DOI: 10.17026/dans-29h-ac95
title: Sea surface salinity and the ocean structure in the tropical Indian Ocean

description: The variability of the tropical Indian Ocean (TIO) is crucial to the global water cycle and heat transportation. At the seasonal time scale, the variability of the TIO is closely related to the monsoon circulation. Interannually, on the one hand, it is influenced remotely by the variability of the Pacific Ocean through atmospheric bridge and Rossby wave. On the other hand, the variability of TIO is independent of El Niño-Southern Oscillation (ENSO) and varies as a dipole pattern along the equator, known as the Indian Ocean dipole (IOD). In addition, the unique distribution of the TIO with shallower thermocline in the west, due to the unstable easterlies along the equator, makes the variability of the TIO differ from that in the Pacific and the Atlantic Ocean with deeper thermocline in the west. However, the TIO variability is still hard to precisely predict and the difficulty is well acknowledged nowadays.To this end, the main aim of this dissertation is to better understand the seasonal and interannual variability of the TIO. Specifically, the first objective is to distinguish the relative impacts of heat flux and wind stress on the interannual variability of upper-ocean temperature in the TIO. The second objective focuses on studying the relationships among the parameters in the mixed layer and thermocline with the barrier layer thickness (BLT) in the TIO. The last objective of this dissertation is to investigate the role of sea surface salinity (SSS) in the onset of South Asian Summer Monsoon (SASM). To achieve this aim, the observation datasets including float and satellite data, reanalysis data, and simulations from a high-resolution model, are employed.In Chapter 2, we compared the ocean general circulation model (OGCM) hindcast with the reanalysis and observation data and found that they were consistent in the simulation of the climatological mean and interannual variability of the upper-ocean temperature vertical structure in the TIO. Therefore, OGCM was adopted in this study to study the variability of the TIO. Two sensitivity simulations were designed to study the relative contribution of heat flux and wind stress to the variability of TIO: one was set with only heat flux varying interannually, and another one assumed only wind stress varying interannually. The results show that the impacts of heat flux and wind stress on the interannual variability of ocean temperature in the TIO have a depth-dependent feature. Specifically, heat flux mainly dominates the interannual variability of ocean temperature above the depth of approximately 30 m in the TIO, while wind stress contributes most to the interannual variability of ocean temperature below the depth of 30 m. This depth-dependent feature has also been observed for sub-areas and different seasons in the TIO. Therefore, we define the depth where the dominant force switches from heat flux to wind stress in the TIO as the “crossing depth”. Shallower crossing depth indicates that heat flux only controls the interannual variability of ocean temperature near the surface while wind stress is the dominant driving force for the interannual temperature variability in the upper-ocean, such as the Seychelles-Chagos Thermocline Ridge (SCTR) and the eastern part of the Indian Ocean Dipole (IODE).In chapter 3, we assessed the driving forces for the variability of the barrier layer thickness (BLT) in TIO. The barrier layer is defined as a thinner layer between the bottom of the mixed layer and the top of the thermocline. The BLT in the TIO has significant seasonal and interannual variabilities. The results of this study show that the sea surface temperature (SST) barely contributes to the variability of BLT but the main forcing on the BLT from the mixed layer can be explained by the variability of SSS. At the seasonal time scale, the dominating drivers of the BLT variability are different in the western and eastern TIO. In the western TIO, SSS exerts a negative correlation with the BLT variability during boreal autumn, winter, and spring while shows minimal impacts on the BLT variability in summer. In the eastern TIO, the thermocline is the dominant driver for the BLT variability, and it has positive correlations with BLT in all four seasons. At the interannual time scale, the variability of BLT in the TIO is affected by the IOD and ENSO events. Particularly, in the eastern TIO, thinner BLT could be detected mainly induced by the anomalous thermocline during the positive IOD and El Niño years. In the western TIO, deepening thermocline due to El Niño induced anomalous wind stress results in thicker BLT. But the correlation between BLT and El Niño does not become weaker with the weakening relationship between thermocline and El Niño. This is because of the variation of SSS.In Chapter 4, we investigated the role of sea surface salinity anomalies (SSSAs) in the onset of the SASM. Positive SSSAs appears in the western TIO before the onset of SASM. This SSSAs has two maxima in the symmetry of the equator with one in the southern TIO and another one in the northern TIO. The location of the southern maxima corresponds to the SCTR. It is found that the SSSAs respond more sensitively to the change of the atmospheric circulation than sea surface temperature anomalies (SSTAs). The positive SSSAs in the SCTR, induced by the anomalous wind stress curl and associated with thinner BLT anomalies, provides a favorable environment for the decreasing SSTAs during spring. Meanwhile, this SSSAs-BLT-SSTAs process also develops in the SSSAs maxima of the northern TIO but happens later than that in the southern TIO. Therefore, this time-lag between the northern and southern TIO strengthened the north-to-south SST gradient, which in turn, promotes the northward crossing current for the onset of SASM.To conclude, this Ph.D. research primarily investigates the variability of the TIO by distinguishing the main driver of the interannual upper-ocean temperature variability, studying the relationships between SSS and thermocline with BLT at seasonal and interannual scales and discovering the role of SSS in the onset of SASM. However, more efforts are highly demanded in the near future to investigate more sophisticated air-sea interaction mechanism within the TIO. For example, the theory of crossing depth is only adapted in the area of (15°S-20°N). Thus, how to distinguish the relative impacts of heat flux and wind stress on the upper-ocean variability in the area to the south of 15°S is still to be unraveled. In addition, the role of SSS in the interannual variability of the TIO needs more precise datasets to investigate and the numerical modeling with more sophisticated air-sea interactions should be taken into consideration.