Browsing by Author "Fischer, MJ"
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- ItemA 1,000 year rainfall record for SE Australia using speleothem hydrological proxies(Geological Society of Australia, 2010-07-04) McDonald, J; Drysdale, RN; Hodge, E; Hua, Q; Fischer, MJ; Treble, PC; Greig, A; Hellstrom, JCCave drip water studies at Wombeyan Caves (34°19’S, 149°59’E) demonstrated a marked hydrochemical response to wet/dry (McDonald et al., 2004; 2007). Three 20th Century coeval active speleothems (WM4, WM6 and WM20) were geochemically analysed and changes in trace element and stable isotope values were related to the instrumental record. These climate‐geochemical relationships were interpolated to two longrecord speleothems (WM7 and WM9) which grew deeper within the same cave system. Two major difficulties were encountered. Unlike other sub‐annual/annual climate records derived from speleothems using trace elements (Treble et al., 2003; Johnson et al., 2006); stable isotopes (Treble et al., 2005; Johnson et al., 2006) and annual laminae (Borsato et al., 2007) where annual cycles were shown to be consistent, at Wombeyan frequent droughts perturb predicted rainfall patterns and rainfall variability is high. Here, droughts can span several years, or extend over winter, diminishing or obliterating the expected winter recharge signal. Alternatively, wet summers can sustain ‘unseasonal’ high discharge and lessen the expected prior calcite precipitation (PCP) signal. Thus an annual wet/dry geochemical signal is often absent. Secondly, due to the young age of the speleothems and very low uranium concentrations (~10 ppb) the use of U‐series disequilibrium dating was ineffective to produce a robust chronology. Trace element cycle counting only gave the minimum age due to the non‐expression of many cycles. The 14C bomb pulse was successfully defined in two modern stalagmites (WM4 and WM6) and maximum 14‐C activity was around 134 per cent modern carbon (pMC) for both speleothems, indicating rapid transfer of 14‐C from atmosphere, to soil, to drip water during the bomb‐pulse period. A dampened 14C bomb pulse was detected in WM7 (where pMCmax was 112 per cent modern carbon) reflecting the greater degree of mixing within the thicker bedrock. Carbon‐14 AMS analyses were utilised together with trace element cycle counting to obtain preliminary chronological control. Despite these difficulties, palaeohydrological records using multiple proxies: Mg/Ca (aridity), P and Y (wet), δ18O (dominant air mass and rainfall amount signals), and based on 14C ages were constructed for the longerrecord, slower growing stalagmites. Over the last 1,000 years there have been several sustained episodes of wet/arid or highly variable phases. A sustained wet phase occurred ~ 900–1300 AD and followed by ~ 200 years of highly variable wet/dry conditions. From ~ 1500 to 1800 AD a dry phase is indicated. The last 150 years support a drying phase, but the negative IPO (1944–1978 [wettest period in 20thC]) is not indicated by a negative anomaly and further, sustained positive δ18O anomalies in this period suggest that other factors maybe influencing this part of the record (temperature?). The δ18O record indicates changing frequency from ~ centennial to pentadecadal time‐scales in the longer‐time scale oscillations. Within the longer‐time scale oscillations, higher resolution (~ 2–5 years) variability is evident replicating the trend shown by modern annually resolved stalagmites at this site.
