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Influence of East Asian Monsoon and El Niño Southern Oscillation (ENSO) to Holocene Hydroclimate deduced from Northwest Pacific corals
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| Title: | Influence of East Asian Monsoon and El Niño Southern Oscillation (ENSO) to Holocene Hydroclimate deduced from Northwest Pacific corals |
| Other Titles: | 完新世のサンゴ骨格記録を用いた東アジアモンスーンがENSOに与える影響の解明 |
| Authors: | Garas, Kevin Lariosa Browse this author |
| Issue Date: | 30-Jun-2023 |
| Publisher: | Hokkaido University |
| Abstract: | Geochemical records are important evidence of past climate and environmental changes beyond historical and instrumental records. This dissertation will focus on the interaction of East Asian monsoon (EAM) and El Niño Southern Oscillation (ENSO) in modulating the interannual and seasonal sea surface temperature (SST), sea surface salinity (SSS), and rainfall variabilities in the subtropical and tropical Northwest Pacific during the Holocene. Investigating how the interannual ENSO affects the monsoon is vital in seasonal rainfall prediction. Due to the complex teleconnections of equatorial Pacific SST anomalies (SSTA) and the East-Southeast Asian climate, few paleoclimate records could explain the mechanism of EAM-ENSO interaction in the Holocene.
This research explores the potential of corals from Kikai Island, Japan and NW Luzon Island, Philippines. There are 40 Porites colonies collected from the two study areas. A series of screening of fossil corals was done using thin section analysis, X-ray Diffraction (XRD), and Scanning Electron microscopy (SEM). Fossil corals with excellent preservation were dated using Accelerator Mass Spectrometer to measure the radiocarbon age dates. Corals were cut into 5-mm slabs, cleaned, and prepared for X-radiography, microsampling, and geochemical analysis. Coral growth parameters such as linear extension rate, skeletal density, and calcification rate were calculated from the X-ray photographs of corals. Sr/Ca ratio was measured with an Inductively Coupled Plasma-Atomic Emission Spectrometry (iCAP 6300 ICP Spectrometer). Similar coral powder samples were analyzed using Isotope Ratio Mass Spectrometer (MAT Finnigan 253) with an automated carbonate device Kiel IV for oxygen isotope analysis.
Results of thin section analysis, XRD, and SEM have shown that six fossil Porites have excellent to good fossil preservation. These samples have <1% calcite content based on phase mineral estimates from Reference Intensity Ratio (RIR) Method. Corals with excellent preservation are devoid of dissolution, fine- to medium-sized acicular aragonite and rhombohedral calcite overgrowths. Well-preserved fossil corals from Kikai Island yielded radiocarbon age dates of 3235±20 (KIKJ-20160907-II) and 5712±24 (KIST-20160518-I) years BP. Well-preserved fossil corals from NW Luzon Island yielded radiocarbon age dates of 6285±79 (CuPf-180218-1), 6144±77 (CuPf-180218-4), 4336±21 (DMT-2), and 4200±20 (DMT-1) years BP. The annual linear extension rates of modern corals from Kikai and NW Luzon islands were plotted against the annual Sr/Ca and δ18Ocoral for each coral colony to investigate the effect of growth rate on coral geochemistry. The absence of a strong linear correlation between these variables suggests that growth minimally affects the variation of these two climate proxies.
The ordinary least square (OLS) regression analysis demonstrated the dependence of coral Sr/Ca, δ18Ocoral, and δ18Osw on SST, SSS, and rainfall. The monthly-resolved Sr/Ca records of modern corals from Kikai Island and NW Luzon Island are negatively correlated to the AVHRR SST from 1989-2015 (slope: -0.0643 mmol/mol°C-1) and OISST from 2011-2017 (slope: -0.0057 mmol/mol°C-1) respectively. The 5-point running average of δ18Osw and monthly in-situ rainfall data of Kikai Island from 2007-2015 were regressed and have shown statistically significant negative linear correlation (r=0.55; p<0.000; n = 93). The monthly-resolved δ18Osw from NW Luzon Island coral and SODAv3.3.1 SSS from 2011-2017 have statistically significant positive linear correlation (r=0.50; p<0.000; n = 48). The regression equations were used to interpret and quantify the hydroclimatic changes in fossil coral geochemical records.
