Low-frequency Rossby Wave In The Tropical Indian Ocean And Its Influence On The Thermal Structure Of The Upper Layer

Rossby Wave is one of the most important waves in geophysical fluid dynamics. Long baroclinic Rossby Wave plays very important role in dynamic ocean processes. It maintains and intensifies the strong western boundary currents in the ocean. Rossby Wave is an important mechanism of energy transfer in the world ocean. The variation signal carried by it propagates to the interior ocean, having significant impacts on the air-sea coupled system. Tropical Indian Ocean is the original region of Asian monsoon and has a considerable influence on its interannual variability. Therefore, study on the tropical Indian Ocean is of great scientific value and social benefits.In this dissertation, the characteristics and generation mechanism of low-frequency baroclinic Rossby Wave in the tropical Indian Ocean, as well as its influence on the thermal structure of the upper layer are investigated, by analyzing in situ and satellite data including sea surface height from Altimeter missions (TOPEX/Poseidon, Jason-1, Jason-2, and so on), WOA05 (World Oceanic Atlas 2005) from National Ocean Data Center, ocean temperature dataset from Scripps Institution of Oceanography, Argo profiles from China Argo Data Center, reanalysis products from National Center for Environmental Prediction/National Centers for AtmosphericResearch (NCEP/NCAR), wind field dataset from Florida State University, and so on. By adopting correlation analysis method together with employing 1.5-layer reduced gravity model, the generation mechanism of low-frequency Rossby Wave mainly in the south tropical Indian Ocean are thoroughly investigated. Results are as follows.(1) There are two distinct forms of Rossby Wave. One is boundary generated with phase speed about 13 cm/s along 12°S consistent with that of linear theory, and the signature can reach 80°E at the farthest. The other is wind forced and can be enhanced along the path of its westward propagation, with a phase speed exceeding 20 cm/s along 12°S.(2) The wind forced Rossby Wave is mainly generated in the interior south tropical Indian Ocean (70°E–95°E, 15°S–5°S), whereas the geostrophic adjustment associated with Indonesian throughflow (ITF) variation contributes to the one originated from the east boundary. These two classes of low-frequency Rossby Wave are both closely related to El Ni?o-Southern Oscillation (ENSO) events. In other words, ENSO can give rise to the low-frequency Rossby Wave in the south tropical Indian Ocean both through ocean and atmosphere remote forcing mechanisms simultaneously.(3) As an important contributor of the east boundary fluctuation, the variation of ITF is closely related to the different phase of ENSO event. Genarally, it is weak (strong) during El Ni?o (La Ni?a) year, in agreement with previous studies. Results of examination further show that ITF variability has a feature of phase locking to the summer time, it is strong (weak) from spring to autumn in the El Ni?o (La Ni?a) developing year with the strongest (weakest) flow during summer; and continuously weak (strong) from mature phase of El Ni?o (La Ni?a) through the next autumn with the weakest (strongest) flow during summer, which is corresponding to the variation of wind field during the summer time.The southwestern tropical Indian Ocean (SWIO) (50°E–75°E, 15°S–5°S) and the southeastern Indian Ocean along Sumatra-Java coast are two regions of activity where sea surface height anomalies (SSHA) and sea surface temperature anomalies (SSTA) are significantly correlated to each other, implying that internal dynamic processes govern the relationship between the surface and the subsurface variability. Moreover, the SWIO, with shallow mean thermocline, is a distinct region where Rossby Wave can affect the upper layer. In this dissertation, the impact of Rossby Wave on the thermal structure of the upper layer is investigated by analyzing the three consecutive positive Indian Ocean Dipole (IOD) events during 2006–2008, using vertical normal mode decomposition method. Main results are as follows.(1) Generally speaking, ocean dynamical processes in the tropical Indian Ocean are mainly dominated by the first and the second vertical baroclinic modes, of which the first mode is primary. The explained variance of the first mode is 2–3 times that of the second in the SWIO, and it also reaches 2 times in the equator and southeast Indian Ocean. Moreover, the higher baroclinic modes should be also taken into account in explaining the variability in the equatorial Indian Ocean.(2) The strong warm downwelling Rossby Wave can influence the vertical stratification structure in the SWIO, enhance the contribution of the second baroclinic mode motion which displays a significant dynamic signal in the upper thermocline layer, enlarging the downward vertical displacement of each isotherm significantly. Consequently, the thermal structure of the upper level can be largely modulated by this vertical displacement through mixing processes. However, since the vertical stratification becomes stronger under the influence of the cold Rossby Wave in the upper thermocline layer, which in turn restricts the variability in the upper layer related to the second baroclinic mode, the SWIO region is still governed by the first baroclinic mode motion in this case.(3) The features of surface and subsurface dipole are different. One of the main features of suface dipole is that the eastern cooling is confined to the south of equator off Sumatra-Java coast, whereas the subsurface eastern cooling is equatorial symmetric. In western tropical Indian Ocean, surface warming was basically equatorial symmetric, whereas the subsurface warming was asymmetric about equator: the subsurface warming is more strongly developed in south of equator than in north of equator, with high anomalous center locating at SWIO.(4) During the positive IOD event, cold SSTA first appears off Java coast. The increased latent heat flux induced by the along shore southeasterly as well as coastal upwelling are responsible for the initial cooling. Overlying atmosphere response to the zonal SST gradient associated with this cooling is in turn an important forcing mechanism of the ocean dynamical activity. One or two months later, upwelling along the Sumatra coast, cold Kelvin Wave propagated from the equatorial, together with the reflected Rossby Wave at the eastern boundary drive the initial cooling to spread into northwest, accelerating the development of positive IOD event. (5) Equatorially symmetrical STA is initially established in the eastern Indian Ocean off Sumatra, and it is enforced by the reflected Rossby wave there after, the cold STA signal also propagates southeastward in form of coastal Kelvin Wave, which can reach the Java coast.(6) The westward propagated downwelling Rossby Waves both in the north and south Indian Ocean are important mechanism of the western Indian Ocean warming in 2006 and 2008 IOD events. And Rossby Waves arrived at the western Indian Ocean during early 2007 do not stop propagating westward slowly untill summer, which largely contributes to west warming of 2007 IOD. Latent heat flux is also a considerable factor to the western Indian Ocean warming during 2006 IOD event.