World Ocean Atlas Research Articles

Iron limitation study in the North Indian Ocean using model simulations

There have been many studies on iron as a limiting nutrient for productivity in the World Ocean, but only a few studies have been done in the Arabian Sea on iron limitation. Sensitivity of primary productivity, chlorophyll, nitrate, pCO2 and carbon flux in the Arabian Sea to one of the parameters related to iron limitation has been investigated using a three-dimensional coupled biogeochemical model (TOPAZ) embedded in Modular Ocean Model (MOM) in the global domain, for climatology and interannual variability. Initially, the model results are evaluated for many of the biogeochemical components using data from World Ocean Atlas, satellites and cruises in the Arabian Sea. It is noticed that model results capture spatial and temporal variations of some of the biogeochemical components and fluxes in the north Indian Ocean. Subsequently, it is shown that micronutrient iron plays a significant role on the growth of phytoplankton, utilization of nutrients, pCO2 in the ocean and carbon flux across the air–sea interface in north and north-west regions of the Arabian Sea.

Estimation of range‐dependent sound‐speed profile with dictionary learning method

A range-dependent dictionary learning method is proposed to retrieve the sound-speed profile (SSP) in a water body, with the whole computational domain decomposed into multiple range-independent subdomains. A dictionary is constructed by using the World Ocean Atlas depth profiles of temperature and salinity. The simulation results verify the efficacy of the dictionary in retrieving the range-independent SSP profile in each subdomain.

Distribution of common modern dinoflagellate cyst taxa in surface sediments of the Northern Hemisphere in relation to environmental parameters: The new n=1968 database

Abstract We present a new version of the standardized Northern Hemisphere “modern” dinoflagellate cyst (“dinocyst”) database, which includes abundances of 71 taxa at 1968 sites across the Northern Hemisphere, cross-referenced with 17 environmental parameters extracted mostly from the 2013 World Ocean Atlas. Several taxa with tropical to warm temperate affinities were added to the previous database version. Dinocyst concentrations in the surface sediments reach 106 cysts g−1, with maximum values along the continental margins and minimum values offshore in distal open ocean settings. Assemblages are characterized by the co-occurrence of phototrophic (n = 41) and heterotrophic taxa (n = 30), with maximum proportions of heterotrophic taxa in high productivity regions, notably at the sea-ice edge and in upwelling regions. The main pattern of assemblage distributions includes north–south and nearshore–offshore gradients, with maximum diversity of species along the continental margins and towards the south, in warm temperate and tropical areas. Canonical correspondence analyses performed on heterotrophic and phototrophic taxa independently yield consistent results, hence suggesting similar, but not identical, ecological affinities for both taxonomic groups. For both groups, sea-surface temperature and sea-ice are the most determining parameters, but the phototrophic taxa seem more sensitive to onshore–offshore gradients marked by salinity changes. Productivity-related parameters also determine dinocyst distribution, especially primary productivity in the fall and winter, with a stronger relationship for the combined dataset of phototrophic and heterotrophic taxa.

Regional'nye osobennosti raspredeleniya chastoty Vyaysyalya – Brenta v moryakh Laptevykh i Vostochno-Sibirskom

Purpose. The aim of the paper is to study buoyancy frequency in the Laptev and East Siberian seas and to assess correlation relations between the intra-year variability of the Vaisal – Brent frequency maximum and the climatic indices reflecting the atmosphere and hydrosphere state. Methods and Results. Based on the World Ocean Atlas 2013 for 1955–2012, the features of spatial and temporal variability of the buoyancy frequency distribution and its maximum depth in the Laptev and East Siberian seas are considered. It is found that the highest values of the Vaisal – Brent frequency are observed in the estuaries of the rivers Lena, Khatanga, Kolyma and Indigirka in summer where they attain 70–86 cycle/hour. In the deep-water northern areas of the seas under study, the majority of the water area is covered with ice throughout the whole year. As a consequence the surface layer of water is heated slightly and stability of stratification is much lower. The buoyancy frequency values in the seasonal pycnocline attain 24–46 cycle/hour. As a result of advection of the transformed Pacific waters in the northern and southeastern regions of the East Siberian Sea, on the depths 25–55 m observed is the layer of constant pycnocline where the Vaisal – Brent frequency values can mount to 21 cycles/hour. Correlation analysis is performed for the relations between the intra-annual variability of the Vaisal – Brent frequency maximum and the climatic indices (North Atlantic Oscillation, Arctic Oscillation, Pacific Decadal Oscillation, Gulf Stream North Wall) reflecting the atmosphere and hydrosphere state. It is shown that relationship between the buoyancy frequency maximum and the two latter indices is the most pronounced. Conclusions. The results of the carried out investigations show that the features of spatial-temporal variability of the buoyancy frequency distribution and its maximum depth in the Laptev and East Siberian seas are conditioned by the river discharge and also by advection in the Arctic region of the Atlantic and transformed Pacific waters inflowing from the adjacent basins.

