Pedro L. Silva Dias

Insolation-driven changes in atmospheric circulation over the past 116,000 years in subtropical Brazil

During the last glacial period, large millennial-scale temperature oscillations—the ‘Dansgaard/Oeschger’ cycles—were the primary climate signal in Northern Hemisphere climate archives from the high latitudes to the tropics. But whether the influence of these abrupt climate changes extended to the tropical and subtropical Southern Hemisphere, where changes in insolation are thought to be the main direct forcing of climate, has remained unclear. Here we present a high-resolution oxygen isotope record of a U/Th-dated stalagmite from subtropical southern Brazil, covering the past 116,200 years. The oxygen isotope signature varies with shifts in the source region and amount of rainfall in the area, and hence records changes in atmospheric circulation and convective intensity over South America. We find that these variations in rainfall source and amount are primarily driven by summer solar radiation, which is controlled by the Earths precessional cycle. The Dansgaard/Oeschger cycles can be detected in our record and therefore we confirm that they also affect the tropical hydrological cycle, but that in southern subtropical Brazil, millennial-scale climate changes are not as dominant as they are in the Northern Hemisphere.

The THORPEX Interactive Grand Global Ensemble

Ensemble forecasting is increasingly accepted as a powerful tool to improve early warnings for high-impact weather. Recently, ensembles combining forecasts from different systems have attracted a considerable level of interest. The Observing System Research and Predictability Experiment (THORPEX) Interactive Grand Globa l Ensemble (TIGGE) project, a prominent contribution to THORPEX, has been initiated to enable advanced research and demonstration of the multimodel ensemble concept and to pave the way toward operational implementation of such a system at the international level. The objectives of TIGGE are 1) to facilitate closer cooperation between the academic and operational meteorological communities by expanding the availability of operational products for research, and 2) to facilitate exploring the concept and benefits of multimodel probabilistic weather forecasts, with a particular focus on high-impact weather prediction. Ten operational weather forecasting centers producing daily global ensemble ...

ECOLOGICAL RESEARCH IN THE LARGE-SCALE BIOSPHERE– ATMOSPHERE EXPERIMENT IN AMAZONIA: EARLY RESULTS

The Large-scale Biosphere-Atmosphere Experiment in Amazonia (LBA) is a multinational, interdisciplinary research program led by Brazil. Ecological studies in LBA focus on how tropical forest conversion, regrowth, and selective logging influence carbon storage, nutrient dynamics, trace gas fluxes, and the prospect for sustainable land use in the Amazon region. Early results from ecological studies within LBA emphasize the var- iability within the vast Amazon region and the profound effects that land-use and land- cover changes are having on that landscape. The predominant land cover of the Amazon region is evergreen forest; nonetheless, LBA studies have observed strong seasonal patterns in gross primary production, ecosystem respiration, and net ecosystem exchange, as well as phenology and tree growth. The seasonal patterns vary spatially and interannually and evidence suggests that these patterns are driven not only by variations in weather but also by innate biological rhythms of the forest species. Rapid rates of deforestation have marked the forests of the Amazon region over the past three decades. Evidence from ground-based surveys and remote sensing show that substantial areas of forest are being degraded by logging activities and through the collapse of forest edges. Because forest edges and logged forests are susceptible to fire, positive feedback cycles of forest degradation may be initiated by land-use-change events. LBA studies indicate that cleared lands in the Amazon, once released from cultivation or pasture usage, regenerate biomass rapidly. However, the pace of biomass accumulation is dependent upon past land use and the depletion of nutrients by unsustainable land-management practices. The challenge for ongoing research within LBA is to integrate the recognition of diverse patterns and processes into general models for prediction of regional ecosystem function.

Impact of tropical heat sources on the South American tropospheric upper circulation and subsidence

