Argo Profiling Float Research Articles

New Insights of the Sicily Channel and Southern Tyrrhenian Sea Variability

The dynamics of the Sicily Channel and the southern Tyrrhenian Sea are highly influenced by the seasonal variability of the Mediterranean basin-wide circulation, by the interannual variability of the numerous mesoscale structures present in the Channel, and by the decadal variability of the adjacent Ionian Sea. In the present study, all these aspects are investigated using in-situ (Lagrangian drifter trajectories and Argo float profiles) and satellite data (Absolute Dynamic Topography, Sea Level Anomaly, Sea Surface Temperature, wind products) over the period from 1993 to 2018. The availability of long time series of data and high-resolution multi-sensor surface currents allow us to add new details on the circulation features and on their driving mechanisms and to detect new permanent eddies not yet described in literature. The structures prevailing in winter are mainly driven by wind, whereas those prevailing in summer are regulated by topographical forcing on surface currents. The strength of the surface structures located at the western entrance of the Ionian Sea and of the mesoscale activity along the northern Sicily coast is modulated by the large-scale internal variability. The vertical hydrological characteristics of these mesoscale eddies are delineated using the Argo float profiles inside these structures.

Horizontal velocity structure of mesoscale eddies in the South China Sea

The horizontal velocity structure of mesoscale eddies is investigated using data from 18-year altimetry data in the South China Sea (SCS). A total of 2,433 cyclonic eddies (CEs) and 3,132 anticyclonic eddies (AEs) are detected in this data set. This analysis shows that the eddy velocity structure is strongly linked to the eddy amplitude, with the band of maximum velocity moving closer to the eddy core as the eddy amplitude increases. By combining the records of Argo float profiles and satellite altimetry data, the eddy feature found in sea level anomaly (SLA) data can also be seen at the subsurface. This feature is related to sub-mesoscale processes accompanying with water exchange and energy cascade at the outer edge of mesoscale eddy. A modified Taylor vortex model is built including this new finding to describe the eddy velocity structure. This vortex model is found to be useful as it better describes the eddy’s life history, the knowledge of which is important for better understanding physical, biological and chemical processes in the ocean.

The Dominant Global Modes of Recent Internal Sea Level Variability

AbstractThe advances in the modern sea level observing system have allowed for a new level of knowledge of regional and global sea level in recent years. The combination of data from satellite altimeters, Gravity Recovery and Climate Experiment (GRACE) satellites, and Argo profiling floats has provided a clearer picture of the different contributors to sea level change, leading to an improved understanding of how sea level has changed in the present and, by extension, may change in the future. As the overlap between these records has recently extended past a decade in length, it is worth examining the extent to which internal variability on timescales from intraseasonal to decadal can be separated from long‐term trends that may be expected to continue into the future. To do so, a combined modal decomposition based on cyclostationary empirical orthogonal functions is performed simultaneously on the three data sets, and the dominant shared modes of variability are analyzed. Modes associated with the trend, seasonal signal, El Niño–Southern Oscillation, and Pacific decadal oscillation are extracted and discussed, and the relationship between regional patterns of sea level change and their associated global signature is highlighted.

Smartfloat: A Multimodal Underwater Vehicle Combining Float and Glider Capabilities

This paper proposes a novel hybrid underwater vehicle, the Smartfloat, which integrates the concept of buoyancy-driven underwater gliders and conventional profiling floats. The vehicle is presented to address the challenges in ocean monitoring, such as the multidisciplinary observations of long-range transport of the mesoscale features with high spatial-temporal resolution. The vehicle combines the mechanisms of Argo profiling float and the underwater glider, with the application of a special designed attitude control system and the ingenious general arrangement. The vehicle can switch the operating mode between drifting mode and gliding mode. Such a multimodal vehicle would merge the benefits of making measurements in a very energy-efficient way when drifting with the currents in Argo mode, and operating as an underwater glider in glider mode when it is needed to cross the ocean eddies, ocean fronts, filaments, or some stirring regions. In this paper, a proof-of-concept platform including its concept of operations, the main components and subsystems design, and the correlative mathematical analysis is introduced in detail. Experiments in field trials are presented to characterize and illustrate each mode of operation and repeated mode transitions. The results demonstrate the feasibility and the good performance of the proposed vehicle.