POTENSI PANAS LAUT SEBAGAI ENERGI BARU TERBARUKAN DI PERAIRAN PAPUA BARAT DENGAN METODE OCEAN THERMAL ENERGY CONVERSION (OTEC)

Energi listrik merupakan suatu hal yang sangat dibutuhkan dalam keberlangsungan kehidupan manusia. Kebutuhan akan energi listrik yang semakin meningkat dari waktu ke waktu memerlukan inovasi baru untuk menghasilkan energi listrik yang ramah lingkungan. Ocean Thermal Energy Conversion (OTEC) adalah metode untuk menghasilkan energi listrik menggunakan prinsip perbedaan temperatur antara laut dalam dan permukaan dengan selisih temperatur minimal sebesar 20C untuk menjalankan mesin kalor. Papua Barat merupakan salah satu provinsi di Indonesia dengan elektrifikasi yang masih rendah. Letak Indonesia yang berada di wilayah tropis dengan perbedaan suhu air laut yang tinggi memiliki potensi untuk memanfaatkan metode OTEC dalam menghasilkan energi listrik. Penelitian ini bertujuan untuk mengetahui potensi pemanfaatan OTEC di perairan sekitar Papua Barat. Data yang digunakan adalah data suhu air laut dalam dan permukaan periode tahun 1955 – 2012 di dua belas titik di perairan sekitar Papua Barat. Data tersebut diperoleh dari World Ocean Atlas 2013 yang kemudian diolah menggunakan perangkat lunak olah data dan Ocean Data View (ODV). Hasil olah data didapatkan nilai efisiensi terbesar yaitu 7,67% dan terkecil yaitu 7,21%.

Evaluation of Arctic Ocean surface salinities from the Soil Moisture and Ocean Salinity (SMOS) mission against a regional reanalysis and in situ data

Abstract. Recently two gridded sea surface salinity (SSS) products that cover the Arctic Ocean have been derived from the European Space Agency (ESA)'s Soil Moisture and Ocean Salinity (SMOS) mission: one developed by the Barcelona Expert Centre (BEC) and the other developed by the Ocean Salinity Expertise Center of the Centre Aval de Traitement des Données SMOS at IFREMER (The French Research Institute for Exploitation of the Sea) (CEC). The uncertainties of these two SSS products are quantified during the period of 2011–2013 against other SSS products: one data assimilative regional reanalysis; one data-driven reprocessing in the framework of the Copernicus Marine Environment Monitoring Services (CMEMS); two climatologies – the 2013 World Ocean Atlas (WOA) and the Polar science center Hydrographic Climatology (PHC); and in situ datasets, both assimilated and independent. The CMEMS reanalysis comes from the TOPAZ4 system, which assimilates a large set of ocean and sea-ice observations using an ensemble Kalman filter (EnKF). Another CMEMS product is the Multi-OBservations reprocessing (MOB), a multivariate objective analysis combining in situ data with satellite SSS. The monthly root mean squared deviations (RMSD) of both SMOS products, compared to the TOPAZ4 reanalysis, reach 1.5 psu in the Arctic summer, while in the winter months the BEC SSS is closer to TOPAZ4 with a deviation of 0.5 psu. The comparison of CEC satellite SSS against in situ data shows Atlantic Water that is too fresh in the Barents Sea, the Nordic Seas, and in the northern North Atlantic Ocean, consistent with the abnormally fresh deviations from TOPAZ4. When compared to independent in situ data in the Beaufort Sea, the BEC product shows the smallest bias (< 0.1 psu) in summer and the smallest RMSD (1.8 psu). The results also show that all six SSS products share a common challenge: representing freshwater masses (< 24 psu) in the central Arctic. Along the Norwegian coast and at the southwestern coast of Greenland, the BEC SSS shows smaller errors than TOPAZ4 and indicates the potential value of assimilating the satellite-derived salinity in this system.