- ItemCalibrating climate-δ18O regression models for the interpretation of high-resolution speleothem δ18O time series(American Geophysical Union, 2008-09-03) Fischer, MJ; Treble, PCProviding estimates of past climate changes on interannual - millenial timescales requires suitable regression models between climate and climate proxies. Many proxies appear to show relationships with climate that are timescale dependent. Any proxy-climate model should be able to replicate the major patterns that are observed at multiple timescales. Here we develop a new climate-isotope regression model for speleothems from a middle latitude site. In the low to middle latitudes, daily variation in precipitation isotopes (within individual months) is largely negatively correlated with daily rainfall amount. On interdecadal timescales, though, this relationship appears to be nonstationary. These two points provide a theoretical basis for a new climate-isotope regression model in which the slope and the intercept of a δ O-18(day) - P-day line for a given month are modulated by organized patterns of climate variability, such as the extratropical zonal waves (including the annular modes). In constructing this new regression model, we show how daily precipitation - δ O-18 relationships can be estimated using only monthly δ O-18 data and daily rainfall amounts. The new regression model provides a consistent picture of O-18 variability over a range of timescales, and this has not been the case with any previous climate-isotope regression model. © 2008, American Geophysical Union
- ItemClimate variability and precipitation isotope relationships in the Mediterranean region(American Geophysical Union, 2012-10-24) Fischer, MJ; Mattey, DThis study investigates the links between Mediterranean precipitation delta O-18 and Mediterranean sea level pressure (SLP) anomalies during the winter months and over the years 1960-present. Previous studies have considered only the influence of the North Atlantic Oscillation (NAO) on rainfall delta O-18 at Mediterranean sites, but Mediterranean winter SLP variability evolves with at least three degrees of freedom, which means that other climate patterns may be equally important in influencing Mediterranean rainfall delta O-18. In this study, Multivariate Linear Regression (MLR) is employed to identify the 'coupled patterns' in the Mediterranean winter SLP and delta O-18 fields. The multivariate linear model is estimated in two different ways, using Principal Components Regression (PCR) and regularized Canonical Correlation Analysis (regCCA), resulting in two different models which are compared. In both models two main patterns are identified, that explain 50% of the shared variance in the SLP and delta O-18 fields. Subspace projection of various regional and Northern Hemisphere climate indices shows that the two main patterns are more closely related to local Mediterranean climate indices than to other Northern Hemisphere climate indices. Analysis of the predicted and residual fields from the two models suggests that the regCCA model provides better predictability for rainfall delta O-18 at central Mediterranean sites, while both models explain relatively less of the rainfall delta O-18 variance at eastern Mediterranean sites. These results can potentially aid the interpretation of the climate-isotope signal preserved in high-resolution natural archives from different parts of the Mediterranean. © 2012, American Geophysical Union.
- ItemA climate-isotope regression model with seasonally-varying and time-integrated relationships(Springer, 2011-12-01) Fischer, MJ; Baldini, LMThis study investigates multivariable and multiscalar climate-delta(18)O relationships, through the use of statistical modeling and simulation. Three simulations, of increasing complexity, are used to generate time series of daily precipitation delta(18)O. The first simulation uses a simple local predictor (daily rainfall amount). The second simulation uses the same local predictor plus a larger-scale climate variable (a daily NAO index), and the third simulation uses the same local and non-local predictors, but with varying seasonal effect. Since these simulations all operate at the daily timescale, they can be used to investigate the climate-delta(18)O patterns that arise at daily-interannual timescales. These simulations show that (1) complex links exist between climate-delta(18)O relationships at different timescales, (2) the short-timescale relationships that underlie monthly predictor-delta(18)O relationships can be recovered using only monthly delta(18)O and daily predictor variables, (3) a comparison between the simulations and observational data can elucidate the physical processes at work. The regression models developed are then applied to a 2-year dataset of monthly precipitation delta(18)O from Dublin and compared with event-scale data from the same site, which illustrates that the methodology works, and that the third regression model explains about 55% of the variance in delta(18)O at this site. The methodology introduced here can potentially be applied to historic monthly delta(18)O data, to better understand how multiple-integrated influences at short timescales give rise to climate-delta(18)O patterns at monthly-interannual timescales. © 2011, Springer.
- ItemCoral microatoll reconstructions of El Niño-southern oscillation: new windows on seasonal and interannual processes(PAGES Past Global Changes, 2013-08-01) McGregor, HV; Woodroffe, CD; Fischer, MJ; Gagan, MK; Fink, DPorites corals are the most commonly used genus for reconstructing El Niño- Southern Oscillation (ENSO). This hermatypic coral is found in all tropical reef environments(Veron 2000) with a variety of growth forms. Climate reconstructions of a century or more have been obtained from the most common, dome-shaped Porites growth form, whereby the colonies, beginning from the substrate, grow outward and upward towards the ocean surface(Knutson et al. 1972). Domed structures, however, are not the only Porites growth form. © 2013, Authors.
