A multi-decadal survey program undertaken at the Collaroy-Narrabeen embayment in SE Australia identifies medium-term (~2-7 year) cycles of both erosion and accretion across the entire embayment ('beach oscillation') and at its two extremities ('beach rotation'). These cycles have been observed to respond to phase shifts in the El Ni??o/Southern Oscillation (ENSO). To investigate wave and climate controls of embayment variability in finer detail, this study combines historical surveys with 45 years of wave data from the ERA-40 reanalysis and four years of high-resolution beach measurements using RTK-GPS and image-derived survey techniques. ENSO and Southern Annular Mode (SAM) controls of wave variability in the Sydney region are first explored. In general, wave heights increase/decrease and wave directions become more easterly/southerly during La Ni??a/El Ni??o phases. A positive correlation is observed between the SAM and summer wave heights, and a negative correlation between the SAM and winter wave directions. Storm variability is observed to be modified by the ENSO, but not the SAM. In particular, La Ni??a phases are generally associated with longer duration, higher energy events from a more easterly direction when compared to those during El Ni??o phases. Wave controls of embayment variability are subsequently investigated. In the short-term (days - months), beach oscillation/rotation is observed to be the most dominant process, accounting for approx. 60%/20% of overall embayment variability. Beach oscillation is related to changes in wave height and storms, whereas beach rotation is related to changes in wave direction and/or wave period. An empirical model that estimates the beach response to individual storm events is developed. In the longer-term (months - years), beach rotation is observed to respond to both wave heights and directions. Larger waves are sheltered somewhat at the southern end, creating an apparent clockwise rotation under energetic wave conditions. Clockwise/anticlockwise rotations are also observed to follow southerly/easterly wave shifts at lags of up to 12 months. Comparisons between the ENSO and beach oscillation/rotation agree with previous observations that El Ni??o/La Ni??a phases are associated with an overall accretion/erosion and clockwise/anticlockwise rotation of the embayment. In general, the SAM shows little influence on embayment variability. While it is clear that beach oscillation is driven by cross-shore processes, to what extent beach rotation is a longshore and/or cross-shore phenomena requires further investigation.

A multi-decadal survey program undertaken at the Collaroy-Narrabeen embayment in SE Australia identifies medium-term (~2-7 year) cycles of both erosion and accretion across the entire embayment ('beach oscillation') and at its two extremities ('beach rotation'). These cycles have been observed to respond to phase shifts in the El Ni??o/Southern Oscillation (ENSO). To investigate wave and climate controls of embayment variability in finer detail, this study combines historical surveys with 45 years of wave data from the ERA-40 reanalysis and four years of high-resolution beach measurements using RTK-GPS and image-derived survey techniques. ENSO and Southern Annular Mode (SAM) controls of wave variability in the Sydney region are first explored. In general, wave heights increase/decrease and wave directions become more easterly/southerly during La Ni??a/El Ni??o phases. A positive correlation is observed between the SAM and summer wave heights, and a negative correlation between the SAM and winter wave directions. Storm variability is observed to be modified by the ENSO, but not the SAM. In particular, La Ni??a phases are generally associated with longer duration, higher energy events from a more easterly direction when compared to those during El Ni??o phases. Wave controls of embayment variability are subsequently investigated. In the short-term (days - months), beach oscillation/rotation is observed to be the most dominant process, accounting for approx. 60%/20% of overall embayment variability. Beach oscillation is related to changes in wave height and storms, whereas beach rotation is related to changes in wave direction and/or wave period. An empirical model that estimates the beach response to individual storm events is developed. In the longer-term (months - years), beach rotation is observed to respond to both wave heights and directions. Larger waves are sheltered somewhat at the southern end, creating an apparent clockwise rotation under energetic wave conditions. Clockwise/anticlockwise rotations are also observed to follow southerly/easterly wave shifts at lags of up to 12 months. Comparisons between the ENSO and beach oscillation/rotation agree with previous observations that El Ni??o/La Ni??a phases are associated with an overall accretion/erosion and clockwise/anticlockwise rotation of the embayment. In general, the SAM shows little influence on embayment variability. While it is clear that beach oscillation is driven by cross-shore processes, to what extent beach rotation is a longshore and/or cross-shore phenomena requires further investigation.

