Browsing by Author "Deshayes, Julie"

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    Climate change impact on ecosystems of Prince Edward Islands: role of oceanic mesoscale processes
    (2019) Asdar, Sarah; Ansorge, Isabel; Penven, Pierrick; Deshayes, Julie
    The subantarctic Prince Edward Islands (PEIs, 47◦S-38◦E) are classified as isolated and hostile regions, in which the terrestrial and marine ecosystems are relatively simple and extremely sensitive to perturbations. The island’s location, between the Subantarctic Front (SAF) and the Polar Front (PF), bordering the Antarctic Circumpolar Current (ACC) provides an ideal natural laboratory for studying how organisms, ecological processes and ecosystems respond to a changing climate in the Southern Ocean. Recent studies have proposed that climate changes reported at the islands may correspond in time to a southward shift of the ACC and in particular of the SAF. This southward migration in the geographic position of major ocean fronts is likely to coincide with dramatic changes in the distribution of species and total productivity of this region. However, there are other sources of variability in the hydrodynamic conditions around the PEIs: upstream of the islands, at the South West Indian Ridge (SWIR), a region of high eddy kinetic activity produces mesoscale features that directly irrigate the PEIs and may impact their marine environment. Based on satellite altimetry in that region, the positions of the SAF and PF were found to be highly variable at interannual and monthly time scales. They also revealed a significant long-term southward trend which was highlighted at the Southern Ocean scale. The mesoscale activity also showed an interannual and intra-annual variability and a decrease in eddy kinetic energy over 24 years was observed in the region. At a more local scale, we highlighted that the archipelago’s environment was impacted by the mesoscale features produced at the SWIR. The temperature, the mixed layer and velocities recorded between the islands were clearly affected by the eddies passing in the vicinity of the PEIs. Moreover, a large signal dominating the main current time series appeared to be a tidal signal, another important driver of variability of the circulation in between the two islands. On a second hand, an idealised model configuration was designed for the PEIs region to study the mesoscale eddy properties and the physical mechanisms of their formation at the ridge. The Eddy Available Potential Energy revealed a maximum of energy around 800 m depth, confirming the deep reaching characteristic of the eddy originated in the region and suggested the presence of a local energy source at this depth. This eddies activity was shown to be the result of a combination of barotropic and baroclinic instabilities occurring at the ridge. Finally, we investigated on the potential consequences of a southward shift of the SAF in the region of the islands. Because the model was idealised, it allowed us to simulate an SAF southward shift by shifting the initial and boundary conditions. The main result was the clear decrease of mesoscale activity in the region which could potentially impact the ecosystems of the PEIs.
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    CMIP5 Model Intercomparison of Freshwater Budget and Circulation in the North Atlantic
    (2014) Deshayes, Julie; Curry, Ruth; Msadek, Rym
    ABSTRACT The subpolar North Atlantic is a center of variability of ocean properties, wind stress curl, and air–sea exchanges. Observations and hindcast simulations suggest that from the early 1970s to the mid-1990s the subpolar gyre became fresher while the gyre and meridional circulations intensified. This is opposite to the relationship of freshening causing a weakened circulation, most often reproduced by climate models. The authors hypothesize that both these configurations exist but dominate on different time scales: a fresher subpolar gyre when the circulation is more intense, at interannual frequencies (configuration A), and a saltier subpolar gyre when the circulation is more intense, at longer periods (configuration B). Rather than going into the detail of the mechanisms sustaining each configuration, the authors’ objective is to identify which configuration dominates and to test whether this depends on frequency, in preindustrial control runs of five climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). To this end, the authors have developed a novel intercomparison method that enables analysis of freshwater budget and circulation changes in a physical perspective that overcomes model specificities. Lag correlations and a cross-spectral analysis between freshwater content changes and circulation indices validate the authors’ hypothesis, as configuration A is only visible at interannual frequencies while configuration B is mostly visible at decadal and longer periods, suggesting that the driving role of salinity on the circulation depends on frequency. Overall, this analysis underscores the large differences among state-of-the-art climate models in their representations of the North Atlantic freshwater budget.
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    Evaluating global ocean reanalysis systems for the greater Agulhas Current System
    (2014) Cooper, Kyle Francis; Backeberg, Bjorn Christoph; Hermes, Juliet C; Deshayes, Julie; Veitch, Jennifer Anne
    Operational oceanography aims to accurately hindcast and forecast the state of the ocean. An international initiative, the Global Ocean Data Assimilation Experiment (GODAE), developed and increased the capacity for global operational oceanography. However, the products from the global initiatives were regionally inapplicable due to low spatial resolutions, and have recently improved through GODAE OceanView. A number of local operational oceanographic initiatives have been setup over the southern African regional ocean, but proved to be unsustainable and ended. Recently, the aim to develop a regional ocean prediction system has arisen, and initial steps have been taken. This thesis aims to address the lack of local capacity in operational oceanography, and contribute to a crucial process in developing a regional ocean prediction system. Here, we validate and investigate the differences between three global reanalysis products, namely MyOcean (GLORYS2V1), HYCOM (U.S Naval Research Laboratory) and BlueLINK (OFAM3). These reanalysis products are validated and investigated over the greater Agulhas Current System, which is a crucial system in Southern African regional ocean. The salient oceanographic features represented in the reanalysis products are initially compared to historical literature of the region and followed by available unassimilated observations (i.e. independent). The results show that the reanalysis products from MyOcean, and the U.S Naval Research Laboratory satisfactorily simulate the major salient oceanographic features of the Agulhas Current System. Bluelink does not correctly portray the structure of the source and retroflection regions, and therefore has limited use over the Agulhas Current System. The differences between the three products indicates that the data assimilate does not sufficiently constrain the models in order for their solutions over the Agulhas System to converge. The evaluation of these global ocean reanalysis products is a critical step toward a regional ocean prediction system over Southern Africa, and building toward the local capacity to accomplish this goal.
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    Spatio-temporal characteristics of the Agulhas leakage: a model inter-comparison study
    (2015) Holton, Lisa E; Deshayes, Julie
    Investigating the variability of the Agulhas leakage, the volume transport of water from the Indian Ocean to the South Atlantic Ocean, is highly relevant due to its potential contribution to the Atlantic Meridional Overturning Circulation (AMOC) and the global circulation of heat, salt and freshwater, hence to the global climate. As observations of the leakage are scattered in time and space, ocean model outputs are unavoidable to describe its mechanisms of variability. However, the Agulhas leakage is part of a system that still represents a challenge to modellers, because of its degree of non-linearity. As a consequence, this thesis reports the study of the mechanisms of variability of the Agulhas leakage in six ocean model outputs of varying resolution, vertical discretization and boundary conditions. Inter-comparing several model outputs allow one to identify which characteristics of variability are robust. It also provides valuable information on the model speci_cs required to correctly represent the mechanisms of variability of the Agulhas leakage and hence its contribution to climate variability. The Agulhas leakage is estimated at the Good Hope line, a section through the centre of the Cape Basin, using a Eulerian threshold integration method based on the thermo- haline properties of water masses along the line. The threshold method also takes into account the thermohaline biases of each model. Other methods have been tested but the former was selected because it yields an Agulhas Leakage that represents 60% of the actual magnitude as determined by a passive tracer method (only available in one simulation) and captures nearly all of the temporal variability.