- ItemThe CoralHydro2k Database: a global compilation of coral δ18O and Sr/Ca records for reconstructing tropical hydroclimate over the common era(American Geophysical Union (AGU), 2021-12-18) Sayani, HR; Walter, R; Felis, T; Cobb, KM; Abram, NJ; Atwood, AR; Arzey, A; Brenner, LD; Dassie, EP; DeLong, KL; Ellis, B; Goodkin, N; Hargreaves, J; Kilbourne, KH; Krawczyk, H; Fischer, MJ; Murty, SA; Moore, AL; Ramos, RDP; Reed, E; Samanta, D; Zinke, JShallow-water corals provide annual to subannual -resolution climate reconstructions from normally data-scarce locations in the tropical to subtropical oceans, enabling us to extend the modern-day observational records back to the preindustrial era, contextualize anthropogenic climate change, and improve the skill of future climate projections. The majority of these coral-based reconstructions utilize oxygen isotope ratios (δ18O), a proxy that tracks the combined change in sea surface temperature (SST) and the oxygen isotopic composition of seawater (δ18Osw) and/or strontium-to-calcium ratios (Sr/Ca), which primarily track SST variability. Paired coral δ18O and Sr/Ca records can be combined to isolate δ18Osw variability, which like salinity reflects changes in the local hydrologic budget. Recently, the PAGES Ocean2k project used published coral records to reconstruct regional SST variability across the tropical oceans (Tierney et al., 2015, Abram et al., 2016). Building on this work, the PAGES CoralHydro2k team has compiled a more comprehensive, machine-readable, and metadata-rich network of paired coral δ18O and Sr/Ca records to help facilitate tropical hydroclimate reconstructions across recent centuries. The CoralHydro2k database currently contains 227 coral proxy records from 120 unique locations that are organized into seven tiers based on the availability of paired proxy data, temporal coverage, and record resolution. The metadata for the new database follows PACTs 1.0 recommendations (Khider et al., 2019), and the database is built using LiPD (McKay and Emile-Geay, 2016) with available R, MATLAB, and Python serializations. Here we describe the structure and spatiotemporal characteristics of this new database and outline a crowdsourced data-submission process to ensure active-curation of records and future updates to the database.
- ItemDetecting trends that are nonlinear and asymmetric on diurnal and seasonal time scales(Springer, 2014-02-22) Fischer, MJ; Paterson, AWTrends in climate time series are often nonlinear and temporally-asymmetric, i.e. the trend is different for different seasons and/or hours of the day. Here a method is developed that allows the nonlinearity and temporal asymmetry of a trend to be investigated simultaneously. First, nonlinear trend components are extracted from a univariate time series, by adapting a nonparametric dimension-reduction method. Then, the nonlinear trend components are substituted into a regression model in which the periodic mean component and the periodic variation in the amplitude of the nonlinear trend are modeled using harmonic functions of the seasonal and diurnal periods. Third, trend patterns in the positive and negative anomalies are investigated, by extending the nonlinear trend model using indicator variables. Fourth, a non-local inferential test is developed to test the statistical significance of the trend patterns. The nonlinear trend model is applied to a simulated time series, as well as to long-term high-resolution temperature records from five Southern Hemisphere sites: Lucas Heights, Sydney Airport, Cape Grim, Macquarie Island and Law Dome. Our method should be generally useful for identifying the effect of both climate-related factors and observation/site-related factors on seasonal and diurnal trends in meteorological data series. © 2014, Springer-Verlag Berlin Heidelberg.
- ItemDeuterium and oxygen-18 ratios in rainfall and streamflow in a major drinking water catchment near Sydney, Australia, during drought(International Atomic Energy Agency, 2007-05-21) Hughes, CE; Fischer, MJ; Stone, DJM; Hollins, SEThe Warragamba catchment near Sydney, Australia, is in the midst of a major drought that is threatening water supplies for Australia's largest city. Over a period of 18 months 227 event based rainfall samples were collected at four locations, 74 streamflow samples were collected from the four major inflowing rivers and their tributaries and and 45 reservoir samples were collected at various depths from Warragamba dam. The samples were analysed for δD and δ18O. These data provide a baseline dataset for establishment of a local meteoric water line for the Sydney region and for use in modelling of flow pathways and weather patterns in the Warragamba catchment.