Estimation of the numerical modeling accuracy of the Black Sea thermohaline fields based on using ARGO profiling floats

The paper considers the matters of numerical modeling estimation accuracy of temperature and salinity three-dimensional fields in the monitoring and forecasting center of the Black Sea. Automatic system of analysis and forecast of the Black Sea state is run in the center in an operational mode. The data of in situ measurements from ARGO profiling floats regularly provided by the European Marine Forecasting Service – Copernicus – were used to solve the problem. Spatial and temporal mismatch between the model data and in situ measurements was eliminated to form a joint sample of these two data sources. Comparison is performed for the time period from 2012 to 2015. The results show quite good simulation accuracy of the ther-mohaline fields, however, the problems of modeling the temperature fields in the 5-30 m layer were detected where there is a fairly narrow seasonal thermocline in late spring, summer and early autumn.

A validation and comparison study of new, compact, versatile optodes for oxygen, pH and carbon dioxide in marine environments

Continuous monitoring of dissolved oxygen, pH and carbon dioxide are of great importance in oceanography. Sensors are the optimal tools for in situ measurements from mobile platforms, like Autonomous Underwater Vehicles (AUVs) or Argo profiling floats, and for shipboard deployments. A validation study of small, versatile, easy-to-use, stand-alone optodes is presented. Each analyte can be read out with the identical optoelectronics which greatly minimizes the costs of the hardware needed. Several deployments were performed to evaluate the applicability of the sensors. The deployments varied in terms of duration (profiling, long-term monitoring 5 days to 8 weeks) and environmental conditions (salinity: 6–33 PSS; temperature: 9–25 °C). A set of sensors was successfully deployed at a mooring buoy, in an aquaculture facility and in the Monterey Bay Aquarium Open Sea Exhibition. They were also integrated in an AUV and a profiling float. The performance of the optodes was evaluated in comparison with commercially available sensors for dissolved oxygen (Aanderaa Data Instruments AS, Sea-Bird Scientific, OxyGuard®), pH (Hach, Satlantic) and carbon dioxide (Turner design). The data collected by our optodes and the commercially available sensors is generally in good agreement showing that the new, compact sensor device in combination with sensor foils (pO2, pH, pCO2) can be a valuable tool for many applications in oceanography. The data also revealed the importance of the calibration strategy since inappropriate calibration resulted in an offset in the measured parameter. The efficiency of simple biofouling protection strategy (copper guard) for prolong measurements in highly dynamic environments was also demonstrated.

Water masses in the far western equatorial Pacific during the winters of 2010 and 2012

Conductivity-temperature-depth (CTD) data obtained during the 2010 La Nina winter and the 2012 normal winter, combined with concurrent Argo profiling float data, provide a quasi-synoptic description of the water mass distributions and their variations in the far western equatorial Pacific Ocean. The water mass connection between the western Pacific and the east Indonesian seas is emphasized. Analysis indicates that the North Pacific Tropical Water (NPTW, S >34.9) carried by the Mindanao Current southward and the South Pacific Tropical Water (SPTW, S >35.1) from the southern hemisphere meet in the area. Observations suggest that the southward transport of the NPTW is stronger in 2010 than in 2012 due to enhanced advection of the Mindanao Current. The distribution of SPTW, which crosses the equator in the northwest direction and retroflects back to the interior Pacific Ocean, is found to retreat from 4°–5°N in 2012 to 2°–3°N in 2010 La Nina peak in the 130°E section. A relatively fresh tropical subsurface water is identified in between the NPTW and the SPTW, moving eastward with the North Equatorial Countercurrent into the equatorial Pacific Ocean. However, the salinity maximum of this subsurface fresh water is found to decrease eastward, suggesting that the salinity maximum is generated either by strong diapycnal mixing or by isopycnal mixing of temporally entrained Indonesian sea water into the area.

