CPClimate of the PastCPClim. Past1814-9332Copernicus PublicationsGöttingen, Germany10.5194/cp-12-595-2016Optimal site selection for a high-resolution ice core record in East AntarcticaVanceTessa R.tessa.vance@utas.edu.auhttps://orcid.org/0000-0001-6970-8646RobertsJason L.MoyAndrew D.https://orcid.org/0000-0002-7664-9960CurranMark A. J.TozerCarly R.GallantAilie J. E.AbramNerilie J.https://orcid.org/0000-0003-1246-2344van OmmenTas D.https://orcid.org/0000-0002-2463-1718YoungDuncan A.https://orcid.org/0000-0002-6866-8176GrimaCyrilBlankenshipDon D.SiegertMartin J.Antarctic Climate & Ecosystems Cooperative Research Centre, University of Tasmania, Private Bag 80, Hobart, Tasmania 7001, AustraliaDepartment of the Environment, Australian Antarctic Division, Hobart, Tasmania 7050, AustraliaCentre for Water, Climate and Land Use, University of Newcastle, Callaghan, New South Wales 2308, AustraliaSchool of Earth Atmosphere and Environment, Monash University, Victoria 2904, AustraliaResearch School of Earth Sciences, The Australian National University, Canberra, Australian
Capital Territory 2601 AustraliaJackson School of Geosciences, University of Texas at Austin, Austin, Texas, USAGrantham Institute and Department of Earth Science and Engineering, Imperial College London, London, UKTessa R. Vance (tessa.vance@utas.edu.au)8March20161235956101October20153November201521January201611February2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://cp.copernicus.org/articles/12/595/2016/cp-12-595-2016.htmlThe full text article is available as a PDF file from https://cp.copernicus.org/articles/12/595/2016/cp-12-595-2016.pdf
Ice cores provide some of the best-dated and most comprehensive
proxy records, as they yield a vast and growing array of proxy indicators.
Selecting a site for ice core drilling is nonetheless challenging,
as the assessment of potential new sites needs to
consider a variety of factors. Here, we demonstrate a systematic approach
to site selection for a new East Antarctic high-resolution ice core
record. Specifically, seven criteria are considered: (1) 2000-year-old
ice at 300 m depth; (2) above 1000 m elevation; (3) a minimum
accumulation rate of 250 mmyears-1IE (ice equivalent); (4) minimal surface
reworking to preserve the deposited climate signal; (5) a site with
minimal displacement or elevation change in ice at 300 m depth;
(6) a strong teleconnection to midlatitude climate; and (7) an appropriately
complementary relationship to the existing Law Dome record (a high-resolution record in East Antarctica). Once assessment of these physical
characteristics identified promising regions, logistical considerations
(for site access and ice core retrieval) were briefly considered. We use
Antarctic surface mass balance syntheses, along with ground-truthing of satellite data by airborne radar surveys
to produce all-of-Antarctica maps of surface roughness, age
at specified depth, elevation and displacement change, and surface air
temperature correlations to pinpoint promising locations. We also use
the European Centre for Medium-Range Weather Forecast ERA 20th
Century reanalysis (ERA-20C) to ensure that a site complementary to
the Law Dome record is selected. We find three promising sites in the
Indian Ocean sector of East Antarctica in the coastal zone from Enderby
Land to the Ingrid Christensen Coast (50–100∘ E). Although
we focus on East Antarctica for a new ice core site, the methodology
is more generally applicable, and we include key parameters for all of
Antarctica which may be useful for ice core site selection elsewhere and/or for
other purposes.
Introduction
Our knowledge of current climate change and our ability to
predict future climate change depends on understanding past natural
climate variability. Considerable uncertainties exist in the reconstruction
of past climate, with ice cores playing an increasingly important role in
understanding both climate impacts and forcings. In Antarctica, there is poor
spatial coverage of high-resolution palaeoclimate data over the last 2000 years, which is
articulated in IPCC synthesis reports as limiting present understanding of
climate processes . The last 2000 years have been
recognised as an important interval of Earth history, as they contain both a significant
natural period, prior to anthropogenic influence, and the full industrial era
. Additionally, high-resolution (e.g. annual or seasonal)
records of climate forcings over the past 2000 years are of importance to the
climate modelling community, such as PMIP (Paleoclimate Modelling Intercomparison Project) , and will
improve our knowledge of the dynamics of the climate system over this epoch.
A recent 2000-year Antarctic-wide temperature reconstruction
revealed regional differences in temperature between East
and West Antarctica during the same epoch. However, only four proxy sites
were available to reconstruct all of East Antarctica, which comprises 84 %
of the total Antarctic land mass . It is unknown how
much of this disparity is a true representation of the two regimes or how
much is an artefact of data sparsity.
Filling in data-sparse regions with new ice core records will
contribute to our understanding of regional- and global-scale climate
processes, but the location of ice core sites requires careful site selection.
Unfortunately, ice core site selection is not governed solely by the local climate
response and its preservation in the ice core record. Rather,
site selection is primarily restricted by both glaciological and
logistical constraints, with optimal site positioning from a climate processes
perspective being constrained to this restricted domain. Minimal layer and basal
complexity is necessary to ensure that interpretation of the ice core record is as
straightforward as possible. Additionally, sites where age models
vary uniformly with depth simplify interpretation, provided radar data exists and
are able to confirm suitable ice stratigraphy, accumulation rates, and basal
conditions e.g.. When high-resolution records are required, an additional
constraint is to minimise surface reworking effects (e.g. wind movement
of snow or loss via ablation) and subsurface
complexity (i.e. the basal ice flow regime). High-accumulation sites minimise
the negative effects of any surface reworking, yet, logistically, this
generally means being restricted to coastal locations, which often have
complex basal topography, flow regimes and frequently inclement weather.
High-accumulation sites can also mean that multi-year drilling campaigns may
be required to achieve the required depth and associated ice age.
In reality, few such sites exist in Antarctica that incorporate a satisfactory level of
all these criteria but with sufficiently high accumulation rates
to resolve annual cycles. Three sites that do largely meet these criteria are the
recently obtained West Antarctic Ice Sheet (WAIS) Divide core in West Antarctica
(79.767∘ S, 112.133∘ W; 1766 m elevation)
, the James Ross Island core in the northern
Antarctic Peninsula region (57.685∘ W, 64.2017∘ S; 1252 m elevation) and the Law Dome core
(112.8069∘ W, 66.7697∘ S; 1370 m elevation)
in coastal East Antarctica.
Airborne surveys across large swathes of East Antarctica
e.g. mean that
it is now possible to extend the climate information gleaned from these
few records using
a consistent, logical approach to choosing a new ice core site in East
Antarctica based on prescribed desired parameters.
This study details the search for an East Antarctic ice core site that will
fulfil a particular prescribed role – that of improving high-resolution
climate reconstruction for the Southern Hemisphere over the last 2000 years.
In particular, a number of studies have suggested that variability related to
a broad latitudinal swathe of the southwest (SW) Pacific and southern Indian oceans, a region
with poor observational records prior to the satellite era, may be recorded
in ice core records from East Antarctica. observed a
relationship between changes in mean wind direction and accumulation
disparities in the East Antarctic Plateau region encompassing Dome C and the
incidence of synoptic blocking events in the SW Pacific, a study that neatly
reinforced that of , who had previously suggested that
blocking in this sector may have a significant effect on the accumulation at
inland sites of East Antarctica, as midlatitude cyclones were “steered”
toward the interior of the continent. showed that the
moisture sources for Law Dome have temperate to subtropical sources, while
observed possible decadal cycling in isotopic
records from a related Law Dome core (DE08) that may have been related to
Pacific variability and a resultant change in meridional winds in the Law
Dome region. demonstrated that there was a decadal change
in meridional winds coherent with phase changes in the Interdecadal Pacific
Oscillation (IPO) in the southern Indian Ocean. This teleconnection to
Pacific decadal sea surface temperature (SST) and wind anomalies was exploited to produce a 1000-year
annually dated IPO reconstruction. What can be surmised from these prior
studies is that coastal to inland East Antarctica is subject to annual to
multidecadal changes in the meridional aspects of the midlatitude
westerlies, and ice core records in this region are capable of recording
these signals. Given the paucity of high-resolution records from this region,
new ice core sites have great potential to shed light on the drivers of these
meridional wind variations. A new ice core record from East Antarctica
complementary to the existing Law Dome record would be of great use for
further study of climate history in the SW Pacific and Indian Ocean sector of the
Southern Ocean and the Southern Hemisphere more generally.
