Carlos Raupp

Rossby waves have been recently recognised for their role in the large-scale spatio-temporal organisation of the solar magnetic activity. Here, we study the propagation of magnetohydrodynamic Rossby waves in a thin layer, representing the solar tachocline. We consider the waves embedded in a meridionally varying background state characterised by a mean zonal flow, which mimics the differential rotation profile of the Sun, and a toroidal magnetic field. Two anti-symmetric toroidal magnetic fields are utilised: one having a global structure with the maximum at around 50o and the other characterised by a narrow band centered at around 20o. We show that for a global structure toroidal magnetic field, the MHD Rossby modes undergo significant meridional propagation, either equatorward or poleward. In addition, the latitude where the waves exhibit a stationary behaviour is sensitive to the strength of the background magnetic field. On the other hand, a narrow band toroidal magnetic field i...

The geomagnetic field is generated by a dynamo process in the Earth’s core and is characterized by a predominant dipole component that has been steadily decreasing in the last few centuries. The physical drivers behind the fluctuations of the geomagnetic dipole field remain poorly understood. One of the possible explanations rely on the interaction between the dipole mode and other multipole terms of the geomagnetic field. To test this hypothesis, we used two millennial scale models based on spherical harmonic fitting of paleomagnetic data, which allowed to reconstruct the geomagnetic field of the past. By performing causality and information statistical analysis, we found significant interactions between the dipole and smaller scale harmonics (quadrupole and octupole) of the geomagnetic field. In particular, both data sets agree that the spherical harmonic $$Y_2^2$$ acts as a source term, whereas the axial dipole term $$Y_1^0$$ consists of the term with least information loss. The results suggest a possible control of core–mantle boundary inhomogeneities on the interaction between the components of the geomagnetic field. Our results also show a net information flux from larger to smaller scales, which is compatible with a direct turbulent cascade view of the geodynamo.

Atmospheric blockings are persistent large-scale climate patterns that lock in a geographic position for days and even weeks. In principle, blockings might involve a large number of waves interacting non-linearly, and a conclusive description for their onset and duration is still elusive. In this paper, we introduce a simplified account for this phenomenon by means of a single-triad of Rossby–Haurwitz waves perturbed by one topographic mode. It is shown that the dynamical features of persistent flow patterns have zero measure in the phase space for an unperturbed triad, but such a measure becomes finite for the perturbed dynamics. By this account, we suggest that static inhomogeneities in the two-dimensional atmospheric layer are required for locking the flow patterns to the planetary rotation.

Climate change is expected to increase the intensity of extreme precipitation events in Amazonia that in turn might produce more forest blowdowns associated with convective storms. Yet quantitative tree mortality associated with convective storms has never been reported across Amazonia, representing an important additional source of carbon to the atmosphere. Here we demonstrate that a single squall line (aligned cluster of convective storm cells) propagating across Amazonia in January, 2005, caused widespread forest tree mortality and may have contributed to the elevated mortality observed that year. Forest plot data demonstrated that the same year represented the second highest mortality rate over a 15‐year annual monitoring interval. Over the Manaus region, disturbed forest patches generated by the squall followed a power‐law distribution (scaling exponent α = 1.48) and produced a mortality of 0.3–0.5 million trees, equivalent to 30% of the observed annual deforestation reported i...

The complex network approach has proved to be a valuable tool for climate and atmospheric sciences in recent years. Here, we show an application of causality ideas in complex networks to infer properties of equatorial wave interactions associated with the Madden–Julian Oscillation (MJO), the dominant component of the atmospheric system on intraseasonal timescales in the equatorial region. We use the normal mode function approach to obtain the time series of baroclinic Kelvin and Rossby mode energies, since both of these wave modes are known to play an important role in the MJO dynamics. The partial directed coherence method reveals the structure of the interaction among those modes and shows that the Kelvin mode is the main driver of the MJO system, transferring energy to the Rossby modes. Investigation on the Kelvin mode information source might help evaluating the state-of-the-art of MJO theories.

The study of tropical tropospheric disturbances has led to important challenges from both observational and theoretical points of view. In particular, the observed wavenumber-frequency spectrum of tropical oscillations has helped bridge the gap between observations and the linear theory of equatorial waves. In this study, we obtained a similar wavenumber-frequency spectrum for each equatorial wave type by performing a normal mode function (NMF) decomposition of global Era–Interim reanalysis data. The NMF basis used here is provided by the eigensolutions of the primitive equations in spherical coordinates as linearized around a resting background state. In this methodology, the global multi-level horizontal velocity and geopotential height fields are projected onto the normal mode functions, characterized by a vertical mode, a zonal wavenumber, a meridional quantum index, and a mode type, namely, Rossby, Kelvin, mixed Rossby-gravity, and westward/eastward propagating inertio-gravity ...

