High-resolution, well-dated climate archives provide an
opportunity to investigate the dynamic interactions of climate patterns
relevant for future projections. Here, we present data from a new, annually
dated ice core record from the eastern Ross Sea, named the Roosevelt Island
Climate Evolution (RICE) ice core. Comparison of this record with climate
reanalysis data for the 1979–2012 interval shows that RICE reliably captures
temperature and snow precipitation variability in the region. Trends over the
past 2700 years in RICE are shown to be distinct from those in West
Antarctica and the western Ross Sea captured by other ice cores. For most of
this interval, the eastern Ross Sea was warming (or showing isotopic
enrichment for other reasons), with increased snow accumulation and perhaps
decreased sea ice concentration. However, West Antarctica cooled and the
western Ross Sea showed no significant isotope temperature trend. This
pattern here is referred to as the Ross Sea Dipole. Notably, during the
Little Ice Age, West Antarctica and the western Ross Sea experienced colder
than average temperatures, while the eastern Ross Sea underwent a period of
warming or increased isotopic enrichment. From the 17th century onwards, this
dipole relationship changed. All three regions show current warming, with
snow accumulation declining in West Antarctica and the eastern Ross Sea but
increasing in the western Ross Sea. We interpret this pattern as reflecting
an increase in sea ice in the eastern Ross Sea with perhaps the establishment
of a modern Roosevelt Island polynya as a local moisture source for RICE.
Introduction
With carbon dioxide (CO2) and global temperatures predicted to continue
to rise, model simulations of the Antarctic/Southern Ocean region show an increase in surface warming for
the coming decades resulting in reduced sea
ice extent, weakened Antarctic Bottom Water formation, intensified zonal
winds that reduce CO2 uptake by the Southern Ocean, a slowing of the
southern limb of the meridional overturning circulation (MOC) and associated
changes in global heat transport, and a rapid ice sheet grounding line retreat
that contributes to global sea level rise (Kusahara and Hasumi,
2013; Spence et al., 2012; Marshall and Speer, 2012; Sen Gupta et al.,
2009; Toggweiler and Russell, 2008; Russell et al., 2006; Downes et al.,
2010; Anderson et al., 2009; DeConto and Pollard, 2016; Joughin and Alley,
2011; Golledge et al., 2015; DeVries et al., 2017). Observations confirm an
ozone-depletion-induced strengthening and poleward contraction of zonal
winds (Thompson and Solomon, 2002b; Arblaster et al., 2011), increased
upwelling of warm, modified Circumpolar Deep Water (Jacobs et al.,
2011), a warmer Southern Ocean (Böning et al., 2008; Gille,
2002; Abraham et al., 2013), meltwater-driven freshening of the Ross Sea
(Jacobs et al., 2002), ice shelf and mass loss, grounding
line retreat (Rignot et al., 2014; Pollard et al., 2015; Paolo et al.,
2015; Joughin et al., 2014), reduced formation of Antarctic Bottom Water
(Rintoul, 2007) and Antarctic Intermediate Water (Wong
et al., 1999), changes in sea ice (regional decreases and increases in the
Amundsen and Ross seas, respectively) (Holland and Kwok, 2012; Sinclair et
al., 2014; Stammerjohn et al., 2012), and dynamic changes in Southern Ocean
CO2 uptake driven by atmospheric circulation (Landschützer
et al., 2015). However, these observational time series are short (Böning
et al., 2008; Gille, 2002; Toggweiler and Russell, 2008) and inter-model
variability indicates that physical processes and their consequences are not
well captured or understood (Braconnot et al., 2012; Sen Gupta et al.,
2009). While the skill of equilibrium simulations steadily improves, the
accuracy of transient model projections for the coming decades critically
depends on an improved knowledge of climate variability, forcings and
dynamic feedbacks (Stouffer et al., 2017; Bakker et al., 2017).
Here we present data from a new, highly resolved and accurately dated
ice core record, spanning the past 2.7 kyr, from the eastern Ross Sea region.
The Roosevelt Island Climate Evolution (RICE) ice core is compared with
existing records in the region to investigate the characteristics and
drivers of spatial and temporal climate variability in the Ross Sea region.
