Coral-based proxy calibrations constrain ENSO-driven sea surface temperature and salinity gradients in the Western Pacific Warm Pool

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Mohtar, A. T., Hughen, K. A., Goodkin, N. F., Streanga, I., Ramos, R. D., Samanta, D., Cervino, J., & Switzer, A. D. Coral-based proxy calibrations constrain ENSO-driven sea surface temperature and salinity gradients in the Western Pacific Warm Pool. Palaeogeography Palaeoclimatology Palaeoecology, 561, (2021): 110037, doi:10.1016/j.palaeo.2020.110037.Constraining past variability in ocean conditions in the Western Pacific Warm Pool (WPWP) and examining how it has been influenced by the El-Niño Southern Oscillation (ENSO) is critical to predicting how these systems may change in the future. To characterize the spatiotemporal variability of the WPWP and ENSO during the past three decades, we analyzed climate proxies using coral cores sampled from Porites spp. from Kosrae Island (KOS) and Woleai Atoll (WOL) in the Federated States of Micronesia. Coral skeleton samples drilled along the major growth axis were analyzed for oxygen isotopes (δ18Oc) and trace element ratios (Sr/Ca), used to reconstruct sea surface salinity and temperature (SSS and SST). Pseudocoral δ18O time series (δ18Opseudo) were calculated from gridded instrumental observations and compared to δ18Oc, followed by fine-tuning using coral Sr/Ca and gridded SST, to produce age models for each coral. The thermal component of δ18Oc was removed using Sr/Ca for SST, to derive δ18O of seawater (δ18Osw), a proxy for SSS. The Sr/Ca, and δ18Osw records were compared to instrumental SST and SSS to test their fidelity as regional climate recorders. We found both sites display significant Sr/Ca-SST calibrations at monthly and interannual (dry season, wet season, mean annual) timescales. At each site, δ18Osw also exhibited significant calibrations to SSS across the same timescales. The difference between normalized dry season SST (Sr/Ca) anomalies from KOS and WOL generates a zonal SST gradient (KOSWOLSST), capturing the east-west WPWP migration observed during ENSO events. Similarly, the average of normalized dry season δ18Osw anomalies from both sites produces an SSS index (KOSWOLSSS) reflecting the regional hydrological changes. Both proxy indices, KOSWOLSST and KOSWOLSSS, are significantly correlated to regional ENSO indices. These calibration results highlight the potential for extending the climate record, revealing spatial hydrological gradients within the WPWP and ENSO variability back to the end of the Little Ice Age.We also thank the crew of the M/V Alucia for assistance during the 2012 coral drilling expedition to FSM, funded by the Dalio Family Foundation through a WHOI Access to The Sea grant to KAH (#25110104). Geochemical analysis was funded by Singapore Ministry of Education Academic Research Fund Tier-2 (# MOE2016-T2-1016) to NFG and KAH, and by the WHOI Summer Student Fellowship Program (00450400) and Coastal Preservation Network 501c to IMS

Drivers of Coral Reconstructed Salinity in the South China Sea and Maritime Continent: The Influence of the 1976 Indo-Pacific Climate Shift

The flow of Pacific water into the Indian Ocean via the South China Sea (SCS) and Maritime Continent (MC) plays an important role in the ocean thermohaline circulation providing the only low-latitude pathway for the inter-ocean exchange of heat and salt. The transport of the SCS and Indonesian throughflows is modulated by the East Asian monsoon and major climate modes associated with the Pacific and Indian Oceans. As an indicator of surface layer buoyancy, sea surface salinity (SSS) is critical to rates of exchange but instrumental records of SSS are short and sparse. Using empirical orthogonal functions, a synthesis of proxy-based reconstructions of SSS from coral δ18O is used to study the role of climate variability on long-term SSS behavior in the region. The leading mode of SSS variability in the boreal winter and summer responds to the influence of the 1976 Indo-Pacific climate shift. At multi-decadal timescales, only the East Asian monsoon and the Indian Ocean Dipole (IOD) retain their signal in winter and summer SSS after 1976. At higher frequencies, winter SSS shifts from having a strong East Asian monsoon signal to a more dominant impact of the IOD and the El Niño Southern Oscillation (ENSO) following the shift. In the summer, only a change in ENSO's influence on SSS variability is observed after 1976. The recent intensification and dominance of the IOD and ENSO in driving SSS variability in the SCS and MC may influence circulation in the regional throughflows and perhaps global thermohaline circulation

