multi-model analyses of projected oceanic response to global warming : North Pacific sea-level rise and equatorial reduction

This thesis consists of an analysis of a sea level rise in the North Pacific and an upwelling change along the equator in response to global warming, with emphases on the three-dimensional structure of subsurface ocean. These investigations are important because sea level rise will alter the habitability of coastal regions and equatorial upwelling change has potentials to alter the El Niño through the non-uniform warming of sea surface temperature and to influence ocean deoxygenation. The outputs from the Coupled Model Intercomparison Project Phase 5 (CMIP5) are used and the multi-model ensemble (MME) mean along with differences among models are analyzed in this study. Future changes in the dynamic sea level (DSL), which is defined as sea level deviation from the global mean sea level, is investigated over the North Pacific, by analyzing data from 30+ CMIP5 models under Representative Concentration Pathway (RCP) 8.5 and RCP4.5 until 2300. The study provides a more comprehensive description of DSL responses to the global warming in this region than previous studies, by using surface and subsurface data. The DSL changes in the North Pacific are characterized by a DSL rise in the western North Pacific around the Kuroshio Extension (KE), consistent with previous studies. Subsurface density analysis indicates that DSL rise around the KE is associated with a decrease in density of subtropical mode water (STMW) and with a northward KE migration. The former is more effective between 2000 and 2100 for both RCP4.5 and RCP8.5 and the latter is more effective between 2100 and 2300 for RCP8.5. The STMW density decrease is related to a large heat uptake to the south and southeast of Japan, while the northward KE migration is associated with the poleward shift of the wind stress field. These features are commonly found in multi-model ensemble (MME) means, and also explain differences among the climate models. The equatorial upwelling change in the Pacific and Atlantic Oceans is investigated using data from 24 CMIP5 models for RCP8.5 scenario until 2100. Three-dimensional velocity components of CMIP5 multi-models are used for the first time in an upwelling study, and allow to divide upwelling velocities into diapycnal and isopycnal velocities. This analysis, combined with inter-model regression, revealed that both the MME mean and the inter-model variability in the upwelling change in the eastern equatorial Pacific, where strong upwelling change occurs, are explained by two mechanisms. One is that the diapycnal upwelling decreases near the surface associated with the weakened Ekman divergence. The other is that the isopycnal upwelling decreases at depths of 75-200 m around the core of the equatorial undercurrent (EUC) due to the EUC flattening. Both the weakened Ekman divergence and the EUC flattening are induced by the locally weakened trade wind over the eastern Pacific basin. In the equatorial Atlantic, both the MME mean change and the inter-model variability of upwellings are significantly related to the weakened trade wind and enhanced stratification although these drivers are not independent. The results for the Pacific Ocean imply that future upwelling reduction may impact the sea surface temperature and marine ecosystem by different mechanisms. The rapid warming of sea surface temperature in the eastern basin may be related to the near surface diapycnal upwelling reduction, while the decrease of net community production, which often measured by marine export production at a depth of 100 m, may be related to the isopycnal upwelling reduction.