Observed Warming of Sea Surface Temperature in Response to Tropical Cyclone Thane in the Bay of Bengal

An unusual near-surface warming was seen in observations from a moored buoy BD11 at 14°N/83°E, and a nearby Argo profiling float in the Bay of Bengal, during the passage of tropical cyclone Thane, during 25–31 December 2011. The cyclone induced a warming of sea surface temperature (SST) by 0.6°C to the right of the track. Heat budget analysis based on moored observations and satellite data rules out the role of horizontal advection and net heat flux in warming the surface layer. We find that vertical mixing/entrainment in response to the cyclone, in conjunction with a pre-storm temperature inversion (subsurface ocean warmer than SST) led to the observed warming. Pre-storm and post-storm salinity and temperature profiles from an Argo float close to the mooring BD11 have higher vertical resolution than the moored data; they suggest vertical mixing of the upper 70 m of the water column. The moored observations show that the thermal inversion, erased by storm-induced mixing, reappears in a few days.

Multiscale Observations of Deep Convection in the Northwestern Mediterranean Sea During Winter 2012–2013 Using Multiple Platforms

AbstractDuring winter 2012–2013, open‐ocean deep convection which is a major driver for the thermohaline circulation and ventilation of the ocean, occurred in the Gulf of Lions (Northwestern Mediterranean Sea) and has been thoroughly documented thanks in particular to the deployment of several gliders, Argo profiling floats, several dedicated ship cruises, and a mooring array during a period of about a year. Thanks to these intense observational efforts, we show that deep convection reached the bottom in winter early in February 2013 in a area of maximum 28 ± 3 . We present new quantitative results with estimates of heat and salt content at the subbasin scale at different time scales (on the seasonal scale to a 10 days basis) through optimal interpolation techniques, and robust estimates of the deep water formation rate of 2.0 . We provide an overview of the spatiotemporal coverage that has been reached throughout the seasons this year and we highlight some results based on data analysis and numerical modeling that are presented in this special issue. They concern key circulation features for the deep convection and the subsequent bloom such as Submesoscale Coherent Vortices (SCVs), the plumes, and symmetric instability at the edge of the deep convection area.

Properties, formation, and dissipation of the North Pacific Eastern Subtropical Mode Water and its impact on interannual spiciness anomalies

The properties, formation, and dissipation of the North Pacific Eastern Subtropical Mode Water (ESTMW), their interannual variability, and impact on spiciness anomalies in the upper permanent pycnocline were investigated using Argo profiling float data in 2005–2015. The core temperature and salinity of ESTMWs were horizontally compensated to a constant density, and core potential density concentrates in a range of 24.5–25.2 kg m−3 with two distinct peaks. ESTMWs showed different spatial distribution and persistence for its core potential density. Denser ESTMWs with a potential density of 24.9–25.2 kg m−3 were formed in winter mixed layer depth maximum centered at 30°N, 140°W and lighter ESTMWs of 24.5–24.9 kg m−3 were formed south and east of it. After formation through shoaling of the winter mixed layer, the former persisted until the following autumn and a small part of it subducted in winter, while the latter dissipated in summer. The formation region of ESTMW corresponded to the summer sea surface density maximum resulting from its poleward sea surface salinity front. Sea surface density maximum maintains weak stratification during summer, preconditioning the deepening of the winter mixed layer and hence the formation of ESTMWs. A relationship between the ESTMW formation region and the summer sea surface density maximum was also found in the North Atlantic and the South Pacific, implying the importance of sea surface salinity fronts and the associated summer sea surface density maximum to ESTMW formation. Interannual variations of ESTMW reflected that of the winter mixed layer in its formation region, and the thickness of ESTMW was related to the Pacific decadal oscillation. ESTMW contributed to the occurrence of spice injection and affected spiciness anomalies in the upper permanent pycnocline through its formation and dissipation.

The Shallow Overturning Circulation in the Indian Ocean

AbstractThe number of in situ observations in the Indian Ocean has dramatically increased over the past 15 years thanks to the implementation of the Argo profiling float program. This study estimates the mean circulation in the Indian Ocean using hydrographic observations obtained from both Argo and conductivity–temperature–depth (CTD) observations. Absolute velocity at the Argo float parking depth is used so there is no need to assume a level of no motion. Results reveal previously unknown features in addition to well-known currents and water masses. Some newly identified features include the lack of an interior pathway to the equator from the southern Indian Ocean in the pycnocline, indicating that water parcels must transit through the western boundary to reach the equator. High potential vorticity (PV) intrudes from the western coast of Australia in the depth range of the Subantarctic Mode Water, which leads to a structure similar to a PV barrier. The subtropical anticyclonic gyre retreats poleward with depth, as happens in the subtropical Atlantic and Pacific. An eastward flow was found in the eastern basin along 15°S at the depth of the Antarctic Intermediate Water—a feature expected from property distributions but never before detected in velocity estimates. Meridional mass transport indicates about 10 Sv (1 Sv ≡ 106 m3 s−1) southward flow at 6°S and 18 Sv northward flow at 20°S, which results in meridional convergence of currents and thermocline depression at about 16°–20°S. These estimated absolute velocities agree well with those of an ocean reanalysis, which lends credibility to the strictly databased analysis.

