Air-sea Exchange Research Articles

Evaluating the Extraction and Quantification of Marine Surfactants from Seawater through Solid Phase Extraction and Subsequent Colorimetric Analyses.

Surfactants are amphiphilic molecules that adsorb to interfaces and affect the interfacial tension. Surfactants in seawater can impact gas-exchange, surface properties, and the composition and fate of sea spray aerosol. The accurate quantification of surfactants and their classes is crucial to constraining the effect of surfactants in seawater and their role in air-sea exchanges. Here, we evaluate and optimize a solid phase extraction (SPE) method paired with colorimetry and UV-vis spectroscopy to quantify the concentrations of anionic, cationic, and nonionic surfactants in seawater. We compare tandem SPE with two-step SPE and different elution volumes and evaluate the impact of different interferents. Improved extraction efficiencies were obtained with an 8 mL acetonitrile elution and with separate ENVI-18 and ENVI-Carb extractions, instead of tandem. With complex surfactant mixtures, the presence of anionic surfactants interfered with the quantification of cationic surfactants and caused underestimations of up to 83%. Using a two-step extraction and analyzing each seawater SPE extract separately during colorimetric quantification help avoid the effects of interferents and ensure more representative quantification of surfactants. With this method, average seawater surfactant concentrations ranged from 0.04 to 0.06 μM. At the highest concentrations, the class composition comprised 23% anionic, 21% cationic, and 56% nonionic surfactants.

Dissolved PAHs in the Beibu Gulf and adjacent waters of the South China Sea: Physical and biochemical processes-driven distributional variations

Polycyclic aromatic hydrocarbons (PAHs) in semi-enclosed gulfs are influenced by physical and biochemical processes, which haven’t been well understood. This study aims to investigate the spatial distribution and vertical profiles of dissolved PAHs in the Beibu Gulf and adjacent waters of the South China Sea, along with hydrological, meteorological, and biochemical variables. Particularly relevant are the effects of atmospheric pressure, salinity, ammonium, chlorophyll-a, as well as riverine inputs (RI), sea currents, and upwelling. In surface seawater, the total concentrations of eight dissolved PAHs (∑8PAHs) were 7.76 ± 2.16ng/L, with a distribution pattern of western Guangdong waters (WGWs) > BG > Qiongzhou Strait (QS). ∑8PAHs in the northern BG (9.10 ± 2.00ng/L) was significantly higher than that in the southern BG (6.65 ± 1.54ng/L) (p < 0.01), suggesting that local anthropogenic activities and unique environmental characteristics significantly influenced PAHs distribution. In water column, PAHs in BG displayed enrichment in surface and bottom but decreased in medium water, while those in WGWs and QS decreased with increasing depth. Source apportionment concluded that PAHs in QS and WGWs were primarily from petroleum sources, and PAHs in BG were mainly from coal combustion. RI, combined with circulation, coastal current, and intrusion of SCS water influenced the surface PAHs distribution in BG, with eddy impacts observed. Specifically, regarding the surface PAHs distribution, differences in atmospheric pressure may influence the air-sea exchange of PAHs, especially positively affecting 4-ring PAHs. Salinity factors further corroborated the contribution of RI to 3-ring PAHs, followed by the regulation of PAHs through biological pumps (ammonia and chlorophyll-a). Moreover, upwelling-induced biodegradation and resuspension affected the vertical distribution of PAHs. While most PAHs posed a negligible risk, coking-generated fluorene posed a moderate risk to ecosystems due to changes in the energy structure, warranting further investigation into its toxicological impacts.

The suppression of ocean waves by biogenic slicks.

Ocean waves are significantly damped by biogenic surfactants, which accumulate at the sea surface in every ocean basin. The growth, development, and breaking of short wind-driven surface waves are key mediators of the air-sea exchange of momentum, heat and trace gases. The mechanisms through which surfactants suppress waves have been studied in great detail through careful laboratory experimentation in quasi-one-dimensional wave tanks. However, the spatial scales over which this damping occurs in structurally complex surfactant slicks on the real ocean have not been resolved. Here, we present the results of field observations of the spatial response of decimetre- to millimetre-scale waves to biogenic surfactant slicks. We found that wave damping in organic material-rich coastal waters resulted in a net (spatio-temporally averaged) reduction of approximately 50% in wave slope variance relative to the open ocean for low to moderate wind speeds. This reduction of wave slope variance is understood to result in a corresponding reduction in momentum input to the wave field. This significant effect had thus far evaded quantification due in large part to the enormous range of scales required for its description-spanning the sea surface microlayer to the ocean submesoscale.

