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Biogeochemistry

Sailing the Seven Seas with Argo

Article S. C. Riser, H. J. Freeland, D. Roemmich, S. Wijffels, A. Troisi, M. Belbéoch, D. Gilbert, J. Xu, S. Pouliquen, A. Thresher, P.-Y. Le Traon, G. Maze, B. Klein, M. Ravichandran, F. Grant, P.-M. Poulain, T. Suga, B. Lim, A. Sterl, P. Sutton, K.-A. Mork, P. J. Velez-Belchi, I. Ansorge, B. King, J. Turton, M. Baringer, S. R. Jayne, Fifteen years of ocean observations with the global Argo array. Nat. Clim. Change 6, 145–153 (2016).

An Argo float beginning its journey after launch from a French research vessel. Source: http://www.argo.ucsd.edu/

For the past 50 years, the ocean has quietly taken up over 90% of the excess heat energy building up in the global climate system.  Given that the ocean tends not to brag about these things, how exactly do we know this incredible fact?  Well, we can thank an army of underwater robots – collectively known as Argo – which has revolutionized our understanding of the ocean.

 

Needle in a Haystack

Observing the ocean is no small feat, given its immensity and ever-changing state.  Traditionally, oceanographic sampling has been conducted by ships, which tend to survey regions that are more accessible – few dare to venture to the Southern Ocean, especially in the winter.  As a result, prior to this century large swaths of the ocean remained unsampled, and even the areas that were traversed were done so infrequently.  Without systematic, high-density observations of the global ocean, it is impossible to accurately assess the role that it plays in Earth’s changing climate.

 

Enter Argo

Argo is a fleet of nearly 4,000 robotic floats which prowl the upper (<1 kilometer depth) ocean, collecting information on the physical state of the ocean.  They measure temperature, salinity, and current velocities and transmit these data to satellites in near real-time.  The Argo program is an international effort, with floats populating every ocean basin.  Data obtained by Argo are freely available and can be accessed from two global data servers based in France and the USA.  These data are routinely used by weather and climate centers for ocean analysis and forecasting.  Although the Argo program is barely two decades old, its data overwhelm all other sources of observations in our global database of physical ocean parameters.  Such information is invaluable in allowing us to track recent changes in ocean heat content and thermosteric (temperature-induced) sea level rise.

The distribution of Argo floats, as of October 2018. The color of the points corresponds to the country that owns the float. Source: http://www.argo.ucsd.edu

 

Free Drifters

A typical sampling cycle for an Argo float. Each float will complete about 140 cycles over its lifetime. Source: http://www.argo.ucsd.edu

While the analyses that Argo floats enable are sophisticated, the concept behind their operating principle is quite simple.  A float is a cylinder 1.3 meters (4.25 feet) in length and 20 centimeters (8 inches) in diameter, and weighs in at about 40 kilograms (88 pounds).  At the top of the float are temperature and salinity sensors, as well as an antenna to transmit the float’s location and data collected to satellites.  At the bottom is a bladder which can be manipulated to alter the buoyancy of the float.  A sampling cycle commences with the float sinking to a programmed depth with an emptied bladder – the float will then drift at this depth since it has the same density as the surrounding seawater.  When it is time to ascend, the bladder is pumped with oil so that the float becomes less dense and can rise to the surface.  The floats are powered by batteries and have an approximate life time of 4 years.

 

A Data Explosion

Since the start of the Argo program in the late 1990s, over 2,100 peer-reviewed scientific papers have been published which make use of Argo data.  Already, Argo has greatly advanced our understanding of the ocean, and therefore of global climate.  For instance, most climate models assimilate temperature observations provided by Argo.  This has improved forecasting of monsoon activity and ocean-atmosphere interactions such as El Niño-Southern Oscillation (ENSO), a recurring climate phenomenon that causes extreme weather in many regions of the world.  ENSO is associated with flooding in some parts of the world and drought in others, particularly affecting developing countries dependent on agriculture and fishing.  In the U.S., it alters rainfall and snowfall patterns and the strength of hurricanes.  Given these impacts, it would be invaluable to be able to forecast the magnitude and onset of ENSO.

Argo data have also definitively shown a global-scale warming of the upper ocean, as well as an increase in salinity gradients, with fresh areas getting fresher and salty areas becoming saltier.  And, Argo data have been put to use in studying the mechanisms of deep ocean circulation.  In this process, which varies from year-to-year, water sinks at high-latitude sites and carries heat and gases into the deep ocean.  The circulation patterns induced by these sinking water parcels are important in influencing century-scale climate variability.

 

The Golden Fleece

Ongoing work is focused on technological developments which will expand the range of science questions that Argo data can address.  Floats equipped with new sensors which can measure biogeochemical variables (e.g., dissolved oxygen, nitrate, chlorophyll, and pH) will enable the study of processes such as ocean acidification.  And, expanding Argo’s sampling range to deeper depths will enable more comprehensive analyses.  Just as in the Greek myth, in which the hero Jason set sail on the Argo in search of the Golden Fleece, contemporary Argo floats act as the vessel which enable us to navigate the 21st century seas.  Here, our Golden Fleece is a better understanding of how the ocean is changing, as a result of both natural causes and human activities.

 

Discussion

One Response to “Sailing the Seven Seas with Argo”

  1. Great summary of Argo – and really like your analogy to the ‘Golden Fleece’. I find the Argo program really impressive from both the perspective of scientific measurements and the fact that so many nations have come together on it. Keep up the great posts!

    Posted by Allison Chua | March 27, 2019, 10:30 am

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