Understanding the Inter-basin Interactions of the Tropical oceans on the Low-frequency Timescales

Abstract

This thesis aims to expand and improve our understanding of the inter-basin interaction between tropical oceans on the low-frequency timescales. Recent interest in inter-basin interactions has increased since previous studies have shown that the Pacific, Atlantic, and Indian oceans are closely connected and interact with each other through the tropical Walker circulation and mid-latitude teleconnections. On the other hand, many studies have mainly focused on Interannual variabilities, and hence further studies are needed to understand their decadal variabilities. Moreover, a decadal variability could modulate the strength or characteristics of their Interannual relationship. Based on these issues, this thesis investigated the relationship between inter-basin interactions on low-frequency timescales. Each chapter addresses the interaction of two or more different oceans, and the summary is as follows. First, the non-stationary relationship between Pacific decadal oscillation (PDO) and Indian Ocean Basin Mode (IOBM) is investigated in Chapter 2. In general, these two variabilities are positively correlated, but in this study, we found that this in-phase relationship is characterized by a non-stationary characteristic in which there are periods with a significant correlation and an insignificant correlation. During the period when the relationship between PDO-IOBM was weak (1976-2006), the sea surface temperature (SST) structure of the negative phase of the PDO was different from that of a typical negative phase of the PDO, which caused different responses of the precipitation and the atmospheric circulation in the Pacific. As a result, the Indian Ocean was not cooling enough, resulting in a weakening of the relationship between the PDO and IOBM. Using two pacemaker experiments that nudged the SST observed in the Pacific and Indian Ocean regions, respectively, we showed that the observational results are mainly due to tropical SST change in the Pacific. In addition, long-term simulations and large-scale ensemble results of the Community Earth System Model (CESM) support that this non-stationary relationship of PDO-IOBM might be caused by internal variability. Next, in Chapter 3, the inter-basin interaction between the Pacific and Atlantic oceans was investigated. Previous studies have shown that long-period variabilities in the Pacific and Atlantic oceans can alternately interact with each other through the tropics, but further research is still needed to determine when these interactions work effectively. This study confirmed that the Pacific-Atlantic interaction changes which depend on their phase combination of PDO and Atlantic multidecadal oscillation (AMO). If the PDO and AMO were in the same phase, the Pacific Ocean influenced the Atlantic Ocean by the Walker circulation caused by the Pacific Ocean. In the tropical Atlantic, a negative SST-precipitation relationship was shown in which the atmosphere affected the ocean. On the other hand, if the two variabilities were in different phases, local convection was strengthened in the tropical Atlantic region, altering the Walker circulation. Consequently, the Atlantic affects the Pacific Ocean. Correspondingly, in the tropical Atlantic Ocean, a positive SST-Precipitation relationship was found in which the ocean affected the atmosphere. Additional pacemaker experiments using the climate model also supported the observational results. The final chapter found that AMO affects the relationship between ENSO and the IOD (Indian Ocean Dipole). The ENSO and IOD generally have a positive correlation; however, its relationship is weak (strong) when AMO is a positive (negative) phase. During a negative phase of AMO, for example, the eastern Pacific (EP)-type El Niño increases, and the upward motion related to the Walker circulation becomes stronger in the central-eastern tropical Pacific region. Thus, the high-pressure anomaly with descending motion in the Maritime continent region and the divergence in the lower-level atmosphere are strengthened. Therefore, the westerly wind anomalies related to El Niño in the Pacific Ocean and the easterly wind anomalies related to the positive IOD in the Indian Ocean appear, resulting in a strengthening of the ENSO-IOD relationship.