Reconstructing surface water mixing and climate using records of sea surface salinity, precipitation and radiocarbon from Southeast Asian corals
Murty, Sujata A.
Date of Issue2018-09-13
Interdisciplinary Graduate School (IGS)
Earth Observatory of Singapore
The East Asian Monsoon and the El Niño Southern Oscillation (ENSO) are both key components of Southeast Asian climate variability that impact precipitation and ocean circulation on seasonal to interannual timescales. In the Indonesian Seas, both climate systems influence the Indonesian Throughflow (ITF), a globally important ocean current that fuels heat and buoyancy fluxes throughout the Indo-Pacific. ENSO impacts the ITF on interannual timescales, while the East Asian Winter Monsoon (EAWM) moves surface waters from the South China Sea (SCS) into the western Indonesian Seas that obstruct surface ITF transport seasonally. However, multi-model projections of climate under anthropogenic warming predict changes in the Asian Monsoon and ENSO systems that are expected to impact regional and global climate and ocean variability. Efforts to constrain past EAWM and ENSO influences on surface water circulation are thus critical in order to better anticipate future changes in the interactions between monsoon-driven circulation in the Indonesian Seas and Indo-Pacific climate. Examining interannual to multi-decadal ocean and climate variability throughout Indonesia requires reconstructions of sea surface salinity (SSS), ocean circulation and rainfall that extend beyond instrumental records. Corals are ideal climate archives that provide continuous climate records at subannual resolution due to high linear extension rates and annual density banding. In this study, high-resolution (~biweekly to ~bimonthly) Porites coral proxy records are presented from the Makassar, northern Lombok, and southern Lombok Straits for 87, 110 and 193 years, respectively. These sites are located along main ITF pathways, allowing for examination of monsoon and ENSO influences. Samples were analyzed for δ18O, δ13C, Sr/Ca, Ba/Ca and Δ14C. Measurements of δ18O were used to reconstruct SSS, Δ14C was used to examine surface ocean circulation, and Ba/Ca was used to reconstruct both ocean circulation and rainfall variability. In the Makassar Strait, coral proxy records indicate that changes in the state of the EAWM and Indo-Pacific climate alter interannual to inter-decadal surface ocean circulation. Temperature-salinity relationships using instrumental data indicate that two distinct SCS source waters mix during the EAWM. However, coral-reconstructed temperature-salinity (T-S) relationships and reconstructed SSS collectively suggest that the EAWM impact on SCS surface water mixing decreases after the late 1970s, coinciding with the 1976 Indo-Pacific climate regime shift. A record of seasonally resolved Δ14C similarly reveals an increasing ENSO influence on surface ocean circulation that coincides with 1) a reduced EAWM influence following the transition to a negative EAWM mean state in the 1960s, and 2) the 1970s shift in Indo-Pacific climate. Farther along the ITF pathway in the Lombok Strait, coral proxy records continue to indicate changes in the EAWM and ENSO impacts on surface water circulation. Coral reconstructed T-S relationships, reconstructed SSS and Ba/Ca variability (used to trace water masses) reveal shifts in the contribution of monsoon-driven SCS source waters that coincide with 1) the transition from a strong to weak EAWM mean state in the 1960s, and 2) increased ENSO activity following the Indo-Pacific climate shift. Overall, the coral records from the Makassar and Lombok Straits document that changes in the strength and state of the EAWM and Indo-Pacific climate alter surface water circulation throughout the Indonesian Seas. The East Asian Monsoon and ENSO additionally influence rainfall over the Lombok Strait. Coral Ba/Ca variability from the southern Lombok Strait reveals significant influences of the East Asian Monsoon and ENSO on rainfall across seasonal to multi-decadal timescales. A secular increase in the coral record additionally correlates to a centennial-scale increase in northern hemisphere surface temperature, supporting model projections of enhanced precipitation under anthropogenic warming. The coral proxy records of rainfall, ocean circulation and SSS presented in this thesis thus collectively reflect the influences and interactions between regional (East Asian Monsoon) and global (ENSO) climate phenomena, with important implications for future changes in Indo-Pacific climate and ocean systems.