Argo Float Research Articles

Observed internal tides in the deep northwestern Pacific by Argo floats

This study aims to extract the observed internal wave signals in the northwestern Pacific from Argo floats with Iridium-telecommunication that can directly measure the hourly temperature, salinity and pressure near 1000 dbar. Vertical displacements and spectral features of internal tides can be interpreted from Argo float data. More than 10,000 parking-phase observations of 252 Argo floats were used to estimate their spatial-temporal characteristics. Vertical displacements attributed to internal tides exceeded 30 dbar near energetic sources, such as the Luzon Strait, Izu ridge, Mariana Arc and Palau. The observed internal tides feature both seasonal and spring-neap cycles corresponding to barotropic tidal variations. The diurnal and semidiurnal peaks in the energy spectra differed with latitude in related to different internal wave properties. Particularly, energetic local-trapped diurnal internal tides (topographic waves) occurred at 33°N near Izu Ridge, which is poleward of diurnal critical latitude. The internal tidal energy was calculated by theoretical fitting the vertical displacements from one layer to the full-depth water. The observed e-folding decay scales of the semidiurnal internal tidal energy were estimated as ∼550–950 km in the deep basin. The Argo parking data provide a unique view for observing the basin-scale pattern and energy sink of deep-ocean internal tides, which could supplement the widely-used surface altimetry observations and contribute to a more accurate parameterization of tidal mixing.

What happened around an inverted V-shaped track turning of the tropical cyclone Madi?

The tropical cyclone (TC) Madi with an inverted V-shaped track happened over the Bay of Bengal (BoB) from Dec. 5 to Dec. 12, 2013. In this paper, the strong oceanic and atmospheric responses during the turning of the inverted V-shaped track of Madi were studied using the multisource satellite remote sensing and Argo float data. The results indicate that the response during the pre-turn phase (00:00 Dec.5 to 06:00 Dec.10) is more acute than that during the post-turn phase (06:00 Dec.10 to 18:00 Dec.12) of Madi. During the pre-turn phase, the cumulative precipitation reached 206 mm, the sea surface temperature (SST) decreased from 27.7 °C to 24.5 °C, and the maximum wind speed (MWS) increased from 7.71 m·s−1 to 43.72 m·s−1. At the turning corner on Dec.10, a symmetrical distribution of temperature drop, with greater precipitation on the left than that on the right. Meanwhile, the deep vertical wind shear was 17.13 m·s−1 with a northwestward moving cyclonic eddy. The barrier layer thickness was reduced from 25.82 m (24.88 m) to 23.8 m (12.38 m) during the pre-turn (post-turn) phase. Moreover, the potential vorticity (PV) caused by the ocean eddy disturbance is closely related to the intensity of TC. This study helps to estimate the sea-air interactions in the western BoB.

Ecosystem impacts of marine heat waves in the northeast Pacific

Abstract. Marine heat waves (MHWs) are a recurrent phenomenon in the northeast Pacific that impact regional ecosystems and are expected to intensify in the future. Prior work showed that these events, including the 2014–2015 “warm blob”, are associated with widespread surface nutrient declines in the subpolar Alaska Gyre (AG) and the North Pacific Transition Zone (NPTZ) but reduced chlorophyll concentrations in the NPTZ only. Here we explain the contrast between these two regions using a global ocean biogeochemical model (MOM6-COBALT) with Argo float and ship-based observations to investigate how MHWs influence marine productivity. We find that phytoplankton and zooplankton production respond relatively modestly to MHWs in both regions. However, differences in the response to seasonal iron and nitrogen limitation between large (>10 µm) and small (<10 µm) phytoplankton size classes explain the differences in ecosystem response to MHWs across the two biomes. During MHWs, reduced nutrient supply limits large phytoplankton production in the NPTZ (−13 % annually) but has a limited impact on the already iron-limited large phytoplankton population in the AG (−2 %). In contrast, MHWs yield a springtime increase in small phytoplankton in both regions due to shallower mixed layers and weaker light limitation. These modest changes are in apparent contradiction with prior estimates suggesting a collapse in net community production during the warm blob. We show, however, that 70 % of the decline in net community production previously calculated from nitrate Argo data can be attributed to artifacts in the method and that only 30 % can be attributed to interannual variability, in line with our model-based results. Although modest, the primary production anomalies associated with MHWs modify the phytoplankton size distribution, resulting in a significant shift towards small phytoplankton production (i.e., lower large-to-small-phytoplankton ratio) and reduced secondary and export production, especially in the NPTZ.