Assessing the impact of multiple altimeter missions and Argo in a global eddy-permitting data assimilation system

Abstract. A series of observing system simulation experiments (OSSEs) is carried out with a global data assimilation system at 1∕4° resolution using simulated data derived from a 1∕12° resolution free-run simulation. The objective is to not only quantify how well multiple altimeter missions and Argo profiling floats can constrain the global ocean analysis and 7-day forecast at 1∕4° resolution but also to better understand the sensitivity of results to data assimilation techniques used in Mercator Ocean operational systems. The impact of multiple altimeter data is clearly evidenced even at a 1∕4° resolution. Seven-day forecasts of sea level and ocean currents are significantly improved when moving from one altimeter to two altimeters not only on the sea level, but also on the 3-D thermohaline structure and currents. In high-eddy-energy regions, sea level and surface current 7-day forecast errors when assimilating one altimeter data set are respectively 20 and 45 % of the error of the simulation without assimilation. Seven-day forecasts of sea level and ocean currents continue to be improved when moving from one altimeter to two altimeters with a relative error reduction of almost 30 %. The addition of a third altimeter still improves the 7-day forecasts even at this medium 1∕4° resolution and brings an additional relative error reduction of about 10 %. The error level of the analysis with one altimeter is close to the 7-day forecast error level when two or three altimeter data sets are assimilated. Assimilating altimeter data also improves the representation of the 3-D ocean fields. The addition of Argo has a major impact on improving temperature and demonstrates the essential role of Argo together with altimetry in constraining a global data assimilation system. Salinity fields are only marginally improved. Results derived from these OSSEs are consistent with those derived from experiments with real data (observing system evaluations, OSEs) but they allow for more detailed characterisation of errors on analyses and 7-day forecasts. Both OSEs and OSSEs should be systematically used and intercompared to test data assimilation systems and quantify the impact of existing observing systems.

Net community production in the South China Sea Basin estimated from in situ O2 measurements on an Argo profiling float

For the first time, the net community production (NCP) was estimated over a complete annual cycle in the basin of the South China Sea (SCS) using in situ oxygen measurements from an Argo profiling float and an oxygen mass balance model. The annual NCP from July 2014 to July 2015 was estimated to be 2.7molCm−2yr−1 (calculated to the deepest winter mixed layer depth of 56m), with the uncertainties ranging from 0.9 ~ 2.2molCm−2yr−1. NCP estimates followed a monsoonal pattern with higher values in the cold season (November to April) when northeast monsoon prevailed and low values in the warm season (June to September) when this area was dominated by the southwest monsoon. Most of the net heterotrophic events occurred in the warm season. The magnitude and seasonal pattern derived from our results agree with previous export production studies based on discrete measurements. Comparison with satellite-derived NCP revealed that the results derived with NPP from Carbon-based Production Model (CbPM) were closer to the Argo measurements than the results derived with NPP from Vertically Generalized Production Model (VGPM) in magnitude; while the VGPM-based approach did a better job in reproducing the seasonal cycle of NCP in this area. This novel approach provides the possibilities to study the carbon cycle in the SCS with a much higher temporal and spatial resolution, as well as more insights for metabolic state in the oligotrophic subtropical gyres.