Coral-based reconstruction of climatic and oceanographic variability across the Luzon Strait for the past ~230 years

The Kuroshio-dominated water exchange across the Luzon Strait impacts the climate and oceanography of the South China Sea and the total transport of the Indonesian Throughflow, inviting interest for research that will improve our understanding of the behavior and drivers of the Kuroshio Intrusion. However, continuous high-resolution observations of the Kuroshio intruding process are limited as comprehensive and systematic monitoring remain logistically challenging and complex. Massive corals provide an inexpensive means of extending high-resolution climatic and oceanographic records of the Kuroshio Intrusion and its drivers beyond the instrumental period, which is critical for evaluating long-term variability, regional trends and predictability under future change. One Diploastrea heliopora and two Porites sp. coral cores were collected on either side of the Luzon Strait – Palaui, northwestern Philippines and Houbihu, southern Taiwan. The coral cores were slabbed and x-rayed to determine sub-annual bands allowing for precise age-control. The slabs were subsampled for Sr/Ca, d18O and Δ14C as proxies for sea surface temperature (SST), sea surface salinity (SSS) and ocean circulation, respectively. Monthly-resolved SST and SSS reconstructions spanning the last 132 and 230 years were generated from Palaui and Houbihu, respectively. Seasonal and annual Δ14C records from both Palaui and Houbihu cover the past ~68 years. Sr/Ca-based records of SST reveal site-specific controls on observed SST variability. The Palaui paired Diploastrea and Porites winter Sr/Ca-SST records exhibit similar sensitivity to the Pacific Decadal Oscillation (PDO). The extended Porites-based winter SST reconstruction since the late 19th century remained sensitive to the PDO, highlighting its potential long-term impact on tropical climate variability. Houbihu winter SST, on the other hand, exhibits hemispheric-scale SST patterns driven by global forcings (i.e. solar and volcanic radiative forcings). The additional ~100 years in Houbihu relative to the Palaui record allow for further examination of conditions at the end of the Little Ice Age (LIA), which indicates cooler SSTs relative to the most recent 30 years. Both coral sites exhibit significant recent rise in SSTs likely tied to anthropogenic warming. Collectively, these findings suggest a change in temperature and extent of the warm pool region in the past 230 years. d18Oc-based records of SSS are robust allowing for investigating the advection signal across the Luzon Strait. Similar to Sr/Ca-SST, each of the Luzon Strait sites reveal site-specific controls on the observed surface salinity variability. The Palaui dry-season SSS record is sensitive to ENSO-related changes in hydrological processes. Seasonal difference between Palaui dry and wet season SSS record reveals the additional impact of the PDO in enhancing advection of Pacific waters to Palaui. In contrast, the Houbihu dry-season SSS record reflects East Asian Winter Monsoon (EAWM)-related changes in hydrological processes, modifying the surface salinity of the source waters. Examining the dry-season SSS difference between Palaui and Houbihu reveals the added influence of the PDO on the delivery of Pacific waters across the strait at decadal timescales. The additional ~100 years of dry-season SSS record in Houbihu indicate high salinity surface water conditions (i.e. drier) at the end of the LIA relative to the present. Inferring from the strong modern relationships between the Houbihu SSS and the EAWM and the PDO, the Kuroshio intrusion at the end of the LIA is likely reduced. These conditions potentially resulted in an altered SCS climate and circulation. Records of Δ14C from both Palaui and Houbihu also reveal site-specific processes that control advection and water mixing at these sites, corroborating findings in the Sr/Ca and d18O-based climate records. Palaui Δ14C relative to Pacific Δ14C values is sensitive to the ENSO-forced migration of the North Equatorial Current (NEC) bifurcation latitude. Houbihu Δ14C relative to SCS Δ14C values reflects EAWM-mediated transport across the Luzon Strait and circulation of the SCS. Overall, the interannual Δ14C difference between our sites and the surrounding seas reveal the dominant influence of ENSO in driving the Kuroshio Intrusion into the SCS at interannual timescales. Across proxy records, this thesis has revealed site-specific drivers of observed climatic variability between sites in the Luzon Strait. This key finding underscores the importance of investigating and identifying processes that are related to EAWM, ENSO and PDO variability independently at each site. Collectively, this information is critical to understand their interaction that in turn controls the Kuroshio intruding process into the SCS.Doctor of Philosoph