Subsurface oxygen maximum in oligotrophic marine ecosystems: mapping the interaction between physical and biogeochemical processes

Abstract. The subsurface oxygen maximum (SOM) is observed in oligotrophic oceans and is associated with different physical and biological processes. This study characterizes the SOM in the Mediterranean Sea at the basin scale and investigates its driving mechanisms by analysing the output of the 1/24∘ resolution biogeochemical reanalysis provided by the Copernicus Marine Service for the 1999–2019 time period. We validated the model-derived oxygen concentration in the epipelagic layer at different spatial and temporal scales, including novel process comparisons with estimates from in situ observations. Moreover, using Biogeochemical Argo (BGC-Argo) float observations, we estimated the model uncertainty in reproducing the SOM concentration and depth in summer (13 mmol O2 m−3 and 13 m, respectively). The western and eastern Mediterranean Sea depicts different SOM signatures in summer, with higher oxygen values and shallower depths in the western Mediterranean. The concentrations and depths (in the ranges of 230–250 mmol O2 m−3 and 30–100 m, respectively) are in agreement with the estimations from the literature and show mesoscale variability patterns. The western Mediterranean also shows a stronger biological activity, specifically oxygen production and consumption, along the whole epipelagic layer and higher oxygen concentrations at the surface throughout the year, but heavy undersaturated waters are associated with winter deep convection in the northwestern Mediterranean Sea. A 1-year analysis conducted on selected areas that are representative of the heterogeneity of summer SOM highlighted that the SOM can actually be sustained by biological production (as in northwestern Mediterranean areas), or it can be a residual of the confinement of spring production (as in the central Ionian area) and vertical motions influence its depth (as in the Levantine subduction area).

The generation mechanism of cold eddies and the related heat flux exchanges in the upper ocean during two sequential tropical cyclones

The impacts of two sequential tropical cyclones (TCs), Kyarr and Maha, [from October 24 to November 06, 2019, over the Arabian Sea (AS)] on upper ocean environments were investigated using multiple satellite observations, Argo float profiles and numerical model outputs. To obtain a realistic TC strength, the Weather Research and Forecasting (WRF) model was used to reproduce Kyarr and Maha. During Kyarr and Maha, three distinct cold patches were observed at the sea surface with a maximum sea surface cooling of approximately 5°C. The comparison between WRF model simulation results and ERA5 wind field showed that the WRF model simulation indicated high simulation accuracy with respect to the SST decrease in the AS under the influence of Kyarr and Maha’s wind stress curls. Meanwhile, concentration of chlorophyll a (chl-a) and positive relative vorticity of sea surface also appeared in the three cold patch areas. Through the use of eddy detection algorithms, three mesoscale cold cyclonic eddies were identified along the track of TC Kyarr, and the locations of these cold eddies were highly correlated with three obvious negative sea surface height anomalies (SSHAs). The radii of the three cold eddies were 69 km, 50 km, and 41 km. With a focus on the thermodynamic responses of the three cold eddy fields to Kyarr and Maha, the central regions of the three cold eddies were explored. The central regions of the three cold eddies exhibited relatively shallow mixed-layer depths (MLDs) and low mixed-layer temperatures (MLTs). The depth integrated heat (DIH) content was also calculated to explore the heat flux exchanges occurring in different layers in the upper 200 m of the centre of each eddy. The results showed that DIH in each eddy centre varied by one order of magnitude, accounting for between 127.3 MJ m-2 and 1220.0 MJ m-2 of heat loss. This study suggests that the effect of long forcing time on intense positive wind stress curls can produce upwelling caused by Ekman response, which is the main influencing factor of the three cold eddies generation mechanism. At the same time, the positive relative vorticity injected into the sea surface also has some contribution. TC-induced vertical mixing and upwelling (strengthened by unstable structures inside the cold eddies) cause substantial redistribution of the DIH, and related heat flux exchanges at different layers occur in the eddy fields.

