ANSTO :: Browsing by Author "McGregor, HV"

Browsing by Author "McGregor, HV"

Now showing 1 - 17 of 17

Coral 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, D
Porites 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.
El Niño-Southern Oscillation (ENSO) from 0-2 ka
(University of Melbourne, 2010-05-31) McGregor, HV; Phipps, SJ; Woodroffe, CD; Gagan, MK; Fink, D
Environmental control of the oxygen isotope composition of Porities coral microatolls
(Elsevier, 2011-07-15) McGregor, HV; Fischer, MJ; Gagan, MK; Fink, D; Woodroffe, CD
Understanding 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.
Equatorial Pacific coral geochemical records show recent weakening of the Walker Circulation
(American Geophysical Union, 2014-11-10) Carilli, JE; McGregor, HV; Gaudry, JJ; Donner, SD; Gagan, MK; Stevenson, S; Wong, HKY; Fink, D
Equatorial Pacific ocean-atmosphere interactions affect climate globally, and a key component of the coupled system is the Walker Circulation, which is driven by sea surface temperature (SST) gradients across the equatorial Pacific. There is conflicting evidence as to whether the SST gradient and Walker Circulation have strengthened or weakened over the late twentieth century. We present new records of SST and sea surface salinity (SSS) spanning 1959–2010 based on paired measurements of Sr/Ca and δ18O in a massive Porites coral from Butaritari atoll in the Gilbert Islands, Republic of Kiribati, in the central western equatorial Pacific. The records show 2–7 year variability correlated with the El Niño–Southern Oscillation (ENSO) and corresponding shifts in the extent of the Indo-Pacific Warm Pool, and decadal-scale signals related to the Pacific Decadal Oscillation and the Pacific Warm Pool Index. In addition, the Butaritari coral records reveal a small but significant increase in SST (0.39°C) from 1959 to 2010 with no accompanying change in SSS, a trend that persists even when ENSO variability is removed. In contrast, larger increases in SST and SSS are evident in coral records from the equatorial Pacific Line Islands, located east of Butaritari. Taken together, the equatorial Pacific coral records suggest an overall reduction in the east-west SST and SSS gradient over the last several decades, and a recent weakening of the Walker Circulation. © 2014, American Geophysical Union. All Rights Reserved.
Interlaboratory study for coral Sr/Ca and other element/Ca ratio measurements
(John Wiley & Sons, Inc, 2013-07-25) Hathorne, EC; Gagnon, A; Felis, T; Adkins, J; Asami, R; Boer, W; Caillon, N; Case, D; Cobb, KM; Douville, E; deMenocal, P; Eisenhauer, A; Garbe-Schönberg, D; Geibert, W; Goldstein, S; Hughen, K; Inoue, M; Kawahata, H; Kölling, M; Cornec, FL; Linsley, BK; McGregor, HV; Montagna, P; Nurhati, IS; Quinn, TM; Raddatz, J; Rebaubier, H; Robinson, L; Sadekov, A; Sherrell, R; Sinclair, D; Tudhope, AW; Wei, GJ; Wong, HKY; Wu, HC; You, CF
The Sr/Ca ratio of coral aragonite is used to reconstruct past sea surface temperature (SST). Twenty-one laboratories took part in an interlaboratory study of coral Sr/Ca measurements. Results show interlaboratory bias can be significant, and in the extreme case could result in a range in SST estimates of 7°C. However, most of the data fall within a narrower range and the Porites coral reference material JCp-1 is now characterized well enough to have a certified Sr/Ca value of 8.838 mmol/mol with an expanded uncertainty of 0.089 mmol/mol following International Association of Geoanalysts (IAG) guidelines. This uncertainty, at the 95% confidence level, equates to 1.5°C for SST estimates using Porites, so is approaching fitness for purpose. The comparable median within laboratory error is <0.5°C. This difference in uncertainties illustrates the interlaboratory bias component that should be reduced through the use of reference materials like the JCp-1. There are many potential sources contributing to biases in comparative methods but traces of Sr in Ca standards and uncertainties in reference solution composition can account for half of the combined uncertainty. Consensus values that fulfil the requirements to be certified values were also obtained for Mg/Ca in JCp-1 and for Sr/Ca and Mg/Ca ratios in the JCt-1 giant clam reference material. Reference values with variable fitness for purpose have also been obtained for Li/Ca, B/Ca, Ba/Ca, and U/Ca in both reference materials. In future, studies reporting coral element/Ca data should also report the average value obtained for a reference material such as the JCp-1. ©2013 American Geophysical Union