Seasonal Characteristics of Air–Sea Exchanges over the South Coast of Matara, Sri Lanka

Air–sea exchanges play a crucial role in intense weather events over Sri Lanka, particularly by providing the heat and moisture that fuel heavy rainfall. We present a year-round dataset of meteorological observations from the southern shoreline of Sri Lanka in the equatorial Indian Ocean for 2017, aiming to investigate its seasonal characteristics and evaluate the performance of reanalysis data in this region. The observations reveal distinct diurnal and seasonal patterns. During the winter and spring, higher shortwave (646.2 W/m2) and longwave radiation (−86.9 W/m2) are coupled with higher temperatures (30.6 °C) and lower humidity (67.4% at noon). In contrast, the Indian summer monsoon period features reduced shortwave (579.8 W/m2) and longwave radiation (−58.6 W/m2), lower temperatures (29.2 °C), higher humidity (over 79.7%), and stronger winds (6.25 m/s). The observations were compared with the ERA5 reanalysis dataset to evaluate the regional performance. The reanalysis data correlated well with the observed data for the radiation, temperature, and sensible heat flux, although notable deviations occurred in terms of the wind speed and latent heat flux. During the impact of Tropical Cyclone Ockhi, the reanalysis data tended to underestimate both the wind speed and precipitation. This dataset will provide vital support for studies on monsoons and coastal atmospheric convection, as well as for model initialization and synergistic applications.

Oceanic eddy with submesoscale edge drives intense air-sea exchanges and beyond

Oceanic mesoscale eddies influence air-sea interaction and atmosphere dynamics through ventilating heat and moisture upward. However, whether the sea surface temperature (SST) gradient on the eddy edge could affect the heat and moisture release is still unknown because of the limited observations and coarse-resolution climate models. Using high-resolution atmospheric simulations, this study compares the atmospheric response to the mesoscale (~ 40 km) and submesoscale (~ 4 km) SST gradients at the edge of an eddy. Results show that submesoscale SST gradient drives stronger surface heat and moisture fluxes, enhancing the vertical mixing intensity by 2–3 times within and above the marine atmospheric boundary layer. As a result, one local precipitation event is found to be an order of magnitude larger overlying the eddy. Our findings highlight the importance of resolving oceanic submesoscale features for accurately predicting atmosphere dynamics and precipitation over the ocean.

A dynamically adaptive Langmuir turbulence parameterization scheme for variable wind wave conditions: Model application

Langmuir circulations and turbulence (LT) are crucial in the upper ocean mixed layer, significantly affecting the air-sea exchange of momentum, heat, and mass. The development of an appropriate LT parameterization scheme is vital for ocean modeling. This study employed the Large-eddy Simulation (LES) and the Physics-informed Neural Network (PINN) to optimize the KC04 Langmuir turbulence scheme by dynamically adjusting E6 as a key parameter determined by winds and waves. The LES simulations under different wind wave states indicated the PINN-inferred values for E6. Modelling results from GOTM in OCSPapa station demonstrated that the optimized scheme outperformed the original KC04 scheme in simulating the vertical eddy diffusivity and temperature, with an ∼6.24% annual reduction in the root mean square error (RMSE) for the temperature and an ∼8.23% reduction in the RMSE during autumn. Furthermore, the optimized scheme resulted in a thicker mixed layer, reaching 4.9 m. This enhanced LT parameterization scheme exhibited the improved robustness for variable spatiotemporal resolutions, significantly improving the modeling accuracy.

