Have you ever wondered how scientists know what is happening deep below the surface of the ocean? There are several types of robots that can dive below the sea surface and bring back data from underwater. One type of robot, called an Argo float, moves through the middle depths of the ocean with the currents and comes to the surface once every 10 days, to tell scientists about the information that it has collected. Currently, there are about 4,000 Argo robots keeping an eye on Earth’s oceans every day. These robots measure ocean temperature and saltiness, and some can also monitor more complex chemical and biological parameters. Argo robots provide another tool in an oceanographer’s toolbox, to help scientists understand how the ocean works and how it impacts not only marine life, but also the whole Earth.

The years since 2000 have been a golden age in in situ ocean observing with the proliferation and organization of autonomous platforms such as surface drogued buoys and subsurface Argo profiling floats augmenting ship-based observations. Global time series of mean sea surface temperature and ocean heat content are routinely calculated based on data from these platforms, enhancing our understanding of the ocean’s role in Earth’s climate system. Individual measurements of meteorological, sea surface, and subsurface variables directly improve our understanding of the Earth system, weather forecasting, and climate projections. They also provide the data necessary for validating and calibrating satellite observations. Maintaining this ocean observing system has been a technological, logistical, and funding challenge. The global COVID-19 pandemic, which took hold in 2020, added strain to the maintenance of the observing system. A survey of the contributing components of the observing system illustrates the impacts of the pandemic from January 2020 through December 2021. The pandemic did not reduce the short-term geographic coverage (days to months) capabilities mainly due to the continuation of autonomous platform observations. In contrast, the pandemic caused critical loss to longer-term (years to decades) observations, greatly impairing the monitoring of such crucial variables as ocean carbon and the state of the deep ocean. So, while the observing system has held under the stress of the pandemic, work must be done to restore the interrupted replenishment of the autonomous components and plan for more resilient methods to support components of the system that rely on cruise-based measurements.

Argo salinity is a key set of in situ ocean measurements for many scientific applications. However, use of the raw, unadjusted salinity data should be done with caution as they may contain bias from various instrument problems, most significant being from sensor calibration drift in the conductivity cells. For example, inclusion of biased but unadjusted Argo salinity has been shown to lead to spurious results in the global sea level estimates. Argo delayed-mode salinity data are data that have been evaluated and, if needed, adjusted for sensor drift. These delayed-mode data represent an improvement over the raw data because of the reduced bias, the detailed quality control flags, and the provision of uncertainty estimates. Such improvement may help researchers in scientific applications that are sensitive to salinity errors. Both the raw data and the delayed-mode data can be accessed via https://doi.org/10.17882/42182 (Argo, 2022). In this paper, we first describe the Argo delayed-mode process. The bias in the raw salinity data is then analyzed by using the adjustments that have been applied in delayed mode. There was an increase in salty bias in the raw Argo data beginning around 2015 and peaking during 2017–2018. This salty bias is expected to decrease in the coming years as the underlying manufacturer problem has likely been resolved. The best ways to use Argo data to ensure that the instrument bias is filtered out are then described. Finally, a validation of the Argo delayed-mode salinity dataset is carried out to quantify residual errors and regional variations in uncertainty. These results reinforce the need for continual re-evaluation of this global dataset.

The static and dynamic performances of the RBRargo3 are investigated using a combination of lab-based and in situ datasets from floats deployed as part of an Argo pilot program. Temperature and pressure measurements compare well to colocated reference data acquired from shipboard CTDs. Static accuracy of salinity measurements is significantly improved using (1) a time lag for temperature, (2) a quadratic pressure dependence and (3) a unit-based calibration for each RBRargo3 over its full pressure range. Long term deployments show no significant drift in the RBRargo3 accuracy. The dynamic response of the RBRargo3 demonstrates the presence of two different adjustment timescales: a long-term adjustment O(120 s), driven by the temperature difference between the interior of the conductivity cell and the water, and a short-term adjustment O(5-10 s), associated by the initial exchange of heat between the water and the inner ceramic. Corrections for these effects, including dependence on profiling speed are developed.

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 1000 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.