Ocean depth–temperature profiles for operational oceanography from a shark-borne transmitter

BackgroundMany regions of the ocean are under-sampled in terms of their biology and physical structure. Increasingly sophisticated animal-borne electronic tags are capable of measuring and transmitting in situ environmental data such as ocean temperature–depth profiles. This has the potential to significantly augment the volume of data acquired from under-sampled regions of the ocean. These data would enhance interpretation of animal behavior and distribution and could be used to inform oceanographic and meteorological models. Building on results obtained from marine mammals and turtles, we present a case study of depth–temperature profiles obtained from a tagged tiger shark.ResultsDuring a 102-day deployment, 1350 geolocations were obtained from a shark from waters around Oahu, Hawaii. Of these, 520 were associated with depth–temperature profiles—some of which were from depths exceeding 500 m. Delay between profile creation and transmission to satellite or land-based receiver averaged 8.9 h (range: 35 s–43 h, median 6.32 h). The profiles were in close agreement with profiles extracted from nearby locations in an operational ROMS model. Land-based receivers played a significant role in augmenting data throughput obtained via satellites.ConclusionsShark-borne transmitters offer a viable option for collecting ocean profiles with reporting latencies that make them suitable for operational oceanography. They can significantly increase sampling frequency (especially subsurface) and sample geographic areas that are otherwise difficult to monitor with Lagrangian methods such as Argo floats. They sample locations and depths that are important to the animal and which in some cases may be biological hotspots. The resolution of the data is comparable with that derived from traditional platforms. By targeting appropriate species of shark, different areas of the ocean could be monitored at significantly higher rates than is currently the case.

Spiciness anomalies in the upper North Pacific based on Argo observations

The density-compensated salinity anomalies (spiciness anomalies) in the upper North Pacific were investigated using Argo float profiles during 2004–2018. The freshening of the subtropical thermocline was found within the Central Mode Water (CMW) and the North Pacific Intermediate Water (salinity minimum). Meanwhile, the increase of salinity was found in the lighter layer within the North Pacific Tropical Water (salinity maximum). The interannual and longer spiciness anomalies were interpreted with three-dimensional evolutions and were linked to the fate of three mode waters (i.e., Subtropical Mode Water (STMW), Eastern Subtropical Mode Water (ESTMW), and CMW) for the first time. In the STMW, the salinity was dominated by a quasi-decadal variability, which was consistent with the KE variability with a 1-year lag, and did not show rapid freshening. In the CMW, the salinity decreased with a quasi-decadal variability, which was weaker and out of the phase compared with the STMW. In the ESTMW, the salinity was dominated by year-to-year variability. The spiciness anomalies originated mainly in the outcrop region of the isopycnals, where they were accompanied by the formation and the subduction of the mode waters. They also propagated and decayed downstream the geostrophic currents. However, a few of the interannual anomalies found in the northern part of the CMW were almost dampened before their spread further south. In addition, some anomalies in the ESTMW appeared and intensified without a connection to the mixed layer. These anomalies occurred far from the outcrop line, suggesting these were caused likely by salt fingering associated with the modification of the mode waters. Furthermore, the propagation of the spiciness anomalies in the western to the central subtropics was significantly faster than the geostrophic current and inclined to the inner side of streamlines. These indicated the transport by eddies in addition to the mean geostrophic currents.

