Browsing by Author "Phipps, SJ"
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- Item10Be in last deglacial climate simulated by ECHAM5-HAM – Part I: climatological influences on 10Be deposition(Copernicus Publications, 2013-11-25) Heikkilä, UE; Phipps, SJ; Smith, AMReconstruction of solar irradiance has only been possible for the Holocene so far. During the last deglaciation, two solar proxies (10Be and 14C) deviate strongly, both of them being influenced by climatic changes in a different way. This work addresses the climate influence on 10Be deposition by means of ECHAM5-HAM atmospheric aerosol–climate model simulations, forced by sea surface temperatures and sea ice extent created by the CSIRO Mk3L coupled climate system model. Three time slice simulations were performed during the last deglaciation: 10 000 BP ("10k"), 11 000 BP ("11k") and 12 000 BP ("12k"), each 30 yr long. The same, theoretical, 10Be production rate was used in each simulation to isolate the impact of climate on 10Be deposition. The changes are found to follow roughly the reduction in the greenhouse gas concentrations within the simulations. The 10k and 11k simulations produce a surface cooling which is symmetrically amplified in the 12k simulation. The precipitation rate is only slightly reduced at high latitudes, but there is a northward shift in the polar jet in the Northern Hemisphere, and the stratospheric westerly winds are significantly weakened. These changes occur where the sea ice change is largest in the deglaciation simulations. This leads to a longer residence time of 10Be in the stratosphere by 30 (10k and 11k) to 80 (12k) days, increasing the atmospheric concentrations (25–30% in 10k and 11k and 100% in 12k). Furthermore the shift of westerlies in the troposphere leads to an increase of tropospheric 10Be concentrations, especially at high latitudes. The contribution of dry deposition generally increases, but decreases where sea ice changes are largest. In total, the 10Be deposition rate changes by no more than 20% at mid- to high latitudes, but by up to 50% in the tropics. We conclude that on "long" time scales (a year to a few years), climatic influences on 10Be deposition remain small (less than 50%) even though atmospheric concentrations can vary significantly. Averaged over a longer period, all 10Be produced has to be deposited by mass conservation. This dominates over any climatic influences on 10Be deposition. Snow concentrations, however, do not follow mass conservation and can potentially be impacted more by climate due to precipitation changes. Quantifying the impact of deglacial climate modulation on 10Be in terms of preserving the solar signal locally is analysed in an accompanying paper (Heikkilä et al., 10Be in late deglacial climate simulated by ECHAM5-HAM – Part 2: Isolating the solar signal from 10Be deposition). © Author(s) 2013.
- Item10Be in late deglacial climate simulated by ECHAM5-HAM – Part 2: Isolating the solar signal from 10Be deposition(Copernicus Publications, 2014-04-01) Heikkilä, UE; Shi, X; Phipps, SJ; Smith, AMThis study investigates the effect of deglacial climate on the deposition of the solar proxy 10Be globally, and at two specific locations, the GRIP site at Summit, Central Greenland, and the Law Dome site in coastal Antarctica. The deglacial climate is represented by three 30 year time slice simulations of 10 000 BP (years before present = 1950 CE), 11 000 and 12 000 BP, compared with a preindustrial control simulation. The model used is the ECHAM5-HAM atmospheric aerosol–climate model, driven with sea-surface temperatures and sea ice cover simulated using the CSIRO Mk3L coupled climate system model. The focus is on isolating the 10Be production signal, driven by solar variability, from the weather- or climate-driven noise in the 10Be deposition flux during different stages of climate. The production signal varies at lower frequencies, dominated by the 11 year solar cycle within the 30 year timescale of these experiments. The climatic noise is of higher frequencies than 11 years during the 30 year period studied. We first apply empirical orthogonal function (EOF) analysis to global 10Be deposition on the annual scale and find that the first principal component, consisting of the spatial pattern of mean 10Be deposition and the temporally varying solar signal, explains 64% of the variability. The following principal components are closely related to those of precipitation. Then, we apply ensemble empirical decomposition (EEMD) analysis to the time series of 10Be deposition at GRIP and at Law Dome, which is an effective method for adaptively decomposing the time series into different frequency components. The low-frequency components and the long-term trend represent production and have reduced noise compared to the entire frequency spectrum of the deposition. The high-frequency components represent climate-driven noise related to the seasonal cycle of e.g. precipitation and are closely connected to high frequencies of precipitation. These results firstly show that the 10Be atmospheric production signal is preserved in the deposition flux to surface even during climates very different from today's both in global data and at two specific locations. Secondly, noise can be effectively reduced from 10Be deposition data by simply applying the EOF analysis in the case of a reasonably large number of available data sets, or by decomposing the individual data sets to filter out high-frequency fluctuations. © Author(s) 2014.
