Search Results

You are looking at 1 - 7 of 7 items for

  • Author or Editor: Antonio D. Moura x
  • Refine by Access: All Content x
Clear All Modify Search
Antonio D. Moura

Abstract

Solutions of the linearized balance equations over a sphere are presented and compared with the Laplace's tidal equations results obtained by Longuet-Higgins. On these lines, this study searches for a partial answer concerning the accuracy of the balance system for describing slow, large-scale motions in the atmosphere. The solutions corresponding to Hough's second class waves [small values of ε = (2 Ωa)2/c 2] are well described by the balance system. At large values of ε there are apparent discrepancies for the Rossby symmetric modes as compared to Longuet-Higgins type 2 solutions. Nevertheless, for the antisymmetric modes the agreement is good. The linearized version of the motions studied by Burger is also a solution of the balance equations, corresponding to small frequencies and negative values of ε. There are also unrealistic solutions (in the light of the balance approximation) with high frequencies and ε < 0.

An integral theorem shows that for ε > 0 only westward propagating waves are solutions of the balance system. In particular, it shows that the equatorial Kelvin wave is not a solution. The westward propagating part of the mixed Rossby-gravity mode is a solution, but with slightly higher frequency when compared to Longuet-Higgins’ results.

A study of the “modified balance” equations derived by Charney shows that they describe well all the equatorial Rossby modes. They also describe the equatorial Kelvin wave at large values of ε. Unfortunately, they have additional unrealistic high-frequency eastward-propagating free wave solutions. An interative numerical method is suggested in the hope of avoiding these spurious solutions.

A two-layer spherical model is used to study the instability properties of a basic state of solid rotation. It shows that the balance and the quasi-geostrophic equations have unstable solutions which are remarkably alike for realistic values of the parameters involved.

Full access
Antonio D. Moura
and
Jagadish Shukla

Abstract

It is proposed that a possible mechanism for the occurrence of severe droughts over northeast Brazil is the establishment of a thermally direct local circulation which has its ascending branch at about 10°N and its descending branch over northeast Brazil and the adjoining oceanic region. The driving for this anomalous circulation is provided by warming due to enhanced moist convection associated with warmer sea surface temperature anomalies over the northern tropical Atlantic, and cooling associated with colder sea surface temperature anomalies in the southern tropical Atlantic. The combined effects of thermally forced subsidence and reduced evaporation and moisture flux convergence produces severe drought conditions over northeast Brazil.

We have examined the monthly mean sea surface temperature anomalies over the tropical Atlantic and rainfall anomalies over two selected stations (Fortaleza, 3°46′S 38°31′W and Quixeramobim, 5°12′S 39°18′W) for 25 years (1948–72). It is found that the most severe drought events are associated with the simultaneous occurrence of warm sea surface temperature anomalies over north and cold sea surface temperature anomalies over the south tropical Atlantic. Simultaneous occurrences of warm sea surface temperature anomaly at 15°N, 45°W and cold sea surface temperature anomaly at 15°S, 5°W were always associated with negative anomalies of rainfall, and vice versa.

A simple primitive equation model is used to calculate the frictionally controlled and thermally driven circulation due to a prescribed beating function in a resting atmosphere. The heating function is designed to simulate a heat source to the north and a heat sink to the south of the equator. This prescribed thermal forcing produces a thermally direct circulation with ascending motion to the north and descending motion to the south. Low-level cyclonic circulation and high-level anticyclonic circulation is generated to the north of the equator and low-level anticylonic circulation and high-level cyclonic circulation is generated to the south of the equator. The analytical solutions agree well with the results of numerical experiments carried out with a multilevel global general circulation model.

We also have carried out a series of numerical experiments to test the sensitivity of the GLAS (Goddard Laboratory for Atmospheric Sciences) model to prescribed sea surface temperature anomalies over the tropical Atlantic. It is found that the sea surface temperature anomaly patterns, which resemble the observed ones during drought years, produce an intensified convergence zone, enhanced rainfall and low-level cyclonic circulation to the north, and reduced rainfall and anticyclonic circulation to the south. The reduction of rainfall over continental northeast Brazil is large enough to give further support to the proposed mechanism.

Since the sea surface temperature anomalies over the tropical Atlantic persist for several months. the proposed mechanism could provide guidance for predicting droughts over northeast Brazil.

Full access
Antonio D. Moura
and
Peter H. Stone

Abstract

A baroclinic stability analysis is performed for a simple family of zonal shear profiles over a sphere, using a two-layer, quasi-geostrophic model. The stability properties and the structure of the most unstable waves are qualitatively similar to those on a β-plane. However, the spherical geometry plays a major role in locating some of the important features of the most unstable waves. In particular, the locations of the maximum wave amplitude, maximum eddy heat fluxes, and maximum convergence of the eddy angular momentum flux are all well correlated with the location of the maximum excess of the vertical shear over the minimum value necessary for local instability on a sphere. Consequently the eddy momentum flux tends to generate a mid-latitude jet even if there is no preexisting mid-latitude jet in the basic state zonal flow. These findings suggest some of the elements needed for parameterizing the meridional variations of baroclinic eddy fluxes accurately.