Site criteria
In this study, seven criteria were proposed in order to locate
an ideal site for a new ice core record. These seven criteria can be divided into
site criteria (characteristics necessary to produce an appropriate
record) and atmospheric circulation criteria (circulation characteristics that
may be recorded at the proposed site). The site criteria were as follows:
2000-year-old ice at 300 m depth; an achievable drilling target
for a small field team in a single (summer) season.
minimal summer snowmelt, as surface melt redistributes water, stable water isotopes
and trace chemistry, hindering the interpretation of an ice core.
Large-scale studies indicate that surface layer melt occurs when monthly
average temperatures are above about -4 to -2 ∘C
, and liquid water formation and
near-surface melt layers in firn and ice occurs when air temperatures
are greater than 0 ∘C .
As the air temperature decreases with altitude (with a
rate determined by the adiabatic lapse rate), melting is reduced
at higher elevations. For example, there was minimal surface melt at the
Law Dome ice core site (1370 m elevation) .
Herein, we use a surface elevation above 1000 m a.s.l as a proxy
for minimal surface melt in East Antarctica. This is consistent with
satellite-based and modelling
studies which show minimal surface melt at these elevations.
sub-annual resolution, which for the purposes of this study
we propose requires a minimum of 250 mmyears-1IE accumulation
where IE stands for ice equivalent using an ice density
of 917 kgm-3 to convert between kgm-2years-1 of water and the ice equivalent;. Preference
will be given to sites that also show relatively uniform annual accumulation (i.e.
accumulation is not skewed toward a particular season).
minimal surface reworking to preserve the originally deposited
climate signal, which we assess based on estimated surface roughness.
location on a ridgeline or dome or at a site where the ice at the target depth
has undergone minimal displacement or elevation change.
The proposed circulation criteria are as follows:
a strong teleconnection
with midlatitude (oceanic) climate. For instance, a clearly defined
climatological flow pattern from the regional Southern Ocean to the site.
an appropriately complementary relationship to existing ice
core records (i.e. a site that adds new information to the existing
ice core array).
Any site selected also needs to fulfil the logistical
requirement of being reasonably accessible for ice core drilling and retrieval.
Methods and data sets
To assess which regions of East Antarctica may fulfil the site criteria,
we exploited the newly available aero-geophysical data from the Investigating Cryospheric Evolution through Collaborativ Aerogeophysical Profiling
(ICECAP) surveys . This allowed us to
make a comprehensive spatial assessment of the East Antarctic ice sheet
to pinpoint regions of interest for closer study.
Nye depth
The age of ice at a given depth can be estimated by
assuming a constant vertical strain rate throughout the ice column (equal
to the ice equivalent accumulation rate divided by the ice thickness).
This Nye age model will underestimate the age for deep
ice but should perform well for ice less than 70 %
of the local ice sheet thickness, and it provides a conservative estimate
of ice age (i.e. actual age should be older at any given depth).
Spatial maps of the age of ice at 300 m were calculated based
on vertical strain rates from the regional atmospheric climate model, RACMO2.1/ANT, surface mass balance
data for the period 1979–2012 , converted to ice
equivalent using an ice density of 917 kgm-3 and the BEDMAP2 ice thickness compilation .
Ice advection
To ease the interpretation of ice core records, ideally the ice should
have all originated at the same spatial location, i.e. there has been
minimal horizontal motion. Additionally, to simplify the interpretation
of stable water isotope records, elevation change in the surface should
be near zero. These conditions are only achieved near ice dome summits
and ice divides (ridgelines). For all other locations, the ice at depth has originated
further upstream and generally at higher elevations.
To assess the spatial variation in horizontal displacement and associated
change in surface elevation as ice is advected towards the ice sheet
margin, ice motion trajectories were calculated. Specifically, the
horizontal displacement of ice was calculated over a 1000-year interval
using a modified version of the Lagrangian streamline tracing routine
of with ice velocities from the MEaSUREs data set
(Making Earth System Data Records for Use in Research Environments; ). The associated change in elevation was estimated from
the digital elevation model (DEM), with the implicit assumption of no significant
change in surface elevation over the last 1000 years.
Calculating surface reworking for all of Antarctica
We are not aware of any surface roughness data set for all of Antarctica,
and a high surface roughness compared to accumulation rate
can hinder the interpretation of high-resolution ice core records. To produce
an all-of-Antarctica map of surface roughness, we use satellite imagery
ground-truthed with ICECAP aero-geophysical data to estimate the spatial
distribution of surface roughness.
Observational surface roughness data – laser altimetry
Surface roughness was estimated from Operation IceBridge, ICECAP and
other University of Texas at Austin Institute of Geophysics Level 2 Geolocated
nadir laser altimetry (Riegl) . Specifically, surface roughness was estimated as the median absolute difference
from a local linear regression (calculated using the robust Theil–Sen
method; ) for 500 m sliding windows averaged onto
a 50 km× 50 km grid.
MODIS Mosaic of Antarctica
We calculated a surface roughness estimate from the
125 m resolution 8 bit greyscale MODIS Mosaic
of Antarctica . To do this we considered
50 km× 50 km sub-windows, with both the sub-window
intensity standard deviation (S) and high-pass-filtered (half-amplitude
wavelength of 450 m) mean intensity (M) as parameters.
Furthermore, as katabatic winds redistribute snow, especially below
the 2000 m elevation contour ,
we have used two different data fits for above and below this
elevation threshold. Therefore, the ice sheet surface elevation (H)
was used both as an explanatory variable in the
data fitting and to segment the data fit. As the estimation of surface
roughness is based on a single temporal snapshot, we have had to assume
that surface roughness does not vary with time.
The laser altimetry data (R) were fitted (Eq. 1) using a re-weighted trimmed least squares
method . Data were separately fitted above and
below the 2000 m elevation contour. As we are more interested in
lower surface roughness values, a constant offset of -1.42 × 10-2m
(calculated from the offset of the least squares fit) was introduced to
the fitted data to bring the lower bounds into agreement.
R=(73.6+0.154M-0.284S-3.28×10-3H)/1000if H≤2000 m, r2=0.54(92.0+1.09M-2.20S-2.50×10-3H)/1000if H>2000 m, r2=0.61
The calculated surface roughness increases with increasing high spatial
frequency content (increasing M) and decreases with increasing height (H)
(general reduction in katabatic wind strength). Additionally, it also decreases with
S, a measure of the longer spatial-scale variability.
Details of existing firn and short ice core records in coastal
East Antarctica that are proximal to the areas of interest identified in this study.
Core siteLatitudeLongitudeBoreholeElevationAccumulationInterval coveredRACMO2.1/ANT(∘ S)(∘ E)depth (m)(m a.s.l)(kgm-2a-1)(kgm-2a-1)Promontory–Cape Darnley region MGA68.64660.22523.621830227.7±81.4a1942–1992137.2LGB0068.65061.117151832270b99.7EO6568.62259.705181870232b190.8LGB1672.81757.33315.182689125.2±40.0c1933–199257.8Mt Brown region LGB69/AWS70.83577.075102.181850286d2002–2003136.6LGB65/DT00171.84677.92251.852325130.7±42.8e1751–199657.8DT08573.36777.01750.652577153.3±61.6f1940–199719.8LGB7070.57676.868451651163g196.5U170.44579.3173.862044197±38h1988–1997157.7MB10/268.91078.8505.13933306±44h1990–1998272.3MB15/269.20379.7895.131447276±42h1990–1998166.8MB18/269.50080.7495.131764251±21h1989–1998118.6U269.68281.4164.651973352±49h1991–199795.3MB8/368.00481.0044.93611267±38h1990–1998408.1MB9/467.72483.6593.16494368±70h1995–1998221.6MB12/468.00583.6495.01981244±8h1989–1998276.5MB15/468.35083.9984.971425408±60h1993–1998207.7MB18/468.77884.306.151800225±20h1985–1998186.6U469.27984.5655.412048461±58h1992–1997182.1MBS69.13185.99910.222078255±24h1979–1998128.8U568.97387.2905.492047329±44h1990–1997254.5MB10/667.23288.2454.79837442±51h1993–1998461.0MB15/667.91188.3464.881406383±56h1992–1998396.8MB18/668.38988.6034.831733242±26h1989–1998282.9U668.83488.93462048315±33h1988–1997220.1MB10/866.91191.7546.11983461±122h1993–1998530.7MB15/867.51892.1865.081550238±32h1988–1998184.2MB18/868.00192.5585.161822324±35h1991–1998187.9U868.42792.8684.962075261±30h1988–1997185.0Bunger Hills region GF1268.48397.18341.62320537i1940–1984255.9
abcdefghi
Existing ice core records in East Antarctica
Some validation of our spatial interpolation and extrapolation can be provided by
examining existing short-core records which have been taken in East
Antarctica during prior glaciological surveys and traverses. The locations of
some of these cores and details of their existing data sets are discussed
below and shown in Table and in the figures where
appropriate. It is worth noting here that, while the majority of these
records are published, for numerous reasons (such as time interval of
coverage) many are not used in calibrating products such as RACMO 2.1/ANT. As
a result there may be discrepancies between observations and model outputs of
RACMO 2.1/ANT, especially in coastal regions with high spatial gradients.
