Natural and anthropogenic forcing of multi-decadal to centennial scale variability of sea surface temperature in the South China Sea

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Goodkin, N. F., Samanta, D., Bolton, A., Ong, M. R., Hoang, P. K., Vo, S. T., Karnauskas, K. B., & Hughen, K. A. Natural and anthropogenic forcing of multi-decadal to centennial scale variability of sea surface temperature in the South China Sea. Paleoceanography and Paleoclimatology, 36(10), (2021): e2021PA004233, https://doi.org/10.1029/2021PA004233.Four hundred years of reconstructed sea surface temperatures (SSTs) from a coral located off the coast of Vietnam show significant multi-decadal to centennial-scale variability in wet and dry seasons. Wet and dry season SST co-vary significantly at multi-decadal timescales, and the Interdecadal Pacific Oscillation (IPO) explains the majority of variability in both seasons. A newly reconstructed wet season IPO index was compared to other IPO reconstructions, showing significant long-term agreement with varying amplitude of negative IPO signals based on geographic location. Dry season SST also correlates to sea level pressure anomalies and the East Asian Winter Monsoon, although with an inverse relationship from established interannual behavior, as previously seen with an ocean circulation proxy from the same coral. Centennial-scale variability in wet and dry season SST shows 300 years of near simultaneous changes, with an abrupt decoupling of the records around 1900, after which the dry season continues a long-term cooling trend while the wet season remains almost constant. Climate model simulations indicate greenhouse gases as the largest contributor to the decoupling of the wet and dry season SSTs and demonstrate increased heat advection to the western South China Sea in the wet season, potentially disrupting the covariance in seasonal SST.This research was supported by a Singapore National Research Fellowship to N.F. Goodkin (NRFF-2012-03) as administered by the Earth Observatory of Singapore and by a Singapore Ministry of Education Academic Research Fund Tier 2 award to N.F. Goodkin, K.A. Hughen, and K.B. Karnauskas (MOE-2016-T2-1-016). D. Samanta was partially supported by a Singapore Ministry of Education Tier 3 award (MOE2019-T3-1-004)

East Asian Monsoon variability since the sixteenth century

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters, 46(9), (2019):4790-4798, doi:10.1029/2019GL081939.The East Asian Monsoon (EAM) impacts storms, freshwater availability, wind energy production, coal consumption, and subsequent air quality for billions of people across Asia. Despite its importance, the EAM's long‐term behavior is poorly understood. Here we present an annually resolved record of EAM variance from 1584 to 1950 based on radiocarbon content in a coral from the coast of Vietnam. The coral record reveals previously undocumented centennial scale changes in EAM variance during both the summer and winter seasons, with an overall decline from 1600 to the present. Such long‐term variations in monsoon variance appear to reflect independent seasonal mechanisms that are a combination of changes in continental temperature, the strength of the Siberian High, and El Niño–Southern Oscillation behavior. We conclude that the EAM is an important conduit for propagating climate signals from the tropics to higher latitudes.Thanks go to G. Williams, W. Tak‐Cheung, and J. Ossolinski. Thanks also go to V. Lee, S. H. Ng for coral sampling, and B. Buckley for conversations. This research was supported by the National Research Foundation Singapore NRF Fellowship scheme awarded to N. Goodkin (National Research Fellowship award NRFF‐2012‐03) and administered by the Earth Observatory of Singapore and the Singapore Ministry of Education under the Research Centers of Excellence initiative. The research was also supported by the Singapore Ministry of Education Academic Research Fund Tier 2 (award MOE2016‐T2‐1‐016). Data are available in Table S1 and the NOAA paleoclimate database

Environmental Calibration of Coral Luminescence as a Proxy for Terrigenous Dissolved Organic Carbon Concentration in Tropical Coastal Oceans

The riverine flux of terrigenous dissolved organic matter (tDOM) to the ocean is a significant contributor to the global carbon cycle. In response to anthropogenic drivers the flux is expected to increase. This may impact the availability of sunlight in coastal ecosystems, and the seawater carbonate system and coastal CO2 fluxes. Despite its significance, there are few long-term and high-resolution time series of tDOM parameters. Corals incorporate fluorescent tDOM molecules from the chromophoric dissolved organic matter (CDOM) pool in their skeletons. The resulting coral skeletal luminescence variability has traditionally been used to reconstruct hydroclimate variation. Here, we use two replicate coral cores and concurrent in-situ biogeochemical data from the Sunda Shelf Sea in Southeast Asia, where peatlands supply high tDOM inputs, to show that variability in coral luminescence green-to-blue ratios (coral G/B) can be used to quantitatively reconstruct terrigenous dissolved organic carbon (tDOC) concentration. Moreover, coral G/B can be used to reconstruct the CDOM absorption spectrum from 230 to 550 nm, and the specific ultraviolet absorbance at 254 nm (SUVA(254)) of the DOM pool. Comparison to a core from Borneo shows that there may be site-specific offsets in the G/B-CDOM absorption relationship, but that the slope of the relationship is very similar, validating the robustness of the proxy. By demonstrating that corals can be used to estimate past changes in coastal tDOC and CDOM, we establish a method to study drivers of land-ocean tDOM fluxes and their ecological consequences in tropical coastal seas over decadal to centennial time scales.ISSN:1525-202

The CoralHydro2k database: a global, actively curated compilation of coral δ18O and Sr/Ca proxy records of tropical ocean hydrology and temperature for the Common Era

The response of the hydrological cycle to anthropogenic climate change, especially across the tropical oceans, remains poorly understood due to the scarcity of long instrumental temperature and hydrological records. Massive shallow-water corals are ideally suited to reconstructing past oceanic variability as they are widely distributed across the tropics, rapidly deposit calcium carbonate skeletons that continuously record ambient environmental conditions, and can be sampled at monthly to annual resolution. Climate reconstructions based on corals primarily use the stable oxygen isotope composition (δ18O), which acts as a proxy for sea surface temperature (SST), and the oxygen isotope composition of seawater (δ18Osw), a measure of hydrological variability. Increasingly, coral δ18O time series are paired with time series of strontium-to-calcium ratios (Sr/Ca), a proxy for SST, from the same coral to quantify temperature and δ18Osw variability through time. To increase the utility of such reconstructions, we present the CoralHydro2k database, a compilation of published, peer-reviewed coral Sr/Ca and δ18O records from the Common Era (CE). The database contains 54 paired Sr/Ca-δ18O records and 125 unpaired Sr/Ca or δ18O records, with 88% of these records providing data coverage from 1800CE to the present. A quality-controlled set of metadata with standardized vocabulary and units accompanies each record, informing the use of the database. The CoralHydro2k database tracks large-scale temperature and hydrological variability. As such, it is well-suited for investigations of past climate variability, comparisons with climate model simulations including isotope-enabled models, and application in paleodata-assimilation projects. The CoralHydro2k database is available in Linked Paleo Data (LiPD) format with serializations in MATLAB, R, and Python and can be downloaded from the NOAA National Center for Environmental Information\u27s Paleoclimate Data Archive at 10.25921/yp94-v135 (Walter et al., 2022)