The dynamics of the Earth's atmosphere is characterized by a wide spectrum of oscillations, ranging from hourly to interdecadal and beyond. The low frequency component of the atmospheric variability cannot be understood solely in terms of linear atmospheric waves that have shorter timescales. A newly proposed mechanism, the precession resonance mechanism, is a regime of highly efficient energy transfer in the spectral space in turbulent systems. Here, we investigate the role of the precession resonance, and the alignment of dynamical phases, in the generation of low frequency oscillations and the redistribution of energy/enstrophy in the spectral space using the barotropic vorticity equation. First, the mechanism and its ability to generate low frequency oscillations are demonstrated in low-order models consisting of four and five nonlinearly interacting Rossby-Haurwitz waves. The precession resonance onset is also investigated in the full barotropic vorticity equation, and the ...

Here the theory of global nonhydrostatic normal modes has been further developed with the analysis of both linear and weakly nonlinear energetics of inertia–acoustic (IA) and inertia–gravity (IG) modes. These energetics are analyzed in the context of a shallow global nonhydrostatic model governing finite-amplitude perturbations around a resting, hydrostatic, and isothermal background state. For the linear case, the energy as a function of the zonal wavenumber of the IA and IG modes is analyzed, and the nonhydrostatic effect of vertical acceleration on the IG waves is highlighted. For the nonlinear energetics analysis, the reduced equations of a single resonant wave triad interaction are obtained by using a pseudoenergy orthogonality relation. Integration of the triad equations for a resonance involving a short harmonic of an IG wave, a planetary-scale IA mode, and a short IA wave mode shows that an IG mode can allow two IA modes to exchange energy in specific resonant triads. These ...

In the present study a simplified multiscale atmosphere–ocean coupled model for the tropical interactions among synoptic, intraseasonal, and interannual scales is developed. Two nonlinear equatorial β-plane shallow-water equations are considered: one for the ocean and the other for the atmosphere. The nonlinear terms are the intrinsic advective nonlinearity and the air–sea coupling fluxes. To mimic the main differences between the fast atmosphere and the slow ocean, suitable anisotropic multispace/multitime scalings are applied, yielding a balanced synoptic–intraseasonal–interannual–El Niño (SInEN) regime. In this distinguished balanced regime, the synoptic scale is the fastest atmospheric time scale, the intraseasonal scale is the intermediate air–sea coupling time scale (common to both fluid flows), and El Niño refers to the slowest interannual ocean time scale. The asymptotic SInEN equations reveal that the slow wave amplitude evolution depends on both types of nonlinearities. An...

O presente trabalho tem por objetivo apresentar um estudo de caso de uma ciclogênese ocorrida sobre o Atlântico Equatorial Sul, próximo ao litoral do Nordeste brasileiro, entre os dias 08/06/2000 e 09/06/2000. Tal fenômeno foi induzido por uma instabilidade baroclínica, que caracterizou-se por uma intensa amplificação de um cavado de onda curta nos altos níveis, na faixa latitudinal de 20°S a 10°S. Portanto, trata-se de um fenômeno ocorrido em latitudes relativamente baixas, onde a causa principal foi exatamente o mecanismo de instabilidade baroclínica, situação a qual ocorre tipicamente em latitudes médias. Utilizando os dados das análises do modelo global do CPTEC, foi estimado o termo de aquecimento/resfriamento diabático através da equação da termodinâmica, bem como os principais parâmetros envolvidos na tradicional análise da instabilidade baroclínica através do modelo quase-geostrófico de duas camadas. Foi concluído que o cisalhamento vertical do vento associado à intensificaç...

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...

... al., 1984a; Frederiksen and Webster, 1988). ... Page 11. Revista Brasileira de Meteorologia Setembro 2004 187 FREDERIKSEN, JS; PJ WEBSTER. Alternative theories of atmospheric teleconnections and low-frequency fluctuations. Reviews of Geophysics, 26 (3), 459-494, 1988. ...

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(ε)] 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(ε) 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 equatorially trapped waves and one barotropic Rossby mode, the spectrum of the thermal forcing is such that only one of the triad components is resonant with the heat source. As a result, to illustrate the role of the diurnal forcing in these interactions in a simplified fashion, two kinds of triads have been analyzed. The first one refers to triads composed of a k = 0 first baroclinic geostrophic mode, which is resonant with the stationary component of the diurnal heat source, and two dispersive modes, namely, a mixed Rossby–gravity wave and a barotropic Rossby mode. The other class corresponds to triads composed of two first baroclinic inertio-gravity waves in which the highest-frequency wave resonates with a transient harmonic of the forcing. The integration of the asymptotic reduced equations for these selected resonant triads shows that the stationary component of the diurnal heat source acts as an “accelerator” for the energy exchanges between the two dispersive waves through the excitation of the catalyst geostrophic mode. On the other hand, since in the second class of triads the mode that resonates with the forcing is the most energetically active member because of the energy constraints imposed by the triad dynamics, the results show that the convective forcing in this case is responsible for a longer time scale modulation in the resonant interactions, generating a period doubling in the energy exchanges. The results suggest that the diurnal variation of tropical convection might play an important role in generating low-frequency fluctuations in the atmospheric circulation through resonant nonlinear interactions.