Observing the oceans - 15 years of Argo measurements
February 2016 - The first Argo floats were deployed in 1999, marking the start of the international Argo programme to build a global array of profiling floats throughout the world's oceans. In 2007 Argo met its initial goal of around 3,000 operating floats providing near global coverage, which continues today. The measurements from Argo have revolutionised our ability to observe the oceans and their effect on our climate.
Argo - the technology
Argo floats are battery-powered autonomous robotic platforms that are programmed to measure ocean profiles of temperature and salinity from 2,000m depth. The typical Argo cycle is shown in figure 1, where the float spends much of its life drifting with the deep (1,000 m) ocean current. The temperature, salinity and pressure (depth) are recorded while the float rises to the sea surface and the data are transmitted via satellite (either using the Argos or Iridium systems). They are deployed from ships (Figure 2) and are designed to be able to make as many as 200 profiles at 10-day intervals, typically operating for around 5 years at sea.
How much data has Argo delivered?
Before Argo we relied on ships or fixed moorings to provide observations of the temperature and salinity beneath the sea surface. As a result the observations were largely concentrated along shipping lines, mostly in the Northern hemisphere, with very limited coverage in the Southern Ocean especially during the austral winter months. During the whole of the 20th century around 0.5 million shipboard observations were collected, in contrast the Argo array is delivering around 120,000 new profiles each year and by November 2012 Argo had collected its millionth profile. The increase in the number of ocean profiles measured over the last 15 years, due to Argo, is shown in Figure 3, which also shows the significant increase in the amount of available salinity data.
Argo is not only delivering more profiles but also providing year round global coverage, as shown in Figure 4. All Argo data are made freely available via the World Meteorological Organization (WMO) Global Telecommunications System (GTS) and the two Argo Global Data Assembly Centres (GDACs) based in the USA and France.
From its very beginning it was recognised that implementing Argo would require an unprecedented level of international cooperation and over 30 countries have contributed to providing and/or deploying floats in building and sustaining the Argo array. In the UK the national contribution to Argo is carried out by a collaborative effort between the Met Office, the National Oceanography Centre and British Oceanographic Data Centre, and funded by the Department of Energy and Climate Change (DECC), the Natural Environment Research Council (NERC) and the Met Office. With the recent expansion of Argo to include bio-geochemical variables, Plymouth Marine Laboratory have also become a partner in the UK Argo Programme. The UK presently contributes around 3.9% of the total number of floats operating.
Ocean climate monitoring
The oceans can store over 1,000 times more heat than the atmosphere, and it is estimated that over 90% of the heat produced by global warming goes into the oceans (IPCC AR5, 2013). Argo data has allowed us to better monitor the ocean heat content over the last 15 years, with reduced uncertainty compared to pre-Argo times, as shown in Figure 5. This indicates that the oceans have continued to take up heat (warm) in recent years, during which there has been a pause (the so-called 'hiatus') in the global atmospheric near-surface temperature rise.
Not only does Argo allow us to better monitor ocean temperatures, but it now allows us to examine variations in salinity. For example at mid-latitudes, surface warming is accompanied by increased evaporation, leading to an increase in salinity of the near-surface waters. Also when comparing basin-scale salinities, the contrasts are found to be strengthening, the Atlantic is becoming saltier and the Pacific fresher.
With over 85% of Argo profiles being available within 24 hours of the floats surfacing, the near real-time data are used in ocean forecasting models, such as the FOAM (Forecasting Ocean Assimilation Model) system run by the Met Office (Blockley et al., 2013), where Argo now provides the dominant source of in situ profiles assimilated into FOAM. Experiments have shown the impact of Argo data, as errors in the model's temperature and salinity fields increase significantly when Argo data are withheld (Lea et. al., 2014). Argo data also has an impact on the Met Office's GloSea (MacLachlan et. al., 2015) seasonal forecasts, since these use FOAM to initialise the ocean part of the seasonal forecasting system.
The future of Argo
The initial 'core' Argo design was to deploy around 3,000 profiling floats at approximately 3 degree spacing throughout the ice-free areas between 60 ºN and 60 ºS of the Atlantic, Pacific, Indian and Southern Oceans where ocean depths are greater than 2,000 m. However, the global Argo array is now expanding into new domains: deeper profiling, marginal seas, high latitudes and bio-geochemical sampling. This will demand more floats than are needed for the core Argo array and around 4,000 floats are presently operating.
Bio-geochemical measurements are important for monitoring the health of the oceans and their response to climate change and around 7% of floats presently carry an additional sensor for dissolved oxygen. Smaller numbers of floats have also been deployed with sensors for chlorophyll, nitrates and pH and these are likely to become more widespread as the technology develops.
The deep oceans represent a significant proportion of the ocean volume, the deep (2,000 - 4,000m depth) ocean accounts for 39.5% and the abyssal (4,000 - 6,000m depth) ocean for 11.7% of the total ocean volume. As shown in Figure 4(b) earlier the deep oceans are believed to be warming, especially in the Southern Ocean, and deep and abyssal Argo floats that can profile to 4,000 m and 6,000 m respectively are now being tested. The expansion of Argo to include deep sampling will be a major evolution of the programme over the next ten years.
References and further information
Blockley, E. W., Martin, M. J., McLaren, A. J., Ryan, A. G., Waters, J., Lea, D. J., Mirouze, I., Peterson, K. A., Sellar, A., and Storkey, D. (2013), Recent development of the Met Office operational ocean forecasting system: an overview and assessment of the new Global FOAM forecasts, Geosci. Model Dev. Discuss., 6, 6219-6278, doi:10.5194/gmdd-6-6219-2013.
International Argo Programme web-site, http://www.argo.ucsd.edu
IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp.
JCOMM in situ Observations Program Support Argo Information Centre web-site, http://www.jcommops.org/argo
Lea, D. J., Martin, M. J. and Oke, P. R. (2014), Demonstrating the complementarity of observations in an operational ocean forecasting system. Q. J. R. Meteorol. Soc. 140: 2037-2049, July 2014 B DOI:10.1002/qj.2281
MacLachlan, C., Arribas, A., Peterson, K. A., Maidens, A., Fereday, D., Scaife, A. A., Gordon, M., Vellinga, M., Williams, A., Comer, R. E., Camp, J., Xavier, P. and Madec, G. (2015), Global Seasonal forecast system version 5 (GloSea5): a high-resolution seasonal forecast system. Q.J.R. Meteorol. Soc., 141: 1072-1084. doi:10.1002/qj.2396
Riser, S. C., Freeland, H. J., Roemmich, D., Wijffels, S., Troisi, A., Belbeoch, M., Gilbert, D., Xu, J., Pouliquen, S., Thresher, A., Le Traon, P-Y., Maze, G., Klein, B., Ravichandran, M., Grant, F., Poulain, P-M., Suga, T., Lim, B., Sterl, A., Sutton, P., Mork, K-A., Vélez-Belchí, P. J., Ansorge, I., King, B., Turton, J., Baringer, M. and Jayne, S. (2016), Fifteen years of ocean observations with the global Argo array. Nature Climate Change (to appear). DOI for your paper is 10.1038/nclimate2872