The impacts of the constrictions of the Indonesian Gateway and the Central American Seaway on ocean circulation are among the keys to understand Pliocene climate evolution, including the intensification of the Northern Hemisphere Glaciation between 3.5 and 2.5 Ma. Plate tectonic reconstructions show that the main reorganization of one such seaway, the Indonesian Gateway, occurred between 4 and 3 Myr ago. Model simulations have suggested that this tectonic reorganization triggered far-reaching effects on ocean circulation and climate, including a switch in the source of waters feeding the Indonesian Throughflow into the Indian Ocean. This PHD thesis aims to elucidade the climatic and oceanographic changes related to the constriciton of the Indonesian Gateway. It presents combined d18O and Mg/Ca ratios of planktonic foraminifera (marine protozoa) from surface and subsurface levels to reconstruct the thermal structure and changes in salinities at four following sensitive core sites in the Indian and Pacific Oceans from ~6 to 2 Myr ago: DSDP Site 214 in the tropical east Indian Ocean, ODP Site 709C in the west tropical Indian Ocean, ODP 763A in the subtropical east Indian Ocean under the influence of the Leeuwin Current, and DSDP Site 590B in the southwest Pacific Ocean at the Tasman Front: In the outflow region of the Indonesian Throughflow (DSDP Site 214), sea surface conditions remained relatively stable throughout the mentioned Pliocene interval, while subsurface waters (300-450 m water depth) freshened and cooled by about 4°C between 3.5 and 2.95 Myr ago. After 2.95 Ma, constantly low subsurface temperatures and fresher conditions suggested a prevailing throughflow of North Pacific source waters through the Indonesian Gateway. These findings supported the hypothesis of Cane and Molnar (2001) that the constriction of the Indonesian Gateway (4-3 Ma) led to a major reorganization in the Indonesian Throughflow. The cooling and shoaling of the thermocline in the tropical Indian Ocean might have contributed to cooling in various (sub)tropical upwelling regions. At Site 763, surface temperatures cooled by ~2°C compared to tropical Indian Ocean sites 214 and 709C during the mid-Pliocene, pointing to a Leeuwin Current, which weakened since ~3.3 Myr ago in line with the hydrographic changes in the Indonesian Throughflow region. Most likely, a reduced surface Indonesian Throughflow led to a diminished poleward heat transport resulting in a weakened Leeuwin Current and a cooling of the Benguela upwelling system. Thereby, by cooling the southern Indian and Atlantic Oceans this mechanism amplified the mid-Pliocene global development towards increased meridional temperature gradients. The Tasman Front Site 590B is influenced by both, the Central American Seaway and the Indonesian Gateway. Gradual cooling of ~2°C, and freshening of the sea surface during ~4.6-4 Ma was related to the closing of the Central American Seaway, which reached a critical threshold during this time and presumably cooled the southwest Pacific through heat piracy by the Northern Hemisphere. After ~3.5 Ma, the ongoing restriction of the Indonesian Gateway might have amplified the southward heading East Australian Current, allowing still warm sea surface temperatures at Site 590B when the global climate gradually cooled. In contrast, the cooling and freshening of the subsurface level in line with a marked increase in the sandfraction points to a fostered northward circulation of Subantartic Mode- and Antartic Intermediate waters, possibly a first step towards the present Antarctic Frontal System.