The mean seasonal trends of all modern and fossil corals revealed the seasonality of SST, SSS, and rainfall. The changes in coral-based SST, SSS, and rainfall records of mid- to late Holocene were likely driven by the changes in the monsoon intensity. The summer, winter, and annual mean Sr/Ca-SST and δ18Ocoral records from fossil Porites in subtropical and tropical NW Pacific were compiled to investigate seasonal SST and SSS change in mid-Holocene to late Holocene. The lower SSS and higher rainfall in subtropical NW Pacific were coeval to the higher SSS and lower rainfall in tropical NW Pacific from 7.0 ka to 5.0 ka. The subtropical and tropical SSTA records consistently show that the period was not significantly different and warmer (+0-2°C) at times than the present. The SSS trend of subtropical NW Pacific shifted positively after 4.9 ka, while the SSS trend in tropical NW Pacific shifted negatively around 4.3 to 4.2 ka. After 5.0 ka, the summer SSTA shows a cooling trend in subtropical (-1.8°C) and tropical (-0.9°C) NW Pacific. The coral evidence supports the widely reported EASM intensification (8.2 ka to 4.7 ka) and weakening (4.7 ka to 3.0 ka) based on the stacked marine and terrestrial sediment records from NW Pacific. Moreover, the coral-derived SST and SSS datasets agree that the mean position of the intertropical convergence zone (ITCZ) migrated southwards from the mid-Holocene towards the late Holocene. The northward ITCZ migration and intensification of EASM resulted in the enhancement of summer rainfall in the subtropics, while the rainfall in tropical NW Pacific was reduced in the mid-Holocene. The southward migration of the ITCZ and weakening of the EASM resulted in the aridification of East Asia and reduced rainfall/higher SSS of subtropical NW Pacific. The southerly position of the ITCZ brought more rainfall in the tropical NW Pacific.
Aside from monsoon, the ENSO affects the rainfall pattern in NW Pacific. However, the lack of high temporal resolution records hinders the reconstruction of ENSO-driven rainfall seasonality. ENSO-related signal in coral-derived δ18Osw was isolated by filtering the 2-8-year bandpass frequency. The δ18Osw threshold values for different ENSO intensities were determined using the statistically significant positive correlation (r=0.75; r2=0.56; p<0.000) of 2-8-year bandpass filtered δ18Osw data and the ERSST v4 NINO 3.4 SSTA from 1982 to 2012. The same bandpass filtering method was used to show the ENSO cycle in Holocene coral records from Kikai Island and NW Luzon Island. Using the threshold values from the regression analysis, El Niño and La Niña years were identified in each fossil coral records. The change of standard deviation of δ18Osw records from Kikai fossil corals revealed an 11% to 49% ENSO reduction relative to the 1989-2015 benchmark. The δ18Osw records from NW Luzon fossil corals revealed a 20% to 50% ENSO reduction relative to the 2011-2017 benchmark.
The distinct rainfall seasonality during neutral and El Niño phase of ENSO was evaluated using modern δ18Osw records. Seasonal rainfall in Kikai Island became more pronounced during El Niño of 1991-92 and 1997-98 because of the reduced rainfall during the developing year and enhanced rainfall during the following summer of the decaying year. The rainfall in NW Luzon generally decreased during the strong El Niño in 2015-2016 relative to the neutral phase. Analyses of 3.2 ka, 4.9 ka, and 5.7 ka corals from Kikai Island show increased subtropical rainfall with increasing ENSO variability. On the other hand, the 4.2 ka, 4.3 ka, and 6.1 ka corals from NW Luzon show that ENSO influenced the seasonal rainfall despite the relatively weak variance during these time-windows. However, the other prevailing climate modes could have masked the ENSO effect, as shown by the possible impact of strong EASM at 6.1 ka.
This research proposes a new way to investigate the complexity of monsoon and ENSO effects on the seasonal hydroclimate of the region. The new coral data from NW Pacific confirmed a mid-Holocene ENSO weakening as suggested by coral climate proxies (Cobb et al., 2013; Grothe et al., 2019) and paleo-ENSO model (Tian et al., 2017). However, the mechanism involving the interaction of the monsoon circulation and ENSO cycles in varying SST boundary conditions is still unclear and needs further research. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第15562号 |
| Degree Level: | 博士 |
| Degree Discipline: | 理学 |
| Examination Committee Members: | (主査) 教授 沢田 健, 教授 栗谷 豪, 講師 渡邊 剛, 講師 山崎 敦子 （名古屋大学大学院環境学研究科） |
| Degree Affiliation: | 理学院（自然史科学専攻） |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/90563 |
| Appears in Collections: | 課程博士 (Doctorate by way of Advanced Course) > 理学院(Graduate School of Science)
学位論文 (Theses) > 博士 （理学）
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