Coral records of temperature and salinity in the tropical western Pacific reveal influence of the Pacific Decadal Oscillation since the late nineteenth century

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ramos, R. D., Goodkin, N. F., Siringan, F. P., & Hughen, K. A. Coral records of temperature and salinity in the tropical western Pacific reveal influence of the Pacific Decadal Oscillation since the late nineteenth century. Paleoceanography and Paleoclimatology, 34(8), (2019): 1344-1358, doi: 10.1029/2019PA003684.The Pacific Decadal Oscillation (PDO) is a complex aggregate of different atmospheric and oceanographic forcings spanning the extratropical and tropical Pacific. The PDO has widespread climatic and societal impacts, thus understanding the processes contributing to PDO variability is critical. Distinguishing PDO‐related variability is particularly challenging in the tropical Pacific due to the dominance of the El Niño–Southern Oscillation and influence of anthropogenic warming signals. Century‐long western Pacific records of subannual sea surface temperature (SST) and sea surface salinity (SSS), derived from coral Sr/Ca and δ18O profiles, respectively, allow for evaluating different climatic sensitivities and identifying PDO‐related variability in the region. The summer Sr/Ca‐SST record provides evidence of a significant SST increase, likely tied to greenhouse gas emissions. Anthropogenic warming is masked in the winter Sr/Ca‐SST record by interannual to multidecadal scale changes driven by the East‐Asian Winter Monsoon and the PDO. Decadal climate variability during winter is strongly correlated to the PDO, in agreement with other PDO records in the region. The PDO also exerts influence on the SSS difference between the dry and wet season coral δ18O (δ18Oc)‐SSS records through water advection. The PDO and El Niño–Southern Oscillation constructively combine to enhance/reduce advection of saline Kuroshio waters at our site. Overall, we are able to demonstrate that climate records from a tropical reef environment significantly capture PDO variability and related changes over the period of a century. This implies that the tropical western Pacific is a key site in understanding multifrequency climate variability, including its impact on tropical climate at longer timescales.The authors would like to thank J. Ossolinski, J. Aggangan, J. Quevedo, R. Lloren, G. Albano, J. Perez, and A. Bolton for their help in acquiring core samples in the field. The detailed comments and suggestions of two anonymous reviewers significantly improved the original manuscript. This research was funded by the National Research Foundation Singapore under its Singapore NRF Fellowship scheme awarded to N. F. Goodkin (National Research Fellow award NRF‐RF2012‐03), as administered by the Earth Observatory of Singapore and the Singapore Ministry of Education under the Research Centers of Excellence initiative and by the Ministry of Education, Singapore through its Academic Research Fund Tier 2 (Project MOE2016‐T2‐1‐016). The coral Sr/Ca and δ18O data generated in this study are available in the supporting information Data Set S1 and are archived at the NOAA NCDC World Data Center for Paleoclimatology (https://www.ncdc.noaa.gov/paleo/study/27271). Other data and resources used in this study were sourced from the following sites: PDO index (http://research.jisao.washington.edu/pdo/PDO.latest); IPO index (https://www.esrl.noaa.gov/psd/data/timeseries/IPOTPI/ipotpi.hadisst2.data); NP index (https://www.esrl.noaa.gov/psd/data/correlation/np.data); PDO and North Pacific SST reconstructions (https://www.ncdc.noaa.gov/data‐access/paleoclimatology‐data); and MTM coherence and phase analysis MATLAB® code (https://www.mathworks.com/matlabcentral/fileexchange/22551‐multi‐taper‐coherence‐method‐with‐bias‐correction)