- ItemDeuterium and oxygen-18 ratios in rainfall and streamflow in a major drinking water catchment near Sydney, Australia, during drought(International Atomic Energy Agency, 2007) Hughes, CE; Fischer, MJ; Stone, DJM; Hollins, SEThe Warragamba catchment near Sydney, Australia, is in the midst of a major drought that is threatening water supplies for Australia's largest city. Over a period of 18 months 227 event based rainfall samples were collected at four locations, 74 streamflow samples were collected from the four major inflowing rivers and their tributaries and and 45 reservoir samples were collected at various depths from Warragamba dam. The samples were analysed for δD and δ18O. These data provide a baseline dataset for establishment of a local meteoric water line for the Sydney region and for use in modelling of flow pathways and weather patterns in the Warragamba catchment.
- ItemEnvironmental control of the oxygen isotope composition of Porities coral microatolls(Elsevier, 2011-07-15) McGregor, HV; Fischer, MJ; Gagan, MK; Fink, D; Woodroffe, CDUnderstanding the influence of climatic and non-climatic factors on geochemical signals in corals is critical for assessing coral-derived records of tropical climate variability. Porites microatolls form large, disk-shaped colonies constrained in their upward growth by exposure at or close to mean spring low water level, and occur on Indo-Pacific reefs. Microatolls appear suitable for paleoclimate reconstruction, however the systematics of the microatoll chemistry-climate relationship are yet to be characterized. In this study, the delta(18)O signal in Porites microatolls from well-flushed reef flats on Kiritimati (Christmas) Island, central Pacific was investigated for intra-coral (growth aspect and extension rate effects) and between-coral effects, and to explore the climate signal contained within their skeletons. Samples for delta(18)O analysis were taken from six individual transects from different positions within Porites microatoll XM22. The results show that: (1) the average standard deviation for the mean delta(18)O values of transects that represent the same time periods is 0.03 parts per thousand, and is within measurement error for a single analysis (0.04 parts per thousand); (2) the average standard deviation for time-equivalent, near-monthly samples along the transects within the same microatoll is 0.07 parts per thousand and; (3) comparison of the average delta(18)O values of records for different microatolls from across Kiritimati Island show only a small between-coral differences of 0.04 parts per thousand and 0.11 parts per thousand for different time periods. These differences in mean delta(18)O are within the range for intra-and inter-colony differences in seasonal and interannual delta(18)O reported for dome-shaped Porites. Based on these results, a stacked microatoll delta(18)O record was constructed for the period 1978-2007 for comparison with published coral delta(18)O records for nearby dome-shaped Porites. There is a systematic offset between the two types of records, which is probably due to variations in delta(18)O seawater across Kiritimati Island. Despite the offset, all records show similar amplitudes for the seasonal-cycle of delta(18)O, and there is a strong correlation (r = -0.71) between microatoll delta(18)O and local sea surface temperature (SST). The delta(18)O-SST slope relationship for microatolls is -0.15 parts per thousand/degrees C, very similar to that reported for fast-growing domed corals (-0.18 parts per thousand to -0.22 parts per thousand/degrees C). Statistical analysis of the stacked microatoll delta(18)O record shows that it is correlated with both local and large-scale climate variables (primarily SST) at semiannual, annual and interannual timescales. Our results show that the signal reproducibility and fidelity of skeletal delta(18)O in coral microatolls is comparable to that observed for more conventional coral growth forms. Longer-lived, and fossil, Porites microatolls, where they have grown in suitably flushed environments, are likely to contain delta(18)O signals that can significantly extend instrumental records of tropical climate variability. Crown copyright (C) 2011 Published by Elsevier Ltd.
