The Argo "oceanographic radiosonde" is a revolutionary concept that enhances the real time capability for the measurement of temperature and salinity through the upper 2,000 m in the ice free global Ocean. The exclusion of the high latitudes was due to the inability of early floats to sample under sea-ice. However, technological advances in float design in recent years now give us this capability. Advancements have come through re-design of hardware (i.e. armoured ice floats with ice-hardened antennae), software (ice-avoidance algorithms and open-water test) and communications (Iridium), allowing the transmission of stored winter profiles. Following the geostrophic principles, along with reference level velocities of the ocean, it contributes to the global description of the variability of the upper ocean thermohaline structure and circulation on seasonal and inter-annual time scales. Under a unique, internationally coordinated effort, it has been established as a global array of about 3,000 floats at a spatial resolution of 3° x 3° grids.
The data from these floats have helped to study the state of the upper ocean and the patterns of ocean climate variability, including heat and freshwater storage and transport (Freeland et al. 2009). The data are collected by Argo floats that spend most of their working life drifting with the currents at depth (they are stabilized at a constant level by being less compressible than sea water) of 1,000 or 2,000 m. At typically 10 day intervals, the floats pump fluid into an external bladder and rise to the surface (taking about 6 h) and measure a profile of temperature and salinity. On surfacing the data are downloaded to the satellites (ARGOS or Iridium) which also obtains a series of float positions. When this task is completed the bladder deflates, the float thus returns to its original density and returns to depth to drift until the (usually 10 day) cycle is repeated. Data from Argo floats are available to users through two streams—real time (with only gross errors corrected or flagged) and delayed-mode (where corrections to salinity values have been estimated by experts familiar with the particular geographical environment). At present the delayed mode data delivery system has yet to be fully implemented. The real time data are placed on the GTS that delivers (mostly meteorological) data to operational centers throughout world. They are also available through two linked Argo Global Data Centres (GDACs) in Brest, France (Coriolis) and Monterey, California (US GO-DAE server). The global distribution of floats reporting on the Argo system as on Sept 2010 is shown in Fig. 3.6.
Argo floats bridge the complementary nature of the direct and remote observing systems, filling the large gaps that exist in the global sampling network, and providing essential information for sub surface ocean state estimation. The combination of Argo and satellite altimetry has enabled a new generation of applications. Global maps of sea level, on time scales of weeks to several years, will be interpreted with full knowledge of the upper ocean stratification. Global Ocean and climate models can be initialized, tested and constrained with a level of information hitherto not available. The drift estimates from such an array would in addition provide useful estimates of deep pressure fields (reference level).
Altimeters, together with the sea level gauge network, provide accurate measurements of time-varying sea surface height (SSH) globally every 10 days. On seasonal and longer time-scales, SSH is dominated by changes in subsurface density. The cause of mean sea level change is mainly due to change in volume and shape of the ocean basins at comparatively long time scales. The change in volume is caused by the changes in sea water density (steric) and mass (eustatic). The change in temperature (thermosteric) and salinity (halosteric) of the water column can change sea water density, whereas melting of glaciers in land and Artic and Greenland ice will change the mass of the water in the Ocean. The shape of the ocean basin changes due to vertical land movement, which is associated with local tectonic activity and post glacial rebound of land. The contribution of the steric and eusatic for the total sea level rise can be quantified using Argo profiling floats and GRACE respectively and which can indirectly compared to altimeter sea level data. On global scales, Argo and Jason, together with satellite gravity measurements, partition global sea level rise into its steric and mass-related components (Willis et al. 2008; Cazenave et al. 2009; Leuliette and Miller 2009; Wunsch et al. 2007).
Applications of Argo data are numerous and varied, including initialization of ENSO forecast models, initialization of short-range ocean forecasts, routine production of high-quality global ocean analyses, and studies of predictability on inter-annual and decadal time scales. A substantial improvement in seasonal forecast skill
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