Intercomparison of fast airborne ozone instruments to measure eddy covariance fluxes: spatial variability in deposition at the ocean surface and evidence for cloud processing

Abstract. The air–sea exchange of ozone (O3) is controlled by chemistry involving halogens, dissolved organic carbon, and sulfur in the sea surface microlayer. Calculations also indicate faster ozone photolysis at aqueous surfaces, but the role of clouds as an ozone sink is currently not well established. Fast-response ozone sensors offer opportunities to measure eddy covariance (EC) ozone fluxes in the marine boundary layer. However, intercomparisons of fast airborne O3 sensors and EC O3 fluxes measured on aircraft have not been conducted before. In April 2022, the Technological Innovation Into Iodine and GV Environmental Research (TI3GER) field campaign deployed three fast ozone sensors (gas chemiluminescence and a combination of UV absorption with coumarin chemiluminescence detection, CID) together with a fast water vapor sensor and anemometer to study iodine chemistry in the troposphere and stratosphere over Colorado and over the Pacific Ocean near Hawaii and Alaska. Here, we present an instrument comparison between the NCAR Fast O3 instrument (FO3, gas-phase CID) and two KIT Fast AIRborne Ozone instruments (FAIRO, UV absorption and coumarin CID). The sensors have comparable precision &lt; 0.4 % Hz−0.5 (0.15 ppbv Hz−0.5), and ozone volume mixing ratios (VMRs) generally agreed within 2 % over a wide range of environmental conditions: 10 &lt; O3 &lt; 1000 ppbv, below detection &lt; NOx &lt; 7 ppbv, and 2 ppmv &lt; H2O &lt; 4 % VMR. Both instrument designs are demonstrated to be suitable for EC flux measurements and were able to detect O3 fluxes with exchange velocities (defined as positive for upward) as slow as −0.010 ± 0.004 cm s−1, which is in the lower range of previously reported measurements. Additionally, we present two case studies. In one, the direction of ozone and water vapor fluxes was reversed (vO3 = +0.134 ± 0.005 cm s−1), suggesting that overhead evaporating clouds could be a strong ozone sink. Further work is needed to better understand the role of clouds as a possibly widespread sink of ozone in the remote marine boundary layer. In the second case study, vO3 values are negative (varying by a factor of 6–10 from −0.036 ± 0.006 to −0.003 ± 0.004 cm s−1), while the water vapor fluxes are consistently positive due to evaporation from the ocean surface and spatially homogeneous. This case study demonstrates that the processes governing ozone and water vapor fluxes can become decoupled and illustrates the need to elucidate possible drivers (physical, chemical, or biological) of the variability in ozone exchange velocities on fine spatial scales (∼ 20 km) over remote oceans.

Three-dimensional wave breaking

Although a ubiquitous natural phenomenon, the onset and subsequent process of surface wave breaking are not fully understood. Breaking affects how steep waves become and drives air–sea exchanges1. Most seminal and state-of-the-art research on breaking is underpinned by the assumption of two-dimensionality, although ocean waves are three dimensional. We present experimental results that assess how three-dimensionality affects breaking, without putting limits on the direction of travel of the waves. We show that the breaking-onset steepness of the most directionally spread case is double that of its unidirectional counterpart. We identify three breaking regimes. As directional spreading increases, horizontally overturning ‘travelling-wave breaking’ (I), which forms the basis of two-dimensional breaking, is replaced by vertically jetting ‘standing-wave breaking’ (II). In between, ‘travelling-standing-wave breaking’ (III) is characterized by the formation of vertical jets along a fast-moving crest. The mechanisms in each regime determine how breaking limits steepness and affects subsequent air–sea exchanges. Unlike in two dimensions, three-dimensional wave-breaking onset does not limit how steep waves may become, and we produce directionally spread waves 80% steeper than at breaking onset and four times steeper than equivalent two-dimensional waves at their breaking onset. Our observations challenge the validity of state-of-the-art methods used to calculate energy dissipation and to design offshore structures in highly directionally spread seas.

Rapid cycling and emission of volatile sulfur compounds in the eastern Indian Ocean: Impact of runoff inputs and implications for balancing atmospheric carbonyl sulfide budget

Volatile sulfur compounds, such as dimethyl sulfide (DMS), carbonyl sulfide (OCS), and carbon disulfide (CS2), significantly influence atmospheric chemistry and climate change. Despite the oceans being an important source of these sulfides, the limited understanding of their biogeochemical cycles in seawater introduces considerable uncertainties in quantifying their oceanic emissions and assessing atmospheric OCS budgets. To address this issue, we conducted a comprehensive field survey in the tropical eastern Indian Ocean (EIO) to examine the spatial distributions, source-sink dynamics, and sea-air exchange fluxes of marine DMS, OCS, and CS2. Our study indicates that nutrients, organic matter, and freshwater input from terrestrial runoff significantly affect most of the source-sink processes of these sulfides in the Bay of Bengal and even the tropical EIO. The resulting sulfide accumulation in seawater combined with high wind speeds establishes the tropical EIO as a considerable direct and indirect atmospheric OCS source. These insights underscore the potentially critical role of marine environments influenced by runoff in contributing to the atmospheric OCS budget. However, by integrating these results with previous field surveys, we believe that actual OCS emissions from tropical oceans exceed some bottom-up box-model simulations, yet fall significantly below those predicted by top-down models, still insufficient to bridge the atmospheric OCS source gap. Our detailed examination of source-sink dynamics offers deeper insights into the marine sulfur cycle and has potential implications for refining future box-models, thus mitigating uncertainties in estimating marine sulfur emissions.