Microatolls and the record of Holocene sea level that can be derived from them
(Australian Geosciences Council, 2012-08-05) Woodroffe, CD; Smithers, SG; McGregor, HV; Fink, D; Lambeck, K
The Great Barrier Reef expedition in 1973 described living and fossil microatolls on the Great Barrier Reef and recognised their significance as sea-level indicators. John Chappell subsequently surveyed and dated Porites microatolls along the mainland and interpreted them in the context of hydro-isostatic adjustment, inferring gradual falling sea level during the past 6000 years. We re-examine the significance of microatolls, providing a detailed account of more than 100 fossil microatolls from Christmas Island in the central Pacific. We relate their elevations to living modern equivalents and compare the sea-level record derived from them with the modelled Holocene isostatically-corrected sea level for this equatorial location. These long-lived massive corals, up to 9 metres in diameter, preserve a biologically-mediated multi-decadal record of sea-level over the past 5000 years. The upper surface of fossil microatolls on the reef flat lie in a narrow elevation range similar to that observed for their modern, living counterparts, implying negligible change of sea level. Further evidence for the lack of major fluctuations in sea level over the period 5000–1000 years BP is provided by corals from the interior of the island. This second population grew prolifically in a large lagoonal setting and is offset from those in open water by about 50 cm. The continuity of microatoll growth precludes significant oscillations of sea level during this time, and accords with geophysical modelling for this site, implying that the ‘eustatic’ contribution from post-6ka ice melt and the isostatic adjustment of the ocean floor to loading cancel each other at this site.
Mid-holocene variability in the marine C-14 reservoir age for northern coastal Papua New Guinea
(Elsevier, 2008-08) McGregor, HV; Gagan, MK; McCulloch, MT; Hodge, E; Mortimer, GE
Changes in oceanic radiocarbon (C-14) reservoir ages through the deglaciation and Holocene can provide important information on ocean circulation as Earth's climate warmed. Here, we present reservoir ages for the western tropical Pacific that span the mid-Holocene transition from less to more frequent El Nino events. Reservoir ages were calculated using paired U-Th and conventional C-14 dating of eight individual fossil coral samples from Koil and Muschu Islands, northern coastal Papua New Guinea (PNG). AMS C-14 and MC-ICPMS U-Th dating of additional samples from six of the fossil corals were used to confirm the TIMS U-Th and conventional C-14 ages. The combined results show average reservoir ages of 185 +/- 30 C-14 yr (n = 4) for 7220-5850 yr BP compared to 420 C-14 yr for a modern coral from Muschu Island. From 5850 to 5420 yr BP reservoir ages increase to modern values. The relatively young reservoir ages from 7220 to 5850 yr BP are best explained by greater influx of well-equilibrated sub-tropical water from the southern branch of the South Equatorial Current (SEC). This is consistent with strengthening trade winds (facilitating air-sea exchange) and a more northerly position of the Intertropical Convergence Zone thought to have occurred at this time. The transition to more modern-like reservoir ages from 5850 to 5420 yr BP suggests modern oceanic circulation patterns were established during this interval. The onset of modern El Nino activity around this time would have served to enhance the intrusion of C-14-depleted equatorial waters via the south equatorial branch of the SEC. Overall, the changes in reservoir age presented here for the western tropical Pacific suggest that Holocene changes in the El Nino-Southern Oscillation state of the tropical Pacific resulted in reorganisation of oceanic circulation in this region. © 2008, Elsevier Ltd.