Site characteristics and relevant climate drivers
In this section, a brief overview is provided of the climatological and
glaciological characteristics of the study site.
Dynamic interaction between tropical and midlatitudinal climate drivers and
South Pacific climate variability
Environmental variability in the Pacific Sector of the Southern Ocean and
Antarctica is dominated by three major atmospheric circulation patterns: the
Southern Annular Mode (SAM), the El Niño–Southern Oscillation (ENSO) and
the Inter-decadal Pacific Oscillation (IPO). The SAM is the leading
empirical orthogonal function (EOF) of the Southern Hemisphere extratropical
geopotential height fields on monthly and longer timescales and describes
the strength and position of the Southern Hemisphere westerly winds via the
relative pressure over Antarctica (∼ 65∘ S) and the
midlatitudes (∼ 45∘ S) (Thompson
and Wallace, 2000; Thompson and Solomon, 2002a). The persistent positive,
summer trend of the SAM (decreasing pressure over Antarctica) has been
linked to stratospheric ozone depletion and an increase in atmospheric
greenhouse gas concentration (Arblaster et al.,
2011; Thompson et al., 2011). The positive SAM is associated with above-average warming of the Antarctic Peninsula, cooler conditions over East
Antarctica due to a reduced poleward pressure gradient and thus diminished
transport of heat and moisture, and a reduction in katabatic flow
(Marshall et al., 2013; Marshall and Thompson, 2016; Thompson and Solomon,
2002a). While the positive summer SAM trend (also weakly expressed during
autumn) along the Antarctic margin is generally associated with an
equatorward heat flux, the western Ross Sea is one of two regions (the
Weddell Sea being the other) to experience an anomalous poleward heat flux
(Marshall and Thompson, 2016) that transports heat and moisture
across the Ross Ice Shelf. No such pattern is observed for regional surface air temperature (SAT) (Marshall and Thompson, 2016), which might be masked by the influence of
regional sea ice variability on local temperatures. The positive SAM has
been shown to contribute at least partially to an increase in total
Antarctic sea ice, while a negative SAM has been associated with a reduced
sea ice (Ferreira et al., 2015; Holland et al., 2017; Bintanja et al.,
2013; Kohyama and Hartmann, 2015; Turner et al., 2017). The future behaviour
of the SAM over the next decades is a topic of active research due to the
competing and seasonally biased influences of projected stratospheric ozone
recovery and greenhouse gas emissions (Bracegirdle et al., 2014; Gillett
and Fyfe, 2013; Thompson et al., 2011).
The Pacific South American (PSA) patterns represent atmospheric Rossby wave
trains initiated by anomalously deep tropical convection during ENSO events,
in particular during austral spring, which originate in the western (PSA1)
and the central (PSA2) tropical Pacific (Mo and Higgins, 1998). PSA1 and
PSA2 are defined as the second and third EOF respectively of
monthly-mean extratropical geopotential height fields, with the negative
(positive) phase resembling El Niño (La Niña)-like conditions
(Mo, 2000). Changes in SAT over West Antarctica have been
linked to PSA1 variability (Schneider et al., 2012; Schneider and Steig,
2008), while the warming of West Antarctica's winter temperatures has been
linked to PSA2 (Ding et al., 2011). The positive polarity of PSA1 is
associated with anticyclonic wind anomalies in the South Pacific centred at
∼ 120∘ W, which have been linked to increased
onshore flow and increased eddy activity (Marshall and Thompson,
2016). In contrast, during the positive phase of the PSA2, the anticyclonic
centre shifts to ∼ 150∘ W in the Ross Sea, creating
a dipole across the Ross Ice Shelf, with increased transport of marine air
masses along the western Ross Ice Shelf and enhanced katabatic flow along
the eastern Ross Ice Shelf (Marshall and Thompson, 2016). Sea ice
feedbacks to the SAM and ENSO forcing in the western Ross Sea (as well as
the Bellingshausen Sea) were found to be particularly strong when a negative
SAM coincided with El Niño events (increased poleward heat flux, less
sea ice) or a positive SAM concurred with La Niña events (decreased
poleward heat flux, more sea ice) (Stammerjohn et al.,
2008). The authors found that this teleconnection is less pronounced in the
eastern Ross Sea.