Analysis of Southern Hemisphere warm season circulation
The proposed circulation criteria for site selection in this study are
centred around finding a site that will produce a record similar and complementary to the existing Law Dome ice core, that is, a site that
records annual- to decadal-scale variability of the western sector of
the southern Indian Ocean, as Law Dome samples the more easterly sector
e.g.. Primarily, this is likely to mean
finding a site geographically removed from the Law Dome region but
with similar glaciological characteristics. We have used investigation
with reanalysis products to shed further light on possible circulatory
source regions for a new ice core site. We explored circulation
characteristics using both the ERA-Interim (ERA-Int; ) (1979–2014)
and ERA 20th Century (ERA-20C; ) (using 1960–2010)
reanalysis products to search for a site with incident circulation
complementary to Law Dome and coherent with decadal-scale anomalies
that may be related to the Interdecadal Pacific Oscillation (IPO – see
below). The ERA-20C incorporates surface pressure and wind measurements
only, while the ERA-Int incorporates all available observations,
including satellite information, and is widely regarded as the most
reliable product for the data-sparse Antarctic region at the surface and
Z500 level . Both products were used to compare
and contrast results in the data-sparse region of interest (SW Pacific
and southern Indian Ocean). The longer, 20th-century reanalysis
product covers some of the most recent, completely negative IPO phase (1944–1975),
along with some of the current negative phase, which is likely to have started in the
late 1990s or early 2000s. We acknowledge that the lack of observations prior to 1979 may give
uncertain results. However, we felt that it was more important to use the
ERA 20th Century product, as it samples negative IPO years that span
the full range of the IPO cycle (i.e. negative years that occur when the IPO is
trending negatively, as well as negative years that occur when the IPO is trending positively).
ERA-Int can likely only sample negative years that are trending negatively, as it is unlikely that
the post-2000s negative phase has begun to trend positively at this stage (although end effects make this difficult
to establish absolutely). However, we acknowledge that ERA-Int is the benchmark product
to use; thus, we conducted the same analyses over the 1979–2014 period using ERA-Interim and include this analysis as a Supplement.
The expression of the IPO in the Indian Ocean
The IPO is the basin-wide expression of the Pacific Decadal
Oscillation . Both modes represent multidecadal sea surface temperature
(SST) anomalies in the Pacific Ocean and have significant and
far-reaching effects on climate (e.g. winds, rainfall and surface air
temperature; e.g.). Limited
studies have shown a signature of the IPO in the Indian Ocean region
e.g.. Recently the Law Dome ice core was used to
produce a millennial-length IPO reconstruction by exploiting IPO-related
multi-decadal wind anomalies in the midlatitudes of the southern Indian
Ocean .
For the analysis here, IPO positive and negative years were computed
from the 13-year filtered Tripole Index for the Interdecadal Pacific
Oscillation (TPI), which represents the IPO (; available
at http://www.esrl.noaa.gov/psd/data/timeseries/IPOTPI/).
TPI values half a standard deviation above or below zero were defined as periods
of positive and negative IPO. The mean atmospheric and surface states for
positive and negative IPO events were examined over the global oceans and
in Australian rainfall percentiles in order to highlight any connections
between the IPO, the East Antarctic coast and Australian rainfall.
This analysis used composites of anomalies in geopotential height at
500 hPa (Z500) to highlight the mean circulation, as well as
SST anomalies and Australian precipitation anomalies. The anomalies
of all data except SSTs were computed relative to the complete length
time series, which varied depending on the analysis. The SST anomalies
are pre-computed in the HadISST3.1.1 data set, relative to 1980–2010
. Composite maps were produced and compared over
1960–2010 for positive and negative TPI years using the ERA-20C. For comparison,
the ERA-Interim reanalysis is also provided from 1979 to 2014 in the
Supplement (see Fig. S2).
Site selection in East AntarcticaAreas that fulfil the minimum site criteria
In order to first provide a preliminary assessment of the
broad regions of the Antarctic continent that may yield high-resolution ice core records, we applied a less conservative
subset of our stated criteria. Specifically, we identified regions
above 1000 m elevation where local accumulation rates were
≥ 200 mmyears-1IE and simultaneously ice age at 300 m
depth was at least 500 years. These criteria are less stringent than the
specified criteria used for site selection for this study and identify
a broad, largely coastal band in East Antarctica, primarily in the
Indo-Pacific sector, and a large proportion of West Antarctica, including
the continental divide that may contain sites for high-resolution ice
cores. As this analysis was outside the core criteria for this study
but may be useful to other researchers, we include this preliminary
assessment in the Supplement (see Fig. S1).
Assessing continental East Antarctica for areas that fulfil the
specified high-resolution, multi-millennial record length site criteria for
this study, with all of Antarctica on the left and the Indian Ocean sector of
East Antarctica on the right. Contour intervals (grey) are 500 m.
Panel (a) shows ice age at 300 m (colour bar) using the
process described previously. Panel (b) shows annual snowfall
accumulation rate, with areas that receive > 250 mmyears-1IE
shown using a solid red boundary, and areas that receive
> 200 mmyears-1IE shown using a dashed red boundary. Panel
(c) is a representation of uniformity of snowfall accumulation
through the year (blue represents more seasonally uniform accumulation).
Labelled boxes in panel (b) identify possible sites after site
criteria 1–3 are assessed; see the “Results” section for more detail.
LawProm: Law Promontory–Enderby Land region; Darnley: Cape Darnley
region; Mt Brown: Mt Brown region; Bunger Hills: Bunger Hills
region.
MODIS-based surface roughness estimates. Panel (a) shows
the calculated average surface roughness in each
50 × 50 km2 grid, from Operation IceBridge, ICECAP and
other University of Texas at Austin Institute of Geophysics Level 2
Geolocated nadir laser altimetry flight-line data
. Panel (b) shows the resultant
estimate of surface roughness from MODIS Mosaic of Antarctica (MOA) imagery,
by calibrating it with the averaged surface altimetry calculated in panel (a) (see the “Method” Section (Sect. )). Panel (c) shows
the estimated (modelled) surface roughness for Antarctica using the laser
altimetry-calibrated MOA imagery, while panel (d) shows estimated
surface roughness as a fraction of estimated annual snowfall accumulation
rate from RACMO2.1. In this instance, blue regions (low fractional roughness)
indicate regions with minimal surface reworking as a fraction of snowfall
accumulation rate and hence potential high-resolution ice coring sites.
Panel (e) is the enlarged coastal East Antarctic section of the
fractional roughness (panel d).
In East Antarctica, applying the more stringent criteria 1 and 3 (2000-year age achievable at 300 m and ≥ 250 mmyears-1IE
respectively) greatly restricts possible sites from the broader region
identified in Fig. S1 to a narrower band inland but
parallel to the coastal margin. In general, these primary criteria also
satisfy site criterion 2, that the site chosen be at an elevation
above 1000 m, as sites below this generally do not yield
old enough ice. The fulfilment of site criteria 1–3 is shown in
Fig. . Panel a of Fig.
identifies the predominantly inland, high-elevation regions where
2000-year-old ice is within 300 m of the surface (darker blue
regions), although there are notable exceptions where this occurs closer
to the coast, such as the Ingrid Christensen Coast, the Mawson Coast
and the Bjerkø Peninsula–Cape Darnley region. When the minimum
requirement of 250 mmyears-1IE accumulation is considered
(Fig. b), most of the previously identified coastal
regions have lower accumulation than this minimum requirement and are
thus ruled out, although there are a few regions that remain. Note that
we include the less stringent criteria of 200 mmyears-1IE, identified in Fig. S1 as a dotted boundary for reference
and comparison.
The region between these accumulation boundaries may in fact be the most fruitful zone, as
the maximum ice sheet thickness inferred by the Nye age–depth model is thicker and therefore expands the possible regions of interest.
In addition, Fig. c shows that
accumulation is generally uniform throughout the year for most of East
Antarctica, with the exception of a few small regions upstream of the
Amery Ice Shelf.