A global monthly climatology of total alkalinity: a neural network approach

Abstract. Global climatologies of the seawater CO2 chemistry variables are necessary to assess the marine carbon cycle in depth. The climatologies should adequately capture seasonal variability to properly address ocean acidification and similar issues related to the carbon cycle. Total alkalinity (AT) is one variable of the seawater CO2 chemistry system involved in ocean acidification and frequently measured. We used the Global Ocean Data Analysis Project version 2.2019 (GLODAPv2) to extract relationships among the drivers of the AT variability and AT concentration using a neural network (NNGv2) to generate a monthly climatology. The GLODAPv2 quality-controlled dataset used was modeled by the NNGv2 with a root-mean-squared error (RMSE) of 5.3 µmol kg−1. Validation tests with independent datasets revealed the good generalization of the network. Data from five ocean time-series stations showed an acceptable RMSE range of 3–6.2 µmol kg−1. Successful modeling of the monthly AT variability in the time series suggests that the NNGv2 is a good candidate to generate a monthly climatology. The climatological fields of AT were obtained passing through the NNGv2 the World Ocean Atlas 2013 (WOA13) monthly climatologies of temperature, salinity, and oxygen and the computed climatologies of nutrients from the previous ones with a neural network. The spatiotemporal resolution is set by WOA13: 1∘ × 1∘ in the horizontal, 102 depth levels (0–5500 m) in the vertical and monthly (0–1500 m) to annual (1550–5500 m) temporal resolution. The product is distributed through the data repository of the Spanish National Research Council (CSIC; https://doi.org/10.20350/digitalCSIC/8644, Broullón et al., 2019).

Retrieval of Internal Solitary Wave Amplitude in Shallow Water by Tandem Spaceborne SAR

The accurate estimation of the upper layer thickness in a two-layer ocean is a crucial step in the retrieval of internal solitary wave (ISW) amplitude from synthetic aperture radar (SAR) data. In this paper, we present a method to derive the upper layer thickness and the consequent ISW amplitude by combining two consecutive SAR images with the extended Korteweg-de Vries (eKdV) equation. An ISW case observed twice by the Chinese C-band SAR GaoFen-3 (GF-3) and the German X-band SAR TerraSAR-X (TS-X) with a temporal interval of approximately 11 min in shallow water to the southeast of Hainan Island in the northwestern South China Sea was used to demonstrate the applicability of the method. Using the in situ measurements of temperature and salinity near the observed ISW, the proposed method yielded an ISW amplitude of −4.52 m, in close proximity to −5.66 ± 1.24 m derived by applying the classic Korteweg–de Vries (KdV) equation based on the continuously stratified theory. Moreover, the climatological dataset of the World Ocean Atlas 2013 (WOA13) was also used with the proposed method in the Hainan case, and the results showed that the method can still provide a reasonable estimate of ISW amplitude in shallow water even when in situ oceanic stratification measurements are absent. The application of our method to derive the ISW amplitude from consecutive SAR images seems highly promising with the increasing emergence of tandem satellites in orbits.

Composition of Indonesia throughflow in North Sulawesi water masses and heat content

In this research the composition of water masses in the North Sulawesi (Sangihe Talaud) waters as part of the Indonesian Throughflow is investigated and analysed by using the Optimum Multi Parameter Analysis (OMP). The data used is the World Ocean Atlas data (WOA 13) in 1° x 1° latitude longitude resolution. Main data are the salinity, temperature and pressure. The Optimum Multiparameter analysis was used to calculate the percentage of each water mass contribution in the study waters. The contributions are 20-60% at 200-500 m depth for The North Pacific Intermediate Water (NPIW), 90-100%at around 0-100 meters depth for The North Pacific Subtropical Water (NPSW),100%at 900-1500 meters depth for the South Pacific Intermediate Water (SPIW)and 80-100% at 1100-1500 meters depth for TheAntartic Intermediate Water (AAIW). Surprisingly, the South Pacific Subtropical Water (SPSW) does not have any contribution and largest heat content comes from the AAIW. Magnitudes for each heat content of water types calculated in this study for NPIW, NPSW, AAIW, SPSW, and SPIW are in the range of 1.5 × 1014∼ 1.7 × 1014 J/m2, 2 × 1012∼ 9.9 × 1012 J/m2, 2.7 × 1014∼ 3.1 × 1014 J/m2, 0 ∼ 4.3 × 1011 J/m2, and 1.8 × 1014∼ 2 × 1014 J/m2, respectively. Eventhough the AAIW has the lowest temperature range but its large contribution in the waters outnumbers the warmer water masses.

