the particular triad used and also on the initial energy partition among the triad components. Some implications of the results for the tropical dynamics are disccussed. In particular, we discuss the implications of the results for El Nino and the Madden-Julian in connection with other scales of time and spatial variability.

Modulation of Shallow-Water Equatorial Waves due to a Varying Equivalent Height Background

AbstractThe dynamics of convectively coupled equatorial waves (CCEWs) is analyzed in an idealized model of the large-scale atmospheric circulation. The model is composed of a linear rotating shallow-water system with a variable equivalent height, or equivalent gravity wave speed, which varies in space. This model is based on the hypothesis that moist convection acts to remove convective instability, therefore modulating the equivalent height of a shallow-water system. Asymptotic solutions are derived in the case of a small perturbation around a constant coefficient, which is assumed to be a mean moist equivalent height derived from satellite observations. The first-order solutions correspond to the free normal modes of the linear shallow-water system and the second-order flow is derived solving a perturbation eigenvalue problem. The asymptotic solutions are documented in the case of a zonally varying equivalent height and for wavenumbers and frequencies that are consistent with observations of CCEWs. This...