Chapter 1 - Fundamental Knowledge

Early attempts at understanding scintillations in sound transmission through the ocean assumed that ocean fine structure is homogeneous and isotropic. It is, in fact, dominated by internal waves, which are neither homogeneous nor isotropic. Sound Transmission Through a Fluctuating Ocean (Flatté et al., Cambridge University Press, Cambridge, England 1979) combined recently developed internal-wave models with path-integral methods to predict the fluctuations of resolved acoustic multipaths. At that time, Ocean Acoustic Tomography (Munk et al., Cambridge University Press, Cambridge, England 1995), which uses ray travel times to determine large-scale ocean structure, had just been proposed. The wideband acoustic sources and receivers required to make travel-time measurements for tomography also provided the technology needed to quantify acoustic fluctuations. Conversely, internal-wave-induced scattering limits travel-time measurement precision. Tomography experiments at 25-km range provided some of the earliest tests of path-integral predictions. Subsequent tomography experiments provided data at ever-increasing ranges and decreasing frequencies. The Acoustic Thermometry of Ocean Climate project made measurements at megameter ranges with 1400-m vertical line array receivers. The next step is a modular distributed VLA capable of spanning the full water column that is under development to enable separation of acoustic modes using spatial filtering and to fully characterize deep-water acoustic time fronts.

Ocean Acoustic Tomography (OAT) is an efficient and economical marine acoustic observation technique. Targeted observation is an appealing procedure to reduce the uncertainty of ocean environment prediction through additional observation. This study aimed to assess the validity of OAT as an observation method for targeted observation. OAT based on Niche Genetic Algorithm was employed to extract sound speed and temperature profiles from acoustic transmission time, utilizing data from the 2019 Yellow Sea experiment. The inversion results were compared with measurement data, which are found to be accurate and reliable. To further evaluate OAT as targeted observation method, the vertical bias structure of OAT was added on synchronous measurement data in the sensitive area of targeted observation to simulate OAT observation in sensitive area. This simulated data was then incorporated into a 3D-Var assimilation system to improve the short-term prediction of the target region. Comparing the predictions derived with the measurement data at the verification time, it shows that the simulated OAT observation improved the quality of target region prediction, indicating that OAT can be an effective observation method for targeted observation. An Observing System Simulation Experiment was conducted to assess the impact of OAT characteristics on prediction improvement. The results show that both adding observation nodes and extending the observation duration have positive effects, while extending the observation duration performs better.

The North Atlantic Ocean has a crucial role in the regional and global climate, with significant socio-economic impacts related to droughts, hurricanes and changes in the weather pattern. The Atlantic Meridional Overturning Circulation (AMOC) is the main component of ocean heat transport to the North Atlantic as part of the Earth’s major circulation system. The AMOC is considered a tipping element of the planet’s climate, and abrupt and strong changes in this global-scale circulation could lead to shifts in European and North American climates and warming in the Southern Ocean and Antarctica. Hence, it is critical to better comprehend the drivers influencing the AMOC state and its variability.   The direct connection between the AMOC and the deep-water formation is being challenged by recent studies, suggesting this relationship cannot explain alone the broad spectrum of AMOC variations. Moreover, the Gulf Stream is responsible for a substantial part of meridional heat transport and is considered part of the AMOC system in the North Atlantic. Recent studies show the latitudinal displacement of the current could affect the AMOC variability. Furthermore, the advection at intermediate level (800-1200m) of the Mediterranean outflow is suggested to be a possible key contributor to the strength and variability of the AMOC, although large uncertainty remains concerning the pathway of these waters into the North Atlantic and its contribution in the precondition phase of the deep-water formation in the Labrador and Greenland Island Norwegian sea. Understanding the influence of each of these drivers is vital.   The scarcity of direct observational records in space and time poses an important challenge to disentangle the AMOC drivers and their variability in the North Atlantic, especially since the North Atlantic Ocean presents unusual variability at decadal and multi-decadal scales. Previous studies have used either short observation records, low-resolution numerical models or indirect (proxy) observations to investigate the AMOC. Long-term and three-dimensional datasets are needed to understand the ocean’s changes at large space and time scales. Therefore, the aim of our work is to investigate the multi-drivers of the AMOC variability using a large ensemble (32 members) of global historical ocean reanalyses covering the period from 1961 to 2022. We focus on the deep-water formation, Gulf Stream shift and Mediterranean Outflow Water (MOW) as multi-drivers of the AMOC in the North Atlantic. It is the first time that a large ensemble of 60-year observation-based ocean dataset is used to investigate multiple drivers responsible for changes in the AMOC and the North Atlantic’s decadal climate. We calculated indexes for each driver and used multivariate regression methods (e.g. LASSO) to explore the role of multi-drivers. Preliminary results show that at the interannual time scale, the variability of the Gulf Stream is well correlated with the AMOC variability and at a longer time scale, the deep-water formation and Mediterranean outflow have a close correlation with the AMOC transport. Further investigation is ongoing to quantify the relative contribution from these drivers to the AMOC variability at different time scales.

