School of Earth Sciences - Theses
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ItemThe response of nonlinear feedbacks associated with the Indian Ocean Dipole to global warming( 2015)The Indian Ocean Dipole (IOD) is an interannual mode of variability in the tropical Indian Ocean which has the potential to severely impact the surrounding countries. A prominent feature of the IOD is its positive skewness, where positive IOD events tend to be larger than negative events resulting in stronger impacts. Models from the Coupled Model Intercomparison Project phase 5 (CMIP5) show a threefold increase in the frequency of extreme events by 2100. However, the contribution of feedbacks associated with the IOD to this skewness and how they respond to increasing greenhouse gases are not well understood. The nonlinearity of four feedbacks (the Bjerknes feedback, sea surface temperature (SST)-cloud-radiation feedback, wind-evaporation-SST feedback, and nonlinear dynamic heating) was investigated and their responses to a warmer climate examined using CMIP5 models. A single model was first used to examine the nonlinearity of the four feedbacks, allowing the cause and effect of each feedback process to be determined. Following this, the analysis was applied to a multi-model ensemble to determine the cause of positive IOD skewness. The role of nonlinear dynamic heating in increasing the frequency of extreme events was then examined. This provides a better understanding of IOD dynamics and future behaviour. The positive Bjerknes feedback controls IOD skewness through the thermocline feedback. In a warmer climate, models show that the skewness of the IOD weakens and the asymmetry of the thermocline feedback displays a significant relationship with this reduced skewness. This decreased asymmetry occurs due to the mean state change of the tropical Indian Ocean where the Walker circulation exhibits weakening. The weaker mean westerly winds allow the climatological thermocline to shoal in the east, reducing the asymmetry of the thermocline feedback and thereby IOD skewness. The other feedbacks do not show as strong a relationship with IOD skewness. The negative SST-cloud-radiation feedback shows stronger damping of positive IODs and this is detrimental for IOD skewness. Under greenhouse warming there appears to be a shift amongst most models, with weaker damping of positive IODs and stronger damping of negative IODs. This is unfavourable for the reduction in skewness that occurs under increasing greenhouse gases. The positive wind-evaporation-SST feedback is not well simulated in coupled models. Therefore the relationship between IOD skewness and the wind-evaporation-SST feedback is uncertain and there does not appear to be a significant change in the asymmetry of this feedback in a warmer climate. Nonlinear dynamic heating involves advection of heat by oceanic currents, reinforcing positive IODs but damping negative IOD events. Amongst the models analysed, this process does not appear to contribute significantly to IOD skewness; however it does play an important role in the generation of extreme events and the projected increase. Nonlinear zonal and vertical advection during moderate events becomes more extreme-like due to the mean state change of the tropical Indian Ocean. This means that conditions during future moderate positive IOD events are similar to present-day extreme events, facilitating the projected increase of extreme positive IODs in a warmer climate.
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ItemMesoscale circulation and variability of the Indian Ocean( 2011)The introduction of satellite altimetry in the 1990's and the increasing sophistication of ocean modelling have seen mesoscale oceanography become an increasingly prominent area of research over the last few decades. Evidence is continuing to emerge that the mesoscale variability does not average to zero over extended periods (i.e. the available observational record which is now multi-decadal) and plays an important role in the general circulation of the ocean, with implications for models from short-range through to climate-scales. This study examines mesoscale variability of the Indian Ocean based on a range of observational and model products including a new type of product, which is based on the merging of observations and high-resolution models in the form of a multi-year ocean reanalysis (BlueLink ReANalysis – BRAN2.1). Due to the computational limitations and the grid resolution of this product, we give special emphasis to the southeast Indian Ocean where the model is eddy-resolving. This study has identified quasi-zonal alignment of the mesoscale variability for surface salinity and temperature in the eddy-resolving model domain. Specifically quasi-zonal alignments are found in the orientation of spatial correlation ellipses. The statistical properties have a 2-3° meridional width that are consistently zonal in the tropics, but in the subtropics tend to incline towards the equator. Similar quasi-zonal alignments are also found in spatial anomalies of mean surface prognostic variables for salinity, temperature, as well as surface density. The statistical properties of the reanalysis for surface temperature are verified using a merged product from multiple satellites of high-resolution surface temperature observations from AVHRR. The seasonal variations of the time-mean surface quasi-zonal alignment in physical properties and its link to the coastal current systems of the southeast Indian Ocean are detailed in this study. Consistent quasi-zonal or “arterial” ocean currents of the southeast Indian Ocean are revealed using a colorwheel visualisation tool. In particular, we have identified five eastward jets in the broad surface geostrophic flow regime of the southeast Indian Ocean. Mean meridional sections of ocean currents show that the zonally coherent mean features seen at the surface extend into the abyssal ocean though decline in magnitude with depth; and are similarly quasi-zonal. The seasonality of the surface layer eastward transports in the southeast Indian Ocean is found to be positively correlated with the phase of the Leeuwin Current. Mean westward flows are seen at mid-depth with greater arterial structure. Mesoscale eddies are found to follow regularly along the seasonally persistent mean westward feature connecting the Naturaliste and Broken Plateau. Time-varying properties of quasi-zonal structures in the southeast Indian Ocean show equatorward meridional drift between 25°S-15°S and poleward meridional movements are noted between the equator and 14°S. Meridional displacements of quasi-zonal features in the southeast Indian Ocean are found to correspond with the sign and strength of the mean meridional velocity. Quasi-zonal signatures are found to be also present in the 10-year mean spatial anomalies of the overlying wind-stress field observed from the QuikSCAT satellite observations. Statistical relationships of quasi-zonal features in wind-stress and observed sea surface temperature are derived.