Full access
Paulo Nobre
,
Antonio D. Moura
, and
Liqiang Sun

This study presents an evaluation of a seasonal climate forecast done with the International Research Institute for Climate Prediction (IRI) dynamical forecast system (regional model nested into a general circulation model) over northern South America for January–April 1999, encompassing the rainy season over Brazil's Nordeste. The one-way nesting is one in two tiers: first the NCEP's Regional Spectral Model (RSM) runs with an 80-km grid mesh forced by the ECHAM3 atmospheric general circulation model (AGCM) outputs; then the RSM runs with a finer grid mesh (20 km) forced by the forecasts generated by the RSM-80. An ensemble of three realizations is done. Lower boundary conditions over the oceans for both ECHAM and RSM model runs are sea surface temperature forecasts over the tropical oceans. Soil moisture is initialized by ECHAM's inputs.

The rainfall forecasts generated by the regional model are compared with those of the AGCM and observations. It is shown that the regional model at 80-km resolution improves upon the AGCM rainfall forecast, reducing both seasonal bias and root-mean-square error. On the other hand, the RSM-20 forecasts presented larger errors, with spatial patterns that resemble those of local topography. The better forecast of the position and width of the intertropical convergence zone (ITCZ) over the tropical Atlantic by the RSM-80 model is one of the principal reasons for better-forecast scores of the RSM-80 relative to the AGCM. The regional model improved the spatial as well as the temporal details of rainfall distribution, and also presenting the minimum spread among the ensemble members. The statistics of synoptic-scale weather variability on seasonal timescales were best forecast with the regional 80-km model over the Nordeste. The possibility of forecasting the frequency distribution of dry and wet spells within the rainy season is encouraging.

Full access
Jacques Servain
,
Antonio J. Busalacchi
,
Michael J. McPhaden
,
Antonio D. Moura
,
Gilles Reverdin
,
Marcio Vianna
, and
Stephen E. Zebiak

The tropical Atlantic Ocean is characterized by a large seasonal cycle around which there are climatically significant interannual and decadal timescale variations. The most pronounced of these interannual variations are equatorial warm events, somewhat similar to the El Niño events for the Pacific, and the so-called Atlantic sea surface temperature dipole. Both of these phenomena in turn may be related to El Niño-Southern Oscillation variability in the tropical Pacific and other modes of regional climatic variability in ways that are not yet fully understood. PIRATA (Pilot Research Moored Array in the Tropical Atlantic) will address the lack of oceanic and atmospheric data in the tropical Atlantic, which limits our ability to make progress on these important climate issues. The PIRATA array consists of 12 moored Autonomous Temperature Line Acquisition System buoy sites to be occupied during the years 1997–2000 for monitoring the surface variables and upper-ocean thermal structure at key locations in the tropical Atlantic. Meteorological and oceanographical measurements are transmitted via satellite in real time and are available to all interested users in the research or operational communities. The total number of moorings is a compromise between the need to put out a large enough array for a long enough period of time to gain fundamentally new insights into coupled ocean–atmosphere interactions in the region, while at the same time recognizing the practical constraints of resource limitations in terms of funding, ship time, and personnel. Seen as a pilot Global Ocean Observing System/Global Climate Observing System experiment, PIRATA contributes to monitoring the tropical Atlantic in real time and anticipates a comprehensive observing system that could be operational in the region for the 2000s.

Full access
Liqiang Sun
,
Huilan Li
,
Stephen E. Zebiak
,
David F. Moncunill
,
Francisco D. A. D. S. Filho
, and
Antonio D. Moura

Abstract

The International Research Institute for Climate Prediction (IRI) and Ceará Foundation for Meteorology and Water Resources (FUNCEME) in Brazil have developed a dynamical downscaling prediction system for Northeast Brazil (the Nordeste) and have been issuing seasonal rainfall forecasts since December 2001. To the authors’ knowledge, this is the first operational climate dynamical downscaling prediction system. The ECHAM4.5 AGCM and the NCEP Regional Spectral Model (RSM) are the core of this prediction system. This is a two-tiered prediction system. SST forecasts are produced first, which then serve as the lower boundary condition forcing for the ECHAM4.5 AGCM–NCEP RSM nested system. Hindcasts for January–June 1971–2000 with the nested model, using observed SSTs, provided estimates of model potential predictability and characteristics of the model climatology. During 2002–04, the overall rainfall forecast skill, measured by the ranked probability skill score (RPSS), is positive over a majority of the Nordeste. Higher skill is found for the March–May (MAM) and April–June (AMJ) seasons with forecast lead times up to 3 months. The skill of the downscaled forecasts is generally higher than that of the driving global model forecasts.

Full access

THE PIRATA PROGRAM

History, Accomplishments, and Future Directions *

Bernard Bourlès
,
Rick Lumpkin
,
Michael J. McPhaden
,
Fabrice Hernandez
,
Paulo Nobre
,
Edmo Campos
,
Lisan Yu
,
Serge Planton
,
Antonio Busalacchi
,
Antonio D. Moura
,
Jacques Servain
, and
Janice Trotte

The Pilot Research Moored Array in the tropical Atlantic (PIRATA) was developed as a multinational observation network to improve our knowledge and understanding of ocean-atmosphere variability in the tropical Atlantic. PIRATA was motivated by fundamental scientific issues and by societal needs for improved prediction of climate variability and its impact on the economies of West Africa, northeastern Brazil, the West Indies, and the United States. In this paper the implementation of this network is described, noteworthy accomplishments are highlighted, and the future of PIRATA in the framework of a sustainable tropical Atlantic observing system is discussed. We demonstrate that PIRATA has advanced beyond a “Pilot” program and, as such, we have redefined the PIRATA acronym to be “Prediction and Research Moored Array in the Tropical Atlantic.”

Full access