Late Pleistocene deep-water circulation in the subantarctic eastern Atlantic

Ocean acoustic tomography (OAT) is a method proposed by Munk and Wunsch (1979) to observe the ocean using acoustic transmissions. Although the ocean is nearly opaque to most electromagnetic radiation, acoustic waves propagate well, and low frequencies (less than 100 Hz) can propagate usefully for more than 10 megameters (Munk, Worcester, and Wunsch, 1995). The term "tomography" was chosen to highlight the analogy to medical computed axial tomography (CAT), although because ocean instrumentation and access are expensive, the number of data available in OAT has been far smaller than in medical applications. For this reason, the reconstruction of the unknown field depends critically on utilizing all information, including prior guesses of first and second moment statistics of the data and unknowns and, in some cases, dynamical models of the ocean.

An ocean environment monitoring scheme is designed to be placed in South China Sea, aiming at mesoscale eddy observation. In this paper, we use acoustic moorings combined with PIES (pressure inverted echo sounder) array to inverse three-dimensional sound speed field to infer the structure of mesoscale eddy. Ocean Acoustic Tomography (OAT) with acoustic moorings is widely used for inversion. Single Empirical Orthogonal Function Reconstruction (sEOF-R) is used for inversion with PIES array. We combine them together using convex optimization for empirical orthogonal function parameters inversion. The simulation results show that our proposed method achieves lower RMSE and performs better in the structure estimation of mesoscale eddy than methods with only one kind of equipment and existing methods to combine data from different kinds of equipment.

A tomography system consisting of aircraft-deployed shots as sources and a vertical line array moored by a receiver ship was used to survey a large area (120×120 km) where a mesoscale warm eddy appears frequently. The position of the shots was measured by the GPS on the aircraft and the detonation time (or depth) was monitored by sonobuoys which had been deployed close to the shot position. Results show that individual multipaths between a shot and the receiver contain substantial information for a vertical slice of the ocean with a warm eddy. With high accuracy, GPS positioning of shot locations, and receiver position, this rapidly deployable system can be used for monitoring the submerged structure of the warm eddy during the period of diffusive SST in summer in the East Sea of Korea. [Work supported by KOSEF.]

Water temperature, salinity concentration, current velocity distribution, etc. in the ocean, are very important physical properties to understand the thermal structure of the ocean. The monitoring of these physical properties could be achieved by either of two kinds of measurement schemes, i.e., one the direct measurements using instruments such as ARGO floats and the other the indirect method using some other parameters such as the sound speed. Ocean acoustic tomography (OAT) was invented to verify the possibility of the latter. Although analysis based on the travel time of sound inversion using the inversion technique (TTI) has been attempted based on the ray theory, it is expected to improve both the resolution and the accuracy of the measurements. In the ocean, sound waves travel through complex structure where the contrast in the velocity could be of the order of less than a percent. Therefore, the application of TTI may not produce satisfactory results. We first state the following two hypotheses to propose a method to improve OAT: 1) full waveform inversion (FWI) could improve the resolution, 2) attenuation is likely to be considered in the velocity inversion. The application of FWI to synthetic examples to test these hypothesis provided us pretty satisfactory results and the validity of our hypothesis have been verified.