Environmental impacts of three Asian dust events in the northern China and the northwestern Pacific in spring 2021

In March 2021, China experienced three dust events (Dust-1, 2, 3), especially the first of which was reported as the strongest one in recent ten years. Their environmental impacts have received great attention, demanding comprehensive study to assess such impacts quantitatively. Multiple advanced measurement methods, including satellite, ground-based lidar, online aerosol speciation instrument, and biogeochemical Argo float, were applied to examine and compare the transport paths, optical and chemical properties, and impacts of these three dust events on urban air quality and marine ecosystem. The results showed that Dust-1 exhibited the largest impacts on urban area, increasing PM10 concentration in Beijing, Shuozhou, and Shijiazhuang up to 7525, 3819, and 2992 μg m−3, respectively. However, due to fast movement of the Mongolian low-pressure cyclone, the duration of northwest wind over the land was quite short (e.g., only 10 h in Beijing), which prevented the transport of dust plume to the northwestern Pacific, resulting in limited impact on the ocean. Dust-2 and Dust-3, though weaker in intensity, were transported directly to the sea, and led to a substantial increase in chlorophyll-a concentration (up to near 3 times) in the northwestern Pacific, comparing to climatological value. This indicates that the impacts of dust events on ocean was not necessarily and positively correlated to their impacts on land. Based on the analyses of land-ocean-space integrated observational data and synoptic systems, this study examined how marine ecosystem responded to three significant Asian dust events in spring 2021 and quantitatively assessed the overall impacts of mega dust storms both on land and ocean, which could also provide an interdisciplinary research methodology for future research on strong aerosol emission events such as wildfire and volcanic eruption.

A comparative analysis of the mesoscale thermohaline features across subarctic frontal zones in the Northern Hemisphere

Mesoscale eddies and their thermohaline features across the two subarctic frontal (SAF) zones, the Oyashio Extension (OE) and the Gulf Stream (GS), in the Northern Hemisphere are investigated and compared using both satellite-based observations and Argo float data. Our results show that the thermohaline properties (temperature, salinity and associated density) induced by mesoscale eddies have significant differences between the south and north sides of each SAF zone. The relationships between the sea surface height anomaly (SSHA) and the sea surface temperature anomaly (SSTA), called the H-T amplitude relationship, are examined and used to depict the mesoscale sea surface characteristics. In both the OE and GS regions, SSTAs on the north sides of the SAFs are more sensitive to the SSHA than that on their southern sides, and this sensitivity is stronger in the GS than in the OE. It is found that in the OE, the difference in the H-T amplitude relationships between the north and south sides is mainly due to the thermal expansion coefficient, which is contingent on the ambient sea surface temperature. In the GS, the difference between the north and south sides in the H-T amplitude relationships is mainly due to the change of the vertical profiles of mesoscale eddies, which are related to the ambient meridional density front.

Simulated Signatures of Greenland Melting in the North Atlantic: A Model Comparison With Argo Floats, Satellite Observations, and Ocean Reanalysis

AbstractIncreased Greenland ice sheet melting has an impact on global mean and regional sea level rise and the ocean circulation. In this study, we explore whether Greenland melting signatures found in ocean model simulations are visible in observations from radar altimetry, satellite gravimetry and Argo floats. We have included Greenland freshwater flux (GF) in the global Finite‐Element‐Sea ice‐Ocean Model (FESOM) for the years 1993–2016. The reference run is computed by excluding Greenland freshwater input. These experiments are performed on a low resolution (ca. 24 km) and a high resolution (ca. 6 km) eddy‐permitting mesh. For comparison with the model experiments, we use different observational data, such as Argo floats, satellite observations, and reanalyses. We find that surface GF maps into signatures in temperature and salinity down to about 100 m in the surroundings of Greenland. The simulated melting signatures are particularly visible in steric heights in Baffin Bay and Davis Strait. Here, we find an improvement of the mean square error of up to 30% when including GF. For the Nordic part of the Nordic Seas, however, we find no improvement when including GF. We compare steric heights with reanalysis data and a new setup of the inversion method from gravimetric and altimetric satellite data. We cannot confirm that the GF signatures on variables such as temperature and salinity are visible in the observations on the time scales considered. However, we find that increased model resolution often causes larger improvements than occur due to including the simulated melting effect.

