Mixing it up in the Makassar Strait: Seasonal Reconstructions from Corals of Sea Surface Temperature and Salinity Allow for Records of Source Water Mixing in the Makassar Strait

Abstract:

The Indonesian seas provide a maze of passages through which relatively warm and fresh water flows from the equatorial Pacific to the Indian Ocean forming the Indonesian Throughflow (ITF). As it flows, the ITF provides a transport pathway affecting heat and buoyancy fluxes throughout the region, interacting with both the Asian monsoon and the El Niño Southern Oscillation (ENSO). The present and past behavior of the ITF is therefore one of the keys to understanding regional and global climate variability. Past ITF behavior has been previously recorded in massive coral colonies, which serve as continuous, high-resolution climate archives because of their high linear extension rates and annual banding pattern. Here we present Sr/Ca-SST and δ¹⁸O-SSS calibrations from three Porites spp. coral colonies located in the southern Makassar Strait, through which roughly 75-80% of all ITF waters flow. At interannual timescales, we find that in all three corals Sr/Ca significantly (all p-values<0.05) correlates with sea surface temperature (SST) during the SE monsoon from July to September, while δ¹⁸O is correlated with sea surface salinity (SSS) during the NW monsoon from December to March (all p-values<0.05). HADI SST and SODA SSS instrumental temperature-salinity diagrams from our coral sites indicate that the southern Makassar Strait is a region of surface seawater mixing between end member water masses of the ITF during the SE monsoon. During the NW monsoon, however, end members of the South China Sea Throughflow (SCSTF) instead mix in the surface waters. The strength of our interannual calibrations suggests that it may be possible to reconstruct the ITF and SCSTF surface mixing patterns during their respective monsoon seasons. To preliminarily assess this, we present 150-year seasonally resolved records of Sr/Ca and δ¹⁸O to reconstruct surface water mixing in the southern Makassar Strait.