It came as a great shock in the 1960s that the oceans, like the atmosphere, have an active weather (i.e. ocean storms are called eddies). The traditional expedition mode of individual research vessels making independent measurements was no longer adequate. Ocean Acoustic Tomography was developed in direct response to the ‘eddie revolution’. Sound travels faster in warmer water; acoustic waves transmitted from ship to ship give information about the temperature and currents in the intervening waters. The transmission scale has widened over the years from 100 to 1000 to 10 000 km, approaching the antipodal scale of Ewing’s 1960 transmission from Perth (Australia) to Bermuda. In 1991 we successfully transmitted from a source ship on Heard Island in the Indian Ocean to receiver ships in the north and south Atlantic and Pacific Oceans. Brian Dushaw is planning to repeat Ewing’s experiment; he expects a reduction in travel time of approximately 10 s as confirmation of global ocean warming over the last 50 yr.Sea level rose 15 cm in the 20th century. The rate has since doubled; values up to 2 m by 2100 are now being quoted. To make accurate predictions we must understand the melting processes of continental ice sheets. Floating ice sheets from Antarctic and Greenland glaciers cover huge ocean caverns that have not to date been accessible to observation. We propose probing these caverns with sound waves to study the ocean dynamics at the underside of the floating ice sheets: a daring venture.

At subduction zones, cold lithospheric plates dive deep into the hotter Earth’s mantle. Earthquakes can occur at depths of hundreds of kilometers in these cold subducted slabs, apparently related to their thermal structures. Seismic tomography provides a first-order information on slab morphology but cannot discriminate « cold » from « warm » slabs partly due to the inhomogeneous repartition of seismic sources and surface sensors. This study investigates the potential of the gravity data from the GOCE mission to infer deep slabs’ inner thermal structures (> 200 km depth). Thermal structures of slabs with various morphologies are derived from dynamic subduction zones models. We convert temperature field into density assuming mineralogical phases at thermodynamical equilibrium for pyrolite mantle using HeFESTo model (Stixrude and Lithgow-Bertelloni 2011). We then use the freeware DynG3 (Cadio et al. 2011) to predict surface and CMB deflections due to slab dynamic sinking – depending on the radial mantle viscosity – and calculate the corresponding synthetic signals (geoid, gravity disturbance, gravity gradients). Our parametric study considers various radial mantle viscosity profiles, slab morphologies and slabs inner thermal structures (SITS). As expected, geoid and gravity gradients are sensitive to density anomalies at different depth ranges. We highlight linear relationships between both these signal for a given viscosity profile and a given slab’s morphology : First, the colder an isothermal slab, the higher the geoid and gravity gradients anomalies. Second, for a given shallow temperature, the colder the deep slab (>500 km), the lower the gravity gradient anomaly and the higher the geoid anomaly. This last, counter-intuitive, result is explained by the fact that the long wavelength component associated to deep density anomaly overprints, for colder slabs, the short wavelength component associated to surface deflection. Thus, for a known viscosity profile and slab morphology, both shallow (~ 200-500 km depth) and mean slab thermal structures could be inverted from the combination of geoid and gravity gradients anomalies.

The atmosphere and ocean communicate via the air-sea interface through the exchange of heat, mass, and momentum. This exchange is maximized in stormy regions such as western boundary currents, like the Gulf Stream in the North Atlantic Ocean. The Gulf Stream experiences frequent storms, many of which occur in the wintertime, bringing strong winds and cold air masses that invigorate exchange. This dissertation focuses on novel in-situ observations collected by Uncrewed Surface Vehicles (USVs) to study the bulk air-sea exchange of carbon dioxide (CO2) and heat in the Gulf Stream region. Much of our knowledge of global ocean CO2 uptake is from the analysis of in-situ ocean CO2 data stored in large repositories (with minimal wintertime observations) and machine learning models that use these data try to reconstruct global ocean CO2 uptake. In Chapter 1, we analyze new observations collected in the Gulf Stream region by a USV in February 2019 in comparison to observations collected by a research vessel in January 2006. The sea surface temperatures (SSTs) progressively cooled on the south side of the Gulf Stream, leading the surface waters there to be undersaturated in the partial pressure of carbon dioxide (pCO2) relative to the atmosphere. This undersaturation is not captured in coarse ocean pCO2 reconstructions that are based on sparse data. We also identify that the progressive cooling of SSTs does not explain the resulting ocean pCO2 signature alone. Pairing the USV observations with those collected by a profiling Argo float, we determined that the USV recorded active subtropical mode water formation by detecting the highest ocean pCO2 values on the subtropical side of the Gulf Stream corresponding to some of the coolest temperatures, likely the result of deep vertical mixing of previous years subtropical mode water. We show that the Gulf Stream is an ideal spot for the ocean uptake of large quantities of atmospheric CO2 because of the cooling temperatures, strong winds, and low Revelle factor. Next, we evaluate the importance of collecting all of the parameters necessary to calculate the CO2 exchange on the same platform, as opposed to using a combination of in-situ and data-based products. Chapter 2 uses data collected by five USVs in the Gulf Stream region to test this. We first evaluate the quality of the data between the individual USV platforms and against a regularly maintained mooring array consisting of three surface moorings that measure the same parameters as