(a) Overview map of the Ross Sea region and eastern West
Antarctica. Antarctic ice velocity derived from the Advanced Land Observing Satellite (ALOS) with the Phased Array type L-band Synthetic Aperture Radar (PALSAR), the European Space Agency (ESA) Envisat Advanced Synthetic Aperture Radar (ASAR), European Remote Sensing (ERS) satellites, ERS-1 and 2, and the the Canadian Space Agency (CSA) RADARSAT-1 and 2 satellite radar interferometry colour-coded on a
logarithmic scale (Rignot et al., 2011). Coloured dots indicate the
locations of ice core drilling sites used in this paper: RICE (blue),
West Antarctic Ice Sheet Divide ice core (WDC; red), Siple Dome (green), TALDICE and Talos Dome TD96 (purple), Taylor Dome
(orange). (b) Overview map of Roosevelt Island derived from the Moderate-resolution Imaging Spectroradiometer (MODIS) satellite
images (Scambos et al., 2007). The maps were created using the
Antarctic Mapping Tool (Greene et al., 2017). The coloured dots
indicate the location of the RICE drill site (blue) and the nearest ERA-Interim
(ERAi)
grid point (yellow).
The IPO, an ENSO-like climate variation
on decadal timescales (Power et al., 1999), is closely related
to the Pacific Decadal Oscillation (PDO) (Mantua and Hare, 2002).
While the PDO is defined as the first EOF of sea surface temperature (SST)
variability in the Northern Pacific, the IPO is defined by a tripole index
of decadal-scale SST anomalies across the Pacific (Henley et al.,
2015). A warm tropical Pacific and weakened trade winds are associated with
a positive IPO, while a cooler tropical Pacific and strengthened trade winds
are characteristic of a negative IPO. The phasing of the IPO and PDO have
been shown to influence the strength of regional and global teleconnections
with ENSO (Henley et al., 2015). An in-phase IPO amplification of
ENSO events has been linked to a strengthening of global dry/wet
anomalies, in contrast to periods when the IPO and ENSO are out of phase,
causing these anomalies to weaken or disappear entirely (Wang et
al., 2014). In addition, a negative IPO leads to cooler SSTs in the Ross,
Amundsen and Bellingshausen seas, while a positive IPO is associated with
warmer SSTs (Henley et al., 2015). The centre of anticyclonic
circulation linked to precipitation at Roosevelt Island
(Emanuelsson et al., 2018) moves eastward during the
negative IPO from ∼ 120∘ W during the positive IPO
to ∼ 100∘ W (Henley et al., 2015). It has
been suggested that the negative IPO, at least in part, is responsible for
the hiatus in global surface warming during 1940–1975 and 2001–2009
(England et al., 2014; Kosaka and Xie, 2013; Meehl et al., 2011).
The Amundsen Sea Low (ASL), a semi-permanent low-pressure centre in the
Ross–Amundsen Sea, is the most prominent and persistent of three
low-pressure centres around Antarctica, associated with the wave number 3
circulation (Raphael, 2004; Raphael et al., 2016; Turner et al., 2013). The
ASL is sensitive to ENSO (especially during winter and spring) and SAM (in
particular during autumn) and influences environmental conditions in the
Ross, Amundsen and Bellingshausen seas and across West Antarctica and the
Antarctic Peninsula (Ding and Steig, 2013; Raphael et al., 2016; Steig et
al., 2012; Marshall and Thompson, 2016; Bertler et al., 2004; Turner et al.,
2013; Thomas et al., 2009). Seasonally, the ASL centre moves from
∼ 110∘ W in during austral summer to ∼ 150∘ W
in austral winter (Turner et al., 2013). A positive SAM and/or La Niña
event leads to a deepening of the ASL, while a negative SAM and/or El
Niño event causes a weakening (Turner et al., 2013). The IPO, through its
effect upon ENSO and SAM variability, also influences the ASL and sea ice
extent in the Ross and Amundsen seas (Meehl et al., 2016). Blocking events in
the Amundsen Sea (Renwick, 2005) are sensitive to the position of the ASL and
are dominant drivers of marine air mass intrusions and associated
precipitation and temperature anomalies at Roosevelt Island (Emanuelsson et
al., 2018).