Potential of CO2 sequestration by olivine addition in offshore waters: A ship-based deck incubation experiment

Ocean alkalinity enhancement is considered as an effective atmospheric CO2 removal approach, but currently, little is known about the carbon sequestration potential of implementing olivine addition in offshore waters. We investigated the effect of olivine addition on the seawater carbonate system by carrying out a deck incubation experiment in the Northern Yellow Sea; the dissolution rate of olivine was calculated based on the increase in seawater alkalinity (TA), and the CO2 sequestration potential was evaluated. The results showed that the dissolution of olivine increased seawater TA and decreased partial pressure of CO2, resulting in oceanic CO2 uptake from the atmosphere through sea-air exchange; it also increased seawater pH and mitigated ocean acidification to a certain extent. The addition of 1 ‰ olivine had a more significant effect on the seawater carbonate system than 0.5 ‰ olivine addition. The average dissolution rate constant of olivine was 1.44 ± 0.15 μmol m−2 d−1. Assuming that olivine settles completely on the seabed due to gravity, the theoretically maximum amount of CO2 removed by applying 1 tonne of olivine per square meter area in the Northern Yellow Sea is only 2.0 × 10−4 t/m2. Therefore, when olivine addition is implemented in the offshore waters, it is necessary to consider reducing the olivine size, prolonging the settling time of olivine in the water column; and spreading olivine in well-mixed waters to prolong the residence time through repeated resuspension, thus increasing its potential in carbon sequestration.

Upwelling Enhances Mercury Particle Scavenging in the California Current System.

Coastal upwelling supplies nutrients supporting primary production while also adding the toxic trace metal mercury (Hg) to the mixed layer of the ocean. This could be a concern for human and environmental health if it results in the enhanced bioaccumulation of monomethylmercury (MMHg). Here, we explore how upwelling influences Hg cycling in the California Current System (CCS) biome through particle scavenging and sea-air exchange. We collected suspended and sinking particle samples from a coastal upwelled water parcel and an offshore non-upwelled water parcel and observed higher total particulate Hg and sinking flux in the upwelling region compared to open ocean. To further investigate the full dynamics of Hg cycling, we modeled Hg inventories and fluxes in the upper ocean under upwelling and non-upwelling scenarios. The model simulations confirmed and quantified that upwelling enhances sinking fluxes of Hg by 41% through elevated primary production. Such an enhanced sinking flux of Hg is biogeochemically important to understand in upwelling regions, as it increases the delivery of Hg to the deep ocean where net conversion to MMHg may take place.

A Framework for Constraining Ocean Mixing Rates and Overturning Circulation from Age Tracers

Abstract The age of seawater refers to the amount of time that has elapsed since that water encountered the surface. This age measures the ventilation rate of the ocean, and the spatial distribution of age can be influenced by multiple processes, such as overturning circulation, ocean mixing, and air–sea exchange. In this work, we aim to gain new quantitative insights about how the ocean’s age tracer distribution reflects the strength of the meridional overturning circulation and diapycnal diffusivity. We propose an integral constraint that relates the age tracer flow across an isopycnal surface to the geometry of the surface. With the integral constraint, a relationship between the globally averaged effective diapycnal diffusivity and the meridional overturning strength at an arbitrary density level can be inferred from the age tracer concentration near that level. The theory is tested in a set of idealized single-basin simulations. A key insight from this study is that the age difference between regions of upwelling and downwelling, rather than any single absolute age value, is the best indicator of overturning strength. The framework has also been adapted to estimate the strength of abyssal overturning circulation in the modern North Pacific, and we demonstrate that the age field provides an estimate of the circulation strength consistent with previous studies. This framework could potentially constrain ocean circulation and mixing rates from age-like realistic tracers (e.g., radiocarbon) in both past and present climates. Significance Statement The age of seawater—the local mean time since local water from different pathways was last at the surface—is a valuable indicator of ocean circulation and the transport time scale of heat and carbon. We introduce a novel constraint that relates total age flow across a density surface to its geometry, which provides new insights into constraining ocean circulation and mixing rates from age-like realistic tracers (e.g., radiocarbon).

