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Argo, an international, global observational array of nearly 4,000 autonomous robotic profiling floats, each measuring ocean temperature and salinity from 0 to 2,000 m on nominal 10-day cycles, has revolutionized physical oceanography. Argo started at the turn of the millennium,growing out of advances in float technology over the previous several decades. After two decades, with well over 2 million profiles made publicly available in real time, Argo data have underpinned more than 4,000 scientific publications and improved countless nowcasts, forecasts, and projections. We review a small subset of those accomplishments, such as elucidating remarkable zonal jets spanning the deep tropical Pacific; increasing understanding of ocean eddies and the roles of mixing in shaping water masses and circulation; illuminating interannual to decadal ocean variability; quantifying, in concert with satellite data, contributions of ocean warming and ice melting to sea level rise; improving coupled numerical weather predictions; and underpinning decadal climate forecasts.

Global estimates of mesoscale vertical velocity remain poorly constrained due to a historical lack of adequate observations on the spatial and temporal scales needed to measure these small magnitude velocities. However, with the wide‐spread and frequent observations collected by the Argo array of autonomous profiling floats, we can now better quantify mesoscale vertical velocities throughout the global ocean. We use the underutilized trajectory data files from the Argo array to estimate the time evolution of isotherm displacement around a float as it drifts at 1,000 m, allowing us to quantify vertical velocity averaged over approximately 4.5 days for that depth level. The resulting estimates have a non‐normal, high‐peak, and heavy‐tail distribution. The vertical velocity distribution has a mean value of (1.9 ± 0.02) × 10−6 m s−1 and a median value of (1.3 ± 0.2) × 10−7 m s−1, but the high‐magnitude events can be up to the order of 10−4 m s−1. We find that vertical velocity is highly spatially variable and is largely associated with a combination of topographic features and horizontal flow. These are some of the first observational estimates of mesoscale vertical velocity to be taken across such large swaths of the ocean without assumptions of uniformity or reliance on horizontal divergence. Plain Language Summary: Vertical velocity in the ocean is a fundamental part of how water circulates throughout the globe. This impacts the temperature, salt, nutrients, and currents that make up the ocean. However, vertical velocities are very small and are, therefore, difficult to measure. In particular, the vertical velocities of ocean events that occur on roughly a weekly to monthly time scale (mesoscale) are poorly understood. We have developed a method for estimating these mesoscale vertical velocities across the globe using an array of autonomous robots called Argo floats. Our results show that vertical velocities vary greatly depending on location, with the largest values occurring where there is a combination of relatively shallow ocean depths and larger horizontal velocities. These estimates are some of the first of their kind to be made from observations across such large swaths of the ocean. Key Points: Five‐day averaged vertical velocities from Argo observations near 1,000 m are non‐normally distributed, with a high peak and heavy tailsMesoscale vertical velocities are on the order of centimeters per day, but high‐magnitude events can be on the order of meters per dayVertical velocities estimated from Argo floats are spatially variable and correlated with topographic features and horizontal surface flow [ABSTRACT FROM AUTHOR]

