Contribution of a constellation of two Wide-Swath Altimetry Missions to Global Ocean Analysis and Forecasting

Swath altimetry is likely to revolutionize our ability to monitor and forecast ocean dynamics. To meet the requirements of the EU Copernicus Marine Service, a constellation of two wide-swath altimeters is envisioned for the long-term (post-2030) evolution of the Copernicus Sentinel 3 topography mission. A series of Observing System Simulation Experiments is carried out to quantify the expected performances. The OSSEs use a state-of-the-art high resolution (1/12°) global ocean data assimilation system similar to the one used operationally by the Copernicus Marine Service. Flying a constellation of two wide-swath altimeters will provide a major improvement of our capabilities to monitor and forecast the oceans. Compared to the present situation with 3 nadir altimeters flying simultaneously, the Sea Surface Height analysis and 7-day forecast error will be globally reduced by about 50 %. With two wide-swath altimeters, the quality of Sea Surface Height 7-day forecasts is equivalent to the quality of SSH analysis errors from three nadir altimeters. Our understanding of ocean currents is also greatly improved (30 % improvements at the surface and 50 % at 300 m depth). The resolution capabilities will be drastically improved and will be closer to 100 km wavelength compared to about 250 km today. Flying a constellation of two wide-swath altimeters thus looks to be a very promising solution for the long-term evolution of the Sentinel 3 constellation and the Copernicus Marine Service.

Do Winds Control the Confluence of Subtropical and Subantarctic Surface Waters East of New Zealand?

<p>The confluence region east of New Zealand is one of only a few places in the world where the Antarctic Circumpolar Current meets the strong southwardflowing boundary current of a subtropical gyre. The convergence of subtropical and subantarctic water creates strong fronts. The fronts have clear signatures in height and temperature that make them appropriate places to investigate ocean/climate variability. The location and extent of the New Zealand confluence should respond to changes in large-scale wind patterns, as changes in South Pacific currents have been linked to wind shifts. However, recent studies have shown that highly energetic eddies, local winds, and the bathymetry may be significant controls of currents and associated fronts. This thesis investigates the temporal and spatial variability of the confluence and evaluates its response to variability in South Pacific winds. Analysis of the 18-year time series, from January 1993 to December 2010, of sea surface height mapped from satellite altimetry was used to investigate the location and extent of fronts and the eddy activity and relate these to the wind forcing. Wind stress data were used with the Island Rule to estimate the winddriven transport of the western boundary currents that feed the confluence. In addition, the climate modes Southern Annular Mode (SAM) and Southern Oscillation Index (SOI) were used to examine the influence of the principal modes of atmospheric variability. Time series of the local wind stress curl and local climate indices were calculated and compared to the intensity of the confluence to test any influence of local forcing. In addition, bathymetric effects were investigated by evaluating evidence for preferred front locations near topographic features. Sea level anomalies in the confluence region are increasing at 3.4 cm decade⁻¹. The sea surface height gradients and the eddy kinetic energy are also increasing at a rate of 0.01 cm km⁻¹ and 23 cm² s⁻² per decade respectively, indicating an intensification of the fronts and eddy activity in the confluence. There is a high and significant correlation (r = 0.84) between the front and eddy signals reflecting baroclinic instabilities inherent in the fronts. Difference in transport anomalies across the confluence derived from the Island Rule are also increasing at 8.8 Sv decade⁻¹. SAM and SOI indices showed little or no correspondence with variability in the confluence intensity and eddy kinetic energy, and the same lack of correspondence was observed in local winds and local indices. While these results suggest a connection between the variability in the confluence and South Pacific winds, there is a preferential location of the strongest fronts and eddy activity northeast of Bounty Plateau and Bollons Seamount, indicating some bathymetric control. The correspondence between basin-scale winds and sea surface height gradients in the confluence region indicates that if wind stress continues to increase, as current trends predict, front intensity and eddy activity will also increase, enhancing the transfer of heat and nutrients that, respectively, influence energy transfer and biological productivity.</p>

Do Winds Control the Confluence of Subtropical and Subantarctic Surface Waters East of New Zealand?

<p>The confluence region east of New Zealand is one of only a few places in the world where the Antarctic Circumpolar Current meets the strong southwardflowing boundary current of a subtropical gyre. The convergence of subtropical and subantarctic water creates strong fronts. The fronts have clear signatures in height and temperature that make them appropriate places to investigate ocean/climate variability. The location and extent of the New Zealand confluence should respond to changes in large-scale wind patterns, as changes in South Pacific currents have been linked to wind shifts. However, recent studies have shown that highly energetic eddies, local winds, and the bathymetry may be significant controls of currents and associated fronts. This thesis investigates the temporal and spatial variability of the confluence and evaluates its response to variability in South Pacific winds. Analysis of the 18-year time series, from January 1993 to December 2010, of sea surface height mapped from satellite altimetry was used to investigate the location and extent of fronts and the eddy activity and relate these to the wind forcing. Wind stress data were used with the Island Rule to estimate the winddriven transport of the western boundary currents that feed the confluence. In addition, the climate modes Southern Annular Mode (SAM) and Southern Oscillation Index (SOI) were used to examine the influence of the principal modes of atmospheric variability. Time series of the local wind stress curl and local climate indices were calculated and compared to the intensity of the confluence to test any influence of local forcing. In addition, bathymetric effects were investigated by evaluating evidence for preferred front locations near topographic features. Sea level anomalies in the confluence region are increasing at 3.4 cm decade⁻¹. The sea surface height gradients and the eddy kinetic energy are also increasing at a rate of 0.01 cm km⁻¹ and 23 cm² s⁻² per decade respectively, indicating an intensification of the fronts and eddy activity in the confluence. There is a high and significant correlation (r = 0.84) between the front and eddy signals reflecting baroclinic instabilities inherent in the fronts. Difference in transport anomalies across the confluence derived from the Island Rule are also increasing at 8.8 Sv decade⁻¹. SAM and SOI indices showed little or no correspondence with variability in the confluence intensity and eddy kinetic energy, and the same lack of correspondence was observed in local winds and local indices. While these results suggest a connection between the variability in the confluence and South Pacific winds, there is a preferential location of the strongest fronts and eddy activity northeast of Bounty Plateau and Bollons Seamount, indicating some bathymetric control. The correspondence between basin-scale winds and sea surface height gradients in the confluence region indicates that if wind stress continues to increase, as current trends predict, front intensity and eddy activity will also increase, enhancing the transfer of heat and nutrients that, respectively, influence energy transfer and biological productivity.</p>