Atmospheric water vapor fluxes were derived from a 1‐year data set of horizontal wind speed and specific humidity assimilated from meteorological observations by the European Center for Medium‐Range Weather Forecast (ECMWF). Vertically integrated horizontal freshwater fluxes were compared to those of two data sets based on a climatology [Oort, 1983] and on simulations with an atmospheric general circulation model (AGCM). Zonal transports agree fairly well at all latitudes outside the tropics, where fluxes are about double for the AGCM data set. Meridional fluxes of the AGCM and ECMWF data sets show close agreement, while the climatological fluxes are generally smaller with a considerable northward shift in the southern hemisphere. Atmosphere‐to‐ocean freshwater fluxes were derived from the three data sets. Not only is there substantial disagreement between the data sets, but their zonal averages over the Atlantic, Pacific, and Indian Ocean basins show little resemblance to the respective restoring freshwater fluxes from a 2‐dimensional ocean model. If the ocean model is forced with the observed and modeled atmospheric fluxes, we find that the mode of ocean circulation is determined mostly by the net flux to the high‐latitude oceans and the amount of freshwater exported from the Atlantic basin. The latitudinal structure of the freshwater fluxes in low‐latitudes and midlatitudes has little influence on the modeled thermohaline circulation. The fluxes derived from the climatology and ECMWF permit North Atlantic Deep Water (NADW) formation, but a strong freshwater input to the Southern Ocean inhibits Antarctic Bottom Water formation. The AGCM transports so much moisture to the Arctic Ocean that NADW formation is shut down, resulting in a ocean circulation mode of southern sinking in all three ocean basins. If NADW is formed in the model, the strength of the Atlantic meridional overturning is determined by the net freshwater export from the Atlantic basin. When this export is artificially increased in the model over a range from 0.2 to 1 Sv, the ratio of overturning to freshwater forcing decreases almost linearly.

The aim of this study was to determine the influence of biogeography, oceanography and ecology on the population structure of different crustacean species with emphasis on their larval development. It is predicted that the duration of larval development influences the dispersal capability and therefore the level of genetic differentiation. In addition, the results should provide answers if there are cryptic species. The two atlanto-mediterranean crab species Xantho hydrophilus (Herbst, 1790) and Xantho poressa (Olivi, 1792) and the European prawn Palaemon elegans (Rathke, 1837) were included as marine species. Xantho has a shorter larval development with four zoeal stages than Palaemon with nine larval stages. Therefore, a higher gene flow within Palaemon was expected. We used also the amphidromous species Xiphocaris elongata (Guérin-Méneville, 1856) which occurs in freshwater systems of the Caribbean islands. The life cycle of amphidromous X. elongata is intermediate between freshwater and marine species and thus, high genetic differentiation was predicted. A comparative population genetics analyses was conducted to reveal differences in the population structure due to the distinct larval development and the amphidromous life-cycle. In the case of X. hydrophilus, results show a restricted gene flow between the populations of the Atlantic and the Mediterranean Sea. However, the taxonomic status of the Mediterranean subspecies X. h. granulicarpus is not valid as long as no single mutation step was detected to distinguish constantly between the two subspecies X. hydrophilus and X. h. granulicarpus. In addition, morphological transitional forms of X. hydrophilus are found within the Western Mediterranean Sea. X. poressa is suggested to reproduce panmictically because no differences are found between the populations of the Mediterranean Sea and the Atlantic Ocean and a mark recapture experiment showed that the species reaches high population densities. This species is well adapted to its habitat due to its variability in colouration and size which is the result of passive defence (camouflage). A pattern of allometric growth in the carapace shape was observed. Phylogenetic analyses revealed that X. sexdentatus is very closely related to X. hydrophilus. This could be either due to hybridization or introgression. Despite the fact that Palaemon elegans has nine larval stages, there is surprisingly high genetic differentiation. Normally such strong differences are only found between species and thus a cryptic species within the P. elegans complex is suggested. Three types of haplotype groups are found: Type I from the Atlantic and the Alboran Sea, Type II only within the Mediterranean Sea and Type III within the Mediterranean, Black, Caspian, Baltic and East Sea. Type III could be a relict of the Messinian crisis due to its high genetic differentiation, while Type II has recolonized the Mediterranean Sea after the crisis and ongoing separation mechanisms have established restricted gene flow between Type I and II. The barrier to gene flow between the Atlantic Type and the two Mediterranean Types is the Almería-Oran-Front. The Almería-Oran-Front is an important phylogeographic break, where restricted gene flow is possible. An isolation-by-distance pattern could be detected within the Atlantic Ocean. Furthermore, there is significant genetic differentiation between the Northern Atlantic and the North-East Atlantic due to the English Channel. These results provide evidence that dispersal play a minor role in determining the genetic structure of a species, compared to the biogeographic history and physical factors. This becomes even more obvious by comparison with the shrimp Xiphocaris elongata, for which a higher genetic differentiation due to the isolated freshwater systems and the amphidromous life cycle was expected. Although there is a high genetic differentiation, no geographic pattern emerges. The amphidromous life cycle enhance the dispersal capability and gene flow. It displays rather patterns of a marine species than of an exclusively freshwater species. The species is characterized by its variability in the rostrum length, but the results give no evidence for a genetic differentiation of the phenotypes. Thus, adaptive plasticity due to predators and in part allometric growth could be an explanation for the variability.