- ItemEvidence for Holocene changes in Australian-Indonesian monsoon rainfall from stalagmite trace element and stable isotope ratios(Elsevier, 2010-03-15) Griffiths, ML; Drysdale, RN; Gagan, MK; Frisia, S; Zhao, JX; Ayliffe, LK; Hantoro, WS; Hellstrom, JC; Fischer, MJ; Feng, YX; Suwargadi, BWTrace element and stable isotope ratios from an active stalagmite (LR06-B1) recovered from Liang Luar Cave on the island of Flores (eastern Indonesia) are used to reconstruct the position of the austral summer inter-tropical convergence zone and Australian-Indonesian summer monsoon variability during the Holocene. Uranium-series dating of the stalagmite shows that it commenced growth 12,640 years ago , with hiatuses spanning 8,560 to 6,420 and 3,670 to 2,780 years ago. Stalagmite Mg/Ca and Sr/Ca ratios correlate significantly with one another, and with δ18O and δ13C, throughout the record. This suggests that the Mg/Ca and Sr/Ca ratios are dominated by prior calcite precipitation, a process whereby degassing in the vadose zone during periods of low recharge causes deposition of calcite and disproportionate loss of Ca2+ ions (relative to Mg2+ and Sr2+) ‘upstream’ of the stalagmite. The degree of initial 234U/238U disequilibrium also appears to have been controlled by recharge to the overlying aquifer. Together with the Mg/Ca, Sr/Ca, and δ18O values, the initial uranium isotope activity ratios ([234U/238U]I) imply a generally drier early Holocene, coincident with a lower sea level and lower Southern Hemisphere summer insolation. Comparison of speleothem δ18O time-series from Flores and Borneo shows that they vary in unison for much of the Holocene. However, there is a significant decrease in the Borneo δ18O record ~6,000 to 4,000 years ago that does not occur in the Flores record. This anomaly may be related to a change in the Australian-Indonesian summer monsoon circulation in response to a protracted positive phase of the Indian Ocean Dipole. Under this scenario, stronger upwelling off of western Indonesia would, based on present-day effects, result in reduced summer convective activity over Flores and a subsequent northward shift of the intertropical convergence zone. © 2010, Elsevier Ltd.
- ItemIncreasing Australian-Indonesian monsoon rainfall linked to early Holocene sea-level rise.(Nature Publishing Group, 2009-09) Griffiths, ML; Drysdale, RN; Gagan, MK; Zhao, JX; Ayliffe, LK; Hellstrom, JC; Hantoro, WS; Frisia, S; Feng, YX; Cartwright, I; Pierre, ES; Fischer, MJ; Suwargadi, BWThe Australian-Indonesian summer monsoon affects rainfall variability and hence terrestrial productivity in the densely populated tropical Indo-Pacific region. It has been proposed that the main control of summer monsoon precipitation on millennial timescales is local insolation(1-3), but unravelling the mechanisms that have influenced monsoon variability and teleconnections has proven difficult, owing to the lack of high-resolution records of past monsoon behaviour. Here we present a precisely dated reconstruction of monsoon rainfall over the past 12,000 years, based on oxygen isotope measurements from two stalagmites collected in southeast Indonesia. We show that the summer monsoon precipitation increased during the Younger Dryas cooling event, when Atlantic meridional overturning circulation was relatively weak(4). Monsoon precipitation intensified even more rapidly from 11,000 to 7,000 years ago, when the Indonesian continental shelf was flooded by global sea-level rise(5-7). We suggest that the intensification during the Younger Dryas cooling was caused by enhanced winter monsoon outflow from Asia and a related southward migration of the intertropical convergence zone(8). However, the early Holocene intensification of monsoon precipitation was driven by sea-level rise, which increased the supply of moisture to the Indonesian archipelago. © 2009, Nature Publishing Group.
