School of Earth Sciences - Theses
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ItemMesoscale circulation and variability of the Indian OceanDivakaran, Prasanth ( 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.