Deploying an Argo float. Image credit: Alicia Navidad, CSIRO.


What have we learnt from 15 years of ocean observations with the global Argo array?

A review of the Argo array recently published in Nature Climate Change considers the progress and provides an outline of how the programme is likely to change.

More than 90% of the heat energy accumulation in the climate system between 1971 and the present has been in the ocean. Thus, the ocean plays a crucial role in determining the climate of the planet.

Observing the oceans is problematic even under the most favourable of conditions. Historically, shipboard ocean sampling has left vast expanses, particularly in the Southern Ocean, unobserved for long periods of time. Within the past 15 years, with the advent of the global Argo array of profiling floats, it has become possible to sample the upper 2,000 m of the ocean globally and uniformly in space and time.

The Argo array is a major component of the Global Ocean Observing System and strives to monitor the evolving temperature and salinity fields of the upper ocean. The profiling floats used in Argo are 2 m-long, freely drifting robotic devices that adjust their depth in the ocean by changing their buoyancy.

IMOS deployments of just 30 floats per annum along with other national and international deployments have resulted in 10% of the global array of 3,900 floats delivering a continuous data stream for the Australian region.

The array provides freely available temperature and salinity data from the upper 2,000 m of the ocean with global coverage. The data are available within 24 hours of collection for use in a broad range of applications that focus on examining climate-relevant variability on seasonal to decadal timescales, multidecadal climate change, improved initialization of coupled ocean–atmosphere climate models and constraining ocean analysis and forecasting systems.

Perhaps the single most powerful metric of the value of Argo is the widespread use of the data produced by the programme: since the beginning of Argo in the late 1990s, more than 2,100 papers in the refereed science literature have used Argo observations, attesting to the array's value in expanding our understanding of the oceans and climate.

Highlights of the research using Argo data include:

  • Most climate models assimilate the subsurface temperature observations from Argo, leading to improved forecasts of intraseasonal waves in the atmosphere, monsoon activity, and ocean–atmosphere interactions such as the El Niño/Southern Oscillation (ENSO).
  • The Argo observations have been particularly useful in examining ocean changes on timescales of decades and longer. A stunning example has used contemporary Argo observations in conjunction with data from the HMS Challenger expedition, carried out in the second half of the nineteenth century. The study reveals a warming of the ocean over the past 135 years of nearly 0.6 °C near the sea surface, tapering to near zero at depths close to 1,000 m.
  • The observed increase in ocean heat content in the upper 700 m over the past 40 years, inferred from Argo and hydrographic observations, is the dominant term in the global inventory of heating changes, with over 90% of the excess heat in the climate system being stored in the oceans. Without Argo, it is unlikely that such a conclusion could have been drawn.
  • The data have also allowed temporal spatial variations in ocean heat content to be discerned, suggesting that most of the increase in heat content in the past decade has occurred in the Southern Ocean (which was poorly sampled before Argo); it has also been noted that ENSO variability in the tropical Pacific has for now somewhat obscured the global increase in sea surface temperature.

When Argo began in the late 1990s, it was by no means clear that the project would be successful in deploying and sustaining an array of 3,000 floats over the global ocean, as the required technology was in its infancy and the degree of international cooperation required was unprecedented in the oceanographic community. Now, in the second decade of the twenty-first century, the float technology is well proven, and over 30 countries are contributing resources to Argo, making it sensible to contemplate expansions of its mission.

The Argo Steering Team has provided a roadmap for how the project might evolve and expand in the next decade, and some of this proposed development is now underway via test deployments or regional pilot arrays. One project is to support an increase in the spatial sampling resolution in particular parts of the world ocean where the ocean is especially turbulent (challenging the array's signal resolving power) and the interaction of the ocean and the atmosphere and the resulting climate impacts are especially strong. Improved technology also allows us to expand Argo into previously unsampled regions, such as marginal seas and the seasonal ice-zone, meaning that the array is more truly global in its coverage than its original design.

The full review article:

Riser, SC et al. 2016 Fifteen years of ocean observations with the global Argo array. Nature Climate Change6, 145–153. doi:10.1038/nclimate2872

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The typical cycle of an Argo float. Image credit: Megan Scanderbeg, Scripps Institution of Oceanography.