In cold polar waters, temperatures sometimes drop below the freezing point, a process referred to as supercooling. However, observational challenges in polar regions limit our understanding of the spatial and temporal extent of this phenomenon. We here provide observational evidence that supercooled waters are much more widespread in the seasonally ice-covered Southern Ocean than previously reported. In 5.8% of all analyzed hydrographic profiles south of 55° S, we find temperatures below the surface freezing point (‘potential’ supercooling), and half of these have temperatures below the local freezing point (‘in-situ’ supercooling). Their occurrence doubles when neglecting measurement uncertainties. We attribute deep coastal-ocean supercooling to melting of Antarctic ice shelves, and surface-induced supercooling in the seasonal sea-ice region to winter-time sea-ice formation. The latter supercooling type can extend down to the permanent pycnocline due to convective sinking plumes—an important mechanism for vertical tracer transport and water-mass structure in the polar ocean.

The Biogeochemical-Argo program (BGC-Argo) is a new profiling-float-based, ocean wide, and distributed ocean monitoring program which is tightly linked to, and has benefited significantly from, the Argo program. The community has recommended for BGC-Argo to measure six additional properties in addition to pressure, temperature and salinity measured by Argo, to include oxygen, pH, nitrate, downwelling light, chlorophyll fluorescence and the optical backscattering coefficient. The purpose of this addition is to enable the monitoring of ocean biogeochemistry and health, and in particular, monitor major processes such as ocean deoxygenation, acidification and warming and their effect on phytoplankton, the main source of energy of marine ecosystems. Here we describe the salient issues associated with the operation of the BGC-Argo network, with information useful for those interested in deploying floats and using the data they produce. The topics include float testing, deployment and increasingly, recovery. Aspects of data management, processing and quality control are covered as well as specific issues associated with each of the six BGC-Argo sensors. In particular, it is recommended that water samples be collected during float deployment to be used for validation of sensor output.

In this study, under-ice ocean data from profiling floats, instrumented seals, and shipboard casts are used to assess wintertime upper-ocean stability and heat availability in the sea ice–covered Southern Ocean. This analysis reveals that the southern Weddell Sea, which features a weak upper-ocean stratification and relatively strong thermocline, is preconditioned for exceptionally high rates of winter ventilation. This preconditioning also facilitates a strong negative feedback to winter ice growth. Idealized experiments with a 1D ice–ocean model show that the entrainment of heat into the mixed layer of this region can maintain a near-constant ice thickness over much of winter. However, this quasi-equilibrium is attained when the pycnocline is thin and supports a large temperature gradient. We find that the surface stress imparted by a powerful storm may upset this balance and lead to substantial ice melt. This response can be greatly amplified when coincident with anomalous thermocline shoaling. In more strongly stratified regions, such as near the sea ice edge of the major gyres, winter ice growth is weakly limited by the entrainment of heat into the mixed layer. Thus, the thermodynamic coupling between winter sea ice growth and ocean ventilation has significant regional variability. This regionality will influence the response of the Southern Ocean ice–ocean system to future changes in ocean stratification and surface forcing.

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.

A CTD (conductivity, temperature, depth) device measures temperature and salinity versus pressure. This document is the Argo quality control manual for CTD and trajectory data. It describes two levels of quality control: - The first level is the real-time system that performs a set of agreed automatic checks. - The second level of quality control is the delayed-mode system. These quality control procedures are applied to the parameters JULD, LATITUDE, LONGITUDE, PRES, TEMP, PSAL, and CNDC.

We report on under‐ice profiling float observations of cold, dense, and oxygenated bottom layers on the continental slope of Mac Robertson Land (60°–72°E) in East Antarctica. This bottom layer water mass, with potential temperature in the range −1.8°C < θ < −0.4°C, is identified as modified shelf water. It is a downslope variety of dense water formed on the Antarctic continental shelf in winter and plays an important role in ventilating the deep Southern Ocean. The seasonal evolution of its thickness and density follows the sea ice cycle of growth and decay, reaching a maximum in October–November. The characteristics and location of this modified shelf water are similar to Cape Darnley Bottom Water, thus suggesting the same primary source in the Cape Darnley polynya region. These float data support recent results that the continental shelf along Mac Robertson Land is a significant source of dense waters in East Antarctica.

