Tr Vance

Interdecadal Pacific variability and eastern Australian megadroughts over the last millennium

The Interdecadal Pacific Oscillation (IPO) influences multidecadal drought risk across the Pacific, but there are no millennial-length, high-resolution IPO reconstructions for quantifying long-term drought risk. In Australia, drought risk increases in positive phases of the IPO, yet few suitable rainfall proxies and short (∼100 years) instrumental records mean large uncertainties remain around drought frequency and duration. Likewise, it is unknown whether megadroughts have occurred in Australias past. In this study, an atmospheric teleconnection in the Indian Ocean midlatitudes linking East Antarctica and Australia is exploited to produce the first accurate, annually dated millennial-length IPO reconstruction from the Law Dome (East Antarctica) ice core. Combined with an eastern Australian rainfall proxy from Law Dome, the first millennial-length Australian megadrought (>5 year duration) reconstruction is presented. Eight megadroughts are identified including one 39 year drought (A.D. 1174–1212), which occurred during an unprecedented century of aridity (A.D. 1102–1212).

A Millennial Proxy Record of ENSO and Eastern Australian Rainfall from the Law Dome Ice Core, East Antarctica

ENSO causes climate extremes across and beyond the Pacific basin; however, evidence of ENSO at high southern latitudes is generally restricted to the South Pacific and West Antarctica. Here, the authors report astatisticallysignificantlinkbetweenENSOandseasaltdepositionduringsummerfromtheLawDome(LD) ice core in East Antarctica. ENSO-related atmospheric anomaliesfrom the central-western equatorialPacific (CWEP) propagate to the South Pacific and the circumpolar high latitudes. These anomalies modulate high- latitude zonal winds, with El Nino (La Nina) conditions causing reduced (enhanced) zonal wind speeds and subsequent reduced (enhanced) summer sea salt deposition at LD. Over the last 1010 yr, the LD summer sea salt(LDSSS)recordhasexhibitedtwobelow-average(ElNino-like)epochs,1000-1260 ADand1920-2009 AD, and a longer above-average (La Nina-like) epoch from 1260 to 1860 AD. Spectral analysis shows the below- average epochs are associated with enhanced ENSO-like variability around 2-5 yr, while the above-average epoch is associated more with variability around 6-7 yr. The LDSSS record is also significantly correlated with annual rainfall in eastern mainland Australia. While the correlation displays decadal-scale variability similar to changes in the interdecadal Pacific oscillation (IPO), the LDSSS record suggests rainfall in the modern instrumental era (1910-2009 AD) is below the long-term average. In addition, recent rainfall declines in some regions of eastern and southeastern Australia appear to be mirrored by a downward trend in the LDSSS record, suggesting current rainfall regimes are unusual though not unknown over the last millennium.

The preservation of methanesulphonic acid in frozen ice-core samples

Ice-core records of methanesulphonic acid (MSA) provide a potentially powerful tool for producing proxy records of sea ice, a critical but poorly understood component of the Earths climate system. However, MSA is able to diffuse through solid ice, and here we examine the effect of two different methods of frozen storage on the preservation of MSA in archived ice-core samples. Re-analysis of archived ice sticks confirms that MSA diffuses out of ice cores archived in this manner. Despite MSA losses of up to 39% after 7 years storage, the ice sticks studied here preserve much of the variability of the original MSA record, suggesting that useful proxy records can be obtained from archived ice sticks. Furthermore, re-analysis of ice-core samples that had been refrozen into discrete bottled samples for storage demonstrates that it is possible to archive ice samples in a way that prevents MSA loss. In this case, accurate records of MSA variability and concentration were preserved even over storage periods of 15 years. This has important implications for the storage of ice cores and subsequent determination of MSA, and demonstrates that ice storage history needs to be considered when interpreting MSA records.

