The Indian Ocean plays an important role in modulating climate over the surrounding region and around the globe, particularly through its major mode of climate variability, the Indian Ocean dipole (IOD). In this thesis, the representation of the Indian Ocean mean state and the IOD in the HiGEM high resolution coupled climate model has been investigated. To understand air-sea coupled processes during the IOD, it is first necessary to evaluate the representation of the Indian Ocean mean state in HiGEM. Although HiGEM has a very good representation of the tropical Indian Ocean, there are some regional biases in the model. Firstly biases in the Arabian Sea. HiGEM has biases in various parameters over the northern Arabian Sea during boreal winter. A major result of this thesis is that a winter cold SST bias in the Arabian Sea in HiGEM is shown to be common among most of the CMIP3 models, relating to cold dry air advection from north-west India! Pakistan during the winter monsoon. This also feeds enhanced convergence over the western equatorial Indian Ocean, potentially interacting with biases along the equator and the IOD. Secondly biases in mean equatorial winds, reversing the tilt of the equatorial thermocline during SON and DJF, which may lead to biases in the evolution of the IOD. To test the impact of the equatorial mean state biases in HiGEM on the IOD, an ensemble experiment correcting ocean surface wind stress over the equatorial Indian Ocean has been performed. When applied the wind stress correction to the ocean wind stress, it reduced the biases in the Cold SST and thermocline depth in the east equatorial Indian Ocean (EElO). This suggests that the seasonal evolution of the IOD is sensitive to the model mean state. As the anthropogenic warming can spin down the tropical circulations, the impact of climate change on the IOD in HiGEM is also investigated. Analysis showed an earlier initiation and termination of the roD in HiGEM in a warm climate compared to the HiGEM control run. This shift in initiation and termination in the seasonal evolution of the IOD is consistent with the change in the Indian Ocean mean state in a wann climate. This study shows the importance of understanding the Indian Ocean processes and their representation in models for regional climate studies.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:630470 |
| Date | January 2013 |
| Creators | Marathayil, Deepthi |
| Publisher | University of Reading |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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