As previously stated, our interest is in obtaining an ice core that
reflects features of the circulation and climate of the western sector
of the southern Indian Ocean. As a result, we focus on the region between Enderby Land and the
Bunger Hills. We identify four regions that may contain sites that fulfil
our initial three site criteria, which we then investigate further for fulfilment of
our remaining criteria (see below).
These regions are Law Promontory–Enderby Land, Cape Darnley, Mt Brown and the Bunger
Hills (Figs. – and , fuchsia boxes).
Assessing surface roughness and estimating surface reworking
Figure shows the calibration of the surface roughness estimated from
MODIS satellite imagery against the ICECAP laser altimetry data to
produce a whole-of-Antarctica estimate of surface roughness. Figure c
shows that our four selected regions of interest have minimal surface roughness. Figure d, e indicate that large swathes
of East Antarctica have surface roughnesses that are much less than
the average annual accumulation, although this is not the case in the
Lambert Glacier basin, inland of the Amery Ice Shelf. Nonetheless, our
four selected regions of interest appear to have a relatively smooth
surface compared to the accumulation rate (Fig. e). Additionally,
a prior analysis of drifting snow suggests that these
regions have a favourable drifting-snow-to-accumulation ratio, allowing
for the preservation of annual layers within the ice core.
Existing firn and ice core records
An array of shallow firn (5–10 m) and ice cores (50–100 m)
exists from previous glaciological traverses and survey work in East
Antarctica (Figs. , , Table ),
some of which are near or within the regions specified above.
These existing firn and ice cores have been analysed for a series of
glaciochemical measurements. These include electrical conductivity, water stable isotopes (δ18O), hydrogen peroxide
(H2O2), density, snow accumulation and trace ion chemistry.
Existing records near the Law Promontory and Cape Darnley regions
Ice core and historical in situ measurements in
the Law Promontory and Darnley regions west of the Amery Ice
Shelf are limited to four records on the eastern boundary of the
Law Promontory region. These were collected as part of the Australian National Antarctic Research Expedition (ANARE) Lambert Glacial basin traverse program
and the Chinese National Antarctic Research Expedition (CHINARE) inland traverses .
Accumulation rates from cores and stake measurements along the
west Lambert Glacial basin traverse route 150 to 250 km inland
are highly variable ranging from 0 to 500 kgm-2a-1
and averaging ≈ 230 kgm-2a-1 (; Fig. 2). The high variability is likely due to the steepness
of the ice sheet topography and associated strong katabatic winds
in this area , while further inland from 350 to
550 km accumulation rates are less variable and average
350 kgm-2a-1. Repeat annual accumulation
rate measurements in the west Lambert Glacial basin show similar spatial
variability but also indicate large temporal variability (Qin et al.,
2000). The model accumulation rate (Fig. b)
underestimates the observed snow accumulation rates as well as the
spatial variability . Therefore, a
combination of further analysis of existing data and modern radar site
survey techniques would be required prior to drilling.
Elevation change (metres; (a)) and ice
advection (kilometres; (b)) over 1000 years, calculated using a modified version
of the Lagrangian streamline tracing of , with ice
velocities from the MEaSUREs data set . Blue areas show
comparatively little elevation change and/or advection over 1000 years. The
right-hand panels are the enlarged coastal East Antarctic section of each
panel. Stars indicate historical ice core locations for short
records.
Existing records near the Mount Brown region
The Mount Brown region contains the most comprehensive and detailed
spatial distribution of firn and ice cores in Wilhelm II Land
.
Twenty-one shallow firn cores were collected between 79.3 and
92.9∘ E, ranging between 500 and 2100 m elevation
(Figs. , ; Table ). Many of the cores display well-resolved annual
layers, which record regional climate signals .
For example, at Mount Brown South, Foster et al. (2006)
reported a correlation between methanesulfonic acid (MSA) and sea-ice
extent. Similar to Law Dome, the Mount Brown South MSA sea-ice proxy
record is a regional indicator of sea-ice extent .
Accumulation records from these cores suggest that RACMO 2.1/ANT in
general underestimates the accumulation rates in this region, and
also the spatial variability, although this variability is likely to be
less pronounced in this region than in the west Lambert Glacial basin
described earlier.
Existing records near the Bunger Hills region
There is limited published ice core data from within or proximal to
this region. Ice core data for site GF12 (see map) suggests that the snow
accumulation is 536 kgm-2a-1, with well-resolved
annual layers . Initial comparisons
between Law Dome and GF12 (not shown) suggest that they share a similar
regional climate signal. In addition, RACMO 2.1/ANT significantly
underestimates accumulation in the vicinity of GF12, probably due
to the lack of published data. This means that sites further inland
from GF12 could be suitable for high-resolution 2 kyr ice core records,
albeit with a climate signal similar to that of Law Dome.
Elevation and advection
Figures and a show that due to ice advection,
the elevation at deposition varies down the length of an ice core record.
These figures also show that these elevation artefacts can be highly variable in the coastal zone. This
is due to the increasing horizontal ice velocities in the coastal region,
along with the steeper topography at the margins of the ice sheet. In
the Law Promontory region, there is significant elevation change in the
NE coastal segment as well as in the western inland region. There are
subregions with minimal horizontal displacement of ice over a 1000-year
time frame, although these regions are not as extensive as in the other
identified regions of interest. In the Cape Darnley region, there is
minimal elevation change except in the far south of this box, and displacement
of ice over the 1000-year time frame is minimal. Nonetheless, there is
evidence of larger displacement change in the same far southern area. In the
Mt Brown region, three clear zones of minimal elevation change over 1000 years and associated minimal displacement are identified: a ridge in the
far western portion, a less pronounced ridge in the central portion and
the area near the existing Mt Brown South record (indicated in Fig. a) in the centre of the eastern half of this region. There are also
clear regions of high elevation (and displacement) change over time,
but these are generally restricted to the coastal margins below 1000 m a.s.l. The majority of the Bunger Hills region shows minimal
elevation change and/or displacement over the 1000-year time frame with
the exception of a small portion in the eastern segment, which is probably
related to drainage of the numerous glaciers to the west of Law Dome.
Representative temperature from reanalysis
Figure is a representation of the approximate distance required to
obtain partial decorrelation (decoupling) of the surface temperature
signal. Specifically, the orange spectrum shows that a particular point
contains a temperature signal representative (r≥0.7) of a broad
(greater than 600 km) area. For example, the Bunger Hills region is
broadly within the same representative surface air temperature regime
as Law Dome, which has a temperature radius of ∼ 550 km. The
other three identified regions show that they are likely to produce
temperature proxy records representative of large areas (frequently > 750 km)
in regions that are currently poorly sampled for millennial-length temperature
proxy records.
A spatial plot of the areal extent that a temperature record is
representative of, calculated from ERA-Interim surface (2 m)
temperature. Equivalent radius is the correlation length, i.e. the distance
from a given point, with which surface temperature is highly correlated
(> 0.7). The right-hand panel is the enlarged coastal East Antarctic
sector.
Decadal-scale circulation and complementarity to Law Dome
Anomalies in geopotential height (GPH), SST and rainfall percentiles for the TPI positive
and negative composites produced following the method previously outlined are
shown in Fig. for 1960–2010 (ERA 20C). A total of 14 positive years make up the positive TPI composite, while 23 negative years make up the
negative TPI composite. In addition, Fig. S2 shows the same analysis
using ERA-Interim (1979–2014), which has 15 years for each composite.
Figure shows the difference between the mean state of the surface
ocean and atmosphere during TPI-defined IPO positive and negative years for the Southern
Hemisphere warm season (November–March) which showed the strongest pattern,
as well as the average response of Australian rainfall, with reductions
in rainfall of 5–15 % during IPO positive and increases of up to
20 % or more during the wetter IPO negative phase. The SSTs show
the canonical IPO pattern in the tropical Pacific, with the warmer tongue
of SSTs in the central and east equatorial Pacific flanked by cooler SSTs
west and further from the Equator. In the high latitudes southeast (SE) of Africa, IPO
positive years are associated with higher geopotential heights compared
to IPO negative years. Conversely, IPO positive years are associated
with regions of lower heights to the southwest of Australia. Although
the magnitude of these circulation anomalies differs depending on the
data set used, both sets of analysis consistently indicate that positive
IPO phases are associated with a stronger southerly component of the
atmospheric circulation, while negative IPO phases are associated with
a more northerly component of this circulation.
Panel (a) shows a further enlargement of
Fig. b
(elevation change in metres), with the existing Mt Brown South ice core record
identified (fuchsia dot), while panel
(b) shows a further enlargement of Fig. b (annual snowfall accumulation
with < 250 and < 200 mmyears-1IE shown
as solid red and dashed red lines respectively).