Methanesulphonic acid loss during ice-core storage: recommendations based on a new diffusion coefficient

The loss of methanesulphonic acid (MSA) from stored ice cores can be significant over typical storage times, with diffusion to the ice-core surface controlling the loss. Methods for minimizing this loss are discussed and it is shown how measurements can be corrected by calculating the amount of MSA lost. A revised diffusion coefficient for MSA in solid ice, (4.1 x 10(-13)) +/- (2.5 x 10(-14)) m(2) s(-1), is derived to improve such MSA loss corrections.

Biological responses to the press and pulse of climate trends and extreme events

The interaction of gradual climate trends and extreme weather events since the turn of the century has triggered complex and, in some cases, catastrophic ecological responses around the world. We illustrate this using Australian examples within a press–pulse framework. Despite the Australian biota being adapted to high natural climate variability, recent combinations of climatic presses and pulses have led to population collapses, loss of relictual communities and shifts into novel ecosystems. These changes have been sudden and unpredictable, and may represent permanent transitions to new ecosystem states without adaptive management interventions. The press–pulse framework helps illuminate biological responses to climate change, grounds debate about suitable management interventions and highlights possible consequences of (non-) intervention.A press–pulse framework is used to understand the interactive ecological effects of gradual climate trends and extreme weather events. Australian case studies include population collapses, loss of relictual communities and novel ecosystems.

Correlation confidence limits for unevenly sampled data

Estimation of correlation with appropriate uncertainty limits for scientific data that are potentially serially correlated is a common problem made seriously challenging especially when data are sampled unevenly in space and/or time. Here we present a new, robust method for estimating correlation with uncertainty limits between autocorrelated series that does not require either resampling or interpolation. The technique employs the Gaussian kernel method with a bootstrapping resampling approach to derive the probability density function and resulting uncertainties. The method is validated using an example from radar geophysics. Autocorrelation and error bounds are estimated for an airborne radio-echo profile of ice sheet thickness. The computed limits are robust when withholding 10%, 20%, and 50% of data. As a further example, the method is applied to two time-series of methanesulphonic acid in Antarctic ice cores from different sites. We show how the method allows evaluation of the significance of correlation where the signal-to-noise ratio is low and reveals that the two ice cores exhibit a significant common signal. HighlightsCorrelation confidence limits can be calculated for unevenly sampled data.Employs Gaussian kernel method used with bootstrapping resampling.Two different studies using highly autocorrelated data validates method.

Author Correction: Biological responses to the press and pulse of climate trends and extreme events

In the version of this Perspective originally published, affiliations 1 and 4 ware incorrect, and should have read: “1Antarctic Climate & Ecosystems CRC, University of Tasmania, Hobart, Tasmania, Australia” and “4Centre for Water, Climate and Land (CWCL), University of Newcastle, Callaghan, NSW, Australia”. These have been corrected in the online versions of this Perspective.

Reconstructions of the southern annular mode (SAM) during the last millennium

The leading mode of atmospheric variability in the Southern Hemisphere is the Southern Annular Mode (SAM), which affects the atmosphere and ocean from the mid-latitudes to the Antarctic. However, the short instrumental record of the SAM does not adequately represent its multi-decadal to centennial-scale variability. Long palaeoclimatic reconstructions of the SAM would improve our understanding of its low frequency behavior and its effects on regional temperature, rainfall, sea ice, and ecosystem processes. In this progress report, we review three published palaeoclimatic reconstructions available for understanding multi-decadal to centennial-scale variability of the SAM. Reconstructions reviewed here show similar patterns of decadal SAM variability during the last two centuries, but earlier centuries are less coherent. Reconstructions clearly maintain similar trends towards more positive SAM states since the onset of significant anthropogenic climate forcing from rising greenhouse gas (GHG) concentrations and ozone depletion and these excursions appear unprecedented over at least the last 500 years. We describe how new multi-proxy reconstructions of the SAM could further improve our understanding of its long-term variability and effects across all geographic sectors of the Southern Hemisphere. Here, we recommend careful selection and development of proxies in SAM-sensitive regions and seasons. In particular, proxies related to cool-season conditions and from the poorly-sampled Indian Ocean sector would allow for a true circumpolar and year-round reconstruction of past SAM variability.