A multi-decadal survey program undertaken at the Collaroy-Narrabeen embayment in SE Australia identifies medium-term (~2-7 year) cycles of both erosion and accretion across the entire embayment ('beach oscillation') and at its two extremities ('beach rotation'). These cycles have been observed to respond to phase shifts in the El Ni??o/Southern Oscillation (ENSO). To investigate wave and climate controls of embayment variability in finer detail, this study combines historical surveys with 45 years of wave data from the ERA-40 reanalysis and four years of high-resolution beach measurements using RTK-GPS and image-derived survey techniques. ENSO and Southern Annular Mode (SAM) controls of wave variability in the Sydney region are first explored. In general, wave heights increase/decrease and wave directions become more easterly/southerly during La Ni??a/El Ni??o phases. A positive correlation is observed between the SAM and summer wave heights, and a negative correlation between the SAM and winter wave directions. Storm variability is observed to be modified by the ENSO, but not the SAM. In particular, La Ni??a phases are generally associated with longer duration, higher energy events from a more easterly direction when compared to those during El Ni??o phases. Wave controls of embayment variability are subsequently investigated. In the short-term (days - months), beach oscillation/rotation is observed to be the most dominant process, accounting for approx. 60%/20% of overall embayment variability. Beach oscillation is related to changes in wave height and storms, whereas beach rotation is related to changes in wave direction and/or wave period. An empirical model that estimates the beach response to individual storm events is developed. In the longer-term (months - years), beach rotation is observed to respond to both wave heights and directions. Larger waves are sheltered somewhat at the southern end, creating an apparent clockwise rotation under energetic wave conditions. Clockwise/anticlockwise rotations are also observed to follow southerly/easterly wave shifts at lags of up to 12 months. Comparisons between the ENSO and beach oscillation/rotation agree with previous observations that El Ni??o/La Ni??a phases are associated with an overall accretion/erosion and clockwise/anticlockwise rotation of the embayment. In general, the SAM shows little influence on embayment variability. While it is clear that beach oscillation is driven by cross-shore processes, to what extent beach rotation is a longshore and/or cross-shore phenomena requires further investigation.

Tree-ring based climate reconstructions have been critical for understanding past variability and recent trends in climate worldwide, but they are scarce in Australia. This is particularly the case for temperature: only one tree-ring width based temperature reconstruction – based on Huon Pine trees from Mt Read, Tasmania – exists for Australia. Here, we investigate whether additional tree- ring parameters derived from Athrotaxis cupressoides trees growing in the same region have potential to provide robust proxy records of past temperature variability. We measured wood properties, including tree-ring width (TRW), mean density, mean cell wall thickness (CWT), and tracheid radial diameter (TRD) of annual growth rings in Athrotaxis cupressoides, a long-lived, high-elevation conifer in central Tasmania, Australia. Mean density and CWT were strongly and negatively correlated with summer temperatures. In contrast, the summer temperature signal in TRW was weakly positive. The strongest climate signal in any of the tree-ring parameters was maximum temperature in January (mid-summer; JanTmax) and we chose this as the target climate variable for reconstruction. The model that explained most of the variance in JanTmax was based on TRW and mean density as predictors. TRW and mean density provided complementary proxies with mean density showing greater high-frequency (inter-annual to multi-year) variability and TRW showing more low-frequency (decadal to centennial-scale) variability. The final reconstruction model is robust, explaining 55% of the variance in JanTmax, and was used to reconstruct JanTmax for the last five centuries (1530–2010 C.E.). The reconstruction suggests that the most recent 60 years have been warmer than average in the context of the last ca. 500 years. This unusually warm period is likely linked to a coincident increase in the intensity of the subtropical ridge and dominance of the positive phase of the Southern Annular Mode in summer, which weaken the influence of the band of prevailing westerly winds and storms on Tasmanian climate. Our findings indicate that wood properties, such as mean density, are likely to provide significant contributions toward the development of robust climate reconstructions in the Southern Hemisphere and thus toward an improved understanding of past climate in Australasia.

Reconstructed snow accumulation rate observations from five new firn cores show a statistically significant negative trend in accumulation rates over the past four decades across the central West Antarctic ice sheet. A negative temporal trend in accumulation is unexpected in light of rising surface temperatures and simulations run by GCMs. Both the magnitude of the mean accumulation rates and the range of interannual variability observed in the new records compares favorably to older records, suggesting that the new accumulation rate records may serve as a regional proxy for recent temporal trends in West Antarctic accumulation rates. The observed negative trend is likely the result of Southern Hemisphere high-latitude internal atmospheric dynamics, dominated by changes in the austral fall season. The well-documented positive trend in the Southern Annular Mode causes a low pressure center to form over the Amundsen Sea, which in turn produces lower accumulation rates across the western portion of the West Antarctic ice sheet. The new accumulation rate records are compared to several models/reanalyses to test the skill of simulated accumulation rate predictions. While the models/reanalyses and the new observations agree well in both mean and variability, the simulated records do not capture the full negative trend observed in the reconstructed records.

No description available.