Overview of ice core records used in this paper. Locations are
presented in Fig. 1.
NameLocation Elevation (m)Drill depth (m)Age scaleYear recoveredReferenceRICE DeepS 79.3640W 161.7065508.57–764.60RICE172011/12 (0–130 m) and 2012/13 (130–764.60 m)This paperRICE 12/13 BS 79.3621W 161.7005500–19.41RICE172012/13This paperWDCS 79.47W 112.091.7663.405WDC06A-72006–11Steig et al. (2013), Fudge et al. (2016)Siple DomeS 81.65W 148.816201.004Brook et al. (2005)1997–1999Brook et al. (2005)TALDICES 72.78E 159.072.3181.620Severi et al. (2012)2005–2007Stenni et al. (2011), Buiron et al. (2011), Severi et al. (2012)Talos Dome (TD96)S 72.80E 159.062.31689Stenni et al. (2002)1995Stenni et al. (2002)Taylor DomeS 77.70E 159.072.375554WDC06A-71993–1994Steig et al. (1998, 2000), Sigl et al. (2014), PAGES2k Consortium (2017)RICE site characteristics
Roosevelt Island is an approximately 120 km long by 60 km wide grounded ice
rise located near the north-eastern edge of the Ross Ice Shelf
(Fig. 1). Ice accumulates locally on the ice
rise, while the floating Ross Ice Shelf flows around Roosevelt Island. The
ice surrounding Roosevelt Island originates from the West Antarctic Ice
Sheet (WAIS), via the Bindschadler, MacAyeal and Echelmeyer ice streams.
Bedmap2 data (Fretwell et al., 2013) suggest that the marine basins on
either side of Roosevelt Island are roughly 600 m (western basin) and 750 m
(eastern basin) deep and that the thickness of the Ross Ice Shelf at this
location is about 500 m. At the RICE drill site (79.364∘ S, 161.706∘ W, 550 m
above sea level) near the summit of Roosevelt Island, the ice is 764 m thick
and grounded 214 m below sea level. Radar surveys across the Roosevelt
Island ice divide show well-developed “Raymond arches” (Raymond,
1984) of isochrones suggesting a stable ice divide
(Conway et al., 1999). The vertical velocity,
constrained by phase-sensitive radio echo sounder (pRES) measurements, is
approximately 20 cm a-1 at the surface relative to the velocity of 0 cm a-1
at the bed (Kingslake et al., 2014). A small
migration of the divide has occurred in the past few centuries with the
topographic divide off-set by about 500 m to the south-west. It is possible
that the divide migrated as a result of an imbalance in the ice flux on
either side of the divide, by either changes in the snow accumulation
gradient or changes in the efflux across the grounding line due perhaps to
changes in the buttressing by the Ross Ice Shelf. However, the negligible
divide position migration magnitude suggests that neither snow accumulation
gradient nor grounding line efflux have changed very much; this implies that
the buttressing has not changed significantly either.
Ice core data
During two field seasons, 2011–2012 and 2012–2013, a 764 m long ice core to
bedrock was extracted from the summit of Roosevelt Island. The drilling was
conducted using the New Zealand intermediate depth ice core drill Te Wāmua Hukapapa
(“Ice Cores That Discover the Past” in NZ Te Reo native language). The drill
system is based on the Danish Hans Tausen Drill with some design
modification (Mandeno et al., 2013). The upper 60 m of the
borehole was cased with plastic pipe and the remainder of the drill hole
filled with a mixture of Estisol-240 and Coasol to prevent closure. The part
of the RICE ice core record used in this study covers the past 2.7 kyr and
consists of data combined from the RICE-2012/13-B firn core (0.19–12.30 m
depth) and the RICE Deep ice core (12.30–344 m depth). In addition, the
uppermost samples from a 1.5 m deep snow pit were used to extend the firn and
ice core record for the month of December 2012. An overview of the core
quality and processing procedures are summarised by Pyne et al. (2018). Here, we present new water stable-isotope
(deuterium, δD) and snow accumulation records and compare them with
existing records from West and East Antarctica
(Table 1).
RICE age model: RICE17
The RICE17 age model for the past 2.7 kyr is based on an annually dated ice
core chronology from 0 to 344 m which is described in detail by Winstrup et al. (2017).
