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3000 Argo floats in the ocean
In collaboration with Marie Drévillon and Stéphanie Guinehut, Mercator Océan, November 2007
The goal of 3000 Argo floats deployment is today achieved. This worldwide observation network is the essentiel complement to satellite observations for the operational ocean forecasting systems operated at Mercator Ocean. It allows the 2007 El Niña episode to be monitored in the Pacific.
October 2007: 3000 Argo floats deployment
Credit: Jcommops (Click to enlarge)
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The Argo programme is aiming to deploy and maintain a network of around 3000 profiling floats, spread out on a 3° x 3° grid, for measuring temperature and salinity profiles (hence the name 'Argo profiler') at depths of up to 2000 m, and also for measuring the speed of ocean currents at varying depths. These three parameters can then be assimilated into numerical models. The Argo programme was undertaken as a contribution to various programmes for operational oceanography and seasonal and inter-annual climate forecasting. Each float rises to the ocean surface every ten days to transmit its data to the Argos satellites (other systems such as Iridium are currently being studied), before diving down to 2000 m again. The resulting data is published (free of charge and with no restrictions on use) in real time on the Global Transmission System (GTS) and on Internet. The European Union and 17 other countries are participating in the Argo network. The French contribution is the Coriolis project which consists of instrumentation (development of the Provor model), deployment of instruments and data processing (Coriolis is actually the name of one of the two Global Argo Data Centres).
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Measurement cycle for floating Argo buoy (credit: Ifremer)
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Argo and la Niña 2007
A la Niña episode is occuring since February 2007 in the eastern Pacific. La Niña is characterized by stronger Trade winds than normal. These winds push warmer surface waters westwards leading to deep waters upwelling in the eastern part of the basin. During a la Niña episode, the western Pacific becomes warmer than normal and becomes colder in the East. There is a great number of meteorological consequences throughout the world: drought in the South of North America, North of Mexico and California, strong precipitations in South-East Asia, East of Australia, India and South Africa. NOAA is describing the current episode as moderate. Most of the climate models are predicting this episode to last until the beginning of 2008.
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TAO network temperature observations: section across the Eastern Pacific.
Beginning of August 2007, temperature anomalies show the beginning of la Niña (cold anomaly). (Click to enlarge)
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Same plots from Mercator PSY3V2 system assimilating satellite altimetry, sea surface temperature and in situ data including Argo. The la Niña amplitude seems to be overestimated by the model. NB reference for anomaly is Levitus 2005.
(Click to enlarge)
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Same plots from Mercator PSY3V1 system assimilating only altimetry. Vertical temperature gradients are less tight. This results in an opposite anomaly compared to PSY3V2 anomaly, at 200m (same Levitus 2005 reference). Without in situ data, the system doesn't see the deep cold anomaly characterising la Niña.
(Click to enlarge)
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Gaps between in situ and simulation are decreasing
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On the opposite plot, dots are representing Coriolis in situ data going to be assimilated by the Mercator PSY3V2 system during the 21 to 28 June 2007 week. Difference between measured temperature and simulated temperature at 318 m is shown with a colour code. These differences are computed before assimilation. Values below 0.5° are prevailing showing that Mercator model is close to observations. The high values in high energetic areas suchs as Kuroshio are normal.
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On the right hand side, same plot after assimilation: differences have decreased showing the benefit of in situ data assimilation.
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Pacific subtropical mode water
Measured salinity section
(Click to enlarge)
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Simulated salinity section with in situ data
assimilation (Click to enlarge)
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Simulated salinity section without in situ data
assimilation (Click to enlarge)
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Subtropical Pacific mode water, between 100 and 300 meters is characterized by a high salinity core (36.9 psu). It is a Central Pacific Water component formed by subduction in the subtropical gyre. This water is better simulated thanks to the in situ data assimilation. Fox and Haines (2002, J. Phys. Oceanogr) have shown that the in situ data assimilation allow the models to better represent mode waters.
Useful links
About Argo
About la Niña
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