Abstract
As salinity and temperature of South Pacific Tropical Water (SPTW) increased due to climate change, there is interest in its variability and downstream impacts on the equatorial region. SPTW was characterized by a vertical salinity maximum 35.6-36.5 psu located at 8°S-25°S, 160°W-110°W and lying in the upper thermocline between 24 and 25 σθ. This dissertation examines variability in SPTW that were affected by different forcing mechanisms, and the downstream impacts of this variability on circulation as these waters moved equatorward. Kinematic method and tracer method for calculating subduction rates, complex empirical orthogonal function (CEOF) analysis for mapping spiciness anomalies and propagation patterns were used. Argo, hydrographic and tracers were the main data. The output from the Community Climate System Model 4 (CCSM4) was compared to observations and was used to examine coupled air-sea processes. The Connectivity Modeling System (CMS) was used to track SPTW anomalies and the results were used to examine downstream equatorial impacts. This dissertation finds new connections between the downstream impacts of variability in SPTW and the atmosphere over the equatorial region. First, SPTW annual subduction rates were calculated using two methods, and their variability at interannual and potentially longer time scales was examined. The kinematic method used monthly averaged data from the Argo product, 2005 to 2018. The SPTW annual subduction rates over this period averaged 25 m yr-1 and varied from 14 to 47 m yr-1. Furthermore, subduction and lateral induction rates were negatively correlated with the Nino 3.4 index. The tracer method using chlorofluorocarbon (CFC-12) data from the 1990s generated an average subduction rate of 35 5 m yr-1 over the same SPTW volume as that estimated from the kinematic method. Differences between the two methods and variability in vertical pumping may explain the differences in SPTW subduction rates between the two decades. After subduction, SPTW was transported westward by the South Equatorial Current (SEC). The SEC exhibited two three-year cycles and one four-year cycle, with each cycle containing an El Niño-Southern Oscillation event. Rossby waves played an essential role in these cycles as well as in the westward extension of the SPTW. These results suggested that variability in SPTW subduction rates and hydrographic properties, which was transported westward by the SEC, has the potential to influence the downstream circulation in the tropical Pacific. Second, temperature (T) and salinity (S) anomalies were shown for the first time to alter density downstream, which provided a mechanism for affecting the equatorial circulation. Two T/S anomalies, with respect to the Argo period mean 2005 to 2014, originated in SPTW from about 20°S, 120°W at 24 - 25σθ isopycnals. T anomalies had a stronger influence on density anomalies than S anomalies. Warm/salty anomalies were produced during El Niño phases and cold/fresh anomalies were produced during La Niña phases. Buoyancy changes induced by warm/salty anomalies caused enhanced vertical exchange. Downstream vertical exchange of water between surface and subsurface was intensified during warm/salty anomaly years and weakened during cold/fresh anomaly years. The anomalies propagated westward with the SEC, and after reaching the western boundary some continue equatorward. Thus, both anomalies appeared to affect the downstream vertical exchange in the tropics. In addition, warm/salty anomalies affected equatorial surface water characteristics. Lastly, the potential impacts of SPTW T/S anomalies downstream on the equatorial region surface waters were investigated. Subducted SPTW in the Community Climate System Model 4 (CCSM4) was compared to Argo data. Density uncompensated T and S anomalies were observed in both CCSM4 and Argo. In CCSM4, the average SPTW subduction area was 7°S-20°S, 155°W-105°W, which was smaller than in the Argo data (8°S-25°S, 160°W-110°W). Subducted density uncompensated T/S anomalies were carried by the SEC into the western subtropical Pacific. During westward propagation, density uncompensated subsurface warm/salty anomalies mixed upward into the surface layer. Using particles in CMS to track T/S anomalies, approximately 37% of the originally tracked SPTW particles crossed 8°S into the equatorial region, of which 90% resurfaced in the mixed layer at the end of the 20 years integration. In CCSM4, there was an increased evaporative flux (0.4 kg m-2 s-1) over the western Pacific warm pool one year after resurfacing of warm/salty anomalies. Thus, resurfacing of density uncompensated anomalies appeared to affect the evaporation rate in the equatorial region, and by doing so have the potential to affect the atmosphere and possibly El Niño.