Recent studies (see references below) examined the impact of the Atlantic Ocean variability and climate change on the U.S. East Coast and the major bays of the Mid-Atlantic Bight: the Chesapeake Bay, the Delaware Bay and the New York Bay. Variations in the North Atlantic Oscillation (NAO), the Atlantic Meridional Overturning Circulation (AMOC) and the Gulf Stream (GS) can affect the weather and the climate over coastal regions – a remote impact that is difficult to predict. Analysis of various observations, including coastal sea level, water temperature in bays and estuaries, river discharges and ocean currents show that significant portion of the coastal variability is linked to remote forcing influenced by NAO, AMOC and the GS. For example, surface currents from high-frequency radar measurements near the mouth of the three Mid-Atlantic bays mention above show variations that are driven by a combination of local estuarine dynamics, coastal wind-driven dynamics and remote forcing from the Atlantic Ocean. AMOC and the GS can affect water exchange between bays and the open ocean, and variations in NAO shift the wind pattern and storm track over the coast. Climate change over the northeastern U.S. caused increased precipitation and river discharges into bays and resulted in increased outflows from bays toward the Atlantic Ocean. The impact of extreme events such as hurricanes and winter storms can also be seen in the outflows from bays. A better understanding of remote forcing on the coast will help in predicting impacts of climate change and coastal sea level rise on the highly populated U.S. coasts.

The Fortuna Basin in southeast Spain, part of the Betic Strait, was one of the connecting pathways between the Atlantic Ocean and the Mediterranean Sea during the late Miocene. The basin contains a remarkable record of Messinian marine sediments. Two well‐laminated, marly, diatomaceous sections were examined: the 1‐m‐thick El Fenazar section (F section) and the 36‐m‐thick Rio Chicamo Diatomite Member (RCD section). Both sections are intercalated with gypsum horizons. The diatoms in the Rio Chicamo Diatomite suggest an Atlantic origin and strong coastal upwelling associated with high productivity. On the basis of event stratigraphy, the diatomites correlate to the intra‐Messinian inundation of the Mediterranean Sea during the Messinian salinity crisis. Four biostratigraphically useful species, Nitzchia jouseae, Thalassiosira oestrupii/T. praeoestrupii, Denticulopsis hustedtii, and Asterolampra acutiloba, occur in the west Pacific between ∼5.35 and 4.5 Ma (lower reversed Gilbert chron). This interpretation places the subsequent Upper Evaporite deposition into the lower reversed Gilbert chron. Our results indicate (1) the existence of a marine connection between the Atlantic Ocean and the Mediterranean Sea through the Betic Strait (southeast Spain) during the Messinian salinity crisis, (2) a possible earlier first appearance of Nitzchia jouseae in the Betic Strait compared to the Atlantic and Pacific oceans according to the stratigraphy of the Fortuna Basin, and (3) the probability that many of the “Tripolilike” and younger diatomites in the Mediterranean are local deposits and diachronous with each other.