The cumulated age uncertainty for the past 100 years is ≤±2 years, for the past 1000 years ≤±19 years and for
the past 2000 years ≤±38 years, reaching a maximum uncertainty
of ±45 years at 344 m depth (2.7 kyr). The RICE17 timescale is in good
agreement with the WD2014 annual-layer counted timescale from the WAIS
Divide ice core dating to 200 CE (280 m depth). For the deeper parts of the
core, there is likely a small bias (2–3 %) towards undercounting the
annual layers, resulting in a small age offset compared to the WD2014
timescale.
Snow accumulation reconstruction
Ice core annual layer thicknesses provide a record of past snow accumulation
once the amount of vertical strain has been accounted for. At Roosevelt
Island, repeat pRES measurements were performed across the divide, providing
a direct measurement of the vertical velocity profile
(Kingslake et al., 2014). This has a key advantage over most
previous ice core inferences of accumulation rate because vertical strain
thinning through the ice sheet is measured directly, rather than needing to
use an approximation for ice flow near ice divides (e.g. Dansgaard and
Johnsen, 1969; Lliboutry, 1979). Uncertainty in the accumulation-rate
reconstruction increases from zero at the surface (no strain thinning) to
±10 % at 78 m true depth (1712 CE). Below 78 m, the uncertainty
remains constant at ±10 %. A detailed description of the
accumulation-rate reconstruction is provided by Winstrup et al. (2017).
Spatial correlation fields exceeding ≥ 95 % significance
between (a) ERAi annual SAT at the RICE site with ERAi annual SAT in
the Antarctic/Southern Ocean region and (b) ERAi annual SAT and
annually averaged RICE δD data; (c) as for (b) but
with optimised RICE δD data alignment within the dating uncertainty.
The correlation was performed using http://climatereanalyzer.org/,
University of Maine, USA. Comparison of the ERAi SAT time series with
(d) RICE δD data and (e) optimised RICE δD
data alignment. Panel (f) scatter plot between RICE δDo and ERAi SAT. The coloured dots indicate the locations of the
drill sites – RICE (blue), Siple Dome (green), WDC (red), TALDICE (pink) and
Taylor Dome (orange).
Water stable-isotope data
The water stable-isotope record was measured using a continuous-flow laser
spectroscopy system with an off-axis integrated cavity output spectroscopy
(OA-ICOS) analyser, manufactured by Los Gatos Research (LGR). The water for
these measurements was derived from the inner section of the continuous flow
analysis (CFA) melt head, while water from the outside section was collected
for discrete samples. A detailed description and quality assessment of this
system is provided by Emanuelsson et al. (2015).
The combined uncertainty for deuterium (δD) at 2 cm resolution is
±0.85 ‰. A detailed description of the isotope
calibration, the calculation of cumulative uncertainties, and the assignment
of depth is provided by Keller et al. (2018).
RICE data correlation with reanalysis data – modern temperature, snow
accumulation and sea ice extent trends
The ERA-Interim (ERAi) reanalysis data set of the European Centre for
Medium-Range Weather Forecasts (ECMWF; Dee et al., 2011), along with
firn and ice cores, snow stakes and an automatic weather station (AWS) are
used to characterise the meteorology of Roosevelt Island. ERAi data have
been extracted for the RICE drill site from the nearest grid point (S
79.50 ∘, W 162.00 ∘; Fig. 1b) for the common time
period between 1979 and 2012. The year 1979, the onset of the ERAi
reanalysis, occurs at 13.42 m depth in the firn. For this reason, the period
1979–2012 is predominantly captured in the RICE 12/13 B firn core. Data from
the RICE AWS suggest that precipitation at RICE can be irregular, with large
snow precipitation events dominating the accumulation pattern
(Emanuelsson, 2016). Therefore, we limit the analysis in this study
to annual averages and longer-term trends.