- ItemClimate variability over the last 35,000 years recorded in marine and terrestrial archives in the Australian region: an OZ-INTIMATE compilation(Elsevier Science Ltd., 2013-08-15) Reeves, JM; Barrows, TT; Cohen, TJ; Kiem, AS; Bostock, HC; Fitzsimmons, KE; Jansen, JD; Kemp, J; Krause, C; Phipps, SJ; Petherick, LMThe Australian region spans some 600 of latitude and 500 of longitude and displays considerable regional climate variability both today and during the Late Quaternary. A synthesis of marine and terrestrial climate records, combining findings from the Southern Ocean, temperate, tropical and arid zones, identifies a complex response of climate proxies to a background of changing boundary conditions over the last 35,000 years. Climate drivers include the seasonal timing of insolation, greenhouse gas content of the atmosphere, sea level rise and ocean and atmospheric circulation changes. Our compilation finds few climatic events that could be used to construct a climate event stratigraphy for the entire region, limiting the usefulness of this approach. Instead we have taken a spatial approach, looking to discern the patterns of change across the continent. The data identify the clearest and most synchronous climatic response at the time of the Last Glacial Maximum (LGM) (21 +/- 3 ka), with unambiguous cooling recorded in the ocean, and evidence of glaciation in the highlands of tropical New Guinea, southeast Australia and Tasmania. Many terrestrial records suggest drier conditions, but with the timing of inferred snowmelt, and changes to the rainfall/runoff relationships, driving higher river discharge at the LGM. In contrast, the deglaciation is a time of considerable south-east to north-west variation across the region. Warming was underway in all regions by 17 ka. Post-glacial sea level rise and its associated regional impacts have played an important role in determining the magnitude and timing of climate response in the north-west of the continent in contrast to the southern latitudes. No evidence for cooling during the Younger Dryas chronozone is evident in the region, but the Antarctic cold reversal clearly occurs south of Australia. The Holocene period is a time of considerable climate variability associated with an intense monsoon in the tropics early in the Holocene, giving way to a weakened monsoon and an increasingly El Nino-dominated ENSO to the present. The influence of ENSO is evident throughout the southeast of Australia, but not the southwest. This climate history provides a template from which to assess the regionality of climate events across Australia and make comparisons beyond our region.© 2013, Elsevier Ltd.
- ItemHolocene El Niño–Southern Oscillation variability reflected in subtropical Australian precipitation(Springer Nature, 2019-02-07) Barr, C; Tibby, J; Leng, MJ; Tyler, JJ; Henderson, ACG; Overpeck, JT; Simpson, GL; Cole, JE; Phipps, SJ; Marshall, JC; McGregor, GB; Hua, Q; McRobie, FHThe La Niña and El Niño phases of the El Niño-Southern Oscillation (ENSO) have major impacts on regional rainfall patterns around the globe, with substantial environmental, societal and economic implications. Long-term perspectives on ENSO behaviour, under changing background conditions, are essential to anticipating how ENSO phases may respond under future climate scenarios. Here, we derive a 7700-year, quantitative precipitation record using carbon isotope ratios from a single species of leaf preserved in lake sediments from subtropical eastern Australia. We find a generally wet (more La Niña-like) mid-Holocene that shifted towards drier and more variable climates after 3200 cal. yr BP, primarily driven by increasing frequency and strength of the El Niño phase. Climate model simulations implicate a progressive orbitally-driven weakening of the Pacific Walker Circulation as contributing to this change. At centennial scales, high rainfall characterised the Little Ice Age (~1450–1850 CE) in subtropical eastern Australia, contrasting with oceanic proxies that suggest El Niño-like conditions prevail during this period. Our data provide a new western Pacific perspective on Holocene ENSO variability and highlight the need to address ENSO reconstruction with a geographically diverse network of sites to characterise how both ENSO, and its impacts, vary in a changing climate. © The Author(s) 2019, corrected publication 2021
- 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.
- 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.
- ItemQuantifiying 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, JXThe 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.
- ItemSolar 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, TDThe 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.
- ItemA 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, CDPalaeoclimate 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.