Assessing the Skill of the Improved Treatment of Riverine Freshwater in the Community Earth System Model (CESM) Relative to a New Salinity Climatology

AbstractRecent studies have explored the sensitivity of global ocean model simulations to the treatment of riverine freshwater and the representation of estuarine processes via an estuary box model applied within Community Earth System Model (CESM). This study builds on these efforts by assessing the model skill score relative to a new salinity climatology. The new climatology averages the original observational data of the World Ocean Database directly onto the CESM ocean component tracer grid cells without spatial interpolation, smoothing, or other gap‐filling techniques to mitigate coastal ocean salinity bias present in the World Ocean Atlas. The mean square error for coastal upper ocean salinity relative to climatology is reduced by up to 14%, and the mean square error of near‐surface salinity stratification is reduced by up to 28% near major river mouths in the simulations with improved treatments of river runoff. The improvement in upper ocean bulk salinity is attributed primarily to focusing runoff as point sources thereby avoiding the artificial horizontal spreading of the control run and to applying a locally varying instead of a global constant reference salinity for riverine virtual salt fluxes. The improvements in near‐surface salinity stratification are primarily attributed to adding parameterized estuarine mixing with the estuary box model. Salinity and salinity stratification skill improvements are achieved not just near large rivers but also along the global coast and skill improvements extend far offshore. Despite these improvements, many other sources of model‐climatology mismatch in coastal salinity and stratification remain and merit further attention.

Computation of the vertical and horizontal polarizations of brightness temperature of flat surface water over the Persian Gulf at the L-Band

The brightness temperature is the principle parameter detected by passive microwave radiometers. The radio frequency interference at L-band radiometers (e.g., SMOS, Aquarius and SMAP) impacts the quality of brightness temperature detection in many parts of the world such as the Middle East and the Persian Gulf. In the present work, vertical and horizontal polarizations of brightness temperature over flat surface water of Persian Gulf at L-band were calculated by using physical computations and an empirical model. For this purpose, Rayleigh–Jeans radiation law and Fresnel reflection equations were used and complex permittivity was calculated by Blanch and Aguasca model. Input data for the model calculations are temperature and salinity that were provided from World Ocean Atlas 2013. The calculations showed that the brightness temperature distribution in the Persian Gulf experiences significant spatial and seasonal variations. At nadir incidence angle, the vertical and horizontal components of brightness temperature over the Persian Gulf vary in the range 90.5–96.5 °k and 85.5–90.2 °k, respectively. At off-nadir incidence angle, the temporal variability pattern of brightness temperature horizontal polarization is similar to its vertical counterpart but the difference between them significantly increases.

A noval method of constructing shallow water sound speed profile based on dynamic characteristic of internal tides

In order to provide constraint to the number of inversion parameters, sound speed profile is often modeled by empirical orthogonal functions (EOFs). However, the EOF method, which is dependent on the sample data, is often difficult to apply due to insufficient real-time <i>in-situ</i> measurements. In this paper, we present a novel basis for reconstructing the sound speed profile, which can be obtained by using historical data without real-time sample. By deducing the dynamic equations and the state function of water particle, the hydrodynamic mode bases (HMBs) can be calculated from historical data without real-time in-situ measurement, and a method of constructing the sound speed profile is established by using the dynamic characteristics of seawater. The use of the World Ocean Atlas 2013 (WOA13) can obtain the seasonal profiles of temperature and salinity, and then the HMB which represents the dynamic characteristic of internal tides is obtained and analyzed. Unlike EOF, the HMB and its projection coefficients are directly related to the sea dynamic features and have a more explicit physical meaning. According to the orthogonality analysis of hydrodynamic mode, the first-order coefficient can be used to describe the depth change of sound speed iso-lines and the second-order coefficient can be used to describe the change of sound speed gradient. Based on the conductance-temperature-depth profiles and broadband data from underwater explosion collected in the East China Sea experiment of the Asian Seas International Acoustic Experiment, the HMB is tested and compared with the EOF in the sound speed profile reconstruction and matched field tomography. The results show that the sound speed profile in shallow water area can be expressed by the HMB with proper precision. By means of the conventional matched field tomography, the valid sound speed profile can also be obtained in the form of HMB coefficients. The result of transmission loss prediction and tomography from HMB are as good as those from EOF, while the HMB has less dependent on real-time <i>in-situ</i> measurement. The HMB is easy to obtain and closely related to the physical characteristics of seawater, it can be used as an efficient alternative to EOF for monitoring the marine dynamic phenomena in sea areas with insufficient real-time in-situ measurement.