Isotope temperature correlation
The borehole temperature measured in 2012 in two 11 and 12 m deep drill
holes suggest an average annual temperature of -23.5 ∘C. This
stands in stark contrast to the average annual temperature derived from ERAi
data of -27.4 ∘C at the RICE site. Furthermore, the RICE AWS also
recorded an average annual temperature of -23.5 ∘C. In contrast,
the nearby Margaret AWS (located at 67 m a.s.l., just 96 km south-west of
the RICE AWS; data obtained from Antarctic Meteorological Research Center and
Automatic Weather Station Project; https://amrc.ssec.wisc.edu) records
an average annual temperature of -26.6 ∘C. While recorded summer
temperatures at RICE and Margaret AWS agree well, during winter the Margaret
AWS records up to 10–15 ∘C colder 3-hourly temperatures than the
RICE AWS. It is possible that rime build-up at the RICE AWS during winter
(Supplement Fig. S1) might have provided insulation that allowed for residual
heat from the sensors to warm the temperature cavity leading to erroneously
warm readings. Alternatively, it is possible that the Margaret AWS site is
influenced by stronger temperature inversions leading to exceptionally cold
temperatures of -60 ∘C, while the topography of Roosevelt Island
might be less conducive to such conditions. A comparison between
high-resolution borehole temperature measurements conducted at RICE from
November 2013 to November 2014 and AWS data, including snow temperature
measurements, will provide important insights into this temperature off-set.
Until this analysis is concluded, we argue that ERAi data, which agree well
with the Margaret AWS observations, provide the most reliable temperature
time series to calibrate the stable-isotope–temperature relationship.
In Fig. 2a, the ERAi SAT time series extracted for the RICE drill site is
compared with the ERAi SAT spatial grid. The analysis suggests that
temperature variability at RICE is representative of variability across the
Ross Ice Shelf (with the exception of the westernmost margin along the
Transantarctic Mountains), northern Victoria Land, western Marie Byrd Land,
and the Ross and Amundsen seas. Furthermore, the Antarctic Dipole pattern
(Yuan and Martinson, 2001), a negative correlation between the SAT in the
Ross–Amundsen Sea region and that of the Weddell Sea, is also captured in
the data. The locations of the Siple Dome, West Antarctic Ice Sheet Divide ice core (WDC), Talos Dome, TALDICE and
Taylor Dome ice cores fall within the region of positive RICE SAT
correlation.
The correlation between RICE δD data and ERAi SAT (Fig. 2b) remains
positive across the Ross Ice Shelf and northern Ross Sea, but the WDC site
now falls outside the field of the statistically significant correlation. The
time series correlation between the ERAi SAT record and the RICE δD
data (Fig. 2d) is r=0.42(p<0.01). We test the robustness of this
relationship by applying the minimum and maximum age solutions within the age
uncertainty (≤2 year during this time period; Winstrup et al., 2017) to
identify the age model solution within the age uncertainty that renders the
highest correlation. This optimised solution RICE δDo is
shown in Fig. 2e with a correlation coefficient of r=0.66(p<0.001). The correlation of the RICE δDo record with the
ERAi SAT data (Fig. 2c) produces a pattern that more closely resembles the
correlation pattern using the ERAi data itself (Fig. 2a), suggesting that the
δD record provides useful information about the regional temperature
history.
From the comparison between RICE δDo and ERAi SAT
records, we obtain a temporal slope of 3.37 ‰ ∘C-1
(Fig. 2f), which falls within the limit of previously reported values from
Antarctica of ∼ 2.90–3.43 ‰ ∘C-1 for temporal
(interannual) slopes (Schneider et al., 2005) and ∼6.80±0.57 ‰ ∘C-1 for spatial slopes (Masson-Delmotte et
al., 2008). We use this relationship to calculate temperature variations for
the RICE δD record. The average annual temperature calculated for
1979–2012 from ERAi for the RICE site is -27.4±2.4∘C and
-27.5±3.6∘C for the δDo data. Although
the year-to-year SAT variability appears to be well captured in the RICE
δDo record, there are discrepancies in observed trends.
While RICE δDo data suggest an increase in SAT from 1996
onwards, ERAi SAT data do not show a trend (Fig. 2e). It remains a challenge
to determine how well reanalysis products, including ERAi data, and other
observations, including isotope data, capture temperature trends in
Antarctica (PAGES2k Consortium, 2013; Stenni et al., 2017) and thus whether
the observed difference in the trend between ice core δDo
and ERAi SAT is significant or meaningful. We also compare the δDo with records from AWS (Ferrell, Gill and Margaret AWS) and
stations (Byrd, McMurdo, Scott Base, Siple Stations) in the region (Fig. S2
and Table S1, Supplement). The comparison is hampered by the shortness of the
records and gaps in the observations. Only years were used for which monthly
values were reported for each month of the year. No statistically significant
correlation was identified between δDo and available
data.