Middepth Zonal Velocity in the Southern Tropical Indian Ocean: Striation-Like Structures and Their Dynamics

Abstract In this study, the upper-ocean absolute geostrophic currents in the southern Indian Ocean are constructed using Argo temperature and salinity data from the middepth (1000 m) zonal velocity derived from the Argo float trajectory. The results reveal alternating quasi-zonal striation-like structures of middepth zonal velocity in the equatorial and southern tropical Indian Ocean. Specifically, the eastward time-mean flows are located at the equator and 2°, 5°, 8°, 13°, 16°, 18°–19°, and 21°–22°S, with a meridional scale of ∼300 km. The generation mechanisms of the striation-like zonal velocity structure differ between the near-equatorial and off-equatorial regions. The triad of baroclinic Rossby wave instability plays a significant role in near-equatorial striations. In the south, the high potential vorticity (PV) of Antarctic intermediate water and low PV of southern Indian Ocean Subantarctic Mode Water lead to strong baroclinic instability, which increases the eddy kinetic energy in the middepth layer, thus contributing to a turbulent PV gradient. The convergence/divergence of the eddy PV flux generates the quasi-zonal striations. The meridional scale of the striations is controlled by the most unstable wavelength of baroclinic instability, which explains the observations. Significance Statement The middepth zonal velocity resembles a system of eastward/westward jets with a considerably smaller width than the larger-scale ocean surface circulation. Such a phenomenon always occurs in a turbulent ocean that presents eddy or eddy–mean flow interactions. This study used float observations to reveal a robust middepth zonal velocity in the southern tropical Indian Ocean, where the width of the eastward time-mean flows is approximately 300 km. Smaller eddies drive the zonal currents with a smaller width, and the energy of the eddies is released from the unstable vertical structure at middepths. This study provides new insights into the generation mechanism of small-width zonal current structures in the deep ocean.

Glider observations of thermohaline staircases in the tropical North Atlantic using an automated classifier

Abstract. Thermohaline staircases are stepped structures of alternating thick mixed layers and thin high-gradient interfaces. These structures can be up to several tens of metres thick and are associated with double-diffusive mixing. Thermohaline staircases occur across broad swathes of the Arctic and tropical and subtropical oceans and can increase rates of diapycnal mixing by up to 5 times the background rate, driving substantial nutrient fluxes to the upper ocean. In this study, we present an improved classification algorithm to detect thermohaline staircases in ocean glider profiles. We use a dataset of 1162 glider profiles from the tropical North Atlantic collected in early 2020 at the edge of a known thermohaline staircase region. The algorithm identifies thermohaline staircases in 97.7 % of profiles that extend deeper than 300 m. We validate our algorithm against previous results obtained from algorithmic classification of Argo float profiles. Using fine-resolution temperature data from a fast-response thermistor on one of the gliders, we explore the effect of varying vertical bin sizes on detected thermohaline staircases. Our algorithm builds on previous work by adding improved flexibility and the ability to classify staircases from profiles with noisy salinity data. Using our results, we propose that the incidence of thermohaline staircases is limited by strong background vertical gradients in conservative temperature and absolute salinity.

A machine learning approach to quality-control Argo temperature data

A machine learning approach is proposed to identify temperature outliers from Argo float profiles as a complementary procedure to current Argo quality control. A machine learning unsupervised classification (i.e., the Gaussian mixture model, GMM) is applied to cluster the Argo data into classes to construct convex hulls with the smallest polygons encompassing all the data points. Good or bad temperature data are identified as within or outside the polygons based on point-in-polygon analysis implemented by the ray casting algorithm. The South China Sea was selected as an example and results showed that the proposed approach could identify more than 70% of the profiles containing the outliers and mark the outliers automatically at the same time. This highlights the potential of the proposed methodology to be a good complementary quality control method.摘要本文提出了一种基于机器学习的Argo浮标温度异常值检测方法. 该方法采用机器学习无监督算法高斯混合模型对Argo浮标数据进行聚类分析, 并构建包围所有数据点的最小多边形的凸包. 基于射线投影算法实现点在多边形内分析, 通过自动识别数据点位于凸包内外来判断该数据点数据质量的好坏. 本文采用南海区域Argo浮标数据对该方法进行测试, 结果表明该方法可以识别70%以上的包含异常值的温度剖面, 同时自动标记出各异常值点.