L'océan Austral joue un rôle crucial dans la régulation du système climatique en absorbant de grandes quantités de CO2 atmosphérique. Toutefois de nombreuses incertitudes demeurent quant à l'évolution récente du puits de carbone austral notamment en raison du manque d'observations et des lacunes des modèles océaniques dans la représentation de processus dynamiques comme les tourbillons. Depuis quelques décennies notamment, l'efficacité du puits de carbone austral diminuerait en raison d'une intensification des vents liée à une tendance positive du Mode Annulaire Austral (SAM). L'objectif de ces travaux de thèse est de décrire et comprendre la variabilité spatiale et temporelle récente des flux air-mer de CO2 dans l'océan Austral. Pour cela, des simulations de sensibilité aux phases positives du SAM sont réalisées dans une configuration régionale de l'océan Austral (sud de 30°S), basée sur un modèle couplé dynamique-biogéochimie forcé par l'atmosphère et résolvant partiellement la méso-échelle océanique. Dans l'océan Austral, la réponse des flux de CO2 au SAM correspond à un dégazage intense de CO2 dans la zone antarctique dû à une augmentation des concentrations de surface de carbone inorganique dissous (DIC). Cette augmentation est pilotée par la dynamique de la couche de mélange et alimentée par un transport méridien de DIC qui résulte essentiellement de la compétition entre circulation induite par les vents et par les méandres stationnaires. Ces travaux montrent l'apport d'une augmentation de la résolution numérique des modèles pour la simulation des flux de CO2. / By taking up large amounts of atmospheric CO2, the Southern Ocean helps to regulate the climate system. Southern Ocean carbon sink is poorly constrained, in part because data coverage is sparse and also because ocean models that have been used in such assessments fail to explicitly resolve key physical features such as mesoscale eddies. In recent decades, the growth of the Southern Ocean carbon sink may have been partly counteracted due to a loss of natural CO2 from the ocean driven by an intensification of westerlies, related to a positive trend in the Southern Annular Mode (SAM). This thesis focuses on documenting and understanding recent spatial and temporal variability of air-sea CO2 fluxes in the Southern Ocean. Sensitivity to positive phases of the SAM are tested by making simulations with a regional model of the Southern Ocean (south of 30°S) that couples biogeochemistry to the dynamics, is forced by atmosphere reanalysis data, and partially resolves the mesoscale. The resulting response of Southern Ocean CO2 fluxes to the SAM is dominated by a strong CO2 efflux to the atmosphere from the Antarctic Zone due to an increase in surface dissolved inorganic carbon (DIC). This increase is driven by the mixed-layer dynamics and is supplied by a meridional transport of DIC, a competition between the wind-driven circulation and the standing eddy-induced circulation. This work discusses the effect of increasing model resolution on simulated air-sea CO2 fluxes.

No description available.

El Niño Southern Oscillation events have global implications both climatologically and socio-economically. One such climatological teleconnection is manifested in the Amundsen Sea region (ASR). The Amundsen sea low (ASL) is the dominant low pressure system located around the ASR and is important to the climate of Western Antarctica. Therefore, it is important to understand the ASL and any phenomena that may affect it. This thesis focuses on the ENSO--ASR teleconnection under El Niño conditions and the mechanism behind it. The ENSO--ASR teleconnection was explored using the UM version 8.4 (HadGEM3) model. Time--slice experiments with various magnitudes of idealised perpetual ENSO events are imposed. Two sets of `switch on' experiments in which tropical Pacific SSTs were ramped up were also carried out to investigate the transient nature of the teleconnection. The seasonality of the ENSO--ASR teleconnection is known from previous studies to be stronger in winter compared to summer. The mechanism behind the seasonality was explored using the time--slice experiments. The seasonality is found to originate from the seasonal differences in the Southern Hemispheric jets. As the subtropical jet is only present in austral winter, Rossby wave source anomalies can only be generated in the mid--latitudes in winter. Furthermore, the propagation of the Rossby waves is not possible in summer due to the strong polar front jet. The lack of the source and propagation in summer explains the weaker ENSO--ASR teleconnection. A flowchart summarising the mechanism was created and then verified by the transient runs. The linearity of the ENSO--ASR teleconnection within El Niño has not been previously investigated. This is mainly due to insufficient reanalysis data available to overcome the high internal variability in the ASR. In this thesis, the linearity of the teleconnection under El Niño is studied using the time--slice runs. The results indicate linearity (within errorbars) for both the summer and winter seasons up to historically maximum El Niños. However, under extreme El Niños (beyond historic records) in winter, the teleconnection is no longer linear. The UPSCALE dataset was used to investigate the effects of horizontal resolution on the simulation of the ASL climatological state and the ENSO--ASR teleconnection. The UPSCALE dataset consists of ensembles of HadGEM3 simulations at three different horizontal resolutions. The high resolution model was found to better simulate the ASL while the low resolution model was found to better simulate the ENSO--ASR teleconnection.

No description available.