The nonlinear adjustment of the wind and mass fields to idealized tropical heat sources is studied with a simple nonlinear primitive equation model with emphasis on the upper level circulation over South America and neighboring oceanic regions during the austral summer. Numerical experiments are performed with (1) an idealized symmetrical heat source in the Amazon region, (2) the asymmetry induced in source (1) by the SACZ, (3) the effect of the Atlantic ITCZ off the Amazon mouth, (4) the African heat source, (5) the West Pacific source, and (6) the central Pacific source during the warm phase of ENSO. The linear response is obtained through the reduction of the heat source by a factor of 10 and subsequent multiplication of the results by the same factor. Two basic questions are discussed: (1) are localized heat sources important for the development of the observed cyclonic flow in the midequatorial Atlantic and (2) where is the compensating subsidence associated with the Amazon heat source located? The nonlinearity helps organizing a weak cyclonic curvature in the midequatorial Atlantic, with the inclusion of source (2). The basic state generated by the west Pacific source, and primarily by the central Pacific source, has a large impact on the cyclonic curvature on the equatorial Atlantic. The compensating subsidence associated with the Amazon source is concentrated on the southwest side of the source. The SACZ extension helps to enhance the subsidence over the northern Argentina, and the Atlantic ITCZ enhances the subsidence over northeast Brazil and central equatorial Atlantic. Nonlinearity weakens the subsidence at the 500 hPa level inducing a more barotropic structure in the dynamical response to the heating.

Impact of biomass burning aerosol on the monsoon circulation transition over Amazonia

Received 8 January 2009; revised 25 March 2009; accepted 7 April 2009; published 30 May 2009. [1] Ensemble simulations of a regional climate model (RegCM3) forced by aerosol radiative forcing suggest that biomass burning aerosols can work against the seasonal monsoon circulation transition, thus re-enforce the dry season rainfall pattern for Southern Amazonia. Strongly absorbing smoke aerosols warm and stabilize the lower troposphere within the smoke center in southern Amazonia (where aerosol optical depth >0.3). These changes increase the surface pressure in the smoke center, weaken the southward surface pressure gradient between northern and southern Amazonia, and consequently induce an anomalous moisture divergence in the smoke center and an anomalous convergence in northwestern Amazonia (5S-5N, 60W70W). The increased atmospheric thermodynamic stability, surface pressure, and divergent flow in Southern Amazonia may inhibit synoptic cyclonic activities propagated from extratropical South America, and re-enforce winter-like synoptic cyclonic activities and rainfall in southeastern Brazil, Paraguay and northeastern Argentina. Citation: Zhang, Y., R. Fu, H. Yu, Y. Qian, R. Dickinson, M. A. F. Silva Dias, P. L. da Silva Dias, and K. Fernandes (2009), Impact of biomass burning aerosol on the monsoon circulation transition over Amazonia, Geophys. Res. Lett., 36, L10814, doi:10.1029/ 2009GL037180.

Downstream Amplification: A Possible Precursor to Major Freeze Events over Southeastern Brazil

Abstract Many frost events over southeastern Brazil are accompanied by a large-amplitude upper trough of the middle latitudes that extends well into the Tropics. This paper first illustrates that a mechanism of downstream amplification across the Pacific into South America is generally accompanied in these situations. This is manifested by troughs and ridges that propagate eastward. An analysis of these situations during frost events shows that these features of downstream amplification, illustrated on a Hovmoller (x–t) plot, can be decomposed into a family of synoptic-scale waves that propagate eastward and a family of planetary-scale waves that acquire a quasi-stationary character during the freeze event. It is shown that a global model, at a resolution of 70 km, can be used to predict these features on the decomposition of scales during freeze events. It became apparent from these features that the growth of the long stationary waves during the freeze events may be due to scale interaction among wave c...

Mid-Holocene Climate of Tropical South America: A Model-Data Approach

Most of the Early and mid-Holocene paleoclimate studies in tropical South America indicate a drier climate in Amazon and Southeast Brazil and a wetter climate in Venezuela. This pattern has been interpreted as a northward migration of the Intertropical Convergence Zone (ITCZ) due to insolation changes explained by Milancovitch cycles. We show how model simulations and model-data comparisons can help to investigate further the reason of these changes by considering the mid-Holocene period (6 ka). The insolation effect and the vegetation interaction on the seasonal cycle are explored with emphasis on the regional impact on precipitation and on the atmospheric circulation. A major feature of the mean mid-Holocene simulated climate is indeed the decrease of the rainfall in the South Atlantic Convergence Zone (SACZ) region compared to present day, which is confirmed by the proxy data. However, the ITCZ migrates southward during the Southern Hemisphere summer thus enhancing the precipitation in Northeast Brazil. The SACZ and ITCZ displacements are enhanced by the vegetation feedback. The analysis of the transient meridional heat transport and of the baroclinicity of the model climate suggests more intense winter and early spring cold outbreaks in the central region of South America, which seems in agreement with paleoclimate proxies.