The Chlorophyll-a Content Distribution in the Atlantic Ocean in December 2019 — January 2020 according to Ship Measurements at the Different Hydrometeorological Conditions

The horizontal and vertical chlorophyll-a concentrations (Chl) distributions along the ship route of 79th cruise of R/V “Akademik Mstislav Keldysh” in the Atlantic Ocean in December 2019—January 2020 were obtained. Analysis of the observations was conducted for the biogeochemical provinces which were determined by means of the different hydrometeorological conditions for the phytoplankton community functioning.Three methods of Chl measurements were applied: standard extraction method of the seawater samples, the fluorometer underway measurements in the flow-through water system at the depth of 5 m and in-situ profiling fluorometer measurements at stations. Also, measurements of the seawater temperature and salinity in the flow-through water system, the CTD and optical profiling, and ARGO floats were used.The maximum Chl values in the near-surface ocean layer are observed at zones of divergence in the Canary upwelling systems (CNRY) (1.3 mg/m3) and in the Falkland (Malvinas) Current system (4.8 mg/m3), and also in the Drake Strait (1.2 mg/m3). The least Chl values (0.01–0.12 mg/m3) are observed in the tropical, equatorial areas and in the South Atlantic Subtropical Anticyclonic Gyre (SASTG) at the ship route.Vertical Chl profiles with absence of the significant Chl maximum were found. In the CNRY region the Chl values were not changed much within the euphotic zone. While in the region of the Antarctic Circumpolar Current in the Drake Strait and in the offshore stream of the Falkland Current the quasi-homogeneous Chl profiles were observed in the upper layer which is almost 2 times deeper than the euphotic zone.In the profiles with the significant local Chl maximum the most Chl value of 3.5 mg/m3 is observed at the depth about 12 m in the upwelling region on the Patagonian Shelf. In other regions the depth of Chl maximum localization (0.2–0.9 mg/m3) varies widely from 25 to 120 m. In the western part of the SASTG region the absolute Chl maxima were found at the depth of 120 m, that is deeper, then the euphotic zone which is equal to 100 m.

Citizen scientists’ dive computers resolve seasonal and interannual temperature variations in the Red Sea

Dive computers have the potential to provide depth resolved temperature data that is often lacking especially in close to shore, but spatiotemporal assessment of the robustness of this citizen science approach has not been done. In this study, we provide this assessment for the Red Sea, one of the most dived areas in the world. A comparison was conducted between 17 years of minimum water temperatures collected from SCUBA dive computers in the northern Red Sea (23–30° N, 32–39.4° E), satellite-derived sea surface temperatures from the Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) optimal interpolation product, and depth-banded monthly mean in-situ temperature from the TEMPERSEA dataset, which incorporates data originating from several in-situ recording platforms (including Argo floats, ships and gliders). We show that dive computer temperature data clearly resolve seasonal patterns, which are in good agreement in both phase and amplitude with OSTIA and TEMPERSEA. On average, dive computer temperatures had an overall negative bias of (–0.5 ± 1.1) °C compared with OSTIA and (–0.2 ± 1.4) °C compared with TEMPERSEA. As may be expected, increased depth-related biases were found to be associated with stratified periods and shallower mixed layer depths, i.e., stronger vertical temperature gradients. A south-north temperature gradient consistent with values reported in the literature was also identifiable. Bias remains consistent even when subsampling just 1% of the total 9310 dive computer datapoints. We conclude that dive computers offer potential as an alternative source of depth-resolved temperatures to complement existing in situ and satellite SST data sources.