Responses of cold eddies to Typhoon Soulik (2013) off northeastern Taiwan Island

By using multisource reanalysis datasets, satellite data, etc., the influences of Typhoon Soulik on the upper ocean of two cold eddies off northeastern Taiwan Island (hereafter referred to as the P-Cold Eddy and D-Cold Eddy), which are distributed around Pengjia Islet and northeastern Diaoyu Island, respectively, were investigated. The results showed that the P-Cold Eddy was strengthened, and the D-Cold Eddy, which was previously invisible in the surface layer, began to appear. The seawater temperatures in the upper mixed layer of the two cold eddies decreased the most on 13 July, with the sea surface temperatures (SSTs) decreasing by 1.16 °C and 0.97 °C and the mixed layer temperatures decreasing by 1.23 °C and 1.06 °C, respectively. The upper ocean cooling of the P-Cold Eddy was mainly caused by upwelling, whose formation was related to the climb of more Kuroshio subsurface cold water northward forced by the typhoon. The upwelling of the D-Cold Eddy was weak, and the warming effect of the heat pump could be observed at the bottom of its mixed layer. The process of cooling and increasing salinity in the mixed layer of the D-Cold Eddy was mainly associated with the inflow of the upper cooler and saltier seawater from the P-Cold Eddy. The air–sea exchange term was not the main factor in the cooling of the seawater in the mixed layer during the forced stage, but it became dominant during the relaxation stage, and the latent heat flux (QLH) contributed significantly to its change.

Air-sea exchange of PAHs in the Taiwan Strait: Seasonal dynamics and regulation mechanisms revealed by machine learning approach

In this study, to understand the seasonal dynamics of air-sea exchange and its regulation mechanisms, we investigated polycyclic aromatic hydrocarbons (PAHs) at the air-sea interface in the western Taiwan Strait in combination with measurements and machine learning (ML) predictions. For 3-ring PAHs and most of 4- to 6-ring, volatilization and deposition fluxes were observed, respectively. Seasonal variations in air-sea exchange flux suggest the influence of monsoon transitions. Results of interpretable ML approach (XGBoost) indicated that volatilization of 3-ring PAHs was significantly controlled by dissolved PAH concentrations (contributed 24.0 %), and the gaseous deposition of 4- to 6-ring PAHs was related to more contaminated air masses originating from North China during the northeast monsoon. Henry’s law constant emerged as a secondary factor, influencing the intensity of air-sea exchange, particularly for low molecular weight PAHs. Among environmental parameters, notably high wind speed emerges as the primary factor and biological pump’s depletion of PAHs in surface seawater amplifies the gaseous deposition process. The distinct dynamics of exchanges at the air-water interface for PAHs in the western TWS can be attributed to variations in primary emission intensities, biological activity, and the inconsistent pathways of long-range atmospheric transport, particularly within the context of the monsoon transition.

Machine Learning to Characterize Biogenic Isoprene Emissions and Atmospheric Formaldehyde with Their Environmental Drivers in the Marine Boundary Layer

Oceanic biogenic emissions exert a significant impact on the atmospheric environment within the marine boundary layer (MBL). This study employs the extreme gradient boosting (XGBoost) machine learning method and clustering method combined with satellite observations and model simulations to discuss the effects of marine biogenic emissions on MBL formaldehyde (HCHO). The study reveals that HCHO columnar concentrations peaked in summer with 8.25 × 1015 molec/cm2, but the sea–air exchange processes controlled under the wind and sea surface temperature (SST) made marine biogenic emissions represented by isoprene reach their highest levels in winter with 95.93 nmol/m2/day. Analysis was conducted separately for factors influencing marine biogenic emissions and affecting MBL HCHO. It was found that phytoplankton functional types (PFTs) and biological degradation had a significant impact on marine biogenic emissions, with ratio range of 0.07~15.87 and 1.02~5.42 respectively. Machine learning methods were employed to simulate the conversion process of marine biogenic emissions to HCHO in MBL. Based on the SHAP values of the learning model, the importance results indicate that the factors influencing MBL HCHO mainly included NO2, as well as temperature (T) and relative humidity (RH). Specifically, the influence of NO2 on atmospheric HCHO was 1.3 times that of T and 1.6 times that of RH. Wind speed affected HCHO by influencing both marine biogenic emission and the atmospheric physical conditions. Increased marine biogenic emissions in air masses heavily influenced by human activities can reduce HCHO levels to some extent. However, in areas less affected by human activities, marine biogenic emissions can lead to higher levels of HCHO pollution. This research explores the impact of marine biogenic emissions on the HCHO status of the MBL under different atmospheric chemical conditions, offering significant insights into understanding chemical processes in marine atmospheres.

