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CSIRO Marine and Atmospheric Research
Past Seminars

Seminar Abstract

A model-based assessment of the proposed Indian Ocean mooring array

A series of Observing System Simulation Experiments (OSSEs) are performed for the tropical Indian Ocean (+-15 degrees from the equator) using a simple analysis system. The analysis system projects an array of observations onto the dominant empirical orthogonal functions (EOFs) derived from an intermediate resolution (2 degrees x 0.5 degrees) ocean circulation model. This system produces maps of the depth of the 20 degrees C isotherm (D20), representing interannual variability, and the high-pass filtered mixed layer depth (MLD), representing intraseasonal variability, independently.

The OSSEs are designed to assess the suitability of the proposed Indian Ocean surface mooring array (CLIVAR IOP, 2004) for resolving intraseasonal-interannual variability. While the proposed array does a reasonable job of resolving the interannual time-scales, it does not adequately resolve the intra-seasonal time-scales. We conclude that it probably over-samples the region within 3 degrees of the equator and under-samples the region south of the equator.

A procedure is developed to rank the importance of potential observation locations, by determining the observation array that best projects onto the EOFs used in the analysis system. OSSEs using an optimal array clearly outperform the OSSEs using the proposed array. We find that the configuration of the optimal array is sensitive to details of the analysis system, namely the number of EOFs used and the period over which the EOFs are calculated. The optimal array is also different when fixed observations are included that represent an idealised ARGO array.

We therefore compile a composite of optimal arrays for D20 and MLD, using only the most important observations for different configurations of the analysis system and for applications with and without ARGO observations. These composites demonstrate that observations south of 8 degrees S are found to be most important for resolving interannual variability; while observations along the equator, and particularly east of 75 degrees E, are important for resolving intraseasonal variability. Finally we use the composites to determine a single “improved”, albeit less optimal, array. In a series of OSSEs, we show that the “improved” array outperforms the proposed array for all configurations of the analysis system for both intraseasonal and interannual variability.

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CSIRO = Marine Laboratories Auditorium, Castray Esplanade, Hobart

For further information, or to schedule a seminar, contact:
Peter Oke, (Oceanographic seminars) CSIRO Marine Research (03) 6232 5387
Piers Dunstan, (Biological seminars) CSIRO Marine Research (03) 6232 5382
Katrina Nitschke, Antarctic Climate and Ecosystems CRC (03) 6226 2265 & IASOS, University of Tasmania (03) 6226 2509