Non-redfieldian mesopelagic nutrient remineralization in the eastern North Atlantic subtropical gyre

The eastern boundary of the North Atlantic subtropical gyre (NASTG) receives organic materials produced in the nearby Canary upwelling system (CanUS), which has important implications for the metabolic balance of the region. Here we computed absolute geostrophic, Ekman and eddy diffusive transports and net budgets of oxygen, nitrate, and phosphate, with an inverse model applied to annual climatologies from the World Ocean Atlas 2013, in a large box covering the eastern boundary of the NASTG. The surface waters (<100 m) of the box exported 4.2±1.1 mol O2 m−2 yr−1 to the adjacent ocean and atmosphere, and imported 0.40±0.17 mol N m−2 yr−1 and 22.2±9.5 mmol P m−2 yr−1 of nitrate and phosphate, respectively, indicating that net autotrophy prevailed in the euphotic waters of the eastern NASTG. The central, intermediate and deep waters (100 m-bottom) imported 6.7±3.3 mol O2 m−2 yr−1, and exported 1.05±0.35 mol N m−2 yr−1 and 50.2±21.8 mmol P m−2 yr−1 of nitrate and phosphate, respectively, indicative of net remineralization. Mesopelagic remineralization rates exceeded epipelagic net community production, implying a net deficit of carbon and nutrients in the region. Remineralization occurred at high N:P molar ratios (∼28) in central waters, and thus, a excess nitrate (DINxs = NO3-16·PO4) was produced at a rate of 2.2±1.6×1011 mol N yr−1. This excess nitrate was exported to the gyre interior, influencing the biogeochemistry of the whole NASTG and leaving the eastern margin with a strong nitrogen deficit, relative to carbon and phosphorus. According to the available literature, the organic carbon export from the CanUS could be sufficient to account for the carbon deficit, but the source of nitrogen remains elusive.

Baroclinic Tides Simulation in the Red Sea: Comparison to Observations and Basic Characteristics

AbstractThe baroclinic tides in the Red Sea are simulated using a three‐dimensional, nonhydrostatic, high‐resolution Massachusetts Institute of Technology general circulation model. Various observations have been used to validate the simulation results. A good match between the model results and observations from five tidal gauges has been obtained. Tidal amplitude and phase data from 21 tidal stations present high correlation coefficients and low deviations with the model results. Comparisons between model and Oregon State University Tidal Inversion Software data suggest consistent results, with only small discrepancies at the locations of the amphidromic points. Tidal currents from four mooring observations are in good agreement with the simulation results, with discrepancies appearing in shallow areas and those with complex bottom topography. Based on the simulation results, the basic characteristics of baroclinic tides in the Red Sea are analyzed. The properties of barotropic tides, and distribution of the forcing function parameter, indicate that the baroclinic tides are generated mainly in four areas: the Bab‐el‐Mandeb (BAM) Strait, the southern Red Sea, the Gulf of Suez, and the Strait of Tiran. This is confirmed by the spatial distributions of baroclinic tidal kinetic energy and energy flux. The properties of the conversion rate from barotropic tides to baroclinic tides, and the divergence of baroclinic energy flux, further reveal quantitatively that the southern Red Sea features the most of the generated baroclinic energy. The majority of the baroclinic energy disappears within the four areas, either dissipating due to friction and bottom drag or converting back to barotropic energy.