- ItemInterpreting past climate using southwest Australian speleothems(Australian Meterological & Oceanographic Society, 2012-01-31) Treble, PC; Azcurra, CS; Baker, AA; Bradley, C; Wood, A; Fischer, MJ; Fairchild, IJ; Hellstrom, JC; Gagan, MKThere is an identified need to extend our baseline climate information beyond the relatively short duration of instrumental records in Australia. An improved knowledge of natural rainfall variability would assist in our understanding of climate change. SW Australia (SWWA) is one region that has been identified as having a changing climate since the 1970s. Speleothems (cave stalagmites) are an effective archive of past climate variability and caves from the coastal region of SWWA are being studied for paleoclimate records. The modern speleothem record from this region has been assessed and shown to record the post-1970s rainfall decrease (Treble et al., 2003; 2005; Fischer and Treble, 2008). The extension of the speleothem record is currently underway, however, a long-term cave monitoring program was also deemed necessary to separate the climatic from non-climatic signals i.e. to reduce uncertainty when interpreting these records. This presentation outlines what we have learnt about the possible hydrological modification of the climate signal in speleothems. In particular, we present results from a five-year long monitoring study of rainfall and cave drip water O isotopes (!18O) from Golgotha Cave, SWWA. From this study, we have been able to characterize the probable flow paths feeding stalagmites in our monitored cave. These flow paths range from slow diffuse flow of isotopically-averaged rainfall to preferential routing of high-magnitude, 18O-depleted, events along fast flow routes into the cave. Hence, we offer a possible explanation for why paleoclimate records from coeval speleothems in our cave may differ. Our study suggests that this disagreement may simply be due to different flow paths resulting in a bias towards the preservation of high or low magnitude rainfall events.
- ItemAn investigation of the climatic influences on 14C activity in a Holocene stalagmite from Flores, Indonesia(18th INQUA Congress, 2011-07-21) Griffiths, ML; Drysdale, RN; Hua, Q; Hellstrom, JC; Frisia, S; Gagan, MK; Zhao, JX; Fischer, MJ; Ayliffe, LKOver the past decade, a number of speleothem studies have used radiocarbon dating to address a range of palaeoclimate problems. These have included the use of the bomb pulse to anchor chronologies over the last 60 years, the combining of U-series and radiocarbon measurements to improve the radiocarbon calibration curve, and linking atmospheric radiocarbon variations with climate changes. Central to a number of these studies is how to constrain, or interpret variations in, the amount of radioactively dead carbon (i.e. the dead carbon fraction, or DCF) that contributes to a speleothem radiocarbon measurement. In this study, we use radiocarbon measurements, stable-isotope and trace-element geochemistry, and U-series ages from a previously studied speleothem from Flores, Indonesia, to examine DCF variations and its relationship with above-cave climate over the late-Holocene to modern interval. A strong association between the DCF and other hydrologically controlled proxy data clearly shows that more dead carbon is being delivered to the speleothem during periods of higher cave recharge (i.e. lower ?18O, ?13C and Mg/Ca values) and hence a stronger summer monsoon. One possible explanation is a higher contribution from the bedrock under such conditions. Although one might expect a concurrent increase in stable carbon isotope values as DCF increases (not observed here), it is possible that such an increase in ?13C may be more than offset by the effect of increased recharge on the rate of carbon dioxide degassing. But, a higher proportion of bedrock carbon is not the only possible explanation: when the monsoon is stronger, a greater proportion of less mobile ‘older carbon’ may be leached from the soil thus diluting the ‘younger carbon’ fraction. This would produce an ‘apparent’ increase in DCF. Copyright (c) 2011 INQUA 18
- ItemAn investigation of the controls on Irish precipitation δ18O values on monthly and event timescales.(Springer, 2010-11) Baldini, LM; McDermott, F; Baldini, JUL; Fischer, MJ; Mollhoff, MThis two-year study investigates the relative influence of meteorological variables (precipitation amount and temperature), atmospheric circulation, air mass history, and moisture source region on Irish precipitation oxygen isotopes (δ18Op) on event and monthly timescales. Single predictor correlations reveal that on the event scale, 20% of δ18Op variability is attributable to the amount effect and 7% to the temperature effect while on the monthly timescale the North Atlantic Oscillation accounts for up to 20% of δ18Op variability and the amount and temperature effects are not significant. In comparison, multivariate linear regression reveals that the interaction of temperature and precipitation amount explains up to 40% of δ18Op variance at event and monthly timescales. Five-day kinematic back trajectories suggest that the amount-weighted mean δ18Op value of southerly- and northerly-derived events are lower by 2‰ relative to events derived from the west. Because air mass history and atmospheric circulation appear to influence δ18Op in Ireland, Irish paleo-δ18Op proxy records are best interpreted as reflecting a combination of parameters, not just paleotemperature or paleorainfall. © 2010, Springer.