Oceanographic conditions on the continental shelf of the Ross Sea, Antarctica, affect sea ice production, Antarctic Bottom Water formation, mass loss from the Ross Ice Shelf, and ecosystems. Since ship access to the Ross Sea is restricted by sea ice in winter, most upper ocean measurements have been acquired in summer. We report the first multiyear time series of temperature and salinity throughout the water column, obtained with autonomous profiling floats. Seven Apex floats were deployed in 2013 on the midcontinental shelf, and six Air‐Launched Autonomous Micro Observer floats were deployed in late 2016, mostly near the ice shelf front. Between profiles, most floats were parked on the seabed to minimize lateral motion. Surface mixed layer temperatures, salinities, and depths, in winter were −1.8 °C, 34.34, and 250–500 m, respectively. Freshwater from sea ice melt in early December formed a shallow (20 m) surface mixed layer, which deepened to 50–80 m and usually warmed to above −0.5 °C by late January. Upper‐ocean freshening continued throughout the summer, especially in the eastern Ross Sea and along the ice shelf front. This freshening requires substantial lateral advection that is dominated by inflow from melting of sea ice and ice shelves in the Amundsen Sea and by inputs from the Ross Ice Shelf. Changes in upper‐ocean freshwater and heat content along the ice shelf front in summer affect cross‐ice front advection, ice shelf melting, and calving processes that determine the rate of mass loss from the grounded Antarctic Ice Sheet in this sector. Plain Language Summary: Measurements of temperature and salinity in coastal Antarctic waters are generally restricted to summer when sea ice disappears so that ships can operate there. Moored sensors can collect data through winter but are usually deployed below 200–300 m to minimize risk of damage from drifting icebergs. We describe a novel approach, using autonomous profilers deployed by ship and aircraft, to collect data from the seabed to the ocean surface throughout the year. We deployed 13 profilers in the Ross Sea, Antarctica. Most profilers were programmed to sit on the seabed between profiles to minimize drift and to continue profiling even when ice cover prevented their ascent to the surface. As expected, annual changes in upper‐ocean temperature and salinity were closely related to seasonal changes in sea ice. However, the upper ocean continued to freshen even after the sea ice had all melted, suggesting that substantial amounts of freshwater must be coming from ice melting in the adjacent Amundsen Sea. Increasing our understanding of sources of freshwater in the Ross Sea will improve our predictions of this region's changing role in Antarctic sea ice formation, ice loss from the Antarctic Ice Sheet, and Southern Ocean ecosystems. Key Points: First aircraft deployments of profiling floats near an ice shelf front document heat and freshwater evolution in spring and summerMultiyear time series of Ross Sea upper‐ocean hydrography under sea ice reveal annual cycle and interannual variabilitySummer upper‐ocean freshwater budgets require substantial lateral fluxes of ice melt from the Amundsen Sea, and from the Ross Ice Shelf [ABSTRACT FROM AUTHOR]

Nitrate limits productivity in much of the ocean. Nitrate residence time is a few thousand years, and changes in nitrate loss could influence ocean productivity. Major sinks for nitrate are denitrification and anaerobic ammonia oxidation in the oxygen minimum zones (OMZs). The Bay of Bengal OMZ is anomalous because large amounts of nitrate loss do not occur there, while nitrate is removed in the nearby OMZ of the Arabian Sea. Observations of nitrate and oxygen made over 5 years by 20 profiling floats equipped with chemical sensors in the Bay of Bengal and the Arabian Sea are used to understand why nitrate is removed rapidly in the Arabian Sea but not in the Bay of Bengal. Our results confirm that nitrate is poised for removal in the Bay of Bengal. However, highly variable oxygen concentrations inhibit its loss. Nitrate loss is regulated by physical oceanographic processes that introduce oxygen. Plain Language Summary: Denitrification is a microbial process that removes nitrate, a compound essential for phytoplankton production, from seawater. Denitrification occurs in the absence of oxygen. Most denitrification in the oceanic water column occurs in three oxygen minimum zones, the Eastern Tropical North Pacific, the Eastern Tropical South Pacific, and the Arabian Sea. The Bay of Bengal is a fourth oxygen minimum zone, but little nitrate loss occurs there. Our profiling float observations show that the Bay of Bengal is poised for denitrification to occur, as found in prior studies. However, we find that frequent events raise oxygen concentrations above levels where denitrification occurs. The amount of denitrification, relative to the Arabian Sea, is set by fraction of time that oxygen is elevated. Onset of large‐scale denitrification would require a shift in the ocean physics that transport the water with elevated oxygen concentration. Key Points: Nitrate and oxygen were measured with sensors on profiling floats for time periods up to 5 yearsNitrate is removed by denitrification or anammox in the Bay of Bengal at the lowest oxygen concentrationsThe amount of nitrate loss in the Bay of Bengal is limited by events that raise oxygen levels [ABSTRACT FROM AUTHOR]