Partitioning Ocean Motions Into Balanced Motions and Internal Gravity Waves: A Modeling Study in Anticipation of Future Space Missions

AbstractInternal gravity waves (IGWs) and balanced motions (BMs) with scales &lt;100‐km capture most of the vertical velocity field in the upper ocean. They have, however, different impacts on the ocean energy budget, which explains the need to partition motions into BMs and IGWs. One way is to exploit the synergy of using different satellite observations, the only observations with global coverage, and a reasonable spatial and temporal resolution. But we need first to characterize and understand their signatures on the different surface oceanic fields. This study addresses this issue by using an ocean global numerical simulation with high‐resolution (1/48°). Our methodology is based on the analysis of the 12,000 frequency‐wave number spectra to discriminate these two classes of motions in the surface kinetic energy, sea surface height, sea surface temperature, sea surface salinity, relative vorticity, and divergence fields and for two seasons. Results reveal a complex picture worldwide of the partition of motions between IGWs and BMs in the different surface fields, depending on the season, the hemisphere, and low and high eddy kinetic energy regions. But they also highlight some generic properties on the impact of these two classes of motions on the different fields. This points to the synergy of using present and future satellite observations to assess the ocean kinetic energy on a global scale. The 12,000 frequency‐wave number spectra represent a World Ocean Atlas of the surface ocean dynamics not fully exploited in the present study. We hope the use of this World Ocean Atlas by other studies will lead to extend much these results.

The Global Mode‐2 M2 Internal Tide

AbstractThe mode‐2 M2 internal tide is observed from satellite altimetry. It is extracted in two steps: First, the mode‐2 component is separated from modes 1 and 3 by a bandpass filter with cutoff wavelengths of [0.85 1.35]×λ2, where λ2 is the mode‐2 wavelength; and second, three mode‐2 internal tidal waves are extracted by fitting plane waves in each 120 km by 120 km window. The satellite‐observed mode‐2 M2 internal tide underestimates its strength: It contains the phase‐locked component only, missing the time‐varying component; it contains the southbound/northbound component only, missing the eastbound/westbound component. The satellite results provide rich information on the global mode‐2 M2 internal tide. The results show that the mode‐2 M2 internal tide is ubiquitous in the ocean, and its sea surface height amplitudes are O (1 mm). The spatial patterns of mode‐1 and mode‐2 internal tides are very different. Mode 1 mainly originates at steep topographic features such as submarine ridges, but mode 2 is also generated at gentler topographic features such as abyssal seamounts and fracture zones. Mode‐1 beams propagate O (1,000 km), while mode‐2 beams can be tracked for O (100 km). Depth‐integrated energy and flux are calculated using sea surface height amplitudes and conversion functions built from the World Ocean Atlas 2013. The globally integrated energy of the mode‐2 M2 internal tide approximates to 8 PJ, about 22% of mode 1 (36.4 PJ). This work suggests that higher‐mode internal tides play an important role in the tidal energetics, in particular, over abyssal seamounts and fracture zones.

Freshwater structure and its seasonal variability off western Patagonia

Climatological hydrographic data for the southeastern Pacific from the World Ocean Atlas show an extended low-salinity region associated with high rainfall/river runoff along the Chilean Patagonia coast. However, the structure and variability of this extensive freshwater area is poorly understood due to the lack of periodic hydrographic observations. Here, we use 15 years of satellite observations in combination with hydrographic data from Argo profiling floats to increase our understanding of the freshwater structure and its seasonal variability off western Patagonia. Satellite-derived seasonal fields of surface salinity from Aquarius/SAC-D and SMAP (Soil Moisture Active Passive) satellite missions reveal a prominent coastal band of low-salinity water year-round (fresher during spring and summer). Hydrographic profiles from Argo floats confirm this coastal freshening. The major content of freshwater is located off central Patagonia, where the low-salinity surface layer dominates the stratification of the upper ocean. In contrast, the freshwater content plays a minor role in setting up the stratification off northern and southern Patagonia. A warm surface layer greatly increases the stability of the upper 500 m in the northern sector (especially in summer and fall), whereas cold temperatures in the southern region tend to damp zonal density gradients and decrease the stratification. Future studies at interannual scales are suggested considering the melting of Patagonian icefields since the 1960s.