Coral-based proxy calibrations constrain ENSO-driven sea surface temperature and salinity gradients in the Western Pacific Warm Pool

Constraining past variability in ocean conditions in the Western Pacific Warm Pool (WPWP) and examining how it has been influenced by the El-Niño Southern Oscillation (ENSO) is critical to predicting how these systems may change in the future. To characterize the spatiotemporal variability of the WPWP and ENSO during the past three decades, we analyzed climate proxies using coral cores sampled from Porites spp. from Kosrae Island (KOS) and Woleai Atoll (WOL) in the Federated States of Micronesia. Coral skeleton samples drilled along the major growth axis were analyzed for oxygen isotopes (δ18Oc) and trace element ratios (Sr/Ca), used to reconstruct sea surface salinity and temperature (SSS and SST). Pseudocoral δ18O time series (δ18Opseudo) were calculated from gridded instrumental observations and compared to δ18Oc, followed by fine-tuning using coral Sr/Ca and gridded SST, to produce age models for each coral. The thermal component of δ18Oc was removed using Sr/Ca for SST, to derive δ18O of seawater (δ18Osw), a proxy for SSS. The Sr/Ca, and δ18Osw records were compared to instrumental SST and SSS to test their fidelity as regional climate recorders. We found both sites display significant Sr/Ca-SST calibrations at monthly and interannual (dry season, wet season, mean annual) timescales. At each site, δ18Osw also exhibited significant calibrations to SSS across the same timescales. The difference between normalized dry season SST (Sr/Ca) anomalies from KOS and WOL generates a zonal SST gradient (KOSWOLSST), capturing the east-west WPWP migration observed during ENSO events. Similarly, the average of normalized dry season δ18Osw anomalies from both sites produces an SSS index (KOSWOLSSS) reflecting the regional hydrological changes. Both proxy indices, KOSWOLSST and KOSWOLSSS, are significantly correlated to regional ENSO indices. These calibration results highlight the potential for extending the climate record, revealing spatial hydrological gradients within the WPWP and ENSO variability back to the end of the Little Ice Age.Ministry of Education (MOE)Published versionWe also thank the crew of the M/V Alucia for assistance during the 2012 coral drilling expedition to FSM, funded by the Dalio Family Foundation through a WHOI Access to The Sea grant to KAH (#25110104). Geochemical analysis was funded by Singapore Ministry of Education Academic Research Fund Tier-2 (# MOE2016-T2-1016) to NFG and KAH, and by the WHOI Summer Student Fellowship Program (00450400) and Coastal Preservation Network 501c to IMS

Repeat Storm Surge Disasters of Typhoon Haiyan and Its 1897 Predecessor in the Philippines

On 8 November 2013, Typhoon Haiyan impacted the Philippines with estimated winds of approximately 314 km h-1 and an associated 5–7-m-high storm surge that struck Tacloban City and the surrounding coast of the shallow, funnel-shaped San Pedro Bay. Typhoon Haiyan killed more than 6,000 people, superseding Tropical Storm Thelma of November 1991 as the deadliest typhoon in the Philippines. Globally, it was the deadliest tropical cyclone since Nargis hit Myanmar in 2008. Here, we use field measurements, eyewitness accounts, and video recordings to corroborate numerical simulations and to characterize the extremely high velocity flooding caused by the Typhoon Haiyan storm surge in both San Pedro Bay and on the more open Pacific Ocean coast. We then compare the surge heights from Typhoon Haiyan with historical records of an unnamed typhoon that took a similar path of destruction in October 1897 (Ty 1897) but which was less intense, smaller, and moved more slowly. The Haiyan surge was about twice the height of the 1897 event in San Pedro Bay, but the two storm surges had similar heights on the open Pacific coast. Until stronger prehistoric events are explored, these two storm surges serve as worst-case scenarios for this region. This study highlights that rare but disastrous events should be carefully evaluated in the context of enhancing community-based disaster risk awareness, planning, and response.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio

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)