Defining Southern Ocean fronts using unsupervised classification

Oceanographic fronts are transitions between thermohaline structures with different characteristics. Such transitions are ubiquitous, and their locations and properties affect how the ocean operates as part of the global climate system. In the Southern Ocean, fronts have classically been defined using a small number of continuous, circumpolar features in sea surface height or dynamic height. Modern observational and theoretical developments are challenging and expanding this traditional framework to accommodate a more complex view of fronts. Here, we present a complementary new approach for calculating fronts using an unsupervised classification method called Gaussian mixture modelling (GMM) and a novel inter-class parameter called the I-metric. The I-metric approach produces a probabilistic view of front location, emphasising the fact that the boundaries between water masses are not uniformly sharp across the entire Southern Ocean. The I-metric approach uses thermohaline information from a range of depth levels, making it more general than approaches that only use near-surface properties. We train the GMM using an observationally constrained state estimate in order to have more uniform spatial and temporal data coverage. The probabilistic boundaries defined by the I-metric roughly coincide with several classically defined fronts, offering a novel view of this structure. The I-metric fronts appear to be relatively sharp in the open ocean and somewhat diffuse near large topographic features, possibly highlighting the importance of topographically induced mixing. For comparison with a more localised method, we also use an edge detection approach for identifying fronts. We find a strong correlation between the edge field of the leading principal component and the zonal velocity; the edge detection method highlights the presence of jets, which are supported by thermal wind balance. This more localised method highlights the complex, multiscale structure of Southern Ocean fronts, complementing and contrasting with the more domain-wide view offered by the I-metric. The Sobel edge detection method may be useful for defining and tracking smaller-scale fronts and jets in model or reanalysis data. The I-metric approach may prove to be a useful method for inter-model comparison, as it uses the thermohaline structure of those models instead of tracking somewhat ad hoc values of sea surface height and/or dynamic height, which can vary considerably between models. In addition, the general I-metric approach allows front definitions to shift with changing temperature and salinity structures, which may be useful for characterising fronts in a changing climate.

The application of coupled 3d hydrodynamic and oil transport model to oil spill incident in karawang offshore, indonesia

This study presents a coupled hydrodynamic and oil transport numerical model to study the spread of Karawang oil spills at sea due to well-kick failures. This model uses the 3D configuration of ROMS-CROCO in the Java Sea. The model has a resolution of 1 km, 25 vertical layers, and runs from January 2019 to September 2019. Temperature, salinity, sea surface height, ocean currents, and harmonic tides are derived from global models and applied to open boundaries. Hourly atmospheric datasets during model simulation are forced as flux input in the surface. The 3D profile of the flow generated by the model is converted to the GNOME oil transport model format as mover type input to disperse the oil. The hydrodynamic model shows that the result has a good agreement with in-situ data and observation with mean of correlation exceeding r>0.8 for sea surface height and sea surface temperature. Compared with radar satellites, oil spill dispersion shows the same scattered trend as satellite data. Backward modelling shows oil particles returning to the initial spill location. The oil spill was moving westward, and some are stranded on the coast between Karawang and Bekasi.

Characteristics of Ichthyoplankton Communities and Their Relationship With Environmental Factors Above the Ninety East Ridge, Eastern Indian Ocean

The Ninety East Ridge is a submarine north–south oriented volcanic ridge in the eastern Indian Ocean. Surface-layer ichthyoplankton collected in this area from September to October were identified by combined morphological and molecular (DNA barcoding) techniques, and their species composition, diversity, and abundance, and correlations with environmental variables were described. Collections comprised 109 larvae and 507 eggs, which were identified to 37 taxa in 7 orders, 20 families, and 27 genera, and were dominated by the order Perciformes and species Vinciguerria sp., Oxyporhamphus micropterus, and Decapterus macarellus. Species abundances at each station and of each species were relatively low, suggesting that this area or the time of sampling were not of major importance for fish spawning. Waters above Ninety East Ridge had lower species diversity but higher species richness than waters further offshore. A generalized additive model revealed that high abundance of ichthyoplanktonic taxa occurred in areas with low sea surface height and high sea surface salinity, temperature, and chlorophyll a concentration. Of these, sea surface height was most correlated with ichthyoplankton abundance. We provided baseline data on surface-dwelling ichthyoplankton communities in this area to aid in development of pelagic fishery resources in waters around the Ninety East Ridge.