Reconstructing the Three-Dimensional Structure of Loop Current Rings from Satellite Altimetry and In Situ Data Using the Gravest Empirical Modes Method

The three-dimensional structure of Gulf of Mexico’s warm-core rings, detaching from the Loop Current, is investigated using satellite altimetry and a large set of ARGO float profiles. Reconstruction of the Loop Current rings (LCRs) vertical structure from sea surface height observations is made possible by the use of the gravest empirical modes method (GEM). The GEMs are transfer functions that associate a value of temperature and salinity for each variable pair {dynamic height; pressure}, and are computed by estimating an empirical relationship between dynamic height and the vertical thermohaline structure of the ocean. Between 1993 and 2021, 40 LCRs were detected in the altimetry and their three-dimensional thermohaline structure was reconstructed, as well as a number of dynamically relevant variables (geostrophic and cyclogeostrophic velocity, relative vorticity, potential vorticity, available potential energy and kinetic energy density, etc.). The structure of a typical LCR was computed by fitting an analytical stream function to the LCRs dynamic height signature and reconstructing its vertical structure with the GEM. The total heat and salt contents and energy of each LCR were computed and their cumulative effect on the Gulf of Mexico’s heat, salt and energy balance is discussed. We show that LCRs have a dramatic impact on these balances and estimate that residual surface heat fluxes of −13 W m−2 are necessary to compensate their heat input, while the fresh water outflow of the Mississippi river approximately compensates for their salt excess input. An average energy dissipation of O [10−10–10−9] W kg−1 would be necessary to balance their energy input.

Warm bias of cold sea surface temperatures in the East Sea (Japan Sea)

The East/Japan Sea (ES) is regarded as a natural laboratory for predicting future changes in the global Meridional Overturning Circulation (MOC) under warming climates, as the ES MOC (EMOC) changes rapidly in comparison with the global MOC. Specifically, intermediate and deep-water masses of the ES are formed in its northern reaches via wind-driven subduction of surface water, and convection from the surface to deep layers during the winter. Accordingly, it is important to investigate the variation of winter sea surface temperatures (SSTs) for characterizing and predicting the EMOC; however, global SST products must be corrected and optimized for the ES, as they fail to incorporate the local marginal sea conditions. Here, a warm bias in cold SST was identified for three SST products, such as optimally interpolated sea surface temperatures (OISSTs), microwave SSTs, and operational SST and sea ice analysis products, suggesting the potential usefulness of a correction method incorporating Argo float data. When comparing OISSTs with 5 m temperature estimates from Argo float data during 2000–2020, under the assumption that the mixed layer depth is deeper than 8 m, a nearly normalized histogram of biases was produced, and the robust warm bias (mean = 0.9°C) was detected in the range of relatively cold SSTs (-2°C to 10°C), yet no significant bias in warm SSTs (> 10°C) was found. To minimize the warm bias in cold SSTs, OISSTs were corrected with an inverse 4th-order polynomial fitting method. Subsequently, the mean bias between the corrected SSTs and top depth temperatures of Argo float data was significantly reduced to less than 0.1°C. Moreover, the warm bias of cold SSTs resulted in severe underestimations of the outcropping area colder than 1°C over the northern region, as well as the occurrence period of 1°C to 5°C SSTs in the north-western ES. These results highlight the importance of local bias correction for SST products, and it is expected that the newly suggested correction method will improve model predictions of EMOC change by enhancing SST data quality in the northern ES.

