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 pro ling oats, it has become possible to sample the upper 2,000 m of the ocean globally and uniformly in space and time. The primary goal of Argo is to create a systematic global network of pro ling oats that can be integrated with other elements of the Global Ocean Observing System. The network 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.

The Argo Program has been implemented and sustained for almost two decades, as a global array of about 4000 profiling floats. Argo provides continuous observations of ocean temperature and salinity versus pressure, from the sea surface to 2000 dbar. The successful installation of the Argo array and its innovative data management system arose opportunistically from the combination of great scientific need and technological innovation. Through the data system, Argo provides fundamental physical observations with broad societally-valuable applications, built on the cost-efficient and robust technologies of autonomous profiling floats. Following recent advances in platform and sensor technologies, even greater opportunity exists now than 20 years ago to (i) improve Argo's global coverage and value beyond the original design, (ii) extend Argo to span the full ocean depth, (iii) add biogeochemical sensors for improved understanding of oceanic cycles of carbon, nutrients, and ecosystems, and (iv) consider experimental sensors that might be included in the future, for example to document the spatial and temporal patterns of ocean mixing. For Core Argo and each of these enhancements, the past, present, and future progression along a path from experimental deployments to regional pilot arrays to global implementation is described. The objective is to create a fully global, top-to-bottom, dynamically complete, and multidisciplinary Argo Program that will integrate seamlessly with satellite and with other in situ elements of the Global Ocean Observing System (Legler et al., 2015). The integrated system will deliver operational reanalysis and forecasting capability, and assessment of the state and variability of the climate system with respect to physical, biogeochemical, and ecosystems parameters. It will enable basic research of unprecedented breadth and magnitude, and a wealth of ocean-education and outreach opportunities.

In the past two decades, the Argo Program has collected, processed, and distributed over two million vertical profiles of temperature and salinity from the upper two kilometers of the global ocean. A similar number of subsurface velocity observations near 1,000 dbar have also been collected. This paper recounts the history of the global Argo Program, from its aspiration arising out of the World Ocean Circulation Experiment, to the development and implementation of its instrumentation and telecommunication systems, and the various technical problems encountered. We describe the Argo data system and its quality control procedures, and the gradual changes in the vertical resolution and spatial coverage of Argo data from 1999 to 2019. The accuracies of the float data have been assessed by comparison with high-quality shipboard measurements, and are concluded to be 0.002 • C for temperature, 2.4 dbar for pressure, and 0.01 PSS-78 for salinity, after delayed-mode adjustments. Finally, the challenges faced by the vision of an expanding Argo Program beyond 2020 are discussed.

SUMMARYThe structure near the top of stratocumulus clouds is investigated with the aid of aircraft data to determine possible processes influencing entrainment. An expression is derived to predict the buoyancy fluctuations which can be produced by mixing at the interface due to evaporative cooling, and the relative influence of cooling by radiation and by evaporation near cloud top is discussed. It is argued that the effects of evaporative cooling are not properly considered in current assessments of the stability of inversions to entrainment. A method for taking this into account in a more realistic manner is proposed.Entrainment rates derived directly from aircraft flux measurements are compared with various prediction mcthods. These include the predictions from four entrainment models. These predictions are found to vary widely for a given set of conditions but are generally smaller than the observationally derived values. Some of thc consequences of underestimating entrainment and various other shortcomings of the models which are exposed by the comparisons are also discussed.

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A multiple mixed layer model of the cloud-topped boundary layer is developed to investigate the diurnal variation of stratocumulus. A simple representation of the microphysical properties of the cloud layer is included which enables high resolution, interactive radiation calculations to be made. A constraint on the buoyancy flux profile is introduced which permits the previously well-mixed layer to separate into two independently driven layers, thereby avoiding many of the unrealistic aspects of single-layer models and enabling the model to reproduce features seen in observational studies. The diagnosis of this decoupling, which generally occurs during the morning, is discussed and is shown to have a seasonal and latitudinal sensitivity due to its dependence on cloud layer short-wave absorption.Comparisons of results with those obtained when separation is not permitted show significant differences. In particular, the cloud layer displays a much enhanced diurnal variation in thickness when separation is allowed, with the minimum occurring in the afternoon. These results are shown to produce much better agreement with observations. The likely consequences of separation for boundary layer evolution are discussed. The surface energy balance can be quite strongly affected, suggesting that it is important to resolve this diurnal variation, especially in areas where stratocumulus is a dominant climatological feature.The sensitivity of the results to model assumptions are investigated and the limitations of the model are also assessed.

Hartman, S. E., Lampitt, R. S., Larkin, K. E., Pagnani, M., Campbell, J., Gkritzalis, T., Jiang, Z.-P., Pebody, C. A., Ruhl, H. A., Gooday, A. J., Bett, B. J., Billett, D. S. M., Provost, P., McLachlan, R., Turton, J. D., and Lankester, S. 2012. The Porcupine Abyssal Plain fixed-point sustained observatory (PAP-SO): variations and trends from the Northeast Atlantic fixed-point time-series. – ICES Journal of Marine Science, 69: 776–783. The Porcupine Abyssal Plain sustained observatory (PAP-SO) in the Northeast Atlantic (49°N 16.5°W; 4800 m) is the longest running open-ocean multidisciplinary observatory in the oceans around Europe. The site has produced high-resolution datasets integrating environmental and ecologically relevant variables from the surface to the seabed for >20 years. Since 2002, a full-depth mooring has been in place with autonomous sensors measuring temperature, salinity, chlorophyll-a fluorescence, nitrate, and pCO2. These complement ongoing mesopelagic and seabed observations on downward particle flux and benthic ecosystem structure and function. With national and European funding, the observatory infrastructure has been advanced steadily, with the latest development in 2010 involving collaboration between the UK's Meteorological Office and Natural Environment Research Council. This resulted in the first simultaneous atmospheric and ocean datasets at the site. All PAP-SO datasets are open access in near real time through websites and as quality-controlled datasets for a range of remote users using ftp sites and uploaded daily to MyOcean and the global telecommunications system for use in modelling activities. The combined datasets capture short-term variation (daily–seasonal), longer term trends (climate-driven), and episodic events (e.g. spring-bloom events), and the data contribute to the Europe-wide move towards good environmental status of our seas, driven by the EU's Marine Strategy Framework Directive (http://ec.europa.eu/environment/water/marine).

A recent workshop on in-situ wave measurement technology noted that: (1) geographical coverage of insitu data is still very limited especially as far as any measure of wave directionality is concerned, and most measurements are taken near coasts in the Northern Hemisphere; (2) present in-situ reports are not standardized resulting in impaired utility; (3) significant differences exist in measured waves from different platforms, sensors, processing and moorings. Three main topics were discussed: (1) how to add wave observing capabilities to drifting buoys; (2) how to assess and improve the quality of observations from the present networks of moored buoys; 3) the addition of wave observation capabilities to future moored buoy networks.