Stability analysis of the western boundary currents in the tropical Indian Ocean

该项目的总体目标是更好地了解热带印度洋西部边界流的不稳定性，即北半球的索马里流和南半球的东非沿岸流。这将特别有助于确定这些洋流的不稳定性是否有助于在西印度洋产生柳艾波，也称为混合Rossby重力波。在太平洋、大西洋和印度洋都观测到了具有热带不稳定波特征的柳艾波。虽然在大西洋和太平洋已经很好地了解了这种产生机制，但在印度洋仍然几乎一无所知。这一知识缺口是这项工作的主要动机。该项目将采用经典的简正波稳定性理论，该理论将指数增长过程视为斜压不稳定理论，以及相对较新的广义稳定性理论，该理论也包括瞬时增长过程。为了研究热带印度洋西部边界流的不稳定性，将使用三个模式：一个改进的准地转模式，最近由Theiss和Mohebalhojeh发展的准地转模式的赤道对应模式，以及区域海洋模式系统(ROMS)。第一个额外的目标是调查准地转模式的赤道对应物是否用于赤道地区的不稳定性研究，这一工具在中纬度地区与准地转模式一样具有指导意义。第二个额外目标是将该项目与PIS的桑给巴尔项目结合起来，桑给巴尔项目是东非沿海海洋学学生的教育项目。热带印度洋西部边界流研究和东非沿海海洋学研究的自然重叠将被PIS用于两个项目，特别是有关学生的共同利益。更好地了解热带印度洋西部边界流的不稳定性，特别是它们对产生Yanai波的贡献，将有助于发展对大西洋和太平洋Yanai波产生的现有理解水平。季风风交替等主要特征必然会影响热带印度洋西部边界流的不稳定。这些不稳定反过来必然影响非洲东海岸的海洋生态系统，并可能促进亚奈波的产生，从而影响赤道印度洋的动量和热量平衡和气候变异性。因此，不稳定性是一个不可分割的部分，受到影响，并进而影响从局部到整个洋盆范围的各种过程。此外，将这项研究与PIS教育桑给巴尔项目相结合，将有助于将学生对东非沿海海洋的研究扩展到更大的西部热带印度洋。

The overall objective of the project is to understand better the instabilities of the western boundary currents in the tropical Indian Ocean, which are the Somali Current in the Northern and the East African Coastal Current in the Southern hemisphere. This will in particular allow for determining whether the instabilities of these currents contribute to the generation of Yanai waves, also known as mixed Rossby gravity waves, in the western Indian Ocean. Yanai waves, which have the characteristics of Tropical Instability Waves, have been observed in the Pacific, Atlantic, and Indian Ocean. While the generation mechanism is well understood in the Atlantic and Pacific Ocean, it is still practically unknown in the Indian Ocean. This knowledge gap is the main motivation for the work.The project will employ the classical normal mode stability theory, which considers exponential growth processes as for instance in the theory of baroclinic instability, and the comparatively new Generalized Stability Theory, which encompasses also transient growth processes. To investigate the instabilities of the western boundary currents in the tropical Indian Ocean three models will be used: A modified form of the quasigeostrophic model, the equatorial counterpart of the quasigeostrophic model recently developed by Theiss and Mohebalhojeh, and the Regional Ocean Modeling System (ROMS).The overall objective inspires two additional objectives. The first additional objective is to investigate whether the equatorial counterpart of the quasigeostrophic model is for instability studies in the equatorial region a tool that is as instructive as the quasigeostrophic model in the mid-latitudes. The second additional objective is to integrate the project with the PIs' Zanzibar Project, which is an educational project for students on coastal oceanography in East Africa. The natural overlap of the study of the western boundary currents of the tropical Indian Ocean and coastal oceanography in East Africa will be used by the PIs for the mutual benefit of both projects and especially the involved students.Better understanding the instabilities of the western boundary currents in the tropical Indian Ocean and in particular their contribution to the generation of Yanai waves would help towards developing a level of understanding that already exists about the generation of Yanai waves in the Atlantic and Pacific Ocean. Dominant features such as the alternating Monsoon winds must affect the instabilities of the western boundary currents in the tropical Indian Ocean. These instabilities in turn must affect the marine ecosystems along the East Coast of Africa and might contribute to the generation of Yanai waves, thus effect the momentum and heat balance in the equatorial Indian Ocean and climate variability. The instabilities are therefore an integral part affected by and in turn affecting processes with scales ranging from local to ocean-basin-wide. In addition, the integration of the study with the PIs educational Zanzibar Project will help expand the students' study of the coastal ocean of East Africa to the larger western tropical Indian Ocean.