Dynamics of resonantly interacting equatorial waves

In this paper we explore some dynamical features on the non-linear interactions among equatorial waves. The shallowwater equation model with the equatorial β-plane approximation is used for this purpose. The Galerkin method is applied to the governing equations with the basis functions given by the eigensolutions of the linear problem. From the phase space expansion of two particular integrals of motion of the system, quadratic to lowest order, some constraints are obtained which the coupling coefficients must satisfy in order to ensure the invariance of such integrals. From the numerical evaluation of the coupling coefficients, these constraints are used to determine the possible resonant triads among equatorial waves. Numerical integrations of the resonant three-wave problem show that the energy of the waves in a resonant triad evolves periodically in time, with the period and amplitude of the energy oscillations dependent on the magnitude of the initial amplitudes of the waves and the way in which the initial energy is distributed among the triad components. The high-frequency modes are found to be energetically more active than the low-frequency modes. The latter tend to act as ‘catalytic’ components in a resonant triad. Integrations of the problem of two resonant triads coupled by a single mode point out the importance of gravity waves in the intertriad energy exchanges, suggesting the significance of these modes in the redistribution of energy throughout the atmospheric motion spectrum. The results also show that the intertriad energy exchanges provided by the highest frequency mode of two triads occur in a longer time-scale than the intratriad interactions. Therefore, these results also suggest the importance of the high-frequency modes in the generation of the low-frequency variability (intraseasonal and even longer term) of the atmospheric flow.

The relationship between South Atlantic SST and SACZ intensity and positioning

This study explores the ocean–atmosphere interaction in the formation and dynamics of the South Atlantic Convergence Zone (SACZ), through the analysis of the heat sources estimated through the outgoing longwave radiation. The results obtained with this study show that the coupled variability between SACZ and the South Atlantic Ocean indicates that in northern positioned SACZ cases (over Southeastern Brazil), westerly anomalies are verified in the low level continental tropical circulation, consistent with the active phase of the South America Monsoon System (SAMS). In these cases, cold anomalies in the subtropical Atlantic Ocean cause an increase in the continent–ocean temperature gradient, favoring an easterly flow in this region, and blocking the SACZ at a northerly position. Easterly anomalies in the tropical continent were verified in the low level circulation in southern positioned cases (over Southern Brazil), consistent with the SAMS break phase. The SST anomaly patterns indicate cold anomalies in the tropics and warm anomalies in the subtropics, which do not favor the development of an easterly flow at low levels over the western tropical Atlantic. In these cases, two situations may occur: the strengthening of the Low Level Jet (LLJ), which prevails in the eastern subtropical South America and convergence with the South Atlantic Subtropical High at its southern position; or the atmospheric unstable conditions caused by ocean warm SST anomalies (in this case the LLJ may be weaker than its climatological intensity).

Resonant Wave Interactions in the Equatorial Waveguide

Weakly nonlinear interactions among equatorial waves have been explored in this paper using the adiabatic version of the equatorial � -plane primitive equations in isobaric coordinates. Assuming rigid lid vertical boundary conditions, the conditions imposed at the surface and at the top of the troposphere were expanded in a Taylor series around two isobaric surfaces in an approach similar to that used in the theory of surface–gravity waves in deep water and capillary–gravity waves. By adopting the asymptotic method of multiple time scales, the equatorial Rossby, mixed Rossby–gravity, inertio-gravity, and Kelvin waves, as well as their vertical structures, were obtained as leading-order solutions. These waves were shown to interact resonantly in a triad configuration at the O(�) approximation. The resonant triads whose wave components satisfy a resonance condition for their vertical structures were found to have the most significant interactions, although this condition is not excluding, unlike the resonant conditions for the zonal wavenumbers and meridional modes. Thus, the analysis has focused on such resonant triads. In general, it was found that for these resonant triads satisfying the resonance condition in the vertical direction, the wave with the highest absolute frequency always acts as an energy source (or sink) for the remaining triad components, as usually occurs in several other physical problems in fluid dynamics. In addition, the zonally symmetric geostrophic modes act as catalyst modes for the energy exchanges between two dispersive waves in a resonant triad. The integration of the reduced asymptotic equations for a single resonant triad shows that, for the initial mode amplitudes characterizing realistic magnitudes of atmospheric flow perturbations, the modes in general exchange energy on low-frequency (intraseasonal and/or even longer) time scales, with the interaction period being dependent upon the initial mode amplitudes. Potential future applications of the present theory to the real atmosphere with the inclusion of diabatic forcing, dissipation, and a more realistic background state are also discussed.