It is encouraging that composite anomalies produced by both ERA-20C
and ERA-Int over their common period of 1979–2010 showed generally
the same atmospheric anomalies. However, the patterns were amplified in
the ERA-20C suggesting it may overestimate the magnitudes of changes
between the IPO phases, although we cannot discount the possibility that
the ERA-20C may record more detail as it samples negative years before and after
IPO minima (i.e. negative years from when the IPO is trending both negatively
and then positively). While caution must be applied to the interpretation of data prior to the introduction of satellite data
in 1979, Fig. suggests that IPO phase changes may result in anomalies
that could be recorded in ice cores in East Antarctica, as suggested
by .
Sea surface temperature, 500 m geopotential height and Australian
rainfall correlation analysis with IPO during the warm season
(November–March) for positive (top) and negative (bottom) years. For this
analysis we used ERA-20C over 1960–2010, defining positive and
negative years as 0.5 SD above or below the average respectively.
See the Supplement for this analysis performed using ERA-Interim
over 1979–2014. Note that the colour bar defines a rainfall increase over
Australia, as well as warm SSTs, as the red spectrum (conversely, a rainfall
decrease and cool SSTs are blue). Identified in fuchsia are regions shown in previous figures: 1 – Law Promontory and Enderby Land; 2 – Cape Darnley; and 3 – Mt Brown. Law Dome is shown as a red star to the east of the three regions.
Discussion
This application of mapping and numerical methods to identify a suitable
ice coring site is useful provided the initial criteria are stringent enough
so as to reduce the possible sites rapidly but not so stringent that no
reasonably accessible site is available. In this study, the requirement
of high annual snowfall combined with a 1000–2000-year
ice age within 300 m of the surface were the two primary criteria
for site selection, as high accumulation generally precludes millennial-length
records within 300 m of the surface.
However, we acknowledge that the numerical analyses performed here
have several shortcomings and assumptions that may be invalid, at least
locally, and are due to the quality and spatial density of the
observational data available. One of the principal shortcomings
is the use of a modelled accumulation product; however, we note that the
product used here (RACMO 2.1/ANT) has exceptional skill in many parts
of the continent. Due to the spatial resolution of the underlying model
(27 km), small-scale topography variations which affect the
local accumulation regime are not resolved. In our region of interest,
in general RACMO appears to underestimate the accumulation rate, the
result of which is younger ice at depth than is predicted. A mitigating
factor is that the Nye age model tends to underestimate age at depth; thus, the impact of the underestimation of accumulation rate may not be
as severe.
There are also several limitations with the surface roughness analysis.
The length scale used to calibrate it against – 500 m – is most
likely too broad and therefore not entirely appropriate for modelling
processes that distort the seasonal signals recorded in the ice core.
The regression for surface roughness obtained for the MODIS imagery
only accounts for around 25 % of the variance in the roughness measured
from the laser altimetry. Additionally, while areas with high surface
roughness are unlikely to be suitable for obtaining an annually resolved
ice core, a smooth surface is not a sufficient condition to define
suitability for high-resolution ice coring. Importantly, blue ice and
wind-glazed areas are both unsuitable for high-resolution ice coring but
are characterised by smooth surfaces and would not be excluded with
this analysis.
We also acknowledge here that this study is only the initial step in selecting a new, high-resolution ice core site. Additional
analysis and ground-truthing from both geophysical surveys and short cores
are required to ultimately confirm the suitability of a particular site.
Optimal site selection – site criteria
Site criteria 1–3 significantly narrowed the suitable sites in East Antarctica
for very high-resolution ice coring. Four likely regions remained after the site criteria were
applied: the Law Promontory and Enderby Land
region inland of the Mawson Coast, the region encompassing Cape Darnley
and the Bjerkø Peninsula, the region encompassing Mt Brown on the Ingrid
Christensen Coast near Davis station, and the Bunger Hills region to the west of
Law Dome. The Bunger Hills region is comparatively close
to Law Dome compared to the other three regions, and limited studies
suggest a similar regional climate signal to Law Dome. Furthermore,
Fig. indicates that a similar representative surface air temperature
regime to Law Dome, while Fig. indicates the Bunger Hills region
has a similar decadal wind signal to Law Dome. On the strength of this,
we rule out Bunger Hills as a site for this study, as our circulation
criteria, which specify a circulation regime complementary to Law Dome,
are unlikely to be fulfilled. Nonetheless, the Bunger Hills region has
strong potential for high-resolution ice core records.
The Law Promontory region (LawProm) shows some promise and has
associated prior records. The 1000–1500 m contour interval
neatly encloses a region of > 250 mmyears-1IE accumulation
and is also positioned on a dome or ridgeline (Fig. a) and hence has
favourable ice dynamics. There is significant elevation change in the
near neighbourhood of this region, suggesting some danger that isotope
records may be less straightforward to interpret, hampering both the
ability to infer temperature and moisture sources at the site and the
precise dating of the record. Nye age modelling suggests that obtaining
1500–2000 years within 300 m of the surface may be possible
with > 250 mmyears-1IE accumulation but that the region
where this is possible is restricted to a very small site, on the ridge, at between 1000 and 1500 m elevation. Unfortunately, no existing ice
core records are known of in the immediate vicinity of this ridge.
Historically, logistical access to this site has been problematic, due
largely to frequently inclement weather. Additionally, this region is
outside of the traditional logistical shipping routes (for Australian
Antarctic station resupply). Nonetheless, another promising site in
this region is the saddle in Enderby Land, behind the Napier Mountains
Dome, at between 1000 and 1500 m elevation, that has accumulation of
200 to above 250 mmyears-1IE. We note that there are numerous
nunataks in this subregion and this may indicate complex basal features
in this region. Detailed aero-geophysical surveys would be required to
further specify an appropriate site in this subregion.
Of the four promising sites considered here, the temperature signal in
the Cape Darnley region (Darnley) is representative of the largest
area in East Antarctica. Figure a shows
that on the easterly facing slope of the Darnley region, ice age
at target depth increases with elevation, and obtaining an ice age
of 1500–2000 years at 300 m is achievable. However, the
requirement of > 250 mmyears-1IE accumulation means that, as
with much of the East Antarctic coast, there is only a very small band at
around 1000 m where both criteria are satisfied (Fig. b). This
very small region is situated almost precisely on the ridgeline and
therefore is likely to have favourable ice dynamics. Figure a shows this
as there is little in the way of major elevation changes over time in
the immediate area. Existing accumulation measurements in this coastal
region indicate that there is reasonably high variability in annual accumulation rates
(Table ). This means that pinpointing an ideal high-resolution site in
the Darnley region would require ground-truthing to confirm site-specific
accumulation rates.
The Mount Brown region already has numerous existing short-core records
for validation of accumulation rate estimates, and these existing
records have also shown compelling evidence of the preservation of
seasonally varying glaciochemical properties. The Mt Brown region has
three promising subregions: two ridges and an area of quiescent ice flow
associated with Mt Brown. The accumulation estimates from RACMO suggest that accumulation rates may be too low for our stated site selection criteria. However, as
stated above, the numerous existing short-core records for this area
indicate higher accumulation rates, sufficient for annual resolution.
Go west: capturing a new southern Indian Ocean climate signal
Pacific decadal variability is a strong driver of climate variability
across the Pacific and in the Antarctic region
e.g.. The incidence of extreme events (e.g. floods
and droughts) changes significantly across the Pacific Basin depending on
the state of the IPO phase. As an example, eastern Australia experiences
increased drought risk during IPO positive phases and increased flood risk
during IPO negative phases . This IPO-related
variability in the Australian hydroclimate is also clear in Fig. , with,
for example, on average 10–20 % higher rainfall during the IPO
negative phase.
As previously described, there is some evidence that IPO-like
decadal variability has hemispheric influences at high latitudes of the Southern
Hemisphere expressed as changes to the zonal wave-3 pattern .
More recently, highlighted differences in the Southern Hemisphere
climatic response by comparing two intervals that coincided with the warm
(1979–1998) and cool (1999–2012) phases of the IPO. Although they did not directly
address decadal variability in the Pacific (i.e. IPO-like variability) as a mechanism for
these changes, they did highlight concurrent changes in the structure of the Southern
Annular Mode and the El Niño–Southern Oscillation between the two epochs. Their results
highlight a connection between tropical Pacific and Southern Hemisphere high-latitude
climate variability on the decadal timescale. Moreover, showed that
these differences occurred in the non-annular spatial component of the Southern Annular
Mode, providing corroborating evidence for changes in the wave-like structures of the
Southern Hemisphere circulation between these two intervals, similar to .