Knowledge of the intensity and spatial-temporal distribution of rainfall over the ocean is critical in understanding the global hydrological cycle. However, rain has proven difficult to measure over the ocean due to problems associated with platform motion and flow distortion combined with the spatial and temporal variability of rainfall itself. Underwater acoustical rain gauges avoid these issues by using the loud and distinctive underwater sound generated by raindrops on the ocean surface to detect and quantify rainfall. Here, the physics and operation of and results from an instrument that uses underwater ambient sound to measure rainfall rate and wind speed are presented. Passive Aquatic Listener (PAL) instruments were mounted on a buoy deployed at Ocean Station P and on 13 Argo profilers that were deployed as part of the US National Aeronautics and Space Administration-sponsored Salinity Processes in the Upper-ocean Regional Study (SPURS) field experiment in the North Atlantic Ocean. The PALs provide near-continuous measurements of rain rate and wind speed during the two-year period over the SPURS study region defined by the Argo profilers. Comparisons of PAL data with rain and wind measured by other techniques, including direct in situ observations and satellite measurements, show good agreement for both rain rate and wind speed. [ABSTRACT FROM AUTHOR]

We examine the variability of near-surface salinity in a 10° x 10° region of the eastern North Atlantic, the location of the first part of the Salinity Processes in the Upper-ocean Regional Study (SPURS-1). The data used were collected over a two-year period, largely by a group of two types of profiling floats equipped with sensors that record high-resolution temperature and salinity measurements in the upper few meters of the water column. By comparing the SPURS-1 measurements to observations in the area from previous decades, we examine variability at time scales ranging from a few hours (mostly consisting of rainfall-driven decreases in salinity) to diurnal cycles in temperature and salinity, seasonal variability and the annual cycle, and finally to decadal-scale changes. The relationship of near-surface salinity to the hydrological cycle suggests a continuous spectrum of variability in this cycle from hours to decades. [ABSTRACT FROM AUTHOR]

The North Atlantic subtropical salinity maximum harbors the saltiest surface waters of the open world ocean. Subduction of these waters gives rise to Subtropical Underwater, spreading the high-salinity signature over the entire basin. The Salinity Processes in the Upper-ocean Regional Study (SPURS) is aimed at understanding the physics controlling the thermohaline structure in the salinity maximum region. A combination of moored and autonomous float observations is used here to describe the vertical water mass interleaving in the area. Seasonal intensification of interleaving in late spring and the abundance of small-scale thermohaline intrusions point to an important role for submesoscale processes in the initial subduction and subsequent evolution of Subtropical Underwater. [ABSTRACT FROM AUTHOR]

This study examines the global variability of the internal wave field near a depth of 1000 m using data from a set of 194 Argo floats equipped with Iridium communications, capable of measuring hourly temperature and pressure during the park phase of their 10-day cycles. These data have been used to estimate vertical isotherm displacements at hourly intervals, yielding a global measure of the heaving due to internal gravity waves. The displacement results have been employed to examine the global variability of these waves and how the displacement power spectrum compares to the canonical Garrett-Munk spectrum. Using the data, the authors find correlations between internal wave intensity and seafloor roughness, proximity to the seafloor, and the magnitude of the local barotropic velocity. The measurements also show large seamount-trapped waves at high latitudes and coastally trapped subinertial waves. These observations provide a rough global census of the nature of these waves that can ultimately be used in studies of ocean mixing. [ABSTRACT FROM AUTHOR]

Using observations from the Argo array of profiling floats, the large-scale circulation of the upper 2000 decibars (db) of the global ocean is computed for the period from December 2004 to November 2010. The geostrophic velocity relative to a reference level of 900 db is estimated from temperature and salinity profiles, and the absolute geostrophic velocity at the reference level is estimated from the trajectory data provided by the floats. Combining the two gives the absolute geostrophic velocity on 29 pressure surfaces spanning the upper 2000 db of the global ocean. These velocities, together with satellite observations of wind stress, are then used to evaluate Sverdrup balance, the simple canonical theory relating meridional geostrophic transport to wind forcing. Observed transports agree well with predictions based on the wind field over large areas, primarily in the tropics and subtropics. Elsewhere, especially at higher latitudes and in boundary regions, Sverdrup balance does not accurately describe meridional geostrophic transports, possibly due to the increased importance of the barotropic flow, nonlinear dynamics, and topographic influence. Thus, while it provides an effective framework for understanding the zero-order wind-driven circulation in much of the global ocean, Sverdrup balance should not be regarded as axiomatic. [ABSTRACT FROM AUTHOR]