- ItemMillennial to seasonal scale views of El Niño-Southern Oscillation from central Pacific corals(Australasian Quaternary Association, 2022-12-06) McGregor, HV; Wilcox, P; Fischer, MJ; Phipps, SJ; Gagan, MK; Wittenberg, A; Felis, T; Kölling, M; Wong, HKY; Devriendt, L; Woodroffe, CD; Zhao, JX; Fink, D; Gaudry, JJ; Chivas, AREl Niño-Southern Oscillation (ENSO) is naturally highly variable on interannual to decadal scales making it difficult to detect a possible response to climate forcing. Despite the high variability, several lines of evidence from tropical corals, mollusc, lake sediments, and foraminifera suggest that 5,000-3,000 years ago ENSO variance was on average reduced by 60-80% compared to the present day. We investigate the seasonal-to-centennial variation in ENSO amplitude and tropical climate during this ENSO ‘quiet period’ 5,000-3,000 years ago using a new Sr/Ca SST record from a 175-year-long 4,300-year-old coral, and new d18O and Sr/Ca results from a similar-aged ~180-year-long Porites sp. coral. Both corals were discovered on Kiritimati (Christmas) Island, an optimal ENSO ‘centre of action’ in the central tropical Pacific. Together, these corals confirm a reduction in ENSO amplitude and that ENSO amplitude is modulated on multi-decadal scales. Composites of month-by-month changes in Sr/Ca-SST show an unprecedented view of ENSO and detail which seasonal-scale features of ENSO are an inherent part of the system, and which are subject to change under altered climate states. We also investigate the millennial timescale changes in ENSO variance using combine coral oxygen isotope (18O) data from central Pacific corals and a suite of forced and unforced simulations conducted using the CSIRO Mk3L and GFDL CM2.1 climate system models. On millennial timescales, the coral data reveal a statistically significant increase in ENSO variance over the past 6,000 years. This trend is not reproduced by the unforced model simulations but can be reproduced once orbital forcing is accounted for. Together these views of past ENSO may contribute to advances in understanding the response of ENSO to future changes in climate forcings.
- ItemMillennial to seasonal scale views of El Niño-Southern Oscillation from central Pacific corals(2022-12-06) McGregor, HV; Wilcox, P; Fischer, MJ; Phipps, SJ; Gagan, MK; Wittenberg, A; Felis, T; Kölling, M; Wong, HKY; Devriendt, L; Woodroffe, CD; Zhao, JX; Fink, D; Gaudry, JJ; Chivas, AREl Niño-Southern Oscillation (ENSO) is naturally highly variable on interannual to decadal scales making it difficult to detect a possible response to climate forcing. Despite the high variability, several lines of evidence from tropical corals, mollusc, lake sediments, and foraminifera suggest that 5,000-3,000 years ago ENSO variance was on average reduced by 60-80% compared to the present day. We investigate the seasonal-to-centennial variation in ENSO amplitude and tropical climate during this ENSO ‘quiet period’ 5,000-3,000 years ago using a new Sr/Ca SST record from a 175-year-long 4,300-year-old coral, and new d18O and Sr/Ca results from a similar-aged ~180-year-long Porites sp. coral. Both corals were discovered on Kiritimati (Christmas) Island, an optimal ENSO ‘centre of action’ in the central tropical Pacific. Together, these corals confirm a reduction in ENSO amplitude and that ENSO amplitude is modulated on multi-decadal scales. Composites of month-by-month changes in Sr/Ca-SST show an unprecedented view of ENSO and detail which seasonal-scale features of ENSO are an inherent part of the system, and which are subject to change under altered climate states. We also investigate the millennial timescale changes in ENSO variance using combine coral oxygen isotope (18O) data from central Pacific corals and a suite of forced and unforced simulations conducted using the CSIRO Mk3L and GFDL CM2.1 climate system models. On millennial timescales, the coral data reveal a statistically significant increase in ENSO variance over the past 6,000 years. This trend is not reproduced by the unforced model simulations but can be reproduced once orbital forcing is accounted for. Together these views of past ENSO may contribute to advances in understanding the response of ENSO to future changes in climate forcings.