Variability and wind forcing of ocean temperature and thermal fronts in the Slope Water region of the Northwest Atlantic

AbstractSubsurface temperatures in the Slope Water region of the Northwest Atlantic from Argo profiling floats and on the adjacent continental shelf from ship‐based measurements are compared with the latitudinal position of the Shelf‐Slope Front (SSF) and the Gulf Stream North Wall (GSNW). The Slope Water and shelf temperature anomalies at 200 m depth are in agreement for the period, 2002–2015. For the period 1978–2015, shelf temperatures are significantly correlated with the SSF position, and to a lesser extent with the GSNW position. Annual SSF position anomalies near the Grand Banks at 50°W–55°W lead anomalies to the west at 65°W–75°W by 1–2 years. Wind stress curl is compared with the annual change in the SSF and GSNW latitudinal positions, rather than with the positions directly. Changes in the mean position of the SSF are related to the wind stress curl pattern in the mid‐Atlantic, with an 8 month lag. It is suggested that a wind pattern favoring a southward shift of the SSF is associated with a southward shift of the zero‐curl line near 40°W, resulting in an expanded subpolar gyre and enhanced flow of Labrador Current Water westward from the Tail of the Grand Banks. However, changes in the GSNW position are related to an NAO‐like wind stress curl pattern in the eastern Atlantic in the winter‐spring period, in agreement with other studies. High sea surface temperatures in the Gulf of Maine and on the Scotian Shelf in recent years can be largely attributed to positive local onshore wind anomalies.

Three‐compartment structure of subsurface‐intensified mesoscale eddies in the ocean

AbstractMesoscale eddies are energetically dominant and pervasive over most of the world's oceans. Among them, many are subsurface intensified with strongest signals in the ocean interior such as mode‐water eddies, which trap water masses with distinctive properties and carry them over long distances. With both Argo profiling floats and atmospheric reanalysis data we showed that the structure of these eddies obeys a universal rule. Hence their three‐dimensional hydrographic fields can be readily reconstructed from very limited information. More interestingly, the volume of water trapped and moved by a mode‐water eddy is much greater than previously thought; it has a three‐compartment structure in the vertical with the mode water being sandwiched between two layers of notably different properties and accounting for only a portion of the total trapped volume.

Long-term change and variation of salinity in the western North Pacific subtropical gyre revealed by 50-year long observations along 137\xb0E

The 137°E repeat hydrographic section for 50 winters during 1967–2016 has been analyzed to examine interannual to interdecadal variations and long-term changes of salinity and temperature in the surface and intermediate layers of the western North Pacific, with a particular focus on freshening in the subtropical gyre. Rapid freshening on both isobars and isopycnals began in the mid-1990s and persisted for the last 20 years in the upper main thermocline/halocline in the western subtropical gyre. In addition, significant decadal variability of salinity existed in the subtropical mode water (STMW), as previously reported for the shallower layers. An analysis of the 144°E repeat hydrographic section during 1984–2013 supplemented by Argo profiling float data in 2014 and 2015 revealed that the freshening trend and decadal variability observed at 137°E originated in the winter mixed layer in the Kuroshio Extension (KE) region and was transmitted southwestward to 137°E 1–2 years later in association with the subduction and advection of STMW. The mechanism of these changes and variations in the source region was further investigated. In addition to the surface freshwater flux in the KE region pointed out by previous studies, the decadal KE variability in association with the Pacific Decadal Oscillation likely contributes to the decadal salinity variability through water exchange between the subtropics and the subarctic across the KE. Interdecadal change in both the surface freshwater flux and the KE state, however, failed to explain the rapid freshening for the last 20 years.

The East Pacific Rise current: Topographic enhancement of the interior flow in the South Pacific Ocean

AbstractObservations of absolute velocity based on Argo float profiles and trajectories in the ocean interior show evidence for an equatorward current, the East Pacific Rise current, between 42°S and 30°S, along the western flank of the East Pacific Rise. The East Pacific Rise current carries predominantly intermediate water masses, including Subantarctic Mode Water and Antarctic Intermediate Water, and deeper waters, from the southern edge of the subtropical gyre toward the Equator. The 2004 to 2014 mean East Pacific Rise current transport extrapolated through the 0–2600 m depth range is 8.1 ± 1.6 sverdrup (Sv) (1 Sv = 106 m3 s−1), consistent with a wind‐driven interior transport influenced by the East Pacific Rise topography. While deep ocean mixing and geothermal heating can both create pressure gradients that support geostrophic flows in the deep ocean, this study indicates that about half of the East Pacific Rise current transport is associated with topographic steering of the deep flow over the East Pacific Rise.