Mid-Pacific microatolls record sea-level stability over the past 5000yr
(Geological Society of America, 2012-10-01) Woodroffe, CD; McGregor, HV; Lambeck, K; Smithers, SG; Fink, D
There has been geographical variation in sea level since rapid postglacial melting of polar ice ceased similar to 6 k.y. ago, reflecting isostatic adjustments of Earth and ocean surfaces to past (and ongoing) redistribution of ice and water loads. A new data set of over 100 fossil microatolls from Christmas (Kiritimati) Island provides a Holocene sea-level record of unparalleled continuity. Living reef-flat corals grow up to a low-tide level. Adjacent fossil microatolls, long-lived Porites corals up to several meters in diameter, occur at similar elevations (+/-0.1 m), and extensive fossil microatolls in the island interior are at consistent elevations within each population. Collectively, they comprise an almost continuous sequence spanning the past 5 k.y., indicating that, locally, sea level has been within 0.25 m of its present position, and precluding global sea-level oscillations of one or more meters inferred from less stable locations, or using other sea-level indicators. This mid-Pacific atoll is tectonically stable and far from former ice sheets. The precisely surveyed and radiometrically dated microatolls indicate that sea level has not experienced significant oscillations, in accordance with geophysical modeling, which implies that the eustatic contribution from past ice melt and the isostatic adjustment of the ocean floor to loading largely cancel each other at this site. © 2012, Geological Society of America
Millennial 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, AR
El 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.
Millennial 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, AR
El 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.
Paleoclimate 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, TD
The 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.
Quantifiying seasonal-scale changes in El Niño southern oscillation for the past millennia
(Australian Meteorological & Oceanographic Society, 2012-01-31) McGregor, HV; Fischer, MJ; Gagan, MK; Woodroffe, CD; Fink, D; Phipps, SJ; Zhao, JX
The El Niño-Southern Oscillation (ENSO) is the greatest source of interannual climate variability, yet model forecasts of the response of this system to global warming are inconsistent. The brevity of the instrumental record and lack of detailed knowledge of ENSO under different background states contribute to the uncertainty. Here we present a sequence of Porites coral microatoll !18O records from Kiritimati (Christmas) Island in the central equatorial Pacific showing ENSO variability during discrete “windows” spaced between 1500 and 6000 years ago (mid- to late Holocene), when background climate conditions were different due to changes in the Earth’s orbit around the sun. Our sequence includes a 175-year monthly-resolved microatoll !18O record showing ENSO variability 4,300 thousand years ago. The record shows a 60% reduction in the ENSO variance, a stronger annual cycle that persisted for the full 175 years of the record, and limited low frequency (multi-decadal) modulation of the ENSO signal. El Niño events were ‘damped’ during their June-December growth phase, but still phaselocked to the seasonal cycle. La Niña events were reduced and together ENSO seasonal phasing was likely similar to that observed during the weak ENSO period of the 1920-1950s.Further, results from corals aged between 1,500 and 2,000 years ago also show reduced ENSO, as well as changes in the contribution of the annual cycle, El Niño and La Niña events to the overall coral !18O signal. Our results show fundamental metrics on the seasonal characteristics of ENSO during the altered background conditions of the mid- to late Holocene. The results suggest that Holocene ENSO responded to changes in orbital forcing and that there was limited, unforced variability. This may have implications for ENSO under future global warming conditions.
