Jacques Servain

Simple climatic indices for the tropical Atlantic Ocean and some applications

Two indices related to the sea surface temperature (SST) variability in the tropical Atlantic are proposed. One index describes the SST averaged over the whole basin (30°N to 20°S, 60°W to 15°E), and the other illustrates a meridional dipole between the northern and southern hemispheres. The computational method for obtaining these indices is intentionally kept simple, the objective being to reproduce the signature of the main results previously provided from more complicated statistical analyses. Monthly time series for both indices are produced from 1964 up to the present time. The whole basin index exhibits principally a sustained warming which has intensified since about 1975, and it has a significant periodicity close to that of the quasi-biennial oscillation. The dipole index exhibits a decadal-scale variation, and its building up seems to be related to other worldwide climatic changes, as for instance El Nino / Southern Oscillation extreme episodes, rainfall variabilities over the Brazilian Nordeste and African Sahel.

The Pirata Program: History, Accomplishments, and Future Directions

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.”

A Pilot Research Moored Array in the Tropical Atlantic (PIRATA)

Abstract 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 Nino events for the Pacific, and the so-called Atlantic sea surface temperature dipole. Both of these phenomena in turn may be related to El Nino-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 location...

Chapter 16 Interannual and Seasonal Variability of the Tropical Atlantic ocean depicted by Sixteen years of Sea-Surface Temperature and wind stress

Abstract Monthly fields of sea-surface temperature and wind stress for the tropical Atlantic are carefully established for January 1964 to December 1979. The availability of these gridded data sets permits a few simple measures of the seasonal and interannual variability of the tropical Atlantic. Throughout most of the basin the amplitude of seasonal SST variability is greater than the amplitude of monthly SST anomalies. Interannual fluctuations of SST are largest in areas of large seasonal SST variability such as coastal and equatorial upwelling zones. In these regions monthly SST anomalies may reach 1°—2°C. As opposed to SST, the largest year-to-year changes in wind stress do not necessarily coincide with the locations of the largest seasonal cycles. The amplitude of monthly wind stress anomalies is greater than the amplitude of the seasonal cycle except in the vicinity of the ITCZ. The range of monthly wind stress anomalies along the equator and eastern boundary, expressed in units of wind speed squared, is 10–20m 2 s −2 . Of particular note are the differing phase relationships with the seasonal cycle for the interannual fluctuations of SST and wind stress within the upwelling zones of the Northern and Southern Hemispheres. Moreover, sample time series of SST and wind stress are characterized, at times, by anomalous events persisting for more than one year. This evidence is suggestive of anomalous conditions in the tropical Atlantic that are not the result of a simple phase shift or change in amplitude of the seasonal cycle.

The relationship between the simulated climatic variability modes of the tropical Atlantic

Two main modes of climatic variability occur in the tropical Atlantic Ocean at inter-annual time-scales: the equatorial mode, similar to the El Nino phenomenon in the Pacific Ocean, and the meridional mode, or dipole-like mode, with no Pacific counterpart. The Atlantic equatorial mode is characterized by the occurrence of alternating warm and cold episodes at the equator, on the eastern side of the basin. These events are associated with abnormal variations in the zonal equatorial slope of the thermocline. The meridional mode is characterized by an inter-hemispheric gradient in the sea-surface temperature (SST). The mean position of the Inter-tropical Convergence Zone (ITCZ) separates positive and negative SST signals. It was recently shown with observational indices that there is significant correlation between these two climatic modes of variability. This study goes one step further, by using a multi-year numerical simulation, where an oceanic general circulation model is forced by the 1979-1993 ECMWF reanalysis. Model computed indices representing the two main modes of variability compare well with observations. The two inter-annual modes of variability are shown to have the same physics as the annual variability does, which is related to the latitudinal displacement of the ITCZ. Furthermore, it is suggested that the ocean dynamics (as opposed to thermodynamic processes) is the principal cause of climate variability in the region.

Heavy Rainfall Episodes in the Eastern Northeast Brazil Linked to Large-Scale Ocean-Atmosphere Conditions in the Tropical Atlantic

Relationships between simultaneous occurrences of distinctive atmospheric easterly wave (EW) signatures that cross the south-equatorial Atlantic, intense mesoscale convective systems (lifespan > 2 hour) that propagate westward over the western south-equatorial Atlantic, and subsequent strong rainfall episodes (anomaly > 10 mm·day−1) that occur in eastern Northeast Brazil (ENEB) are investigated. Using a simple diagnostic analysis, twelve cases with EW lifespan ranging between 3 and 8 days and a mean velocity of 8 m·s−1 were selected and documented during each rainy season of 2004, 2005, and 2006. These cases, which represent 50% of the total number of strong rainfall episodes and 60% of the rainfall amount over the ENEB, were concomitant with an acceleration of the trade winds over the south-equatorial Atlantic, an excess of moisture transported westward from Africa to America, and a strengthening of the convective activity in the oceanic region close to Brazil. Most of these episodes occurred during positive sea surface temperature anomaly patterns over the entire south-equatorial Atlantic and low-frequency warm conditions within the oceanic mixing layer. A real-time monitoring and the simulation of this ocean-atmosphere relationship could help in forecasting such dramatic rainfall events.