Air–sea interactions in stable atmospheric conditions: lessons from the desert semi-enclosed Gulf of Eilat (Aqaba)

Abstract. Accurately quantifying air–sea heat and gas exchange is crucial for comprehending thermoregulation processes and modeling ocean dynamics; these models incorporate bulk formulae for air–sea exchange derived in unstable atmospheric conditions. Therefore, their applicability in stable atmospheric conditions, such as desert-enclosed basins in the Gulf of Eilat/Aqaba (coral refugium), Red Sea, and Persian Gulf, is unclear. We present 2-year eddy covariance results from the Gulf of Eilat, a natural laboratory for studying air–sea interactions in stable atmospheric conditions, which are directly related to ocean dynamics. The measured mean evaporation, 3.22 m yr−1, approximately double that previously estimated by bulk formulae, exceeds the heat flux provided by radiation. Notably, in arid environments, the wind speed seasonal trend drives maximum evaporation in summer, with a minimum winter rate. The higher evaporation rate appears when elevated wind, particularly in the afternoon, coincides with an increase in vapor pressure difference. The inability of the bulk formulae approach to capture the seasonal (opposite from our measurements) and annual trend of evaporation is linked to errors in quantifying the atmospheric boundary layer stability parameter. Most of the year, there is a net cooling effect of surface water (−79 W m−2), primarily through evaporation. The substantial heat deficit is compensated by the advection of heat via northbound currents from the Red Sea, which we indirectly quantify from energy balance considerations. Cold and dry synoptic-scale winds induce extreme heat loss through air–sea fluxes and are correlated with the destabilization of the water column during winter and initiation of vertical water-column mixing.

Seasonal variability and long-term winter shoaling of the upper mixed layer in the southern Baltic Sea

The upper ocean mixed layer plays a crucial role in regulating the exchange between the ocean and the atmosphere. The Mixed Layer Depth (MLD) is a key parameter affecting the air-sea exchanges of momentum and heat and determining the upper ocean temperature. Numerous previous studies have investigated MLD variability in the global ocean or regional seas but no such studies were carried in the Baltic Sea. In this study, we present the first observational assessment of the MLD and its properties in the Southern Baltic Sea including quantification of its seasonal and long-term changes and identification of the multi-year winter shoaling. We calculated monthly maps of MLD in the southern Baltic Sea using a large number of historical CTD profiles collected in 1995–2021 from a combination of different data sets. To test the robustness of the results we compared the MLDs calculated using different threshold methods. Throughout the southern Baltic Sea, across its three basins, a distinct seasonality is evident in the MLD, with values varying from 12 m in July to 60 m in December–March. During winter the water column is well mixed down to the upper halocline depth and the MLD reaches about 45 m in the Bornholm Basin, 50 m in the Slupsk Furrow, and 60 m in the Gdansk Basin. The observed global warming and decadal changes in the salty inflows from the North Sea to the Baltic have had an impact on stratification by increasing water densities in the intermediate and deep layers. Consequently, density gradients have strengthened with depth while the upper ocean mixing has weakened during the winter season. The results reveal a significant winter shoaling of the mixed layer by 4 m per decade, driven by the increased stratification due to rising temperatures and salinity. These changes could have significant impacts on the dynamics and productivity of marine ecosystems.

