Observing the Global Ocean with the Argo Array
Alison R. Gray
Understanding the ocean’s role in the climate system, one of the central problems of oceanography, requires global observations of ocean state. The Argo array, which was in its infancy 15 years ago, represents one of the most substantial advances in our ability to observe the world ocean and today forms a central component of the Global Ocean Observing System (Riser et al., 2016). The autonomous profiling floats that comprise this array evolved from the floats developed in the World Ocean Circulation Experiment of the 1990s (Davis et al., 1992, 2001). Current generation Argo floats measure temperature and salinity in the upper 2,000 m of the global ocean every 10 days and drift at a depth of 1,000 m between profiles. At the surface, the profile data, together with position information, are transmitted to shore via satellite. Floats typically complete more than 200 profiles over five or more years in a cost-effective manner. The array reached its target size of approximately 3,000 floats in 2007 and presently consists of over 3,800 floats, with more than 30 nations making contributions. All data are made freely available in near-real time for use in operational forecasting; the data are also subject to further examination, resulting in a high-quality data set for scientific purposes. The improvements in the spatial and temporal coverage of subsurface ocean observations is remarkable (Figure 1).
Over the past decade, the data collected by the Argo array have revolutionized large-scale physical oceanography and advanced our understanding of the ocean’s role in the climate system. Numerous studies have used Argo data to address one of the primary scientific objectives of the array, namely to quantify upper-ocean climate variability, including heat and freshwater storage and transport. For example, the unprecedented spatial coverage of the data allowed for a detailed analysis of the patterns of upper-ocean heat gain since 2006 (Roemmich et al., 2015). Combining Argo temperature measurements with historical data demonstrated that the ocean has been warming for at least a century (Roemmich et al., 2012). The Argo array has dramatically increased the amount of high-quality salinity measurements in the open ocean, allowing for the first time a comprehensive examination of the salinity structure of the ocean surface and interior. In one such study, changes in surface salinity fields detected with Argo data were shown to indicate substantial intensification of the global hydrological cycle (Durack et al., 2012).
Considerable progress has also been made toward achieving the other scientific goals of the Argo array. The trajectory information provided by the floats has been used to quantify the large-scale circulation of the global ocean (Ollitrault and Colin de Verdiere, 2014; Gray and Riser, 2014) in ways that were previously impossible. Argo data have also been combined with satellite altimetry to determine, for example, the Atlantic meridional overturning circulation (Willis, 2010). Substantial improvements in ocean analysis and forecasting systems have been realized due to the Argo array, and most climate models now depend on these data for initialization and validation.
In addition to proving essential for addressing key questions concerning climate variability in the ocean, Argo data have also been used in an incredibly wide range of applications, many of which were unrecognized at the onset of the program. Indeed, over 2,800 scientific studies using Argo data have been published to date, an accomplishment only made possible by the high-quality and publicly available nature of the data. Some examples include investigations of the spatial variability of mixed layer depths (Holte and Talley, 2009), ocean mixing (Whalen et al., 2012), the internal gravity wave field (Hennon et al., 2014), and horizontal diffusivities (Cole et al., 2015).
The significant scientific achievements of Argo have been enabled by the many engineering and technological innovations contributed by numerous research groups in partnership with float and sensor manufacturers (Riser et al., 2016). For instance, the continuing shift to Iridium satellite communications, which is bi-directional, has resulted in less time at the surface, greater data return, and the ability to alter float missions after deployment. Software algorithms have been developed that allow floats to spend winter under sea ice, greatly expanding our observations of the high-latitude seasonally ice-covered ocean. The design of air-deployable floats has also increased applications in studies of polar sea ice regions, as well as tropical cyclones. The Argo program has also benefited from open communication among participants and strong international collaboration, which have facilitated the development and implementation of improvements and best practices. Capable data management and thorough quality control have been key factors in assuring the scientific successes of the program. The commitment of national and international agencies has been crucial as well.
Argo data continue to be an invaluable asset for scientific studies of large-scale physical oceanography, and sustaining the core array will enable more and more detailed investigations of the ocean’s role in the climate system in the future. Building on more than 15 years of measurements currently available, data from the Argo array will soon be able to address questions of trends and variability in upper-ocean heat and freshwater transport and storage over interannual to decadal time scales. In addition, the trajectory information provided by the floats is becoming more useful due to recent changes to the management of these data, which will lead to better estimates of ocean circulation on global and regional scales. As long as the quality and coverage of the data are ensured, new and creative applications of Argo data will continue to be conceived.
Given the successes of Argo, there is considerable interest in enhancing and expanding the array. Western boundary current regions play a central role in ocean-atmosphere interactions and the transport of heat and other quantities. However, because of the intense turbulence and variability found there, accurately assessing the ocean state in these areas requires greater data density than the current float distribution provides. Similarly, the near-equatorial bands of the world ocean exert a powerful influence in the coupled climate system, so that increased sampling density there will improve predictions of phenomena such as the El Niño-Southern Oscillation that have an enormous impact on societies around the globe. The ice-covered Southern Ocean, although not originally part of the Argo array design, is now accessible due to advances in float technology. Enhancing the array in this region will provide invaluable observations in areas historically undersampled. The marginal seas were likewise excluded from the initial program, but deployments in these areas, which are often vitally important for the surrounding nations, have been increasing. The Argo Steering Team has endorsed these enhancements to the array, and work has begun in each of these regions.
In addition to augmenting the Argo array in these crucial areas, two major expansions are presently being implemented (Figure 2). The ocean’s role in the climate system is not limited to heat and freshwater but encompasses global cycles of carbon, oxygen, nutrients, and productivity. To address questions on these fundamental topics, Biogeochemical Argo seeks to add new sensors to profiling floats to measure additional variables including dissolved oxygen, nitrate, pH, irradiance, and bio-optical properties of seawater. Plans to build a global array of biogeochemical floats have been established (Johnson and Claustre, 2016), and pilot arrays in the Southern Ocean and North Atlantic are being deployed. Just as the core Argo array transformed large-scale physical oceanography, building a global array of biogeochemical floats will likely revolutionize biological and chemical oceanography.
Deep Argo, the second significant expansion of the array, aims to deploy floats that profile the full depth of the ocean, allowing for computation of closed budgets of heat, freshwater, and sea level and investigation of the circulation of the deep ocean. Two different deep floats designs have been developed and are rated for depths up to 4,000 and 6,000 m. Early deployments of deep floats have been successfully carried out, and a design for a global array has been developed (Johnson et al., 2015). The success of both of these expansions will depend on having reliable and cost-effective platforms (in the case of Deep Argo) and sensors (in the case of Biogeochemical Argo).
As we move toward 20 years of ocean observations from the Argo array, sustaining the quality and coverage of the data remains imperative because of the numerous scientific and operational benefits of this component of the Global Ocean Observing System. Continuing to advance basic float technology should be an essential part of the strategy moving forward, as such efforts will lead to increased quality and efficiency. The planned enhancements and expansions of the Argo Program each come with their own set of engineering challenges and opportunities, which will necessitate basic research and experimentation. Many of the lessons learned during the development of the Argo array will be valuable, not just to those working to expand Argo to more regions of the global ocean, to new types of data, and to the deep ocean, but to users of many different types of ALPS. Additionally, efforts to strengthen the integration of Argo data with observations from other ALPS and other parts of the Global Ocean Observing System will improve our ability to understand and predict the ocean and its role in the climate system.
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Alison R. Gray, University of Washington, Seattle, WA, USA, firstname.lastname@example.org