In Situ–Based Pseudo–Wind Stress Products for the Tropical Oceans

Abstract Quality wind stress fields are desired for a wide range of oceanographic and atmospheric studies. An overview is presented of the monthly quick-look and research-quality tropical ocean wind (pseudostress) products produced for the Pacific and Indian Oceans by The Florida State University (FSU) and for the Atlantic Ocean by the French Institut de Recherche pour le Developpement [IRD; formerly Institut Francais de Re-cherche Scientifique pour le Developpement en Cooperation (ORSTOM)]. This review article briefly discusses the current state of tropical wind stress products, including an introduction to the new objective FSU analysis technique and advancements in the IRD method. The primary focus is a detailed discussion of how the FSU and IRD pseudostress fields evolved from early subjective in situ analysis techniques. The historical retrospective introduces the scientific motivation, development, and methodology for each product. Examples of the wide range of scientific research and operational ap...

Tropical Atlantic Contributions to Strong Rainfall Variability Along the Northeast Brazilian Coast

Tropical Atlantic (TA) Ocean-atmosphere interactions and their contributions to strong variability of rainfall along the Northeast Brazilian (NEB) coast were investigated for the years 1974–2008. The core rainy seasons of March-April and June-July were identified for Fortaleza (northern NEB; NNEB) and Recife (eastern NEB; ENEB), respectively. Lagged linear regressions between sea surface temperature (SST) and pseudo wind stress (PWS) anomalies over the entire TA and strong rainfall anomalies at Fortaleza and Recife show that the rainfall variability of these regions is differentially influenced by the dynamics of the TA. When the Intertropical Convergence Zone is abnormally displaced southward a few months prior to the NNEB rainy season, the associated meridional mode increases humidity and precipitation during the rainy season. Additionally, this study shows predictive effect of SST, meridional PWS, and barrier layer thickness, in the Northwestern equatorial Atlantic, on the NNEB rainfall. The dynamical influence of the TA on the June-July ENEB rainfall variability shows a northwestward-propagating area of strong, positively correlated SST from the southeastern TA to the southwestern Atlantic warm pool (SAWP) offshore of Brazil. Our results also show predictive effect of SST, zonal PWS, and mixed layer depth, in the SAWP, on the ENEB rainfall.

Avaliação do impacto de eventos climáticos extremos nos oceanos pacífico e atlântico sobre a estação chuvosa no nordeste do Brasil

Impact of extreme climatic events in the Pacific (El Nino, EN; La Nina, LN) and Atlantic (Meridional SST Gradient, GRADM) on the rainfall ofthree sub-regions over the Northeast Brazil - NEB (Northern NEB, Eastern NEB and Centre/Southern NEB) are evaluated during the period 1948 to 1997. The analysis includes observed precipitation data and simulated rainfall outputs derived from the ARPEGE Atmospheric General Circulation Model (AGCM). The results show that the precipitation over the three sub-regions presents a high variability independent of LN or EN episodes, with a predominance of dry (wet) years in the sub-regions Eastern and Northern of NEB before (after) 1976, i.e. when LN (EL) episodes were more numerous. The interannual rainfall response is stronger when GRADM is positive (inducing dry NEB) or negative (inducing wet NEB), thus showing a stronger climatic influence of the Atlantic vs. the Pacific. Combining simulated influences of Pacific and Atlantic as well as observed precipitation data over the entire period, it was found that during EN years the global tropical ocean is responsible for the rainfall variability over the NEB (especially in the Northern and Eastern sub-regions). On the other hand, during LN years, the Atlantic tropical Ocean is the main contribution, and the tropical Pacific Ocean influence is relatively modest.

Rainfall Response in Northeast Brazil from Ocean Climate Variability during the Second Half of the Twentieth Century

AbstractThe authors investigated the rainfall variability response in northern Northeast Brazil (NNEB) from El Nino/La Nina (EN/LN) events and from the meridional sea surface temperature gradient (MGRAD) over the tropical Atlantic during the period 1948–97. The diagnostic analysis was stratified according to four climatic series of scenarios associated with EN, LN, and positive and negative MGRAD. During ENs, which were more numerous and more intense after the 1970s, the MGRAD was generally not noticeable, and the drought impact in NNEB was mainly due to the warm Pacific influence. Conversely, during LNs, the MGRAD signal was important, but there was an inverse relationship between the third and the fourth quarters of the twentieth century. Thus, before the 1970s the LNs were associated with positive MGRAD, which led to an inverse influence inducing minor changes in seasonal rainfall in NNEB. After the 1970s the LNs were linked to negative MGRAD, which induced a cumulative wet influence in NNEB. The posit...