A review on air–sea exchange of reactive trace gases over the northern Indian Ocean

A review on air–sea exchange of reactive trace gases over the northern Indian Ocean

Distributions and controlling processes of the carbonate system in the Eastern Indian Ocean during autumn and spring

The Eastern Indian Ocean (EIO) is an ideal region to explore the variability and controlling mechanisms of the seawater carbonate system and their potential influence on global climate change due to the distinctive environmental features, while studies in the EIO is far from sufficient. The spatiotemporal distributions of pH, dissolved inorganic carbon (DIC), alkalinity (Alk), and partial pressure of carbon dioxide (pCO2) were investigated in the EIO during autumn 2020 and spring 2021. The respective quantitative contributions of different controlling processes to DIC were further delineated. Significant seasonal variations were observed in the study area. Overall, the surface pH was lower and DIC, Alk, and pCO2 were higher during spring 2021 than during autumn 2020. The pH generally decreased from east to west during autumn 2020, whereas it decreased from north to south during spring 2021. The low values of DIC and Alk that were detected in the Bay of Bengal in these two seasons were mainly attributed to the influence of river inputs. Coastal upwelling during monsoon periods led to higher pCO2 and DIC values near Sumatra and Sri Lanka during spring 2021. The relationships of carbonate system parameters with different types of nutrients and different sized chlorophyll-a in the two seasons indicated the shifts of nutrients utilized by the phytoplankton, and phytoplankton species dominated the carbonate system variabilities. In vertical profiles, carbonate system parameters showed strong correlations with other physical and biogeochemical parameters, and these correlations were more robust during spring 2021 than during autumn 2020. The average sea–air flux of CO2 was 10.00 mmol m−2 d−1 during autumn 2020 and was 16.00 mmol m−2 d−1 during spring 2021, which revealed that the EIO served as a CO2 source during the study period. In addition, the separation of different controlling processes of DIC indicated stronger mixing processes, less CaCO3 precipitation, more intensive sea–air exchange, and weaker photosynthesis during spring 2021 than during autumn 2020.

Technical note: Assessment of float pH data quality control methods – a case study in the subpolar northwest Atlantic Ocean

Abstract. Since a pH sensor has become available that is principally suitable for use on demanding autonomous measurement platforms, the marine CO2 system can be observed independently and continuously by Biogeochemical Argo floats. This opens the potential to detect variability and long-term changes in interior ocean inorganic carbon storage and quantify the ocean sink for atmospheric CO2. In combination with a second parameter of the marine CO2 system, pH can be a useful tool to derive the surface ocean CO2 partial pressure (pCO2). The large spatiotemporal variability in the marine CO2 system requires sustained observations to decipher trends and study the impacts of short-term events (e.g., eddies, storms, phytoplankton blooms) but also puts a high emphasis on the quality control of float-based pH measurements. In consequence, a consistent and rigorous quality control procedure is being established to correct sensor offsets or drifts as the interpretation of changes depends on accurate data. By applying current standardized routines of the Argo data management to pH measurements from a pH / O2 float pilot array in the subpolar North Atlantic Ocean, we assess the uncertainties and lack of objective criteria associated with the standardized routines, notably the choice of the reference method for the pH correction (CANYON-B, LIR-pH, ESPER-NN, and ESPER-LIR) and the reference depth for this adjustment. For the studied float array, significant differences ranging between ca. 0.003 pH units and ca. 0.04 pH units are observed between the four reference methods which have been proposed to correct float pH data. Through comparison against discrete and underway pH data from other platforms, an assessment of the adjusted float pH data quality is presented. The results point out noticeable discrepancies near the surface of &gt; 0.004 pH units. In the context of converting surface ocean pH measurements into pCO2 data for the purpose of deriving air–sea CO2 fluxes, we conclude that an accuracy requirement of 0.01 pH units (equivalent to a pCO2 accuracy of 10 µatm as a minimum requirement for potential future inclusion in the Surface Ocean CO2 Atlas, SOCAT, database) is not systematically achieved in the upper ocean. While the limited dataset and regional focus of our study do not allow for firm conclusions, the evidence presented still calls for the inclusion of an additional independent pH reference in the surface ocean in the quality control routines. We therefore propose a way forward to enhance the float pH quality control procedure. In our analysis, the current philosophy of pH data correction against climatological reference data at one single depth in the deep ocean appears insufficient to assure adequate data quality in the surface ocean. Ideally, an additional reference point should be taken at or near the surface where the resulting pCO2 data are of the highest importance to monitor the air–sea exchange of CO2 and would have the potential to very significantly augment the impact of the current observation network.