Pirata, an in situ observations program in the tropical Atlantic

Pirata enters the Mercator system

By Jacques Servain, Chairman of the Pirata Scientific Steering Committee

Pirata (Pilot Research moored Array in the Tropical Atlantic) is an in situ observation network aiming at the study of the tropical Atlantic's climatic and oceanic conditions. It is similar to the TAO/Triton (Tropical Atmosphere-Ocean Array/Triangle Trans-Ocean buoy Network) program which aims to learn more about large scale climatic phenomena such as El Niño/La Niña. These observation systems are made operational with the daily delivery of satellite data (Argos system). Data enter the global Transmission System which has been used for several decades by meteorological centers for weather forecasting.

Present scheme (red circles) and proposed scheme (blue circles) of the Pirata network

Pirata data are used in atmospheric, oceanic and coupled atmospheric/oceanic simulation models

TAO/Triton data are used by scientists studying climatic conditions variability and are particularly useful to atmospheric and oceanic global circulation models (AGCM and OGCM) and coupled atmosphere/ocean circulation models (CGCM). These data contribute to a good knowledge of initial conditions which are essential to global and regional climatic seasonal forecasting. Concerning the tropical Atlantic and its part in the climate variability, one knows that surface temperature variations are strongly correlated with excessive dryness or rainfall episodes as observed in North-Eastern Brazil and in West Africa.

Moored buoys transmit measurement data to satellites

Deployment of an Atlas buoy from the RV Atalante during Pirata-FR11 campaign (November 2001)

Both TAO/Triton and Pirata networks use the Atlas (Autonomous Temperature Line Acquisition System) buoys conceived by Pmel/Noaa (Pacific Marine Environment Laboratory/National Oceanic and Atmospheric Administration) for the TAO network. Atlas buoys are moored on the bottom of the ocean (10 so far for the Pirata network). Strategic and highly representative oceanic areas are selected for buoys location. These meteorological and oceanic autonomous stations contribute to fill the lack of real-time climatic information in these tropical oceanic areas, which are considered as warm water tanks of the global climatic system. Sensors on these buoys measure the main climatic variables that influence energetic transfer through air-sea interface (wind velocity and direction, temperature, air humidity, solar radiation, rainfall, sea surface temperature). The thermic and saline contents of the superficial oceanic layers (from the surface down to 500m) are very important in the slow fluctuations. They are measured and transmitted in real-time. Each buoy is energitically for 12 months, so an annual electric and mechanic maintenance of the Pirata network is required. That represents a 90 days per year time-vessel investment for the entire Pirata network.

A collaboration between three countries

Pirata is a multinational program with participation of Brazil, France and USA. These three countries share the network implementation and maintenance. Brazil and France take care of the logistics, Brazil carries the responsability of the western part of the network, and France the eastern part. Since the beginning of Pirata program, (September 1997), more than 20 Pirata campaigns have been carried out by French and brazilian ships.

Pirata and the international scientific community

Pirata is recognized, supported and recommended by the international scientific community and the great international climate programs such as Clivar (Climatic Variability and Predictability), Goos (Global Ocean and Observation System), Argo (Array for Real-Time Geostrophic Oceanography). At a strictly french level, Pirata is supported by the National Plan for Climate. Coriolis, which is the french component of Argo, supplies the Mercator project with Pirata data. Presently, a validation of the Pirata data is taking place through the Perene (Pirata Eulerian Essential Network Evaluation) project in which Mercator is involved. The Perene purpose is to measure the impact of Pirata data in multiple numerical simulations , including other assimilated real time data such as altimetry, Argo buoys or XBTs profiles, for a good representation of the tropical Atlantic oceanic state.

Schedule plan

At the end of the first Pirata phase (1997-2001), it has been decided that coordination Pirata scheme between Brazil, France and USA would be extended until the end of 2005. A " Memorandum of Understanding has been signed at a Pirata meeting (Pirata-8, august 2001) by the main organisms supporting the program.The present consolidation phase aims at perpetuating the initial network before it is passed down to international oceanography programs (Goos, Argo, Godae) and in France (Coriolis and Mercator).

Three extensions to the initial network are proposed by several scientific comitees, including Clivar. Two extensions concern the African edge of the Atlantic basin : a north-east extension off the Maroco-Mauritania-Senegal-Guinea coasts, and a south-east extension off the Gabon-Angola-Namibia-South Africa coasts. The third extension would cover the south-west part of the network (off Brazil between Natal and Rio de Janeiro). For each of these extensions, 2 to 5 Atlas buoys would be deployed. These extension projects are presently considered and evaluated by scientific comitees.