Further evidence of Pacific decadal variability,
possibly related to the IPO, has been shown in the Indian Ocean
region and includes local hydroclimate records from Madagascan corals
, tree ring records from Myanmar ,
Antarctic stable water isotope variability , and
an Australian rainfall and IPO reconstruction
from the Law Dome ice core.
Currently, there is limited understanding around tropical Pacific forcing of
high-latitude climate variability. The Law Dome ice core has provided valuable insight
into this tropical–extra-tropical teleconnection .
Therefore, a new, annually resolved ice core that collected a record complementary to
these existing, but limited, studies of Pacific decadal variability in the
Indian Ocean region would be valuable.
Figure shows a signature of IPO-related variability in the
atmospheric circulation in the southern Indian Ocean, SE of the African
continent. This is a similar location to the centre of action of the zonal
wave-3 structure identified in , although displaced slightly
to the west. Even with the uncertainty in the circulation analysis due to the limited
length of the observational data records (see Fig. and Supplement),
travelling west from Law Dome toward our three primary regions suggests that an ice
core here might either sample this centre of action directly or a complementary part of it.
Therefore, a record from one of these regions should provide a record complementary
to Law Dome, possibly with a stronger IPO signal. It should be noted that, in using
data from 1960 to 2010 (1979–2014 Supplement), we are only sampling a small subset
of the known instrumental-period behaviour of the IPO. However, this is a limitation of
all studies characterising decadal variability in the high latitudes of the Southern
Hemisphere and should not preclude the use of these observational results from a site
selection analysis.
As discussed above, have shown that changes in high-latitude climatic anomalies are
coincident with those in El Niño–Southern Oscillation (ENSO) in the tropical Pacific. This suggests that IPO-like
anomalies may be related to the multidecadal variability of the Southern Annular Mode
(SAM), specifically, in its non-annular component . By definition, this
would affect the meridional component of the circulation more than the zonal component,
potentially affecting the advection of heat and moisture onto the Antarctic continent. A
new high-resolution record from this region could complement and enhance the existing SAM
reconstructions from James Ross Island (Antarctic Peninsula) and Law Dome
to produce a hemispheric record of annual SAM variability for the last
1–2 millennia, which incorporates these differences in basin-to-basin behaviour. Evidence
provided here and in the literature e.g. shows that the ability to
capture any IPO-related anomaly in the southwest Indian Ocean is likely to be improved by
choosing a site directly south of the central Indian Ocean, e.g. the Law Promontory–Enderby
Land region. However, we also note that logistical constraints may become
more prohibitive in the more westerly regions, due to weather considerations and the
increasing distance from both Antarctic stations and the normal associated
shipping routes. International collaboration would ameliorate the more prohibitive
aspects of access to these sites.
Conclusions
This study details a systematic method for selection of a new
ice core site – in this instance, one with annual resolution and an ice age
of 1000–2000 years at 300 m depth. This method demonstrates that
having clear, specific and stringent criteria for site selection narrows the
available area for further focus considerably. As a result, specific regions
can be examined in greater detail. We have identified three sites in coastal
East Antarctica spanning the region from Enderby Land to the Ingrid
Christensen Coast that approach the fulfilment of all of our specified
criteria. For targeting high-resolution regions, we have found that sites are
generally restricted to a coastal band within 1000–1500 m elevation.
In addition, the analysis presented here suggests that the three sites we
have identified may sample a region of the Indian Ocean not previously
sampled at high resolution over the last 2 millennia.
This broadening of the geographical extent of the 2000-year ice core array in Antarctica will help refine reconstructions of
both regional and hemispheric climate and global climate indices.
The Supplement related to this article is available online at doi:10.5194/cp-12-595-2016-supplement.
Acknowledgements
Funding for this work was provided by the UK Natural Environment Research
Council grant NE/D003733/1, NSF grant ANT-0733025, the Jackson School of
Geosciences and the G. Unger Vetlesen Foundation. The Australian Antarctic
Division provided funding and logistical support (ASAC 3103, 4077, 4346).
This work was supported by the Australian Government's Cooperative Research
Centres Programme through the Antarctic Climate and Ecosystems Cooperative
Research Centre (ACE CRC). ECMWF ERA-Interim and ERA-20C data used in this
study have been obtained from the ECMWF data server. We acknowledge support
from the joint PAGES and WCRP Polar Climate Predictability Initiative (PCPI)
Research Program. We thank Meredith Nation for discussion related to this
manuscript.
Edited by: E. Wolff
ReferencesAbram, N. J., Mulvaney, R., and Arrowsmith, C.: Environmental signals in a
highly resolved ice core from James Ross Island, Antarctica, J.
Geophys. Res., 116, D20116, 10.1029/2011JD016147, 2011.
Abram, N. J., Mulvaney, R., Vimeux, F., Phipps, S. J., Turner, J., and
England,
M. H.: Evolution of the Southern Annular Mode during the past millennium,
Nature Climate Change, 4, 564–569, 2014.Bamber, J. L., Gomez-Dans, J. L., and Griggs, J. A.: A new 1 km digital
elevation model of the Antarctic derived from combined satellite radar and
laser data – Part 1: Data and methods, The Cryosphere, 3, 101–111,
10.5194/tc-3-101-2009, 2009.
Bindschadler, R.: The environment and evolution of the West Antarctic ice
sheet: setting the stage, Philos. T. R. Soc. A, 364, 1583–1605, 2006.Bisiaux, M. M., Edwards, R., McConnell, J. R., Curran, M. A. J., Van Ommen,
T. D., Smith, A. M., Neumann, T. A., Pasteris, D. R., Penner, J. E., and
Taylor, K.: Changes in black carbon deposition to Antarctica from two
high-resolution ice core records, 1850–2000 AD, Atmos. Chem. Phys., 12,
4107–4115, 10.5194/acp-12-4107-2012, 2012.
Blankenship, D., Kempf, S. D., Young, D. A., Roberts, J. L., van Ommen, T.,
Forsberg, R., and Siegert, M. J.: Icebridge Riegl laser altimeter L2
geolocated surface elevation triplets, Digital media, NASA DAAC at the
National Snow and Ice Data Center, 2012.Bracegirdle, T. and Marshall, G.: The reliability of antarctic tropospheric
pressure and temperature in the latest global reanalyses, J. Climate,
25, 7138–7146, 10.1175/JCLI-D-11-00685.1, 2012.Braconnot, P., Harrison, S., Kageyama, M., Bartlein, P., Masson-Delmotte, V.,
Abe-Ouchi, A., Otto-Bliesner, B., and Zhao, Y.: Evaluation of climate models
using palaeoclimatic data, Nature Climate Change, 2, 417–424,
10.1038/nclimate1456, 2012.
Crueger, T., Zinke, J., and Pfeiffer, M.: Patterns of Pacific decadal
variability recorded by Indian Ocean corals, Int. J. Earth Sci., 98, 41–52, 2009.
Dahe, Q., Jiawen, R., Jiancheng, K., Cunde, X., Zhongqin, L., Yuansheng, L.,
Bo, S., Weizhun, S., and Xiaoxiang, W.: Primary results of glaciological
studies along an 1100 km transect from Zhongshan Station to Dome A, East
Antarctic ice sheet, Ann. Glaciol., 31, 198–204, 2000.D'Arrigo, R. and Ummenhofer, C. C.: The climate of Myanmar: evidence for
effects of the Pacific Decadal Oscillation, Int. J. Climatol., 35,
634–640, 10.1002/joc.3995, 2014.
Das, S. and Alley, R.: Characterization and formation of melt layers in polar
snow: observations and experiments from West Antarctica, J. Glaciol, 51, 307–312, 2005.Dee, D. P., Uppala, S. M., Simmons, A. J., Berrisford, P., Poli, P.,
Kobayashi,
S., Andrae, U., Balmaseda, M. A., Balsamo, G., Bauer, P., Bechtold, P.,
Beljaars, A. C. M., van de Berg, L., Bidlot, J., Bormann, N., Delsol, C.,
Dragani, R., Fuentes, M., Geer, A. J., Haimberger, L., Healy, S. B.,
Hersbach, H., Hólm, E. V., Isaksen, L., Kållberg, P., Köhler, M.,
Matricardi, M., McNally, A. P., Monge-Sanz, B. M., Morcrette, J.-J., Park,
B.-K., Peubey, C., de Rosnay, P., Tavolato, C., Thépaut, J.-N., and
Vitart, F.: The ERA-Interim reanalysis: configuration and performance of the
data assimilation system, Q. J. Roy. Meteor. Soc., 137, 553–597, 10.1002/qj.828, 2011.