Development and Assessment of the Systematically Merged Pacific Ocean Regional Temperature and Salinity (SPORTS) Climatology for Ocean Heat Content Estimations

AbstractA Systematically Merged Pacific Ocean Regional Temperature and Salinity (SPORTS) climatology was created to estimate ocean heat content (OHC) for tropical cyclone (TC) intensity forecasting and other applications. A technique similar to the creation of the Systematically Merged Atlantic Regional Temperature and Salinity (SMARTS) climatology was used to blend temperature and salinity fields from the Generalized Digital Environment Model and World Ocean Atlas 2001 at a 0.25° resolution. The weights for the blending of these two climatologies were estimated by minimizing residual covariances across the basin. Drift velocities associated with eddy variability were accounted for using a series of 3-yr sea surface height anomalies (SSHA) to ensure continuity between the periods of different altimeters. In addition to producing daily estimates of the 20° and 26°C isotherm depths, mixed-layer depth, and OHC, the model produces mapping errors from the optimal interpolation of the SSHA due to gaps in altimeter track coverage and sensor uncertainties.Using SPORTS with satellite-derived sea surface temperature (SST) and SSHA fields from radar altimetry, daily OHC was estimated from 2000 to 2011 using a 2.5-layer model approach. Argo profiling floats, expendable probes from ships and aircraft, long-term Tropical Atmosphere Ocean (TAO) moorings, and drifters provide more than 267 000 quality controlled in situ thermal profiles to assess uncertainty in estimates from SPORTS. This carefully constructed climatology creates an accurate estimation of OHC from satellite-based measurements, which can then be used in TC intensity forecasts in the North Pacific Ocean and analysis of ocean thermodynamics. The SPORTS time and space series extends from 1998 to 2016, forming a 19-yr dataset by the end of 2016.

Climatological Annual Cycle of the Salinity Budgets of the Subtropical Maxima

AbstractSix subtropical salinity maxima (Smax) exist: two each in the Pacific, Atlantic, and Indian Ocean basins. The north Indian (NI) Smax lies in the Arabian Sea while the remaining five lie in the open ocean. The annual cycle of evaporation minus precipitation (E − P) flux over the Smax is asymmetric about the equator. Over the Northern Hemisphere Smax, the semiannual harmonic is dominant (peaking in local summer and winter), while over the Southern Hemisphere Smax, the annual harmonic is dominant (peaking in local winter). Regardless, the surface layer salinity for all six Smax reaches a maximum in local fall and minimum in local spring. This study uses a multidecade integration of an eddy-resolving ocean circulation model to compute salinity budgets for each of the six Smax. The NI Smax budget is dominated by eddy advection related to the evolution of the seasonal monsoon. The five open-ocean Smax budgets reveal a common annual cycle of vertical diffusive fluxes that peak in winter. These Smax have regions on their eastward and poleward edges in which the vertical salinity gradient is destabilizing. These destabilizing gradients, in conjunction with wintertime surface cooling, generate a gradually deepening wintertime mixed layer. The vertical salinity gradient sharpens at the base of the mixed layer, making the water column susceptible to salt finger convection and enhancing vertical diffusive salinity fluxes out of the Smax into the ocean interior. This process is also observed in Argo float profiles and is related to the formation regions of subtropical mode waters.

From regional to local scale modelling on the south-eastern Brazilian shelf: case study of Paranaguá estuarine system

Abstract The applicability of a numerical model following a downscaling methodology was evaluated for the south-eastern Brazilian shelf (regional model) and Paranaguá estuarine system (local model). This approach permits the simulation of different scale processes, such as storm surges and coastal upwelling, and is suitable for operational forecasting purposes. When large areas are covered by regional models, the tidal propagation inside the domain can be significantly affected by the local tidal potential, mainly where the resonance phenomenon is observed. The south-eastern Brazilian shelf is known for the resonance of the third-diurnal principal lunar tidal constituent (M3), the largest amplitudes being found in the Paranaguá estuarine system. Therefore, the significance of the local tidal potential was assessed in this study for the most important tidal constituents inside the estuarine system (including M3). The model validation was performed with tidal gauge data, Argo float profiles and satellite measurements of Sea Surface Temperature. The methodology described in this study can be replicated for other important estuarine systems located on the south-eastern Brazilian shelf. Furthermore, the numerical model was developed within a perspective of operational nowcast/forecast simulations, useful for several activities such as navigation and response to emergencies (e.g., oil spills).