A Multivariable Empirical Algorithm for Estimating Particulate Organic Carbon Concentration in Marine Environments From Optical Backscattering and Chlorophyll-a Measurements

Accurate estimates of the oceanic particulate organic carbon concentration (POC) from optical measurements have remained challenging because interactions between light and natural assemblages of marine particles are complex, depending on particle concentration, composition, and size distribution. In particular, the applicability of a single relationship between POC and the spectral particulate backscattering coefficient bbp(λ) across diverse oceanic environments is subject to high uncertainties because of the variable nature of particulate assemblages. These relationships have nevertheless been widely used to estimate oceanic POC using, for example, in situ measurements of bbp from Biogeochemical (BGC)-Argo floats. Despite these challenges, such an in situbased approach to estimate POC remains scientifically attractive in view of the expanding global-scale observations with the BGC-Argo array of profiling floats equipped with optical sensors. In the current study, we describe an improved empirical approach to estimate POC which takes advantage of simultaneous measurements of bbp and chlorophyll-a fluorescence to better account for the effects of variable particle composition on the relationship between POC and bbp. We formulated multivariable regression models using a dataset of field measurements of POC, bbp, and chlorophyll-a concentration (Chla), including surface and subsurface water samples from the Atlantic, Pacific, Arctic, and Southern Oceans. The analysis of this dataset of diverse seawater samples demonstrates that the use of bbp and an additional independent variable related to particle composition involving both bbp and Chla leads to notable improvements in POC estimations compared with a typical univariate regression model based on bbp alone. These multivariable algorithms are expected to be particularly useful for estimating POC with measurements from autonomous BGC-Argo floats operating in diverse oceanic environments. We demonstrate example results from the multivariable algorithm applied to depth-resolved vertical measurements from BGC-Argo floats surveying the Labrador Sea.

The effects of tropical cyclones on characteristics of barrier layer thickness

The impact of tropical cyclones on the characteristics of barrier layer (BL) thickness is investigated in this study in combination with observations and numerical simulations. Statistical results based on Simple Ocean Data Assimilation reanalysis (SODA) data and Argo float measurements reveal a significant effect of TCs on the BL characteristics. The BL thickness increases remarkably with the TC approaching, reaches its maximum 1–5 days after the TC passage, and recovers to its pre-storm state within 30 days on average. The peak increase in BL thickness is slightly rightward biased relative to the TC track, reaching ∼1.31 m (26.93%) relative to the pre-storm state as revealed by SODA data. The changes in BL thickness show a strong correlation with TC characteristics in terms of intensity and translation speed, tending to be more significant with more intense or slower-moving TCs. The thickening of the isothermal layer on the right of TC tracks and shallowing of the mixed layer act together to determine the BL development. The three-dimensional Price–Weller–Pinkel (3DPWP) ocean model is applied to explore the influence of precipitation on BL thickness. The results show that the precipitation tends to decrease the mixed layer depth while the vertical mixing of TCs deepens the isothermal layer depth, both of which act together to increase the BL thickness.

The Contribution of the Vendée Globe Race to Improved Ocean Surface Information: A Validation of the Remotely Sensed Salinity in the Sub-Antarctic Zone

The Vendée Globe is the world’s most famous solo, non-stop, unassisted sailing race. The Institute of Marine Sciences and the Barcelona Ocean Sailing Foundation installed a MicroCAT on the One Ocean One Planet boat. The skipper, Dídac Costa, completed the round trip in 97 days, from 8 November 2020 to 13 February 2021, providing one measurement of temperature and conductivity every 30 s during navigation. More than half of the ship’s route was in the sub-Antarctic zone, between the tropical and polar fronts, and it passed through areas of oceanographic interest such as Southern Patagonia (affected by glacier melting), the Brazil–Malvinas confluence, the Southern Pacific Ocean, and the entire Southern Indian Ocean. This sailing race gave a rare opportunity to measure in-situ sea surface salinity in a region where satellite salinity measurements are not reliable. Due to the decreased sensitivity of brightness temperature to salinity in cold seas, retrieving sea surface salinity at high latitudes remains a major challenge. This paper describes how the data are processed and uses the data to validate satellite salinity products in the sub-Antarctic zone. The sailing race measurements represent surface information (60 cm depth) not available from drifters or Argo floats. Acquiring measurements using round-the-world sailing races would allow us to analyse the evolution of ocean salinity and the impact of changes in the ice extent around Antarctica.