Delmotte, M., Masson, V., Jouzel, J., and Morgan, V. I.: A seasonal deuterium
excess signal at Law Dome, coastal eastern Antarctica: A southern ocean
signature, J. Geophys. Res., 105, 7187–7197, 2000.
Ding, M., Xiao, C., Li, Y., Ren, J., Hou, S., Jin, B., and Sun, B.: Spatial
variability of surface mass balance along a traverse route from Zhongshan
station to Dome A, Antarctica, J. Glaciol, 57, 658–666, 2011.
Edwards, R. and Sedwick, P.: Iron in East Antarctic snow: Implications for
atmospheric iron deposition and algal production in Antarctic waters,
Geophys. Res. Lett., 28, 3907–3910, 2001.
Foster, A., Curran, M., Smith, B. T, van Ommen, T., and Morgan, V.: Covariation
of sea ice and methanesulphonic acid in Wilhelm II Land, East Antarctica,
Ann. Glaciol., 44, 429–432, 2006.Fretwell, P., Pritchard, H. D., Vaughan, D. G., Bamber, J. L., Barrand, N.
E., Bell, R., Bianchi, C., Bingham, R. G., Blankenship, D. D., Casassa, G.,
Catania, G., Callens, D., Conway, H., Cook, A. J., Corr, H. F. J., Damaske,
D., Damm, V., Ferraccioli, F., Forsberg, R., Fujita, S., Gim, Y., Gogineni,
P., Griggs, J. A., Hindmarsh, R. C. A., Holmlund, P., Holt, J. W., Jacobel,
R. W., Jenkins, A., Jokat, W., Jordan, T., King, E. C., Kohler, J., Krabill,
W., Riger-Kusk, M., Langley, K. A., Leitchenkov, G., Leuschen, C., Luyendyk,
B. P., Matsuoka, K., Mouginot, J., Nitsche, F. O., Nogi, Y., Nost, O. A.,
Popov, S. V., Rignot, E., Rippin, D. M., Rivera, A., Roberts, J., Ross, N.,
Siegert, M. J., Smith, A. M., Steinhage, D., Studinger, M., Sun, B., Tinto,
B. K., Welch, B. C., Wilson, D., Young, D. A., Xiangbin, C., and Zirizzotti,
A.: Bedmap2: improved ice bed, surface and thickness datasets for Antarctica,
The Cryosphere, 7, 375–393, 10.5194/tc-7-375-2013, 2013.Frezzotti, M., Scarchilli, C., Becagli, S., Proposito, M., and Urbini, S.: A
synthesis of the Antarctic surface mass balance during the last 800 yr, The
Cryosphere, 7, 303–319, 10.5194/tc-7-303-2013, 2013.Garreaud, R. and Battisti, D. S.: Interannual (ENSO) and Interdecadal
(ENSO-like) Variability in the Southern Hemisphere Tropospheric Circulation*,
J. Climate, 12, 2113–2123,
10.1175/1520-0442(1999)012<2113:IEAIEL>2.0.CO;2, 1999.
Goodwin, I.: On the Antarctic Contribution to Holocene Sea Level, PhD thesis,
University of Tasmania, 333 pp., 1995.
Goodwin, I. D., Higham, M., Allison, I., and Jalwen, R.: Accumulation
variation in eastern Kemp Land, Antarctica, Ann. Glaciol., 20, 202–206,
1994.
Goodwin, I., van Ommen, T., Curran, M., and Mayewski, P.: Mid latitude winter
climate variability in the South Indian and southwest Pacific regions since
1300 AD, Clim. Dynam., 22, 783–794, 2004.Greenbaum, J. S., Blankenship, D. D., Young, D. A., Richter, T. G., Roberts,
J. L., Aitken, A. R. A., Legresy, B., Schroeder, D. M., Warner, R. C., van
Ommen, T. D., and Siegert, M. J.: Ocean access to a cavity beneath Totten
Glacier in East Antarctica, Nat. Geosci., 8, 294–298,
10.1038/ngeo2388, 2015.Henley, B., Gergis, J., Karoly, D., Power, S., Kennedy, J., and Folland, C.:
A
Tripole Index for the Interdecadal Pacific Oscillation, Clim. Dynam., 45,
1–14, 10.1007/s00382-015-2525-1, 2015.
Higham, M. and Craven, M.: Surface mass balance and snow surface properties
from the Lambert Glacier basin traverses 1990–1994, Article,
Article/Report 9, [Hobart], Antarctic CRC, 1997.Hock, R.: Glacier melt: a review of processes and their modelling, Prog. Phys. Geog., 29, 362–391, 10.1191/0309133305pp453ra, 2005.
Kaczmarska, M., Isaksson, E., Karlöf, L., Brandt, O., Winther, J.-G.,
van de
Wal, R. S. W., van den Broeke, M., and Johnsen, S. J.: Ice core melt features
in relation to Antarctic coastal climate, Antarct. Sci., 18, 271–278,
2006.Kennedy, J. J., Rayner, N. A., Smith, R. O., Parker, D. E., and Saunby, M.:
Reassessing biases and other uncertainties in sea surface temperature
observations measured in situ since 1850: 2. Biases and homogenization,
J. Geophys. Res.-Atmos., 116, D14104,
10.1029/2010JD015220, 2011.
Kiem, A. S. and Franks, S. W.: Multi-decadal variability of drought risk,
eastern Australia, Hydrol. Process., 18, 2039–2050, 2004.Kiem, A. S., Franks, S. W., and Kuczera, G.: Multi-decadal variability of
flood
risk, Geophys. Res. Lett., 30, 1035, 10.1029/2002GL015992, 2003.Lenaerts, J. T. M., van den Broeke, M. R., Déry, S. J., van Meijgaard, E.,
van de Berg, W. J., Palm, S. P., and Sanz Rodrigo, J.: Modeling drifting snow
in Antarctica with a regional climate model: 1. Methods and model evaluation,
J. Geophys. Res.-Atmos., 117, D05108,
10.1029/2011JD016145, 2012a.Lenaerts, J. T. M., van den Broeke, M. R., van de Berg, W. J., van Meijgaard,
E., and Kuipers Munneke, P.: A new, high-resolution surface mass balance
map of Antarctica (1979–2010) based on regional atmospheric climate
modeling, Geophys. Res. Lett., 39, 1–5,
10.1029/2011GL050713, 2012b.
Li, R. X., Xiao, C. D., Sneed, S. B., and Yan, M.: A continuous 293-year
record of volcanic events in an ice core from Lambert Glacier basin, East
Antarctica, Antarct. Sci., 24, 293–298, 2012.Massom, R. A., Pook, M. J., Comiso, J. C., Adams, N., Turner, J.,
Lachlan-Cope,
T., and Gibson, T. T.: Precipitation over the Interior East Antarctic Ice
Sheet Related to Midlatitude Blocking-High Activity, J. Climate, 17,
1914–1928, 10.1175/1520-0442(2004)017<1914:POTIEA>2.0.CO;2, 2004.Masson-Delmotte, V., Delmotte, M., Morgan, V., Etheridge, D., van Ommen, T.,
Tartarin, S., and Hoffmann, G.: Recent southern Indian Ocean climate
variability inferred from a Law Dome ice core: New insights for the
interpretation of coastal Antarctic isotopic records, Clim. Dynam., 21,
153–166, 10.1007/s00382-003-0321-9, 2003.
Morgan, V., Wookey, C., Li, J., van Ommen, T., Skinner, W., and Fitzpatrick,
M.: Site information and initial results from deep ice drilling on Law Dome,
Antarctica, J. Glaciol, 43, 3–10, 1997.Mulvaney, R., Abram, N. J., Hindmarsh, R. C. A., Arrowsmith, C., Fleet, L.,
Triest, J., Sime, L. C., Alemany, O., and Foord, S.: Recent Antarctic
Peninsula warming relative to Holocene climate and ice-shelf history, Nature,
489, 141–144, 10.1038/nature11391, 2012.
Nye, J.: Correction factor for the accumulation measured by the thickness of
the annual layers in an ice sheet, J. Glaciol, 4, 785–788, 1963.
PAGES 2k Consortium: Continental-scale temperature variability during the
past two millennia, Nat. Geosci., 6, 339–346, 2013.
Paterson, W.: The Physics of Glaciers, Butterworth-Heonemann, Oxford, 3 edn.,
496 pp., 1994.