Radiocarbon surface ocean reservoir ages over the past 6,000 years from Porites microatolls, Christmas (Kiritimati) Island, central Pacific Ocean
(Australian Geosciences Council, 2012-08-05) Fink, D; Carilli, JE; McGregor, HV; Woodroffe, CD; Zhao, JX; Fallon, SJ
We present radiocarbon reservoir (ΔR) values over the past 5,000 years based on high-precision paired U-series and AMS 14C in modern and fossil Porites coral micro-atolls from Christmas Island (2N, 157W). The data set (n∼25) allows temporal reconstruction of ΔR with ∼250 year spacing. Christmas Island lies within the Equatorial Counter Current and the NINO3.4 region with a climate frequently punctuated by higher precipitation and warmer SSTs during El-Nino. Along its coastal perimeter, and throughout the internal network of tidal flats and lagoons, which in the late Holocene were flourishing reefal environments, fossil and modern microatolls abound. Microatolls are large discoid colonies with horizontal radial growth axes constrained in upwards vertical growth by spring tide low-water level. Consequently, a single microatoll can extend to ∼9 m in diameter representing an exceptional ∼300 years of continuous growth. Sr/Ca and δ18O in modern microatolls faithfully replicate instrumental climate records (SSTs, rainfall). Results from numerous fossil cores (1500–5500 years BP) shows distinct variability in ENSO variance both in frequency and amplitude domains compared to modern microatolls. These fossil populations are used to generate a temporal pattern of ΔR variability. Strict criteria were followed with respect to secondary calcite (via quantitative XRD), unaltered microstructure (via thin sections) and ensuring splits of identical coral chips for dual U-series and AMS 14C. Preliminary results show a ΔR decrease at about 2000 BP. Further analyses of correlations of ΔR deviations from the long term average with changes in regional ocean surface currents and in paleo-ENSO variance are in progress.
Rapid U-series dating of young fossil corals by laser ablation MC-ICPMS
(Elsevier, 2011-04-01) McGregor, HV; Hellstrom, JC; Fink, D; Hua, Q; Woodroffe, CD
We demonstrate the utility of uranium-series age dating using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA MC-ICPMS) to ‘range find’ a large suite of fossil corals in order to select those for subsequent detailed and high resolution paleoclimatological analyses. The high abundance of unaltered and long-lived (100–200 years) microatolls on Kiritimati Island, Kiribati, offers the exciting prospect of constructing a millennial, if not continuous, coral paleoclimate record for the central Pacific spanning the period from the mid-Holocene to present. However, with a multitude of collected corals and Kiritimati sample sites, we required an efficient, cost-effective and reliable analytical method to optimize selection of quality coral belonging to a prescribed age. Two modern and 41 fossil microatoll corals from Kiritimati were analysed by the range-finding LA MC-ICPMS technique with a subset also analysed by high-precision solution MC-ICPMS and AMS radiocarbon to assess the reliability and accuracy of the laser ablation technique. Coral ages ranged from 6400 to 900 yr BP and laser ablation age precisions, constrained by the low 230Th count rate, ranged from 9 to 45% at the 2SE level. These ages generally agreed with ages measured by solution U-series and AMS 14C. However, conflicting results for (234U/238U)initial in samples analysed by LA- and solution-MC-ICPMS indicates that the laser ablation method may not be as suitable as previously thought for detecting subtle geochemical alteration. With 40–50 sample analyses per day, the laser ablation method offers a rapid, efficient and cost-effective means to histogram coral ages with multi-centennial accuracy, even at such low 230Th count rates typical for young Holocene samples. In addition to in situ corals, the LA MC-ICPMS technique could be further applied to corals recovered from drill core. Copyright © 2011 Elsevier Inc.