Ocean-Ice Shelf Interaction and Possible Bottom Water Formation in Prydz Bay, Antarctica Annie P. S. Wong, Annie P. S. Wong Institute of Antarctic and Southern Ocean Studies, University Of Tasmania, Hobart, AustraliaSearch for more papers by this authorNathaniel L. Bindoff, Nathaniel L. Bindoff Antarctic Co-Operative Research Centre, Hobart, AustraliaSearch for more papers by this authorAndrew Forbes, Andrew Forbes Csiro Division of Marine Research, Hobart, AustraliaSearch for more papers by this author Annie P. S. Wong, Annie P. S. Wong Institute of Antarctic and Southern Ocean Studies, University Of Tasmania, Hobart, AustraliaSearch for more papers by this authorNathaniel L. Bindoff, Nathaniel L. Bindoff Antarctic Co-Operative Research Centre, Hobart, AustraliaSearch for more papers by this authorAndrew Forbes, Andrew Forbes Csiro Division of Marine Research, Hobart, AustraliaSearch for more papers by this author Book Editor(s):Stanley S. Jacobs, Stanley S. JacobsSearch for more papers by this authorRay F. Weiss, Ray F. WeissSearch for more papers by this author First published: 01 January 1985 https://doi.org/10.1029/AR075p0173Citations: 32Book Series:Antarctic Research Series AboutPDFPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShareShare a linkShare onFacebookTwitterLinked InRedditWechat Summary This chapter contains sections titled: Introduction Physical Setting The Observational Program and Hydrographic Sections Water Masses Large Scale Circulation Ocean and Ice Shelf Interaction Bottom Waters Summary Citing Literature Ocean, Ice, and Atmosphere: Interactions at the Antarctic Continental Margin, Volume 75 RelatedInformation

Multiyear under-ice temperature and salinity data collected by profiling floats are used to study the upper ocean near the Wilkes Land coast of Antarctica. The study region is in the seasonal sea ice zone near the southern terminus of the Antarctic Circumpolar Current. The profiling floats were equipped with an ice-avoidance algorithm and had a survival rate of 74% after 2.5 yr in the ocean. The data show that, in this part of Antarctica, the rate of sea ice decay exceeds the rate of sea ice growth. During the sea ice growth period, the water column is weakly stratified because of brine rejection and is only marginally stable. The average winter mixed layer temperature is about 0.12 degrees C above the surface freezing point, providing evidence of entrainment of warmer water from the permanent pycnocline. The average mixed layer salinity increases by 0.127 from June to October. A one-dimensional model is used to quantify evolution of the winter mixed layer under a sea ice cover. The local winter entrainment rate is estimated to be 49 +/- 11 m over 5 months, supplying a heat flux of 34 +/- 8 W m(-2) to the base of the mixed layer in winter. Model output gives a thermodynamic sea ice growth of 28 +/- 15 cm over the same period. The winter ocean-atmosphere heat loss through leads and sea ice is estimated to be 14-25 W m(-2) in this area, which is broadly in line with other winter observations from the East Antarctic region.

An improved method to estimate the time-varying drift of measured conductivity from autonomous CTD profiling floats has been developed. This procedure extends previous methods developed by Wong, Johnson and Owens [2003. Delayed-mode calibration of autonomous CTD profiling float salinity data by θ–S climatology. Journal of Atmospheric and Oceanic Technology, 20, 308–318] and Böhme and Send [2005. Objective analyses of hydrographic data for referencing profiling float salinities in highly variable environments. Deep-Sea Research Part II, 52, 651–664]. It uses climatological salinity interpolated to the float positions and observed θ surfaces and chooses 10 ‘best’ levels that are within well-mixed mode waters or deep homogeneous water masses. A piece-wise linear fit is used to estimate the temporally varying multiplicative adjustment to the float potential conductivities. An objective, statistical method is used to choose the breakpoints in the float time series where there are multiple drift trends. In the previous methods these breakpoints were chosen subjectively by manually splitting the time series into separate segments over which the fits were made. Our statistical procedure reduces the subjectivity by providing an automated way for doing the piece-wise linear fit. Uncertainties in this predicted adjustment are estimated using a Monte-Carlo simulation. Examples of this new procedure as applied to two Argo floats are presented.