Poli, P., Hersbach, H., Tani, D., Deei, D., Thépauti, J.-N., Simmonsi, A.,
Peubeyi, C., Laloyauxi, P., Komorii, T., Berrisfordi, P., Draganii, R.,
Trémoleti, Y., Holmi, E., Bonavitai, M., Isaksen, L., and Fisher, M.: The
data assimilation system and initial performance evaluation of the ECMWF
pilot reanalysis of the 20th-century assimilating surface observations only
(ERA-20C), Tech. Rep. 14, ECMWF, 2013.
Power, S., Casey, T., Folland, C., Colman, A., and Mehta, V.: Inter-decadal
modulation of the impact of ENSO on Australia, Clim. Dynam., 15, 319–324,
1999.
Ren, J. and Qin Dahe, I. A.: Variations of snow accumulation and temperature
over past decades in the Lambert Glacier basin, Antarctica, Ann. Glaciol., 29, 29–32, 1999.Rignot, E., Mouginot, J., and Scheuchl, B.: Ice flow of the Antarctic ice
sheet., Science, 333, 1427–30, 10.1126/science.1208336, 2011.Roberts, J., Plummer, C., Vance, T., van Ommen, T., Moy, A., Poynter, S.,
Treverrow, A., Curran, M., and George, S.: A 2000-year annual record of snow
accumulation rates for Law Dome, East Antarctica, Clim. Past, 11, 697–707,
10.5194/cp-11-697-2015, 2015.Roberts, J. L., Warner, R. C., Young, D., Wright, A., van Ommen, T. D.,
Blankenship, D. D., Siegert, M., Young, N. W., Tabacco, I. E., Forieri, A.,
Passerini, A., Zirizzotti, A., and Frezzotti, M.: Refined broad-scale
sub-glacial morphology of Aurora Subglacial Basin, East Antarctica derived by
an ice-dynamics-based interpolation scheme, The Cryosphere, 5, 551–560,
10.5194/tc-5-551-2011, 2011.
Rousseeuw, P. J.: Least Median of Squares Regression, J. Am. Stat. Assoc.,
79, 871–880, 1984.Scambos, T., Haran, T., Fahnestock, M., Painter, T., and Bohlander, J.:
MODIS-based Mosaic of Antarctica (MOA) data sets: Continent-wide surface
morphology and snow grain size, Remote Sens. Environ., 111,
242–257, 10.1016/j.rse.2006.12.020, 2007.
Sen, P. K.: Estimates of the Regression Coefficient Based on Kendall's Tau,
J. Am. Stat. Assoc., 63, 1379–1389, 1968.Siegert, M. J. and Payne, A. J.: Past rates of accumulation in central West
Antarctica, Geophys. Res. Lett., 31, L12403, 10.1029/2004GL020290,
2004.
Smith, B. and Ruddell, A. R.: Snow accumulation in Wilhelm II Land, East
Antarctica, Article, Article/Report 22, [Hobart], Antarctic CRC, 2001.
Smith, B., Van Ommen, T., and Morgan, V.: Distribution of oxygen isotope
ratios
and snow accumulation rates in Wilhelm II Land, East Antarctica, Ann. Glaciol., 35, 107–110, 2002.Stocker, T., Qin, D., Plattner, G., Tignor, M., Allen, S., Boschung, J.,
Nauels, A., Xia, Y., Bex, V., and Midgley, P.: Climate Change 2013-the
Physical Science Basis: Working Group I Contribution to the Fifth Assessment
Report of the IPCC, 10.1017/CBO9781107415324, 2013.Thompson, D. M., Cole, J. E., Shen, G. T., Tudhope, A. W., and Meehl, G. A.:
Early twentieth-century warming linked to tropical Pacific wind strength,
Nat. Geosci., 8, 117–121,
10.1038/ngeo2321, 2015.Trusel, L. D., Frey, K. E., Das, S. B., Kuipers Munneke, P., and van den Broeke, M. R.: Satellite-based estimates of Antarctic surface meltwater fluxes,
Geophys. Res. Lett., 40, 6148–6153, 10.1002/2013GL058138,
2013.
van den Broeke, M. and Bintanja, R.: The interaction of katabatic winds and
the formation of blue-ice areas in East Antarctica, J. Glaciol,
41, 395–407, 1995.van Ommen, T. D., Morgan, V., and Curran, M. A. J.: Deglacial and Holocene
changes in accumulation at Law Dome, East Antarctica, Ann. Glaciol.,
39, 359–365, 10.3189/172756404781814221, 2004.Van Wessem, J., Reijmer, C., Morlighem, M., Mouginot, J., Rignot, E., Medley,
B., Joughin, I., Wouters, B., Depoorter, M., Bamber, J., Lenaerts, J., Van
De Berg, W., Van Den Broeke, M., and Van Meijgaard, E.: Improved
representation of East Antarctic surface mass balance in a regional
atmospheric climate model, J. Glaciol, 60, 761–770,
10.3189/2014JoG14J051, 2014.Vance, T., Roberts, J., Plummer, C., Kiem, A., and van Ommen, T.:
Interdecadal
Pacific variability and Australian mega-droughts over the last millennium,
Geophys. Res. Lett., 42, 129–137, 10.1002/2014GL062447, 2015.Vance, T. R., van Ommen, T. D., Curran, M. A. J., Plummer, C. T., and Moy,
A. D.: A Millennial Proxy Record of ENSO and Eastern Australian Rainfall
from the Law Dome Ice Core, East Antarctica, J. Climate, 26, 710–725,
10.1175/JCLI-D-12-00003.1, 2013.
Wen, J., Jezek, K. C., Monaghan, A. J., Sun, B., Ren, J., and Huybrechts, P.:
Accumulation variability and mass budgets of the Lambert Glacier-Amery Ice
Shelf system, East Antarctica, at high elevations, Ann. Glaciol., 43,
351–360, 2006.Wright, A. P., Young, D. A., Roberts, J. L., Schroeder, D. M., Bamber, J. L.,
Dowdeswell, J. A., Young, N. W., Le Brocq, A. M., Warner, R. C., Payne,
A. J., Blankenship, D. D., van Ommen, T. D., and Siegert, M. J.: Evidence of
a hydrological connection between the ice divide and ice sheet margin in the
Aurora Subglacial Basin, East Antarctica, J. Geophys. Res.,
117, 1–15, 10.1029/2011JF002066, 2012.
Xiao, C., Qin, D., Bian, L., Zhou, X., Allison, I., and Yan, M.: A precise
monitoring of snow surface height in the region of Lambert Glacier
basin-Amery Ice Shelf, East Antarctica, Sci. China Ser. D, 48, 100–111,
2005.Yeo, S.-R. and Kim, K.-Y.: Decadal changes in the Southern Hemisphere sea
surface temperature in association with El Niño-Southern Oscillation and
Southern Annular Mode, Clim. Dynam., 45, 3227–3242,
10.1007/s00382-015-2535-z, 2015.
Young, D., Wright, A., Roberts, J., Warner, R., Young, N., Greenbaum, J.,
Schroeder, D., Holt, J., Sugden, D., Blankenship, D., van Ommen, T., and
Siegert, M.: A dynamic early East Antarctic Ice Sheet suggested by
ice-covered fjord landscapes, Nature, 474, 72–75, 2011.Young, D. A., Lindzey, L. E., Blankenship, D. D., Greenbaum, J. S.,
de Gorordo,
A. G., Kempf, S. D., Roberts, J. L., Warner, R. C., van Ommen, T., Siegert,
M. J., and Le Meur, E.: Land-ice elevation changes from photon counting swath
altimetry: First applications over the Antarctic ice sheet, J. Glaciol., 61, 17–28, 10.3189/2015JoG14J048, 2015.Zhang, Y., Wallace, J. M., and Battisti, D. S.: ENSO-like Interdecadal
Variability: 1900–93, J. Climate, 10, 1004–1020,
10.1175/1520-0442(1997)010<1004:ELIV>2.0.CO;2, 1997.
Zhang, M. J., Li, Z. Q., Xiao, C. D., Qin, D. H., Yang, H. A., Kang, J. C.,
and Li, J.: A continuous 250-year record of volcanic activity from Princess
Elizabeth Land, East Antarctica, Antarct. Sci., 14, 55–60, 2002.
Zhang, M., Li, Z., Ren, J., Xiao, C., Qin, D., Kang, J., and Li, J.: 250
years of accumulation, oxygen isotope and chemical records in a firn core
from Princess Elizabeth Land, East Antarctica, J. Geogr. Sci., 16, 23–33,
2006.
Zwally, H. J. and Fiegles, S.: Extent and duration of Antarctic surface
melting, J. Glaciol, 40, 463–476, 1994.