Reply to comment by Karnauskas et al. on “Equatorial Pacific coral geochemical records show recent weakening of the Walker circulation”
(American Geophysical Union, 2015-05-18) Carilli, JE; McGregor, HV; Gaudry, JJ; Donner, SD; Gagan, MK; Stevenson, S; Wong, HKY; Fink, D
In our paper describing a new coral record from Butaritari, we hypothesized that comparing the temporal trends in our records to coral records from farther east in the equatorial Paciﬁc may support the evidence for a weakening of a Walker circulation, documented elsewhere in the literature [Power and Smith, 2007; Tokinaga et al., 2012]. Weakening of the Walker circulation is expected under global warming due to an imbalance in the rate of change in different aspects of the hydrological cycle [Vecchi and Soden, 2007]. We thank Karnauskas et al. [2015] for recognizing the value of our Butaritari coral climate reconstruction, and we appreciate their critique of our study. The Karnauskas et al. [2015] analyses strengthen our argument regarding the utility of interisland coral-proxy derived sea surface temperature (SST) gradients as a Walker circulation metric, but we disagree with their interpretation of decadal variability in our records. Here we provide additional analyses, which conﬁrm that our reconstruction [Carilli et al., 2014] shows a long-term weakening of the Walker circulation over 1972–1998. We also document that signiﬁcant decadal variations in Walker circulation strength, and for particular choices of start and end years over which trends are calculated, are able to show slight Walker strengthening. Overall, we conclude that Walker circulation variations are more nuanced than either our original publication [Carilli et al., 2014] or the subsequent Karnauskas et al. [2015] comment would suggest. Karnauskas et al. [2015] also provide a detailed analysis of Equatorial Undercurrent (EUC) activity near the Gilbert Islands and argue that the EUC does not strongly affect Butaritari. Our original publication did not claim to ﬁnd signiﬁcant EUC/Butaritari linkages, and we appreciate the diligence of Karnauskas et al. [2015] for ruling this out as a possibility. © 2015, American Geophysical Union.
Solar and volcanic forcing of the Southern Hemisphere climate over the past 1500 years
(Past Global Changes, 2013-02-13) Phipps, SJ; Ackerley, D; Brown, JR; Curran, MAJ; Fischer, MJ; Gallant, A; Gergis, J; McGregor, HV; Neukom, R; Plummer, C; Stevenson, S; van Ommen, TD
The past 1500 years provides a valuable opportunity to study the role of external forcings in driving the global climate. Significant changes have taken place within the climate system over this period, and proxy data that records these changes covers a wide geographical area and has high temporal resolution. Natural and anthropogenic forcings are also reasonably well constrained. While previous detection and attribution studies have found a significant role of volcanic eruptions in driving the pre-industrial Northern Hemisphere climate, the drivers of the Southern Hemisphere climate are much less well understood. Here, the CSIRO Mk3L climate system model is used to simulate the global climate of the past 1500 years. Different combinations of natural and anthropogenic forcings are applied, including changes in the Earth’s orbital parameters, solar irradiance, volcanic emissions and anthropogenic greenhouse gases. The simulations are then compared with a multi-proxy reconstruction of Southern Hemisphere temperature. We find strong solar and volcanic influences on the Southern Hemisphere climate during the pre-industrial period, with the anthropogenic signal becoming increasingly dominant after 1850 CE. However, the results are sensitive to the specific reconstructions of solar and volcanic activity that are used to drive the model. The choice of volcanic reconstruction is particularly critical, and we find that the dating of major eruptions can impact significantly upon the agreement between the model and the proxy record. If we are to learn all that we can from the climate of recent millennia, a critical challenge is therefore to develop better reconstructions of past climatic forcings − particularly volcanic eruptions.
A weak El Niño/Southern Oscillation with delayed seasonal growth around 4,300 years ago
(Nature Publishing Group, 2013-09-05) McGregor, HV; Fischer, MJ; Gagan, MK; Fink, D; Phipps, SJ; Wong, HKY; Woodroffe, CD
Palaeoclimate records indicate lower El Niño/Southern Oscillation (ENSO) variance during the middle Holocene compared with today, but the mechanisms leading to this muted variability are not clear. A 175-year oxygen isotope record from a Porites coral microatoll in the NINO3.4 region records persistently reduced ENSO variance about 4,300 years ago, and season-specific analyses of the record suggest that insolation played an important role in this change.© 2013, Nature Publishing Group.

Browsing by Author "McGregor, HV"

Results Per Page

Sort Options