Mean circulation and water properties within the Aleutian Basin of the Bering Sea are investigated using hydrographic and subsurface park pressure displacement data from a regional array of 14 profiling CTD floats. After 10 days drifting at 1000 dbar, each float measures temperature and salinity profiles as it rises to the surface and then transmits these data via satellites, which also make several fixes of the float surface positions before it sinks again. Every fourth cycle, the floats sink from 1000 dbar to a 2000-dbar target just prior to ascent to measure deeper profiles. The 1000-dbar displacements estimated from the float surface position fixes reveal a coherent few-centimeters-per-second northwestward flow along the northeastern boundary, the deep signature of the Bering Slope Current. Middepth water property distributions are consistent with cyclonic advection of warm water from the south around the basin, eastward in the Aleutian North Slope Current, and then northwestward in the Bering Slope Current. Geostrophic transport estimates relative to 1000 dbar also show cyclonic motion, although with significant noise, likely owing to the influence of mesoscale eddies. The mean along-slope geostrophic transport of the Bering Slope Current is determined between 0 and 1900 dbar relative to 1000 dbar and then combined with mean along-slope velocities at 1000 dbar. The result is an absolute geostrophic transport estimate with 95% confidence intervals for the along-slope current offshore of the 1000-m isobath and between 0 and 1900 dbar of 5.8 (±1.7) × 10[sup 6] m[sup 3] s[sup -1] . [ABSTRACT FROM AUTHOR]

It is proposed that the cyclonic gyre over the northern half-basin of the Okhotsk Sea is driven by the wind stress curl and that a major part of the East Sakhalin Current (ESC) can be regarded as its western boundary current. Both from the high-resolution ECMWF and Comprehensive Ocean–Atmosphere Dataset (COADS) data, the annual mean wind stress curl is positive over the sea. When the Sverdrup streamfunction is calculated by excluding the shallow shelves, the streamfunction shows a cyclonic pattern over the central basin, which is roughly consistent with the geopotential anomaly distribution from all the available hydrographic data. Profiling floats suggest that the cyclonic gyre extends to at least a depth of 500 m: a relatively intense southward flow (ESC) with an average speed of approximately 10 cm s[sup -1] near the western boundary and slow northward flow with an average speed of approximately 2 cm s[sup -1] in the east. Climatological data show that along zonal sections at 50°–53°N isopycnal surfaces gradually rise from the east to west and sharply drop near the western boundary, suggesting the Sverdrup balance. This feature persists throughout the year. The integrated northward baroclinic transport of 3.5 Sv along 53°N is comparable to the Sverdrup transport of 3.7 Sv, calculated from the annual mean wind stress. Sverdrup balance appears to hold roughly in the baroclinic field in 50°–53°N. A flat-bottom numerical model forced by realistic wind stress reproduces well the cyclonic gyre, with the observed baroclinic features. In the south, the anticyclonic circulation in the Kuril Basin cannot be explained by the wind stress curl inside the Okhotsk Sea in this simplified model. [ABSTRACT FROM AUTHOR]

Intermediate, or deep, convection in a baroclinic flow occurs along slanted paths parallel to the alongflow absolute momentum surfaces. These surfaces are principally tilted due to the vertical shear in velocity but can be further modified by a nonvertical axis of rotation. An inviscid Lagrangian parcel model, using realistic parameters, is utilized to illustrate, qualitatively, the different scenarios resulting from the combined action of inertial and gravitational forces acting on sinking parcels of dense fluid. More quantitative results are derived from a series of numerical experiments using a zonally invariant, high-resolution, nonhydrostatic model. Convection occurring in a flow with tilted absolute momentum surfaces will mix properties along these slanted surfaces. This implies that the fluid can retain a weak vertical stratification while overturning and also, more importantly, that the evolution of the convective layer cannot be described in terms of one-dimensional, vertical mixing. The authors show, for conditions typical of the Labrador Sea, that the convective layer depth difference between that estimated by mixing vertically and one obtained allowing for slantwise mixing can be greater than 100 m; slantwise convection reaches deeper because of the reduced stratification along the slanted paths. An alternative slantwise mixing scheme, based on the assumption of zero potential vorticity of the convected fluid, is proposed. [ABSTRACT FROM AUTHOR]

This response addresses the three comments by A. Polonsky on 'A Global Analysis of Sverdrup Balance Using Absolute Geostrophic Velocities from Argo.' [ABSTRACT FROM AUTHOR]