- ItemA new continuum regression model and its application to climate and rainfall isotope relationships(University of Western Australia, 2013-07-10) Fischer, MJClimate field reconstruction using networks of rainfall-isotope proxies is an example of a problem that requires the estimation of a model that aims to predict one field (Y) using another field (X). The general problem is to estimate a subspace of X that retains useful information for predicting Y. Methods to estimate such subspaces include principal components regression (PCR), partial least squares (PLS), redundancy analysis (RDA), and canonical correlation analysis (CCA), but these methods typically do not estimate the same subspace. One solution is to treat these different methods as end members of a continuous manifold of regression subspaces. By weighting the end member solutions in some way, we can search for the best regression subspace over the manifold. In this study, a new continuum regression model is developed by extending an earlier method known as Principal Covariates Regression (PCovR). PCovR has two end members: PCR and RDA. Here, PCovR is extended by shrinking the covariance matrix of X. As a result, our new method regPCovR includes three end members (PCR, RDA and PLS) and is particularly suited to climate data, where the spatial dimension is larger than the temporal dimension, and where there are missing values in the response field (Y). regPCovR includes both a weighting parameter and shrinkage parameter, which are estimated using crossvalidation. The benefits of regPCovR are illustrated using two examples. In the first example, the problem of predicting the southern Australian winter rainfall (P) field from the regional winter sea level pressure (SLP) field is investigated. The best rank two regression subspace found by regPCovR explains over 50% of the variance in the rainfall field. This subspace thus estimates the relationship between SLP and P better than the end member subspaces. In the second example, PCovR is used to investigate the relationships between the winter SLP and P fields, and rainfall isotope (d18O) data from Australia and New Zealand. Two main patterns are identified, which explain about half the variance in the southern GNIP d18O sites. Subspace projection is used to relate these patterns to various regional and Southern Hemisphere climate indices. regPCovR will be useful for finding subspaces that better capture the relationships between climate and rainfall isotopes, which is a necessary step for quantitative palaeoclimatology. © The Authors
- ItemPaleoclimate data-model comparison and the role of climate forcings over the past 1500 Years(American Meterological Society, 2013-09-01) Phipps, SJ; McGregor, HV; Gergis, J; Gallant, AJE; Neukom, R; Stevenson, S; Ackerley, D; Brown, JR; Fischer, MJ; van Ommen, TDThe past 1500 years provide a valuable opportunity to study the response of the climate system to external forcings. However, the integration of paleoclimate proxies with climate modeling is critical to improving the understanding of climate dynamics. In this paper, a climate system model and proxy records are therefore used to study the role of natural and anthropogenic forcings in driving the global climate. The inverse and forward approaches to paleoclimate data-model comparison are applied, and sources of uncertainty are identified and discussed. In the first of two case studies, the climate model simulations are compared with multiproxy temperature reconstructions. Robust solar and volcanic signals are detected in Southern Hemisphere temperatures, with a possible volcanic signal detected in the Northern Hemisphere. The anthropogenic signal dominates during the industrial period. It is also found that seasonal and geographical biases may cause multiproxy reconstructions to overestimate the magnitude of the long-term preindustrial cooling trend. In the second case study, the model simulations are compared with a coral O-18 record from the central Pacific Ocean. It is found that greenhouse gases, solar irradiance, and volcanic eruptions all influence the mean state of the central Pacific, but there is no evidence that natural or anthropogenic forcings have any systematic impact on El Nino-Southern Oscillation. The proxy climate relationship is found to change over time, challenging the assumption of stationarity that underlies the interpretation of paleoclimate proxies. These case studies demonstrate the value of paleoclimate data-model comparison but also highlight the limitations of current techniques and demonstrate the need to develop alternative approaches. © 2013, American Meteorological Society.
- ItemPrecipitation isotopes in south east Australia: why do they vary?(Australian Institute of Nuclear Science and Engineering (AINSE), 2008-01) Fischer, MJ