Technical note: Two types of absolute dynamic ocean topography

Abstract. Two types of marine geoid exist with the first type being the average level of sea surface height (SSH) if the water is at rest (classical definition), and the second type being satellite-determined with the condition that the water is usually not at rest. The differences between the two are exclusion (inclusion) of the gravity anomaly and non-measurable (measurable) in the first (second) type. The associated absolute dynamic ocean topography (referred to as DOT), i.e., SSH minus marine geoid, correspondingly also has two types. Horizontal gradients of the first type of DOT represent the absolute surface geostrophic currents due to water being at rest on the first type of marine geoid. Horizontal gradients of the second type of DOT represent the surface geostrophic currents relative to flow on the second type of marine geoid. Difference between the two is quantitatively identified in this technical note through comparison between the first type of DOT and the mean second type of DOT (MDOT). The first type of DOT is determined by a physical principle that the geostrophic balance takes the minimum energy state. Based on that, a new elliptic equation is derived for the first type of DOT. The continuation of geoid from land to ocean leads to an inhomogeneous Dirichlet boundary condition with the boundary values taking the satellite-observed second type of MDOT. This well-posed elliptic equation is integrated numerically on 1∘ grids for the world oceans with the forcing function computed from the World Ocean Atlas (T, S) fields and the sea-floor topography obtained from the ETOPO5 model of NOAA. Between the first type of DOT and the second type of MDOT, the relative root-mean square (RRMS) difference (versus RMS of the first type of DOT) is 38.6 % and the RMS difference in the horizontal gradients (versus RMS of the horizontal gradient of the first type of DOT) is near 100 %. The standard deviation of horizontal gradients is nearly twice larger for the second type (satellite-determined marine geoid with gravity anomaly) than for the first type (geostrophic balance without gravity anomaly). Such a difference needs further attention from oceanographic and geodetic communities, especially the oceanographic representation of the horizontal gradients of the second type of MDOT (not the absolute surface geostrophic currents).

Error Characterization of Sea Surface Salinity Products Using Triple Collocation Analysis

The triple collocation (TC) technique allows the simultaneous calibration of three independent, collocated data sources, while providing an estimate of their accuracy. In this paper, the TC is adapted to validate different salinity data products along the tropical band. The representativeness error (the true variance resolved by the relatively high-resolution systems but not by the relatively low-resolution system) is accounted for in the validation process. A method based on the intercalibration capabilities of TC is used to estimate the representativeness error for each triplet, which is found to impact between 15% and 50% the error estimation of the different products. The method also sorts the different products in terms of their resolving spatiotemporal scales. Six salinity products (sorted from smaller to larger scales) used were: the in situ data from the Global Tropical Moored Buoy Array (TAO), the GLORYS2V3 ocean reanalysis output provided by Copernicus, the satellite-derived Aquarius Level 3 version 4 (AV4) and Soil Moisture and Ocean Salinity (SMOS) objectively analyzed (SOA) maps, and the climatology maps provided by the World Ocean Atlas (WOA). This calibration study is limited to the year 2013, a year when all the products were available. This validation approach aims to assess the quality of the different salinity products at the satellite-resolved spatiotemporal scales. The results show that, at the AV4 resolved scales, the Aquarius product has an error of 0.17, and outperforms TAO, GLORYS2V3, and the SOA maps. However, at the SOA resolved scales (which are coarser than those of the Aquarius product because of the large OA correlation radii used), the SMOS product has an error of 0.20, slightly lower than that of GLORYS2V3, Aquarius, and TAO. The WOA products show the highest errors. Higher order calibration may lead to a more accurate assessment of the quality of the climatological products.

Available Potential Energy in Density Coordinates

AbstractThe vertically integrated potential energy of an incompressible stratified fluid formulated in density coordinates can be simply written as a weighted vertical sum of the squares of the vertical displacements of density surfaces, a general expression valid for arbitrary displacements. The sum of this form of potential energy and kinetic energy is then a conserved quantity for the multilayer shallow water model. The formulation in density coordinates is a natural one to find the Lorenz reference state of available potential energy (APE). We describe the method to compute the APE of an ocean state and provide two applications. The first is the classical double-gyre, wind-driven circulation simulated by a shallow water model at high resolution. We show that the eddy kinetic and eddy potential energies are localized in regions of large gradients of mean APE. These large gradients surround an APE minimum found between the two gyres. The second is the time-mean World Ocean Circulation reconstructed from hydrography (World Ocean Atlas) and reference velocities at 1000 db from the Argo float program to obtain an absolute circulation. The total available potential energy exceeds the total mean kinetic energy of the World Ocean by three orders of magnitude, pointing out the very small Burger number of the circulation. The Gulf Stream, the Kuroshio, the Agulhas retroflection, and the confluence regions are four examples that confirm the shallow water model results that large gradients of mean available potential energy can be used as predictors for the presence of high eddy kinetic energy (obtained here from satellite altimetry).