Furthermore, we test the correlation with the near-surface Antarctic
temperature reconstruction by Nicolas and Bromwich (2014; referred to as
NB2014), which uses three reanalysis products and takes advantage of the
revised Byrd Station temperature record (Bromwich et al., 2013) to provide an
improved reanalysis product for Antarctica for the time period
1958–2012 CE. We find no correlation between the NB2014 record and the ERAi
data at the RICE site, nor the RICE δDo data for the
1979–2012 time period, perhaps suggesting some regional challenges. If the
full time period available for the NB2014 data is considered (1957–2012),
the NB2014–RICE δDo correlation becomes weakly
statistically significant with r=0.23 (p=0.09, Table S1).
Regional snow accumulation variability
Temporal and spatial variability in snow accumulation are assessed using 144
snow stakes covering a 200 km2 array. The 3 m long, stainless steel
poles were set and surveyed in November 2010, re-measured in January 2011,
and revisited and extended in January 2012 and November 2013. The
measurements indicate a strong accumulation gradient with up to 32 cm water
equivalent per year (w.e. a-1) on the north-eastern flank decreasing to
9 cm w.e. a-1 on the south-western flank. Near the drill site, annual
average snow accumulation rates range from 22±4 cm w.e. a-1 from
2010 to 2013.
Comparison of snow accumulation data recorded by the RICE AWS (blue)
in cumulative snow height in centimetres and ERAi precipitation values (red)
in centimetres water-equivalent cumulative height for the RICE drill
location. Dotted blue and red lines indicate linear trends for AWS and ERAi
data, respectively.
(a) Spatial correlation between ERAi annual precipitation
at the RICE site with ERAi annual precipitation in the Antarctic/Southern
Ocean region and (b) spatial correlation between ERAi annual
precipitation and annually averaged RICE snow accumulation data. Only fields
exceeding ≥ 95 % significance are shown. The correlation has been
performed using http://climatereanalyzer.org/, University of Maine,
USA. Comparison of the ERAi precipitation time series with (d) RICE
snow accumulation data and (e) RICE snow accumulation with optimised
RICE δD data alignment. Coloured dots indicate locations of WDC
(red), Siple Dome (green), RICE (blue), TALDICE and Talos Dome (purple) and
Taylor Dome (orange).
In addition, snow accumulation was measured using an ultrasonic sensor
mounted on the RICE AWS. The sensor was positioned 140–160 cm above the
ground and reset during each season. The 3-year record shows gaps (Fig. 3)
which represent times when rime on the sensor and/or blowing snow caused
erroneous measurements. This condition was particularly prevalent during
winter. Over the 3 years, the site received an average snowfall of
∼ 75 cm a-1. Assuming an average density of 0.37 g cm-3
(average density from two snow pits, 0–75 cm), the AWS data suggest
∼ 20 cm w.e. a-1 accumulation, which is comparable with the
accumulation rate derived from the ice core (21±6 cm w.e. a-1
for the period 1979–2012; Winstrup et al., 2017). In contrast, ERAi data
suggest an average annual snow accumulation of only 11 cm w.e. a-1.
It has been shown that Antarctic ERAi data capture precipitation variability
but generally underestimate the precipitation total (Wang et al., 2016;
Sinclair et al., 2010; Bromwich et al., 2011). Further, the ERAi data are not
directly comparable to local measurements because they do not capture periods
of snow scouring by wind. Yet, there is a good agreement between the two data
sets with respect to the timing and relative rate of precipitation, which
suggests that neither winters nor summers have been times of significant snow
scouring. We attribute the difference between the ERAi data and our
measurements to the spatial differences between measurements of the snow
stake array, the interpolation field of the nearest ERAi data point and the
actual drill site location, as well as differences in assumed snow densities
or different methodologies in the conversion from precipitation to
water-equivalent units. The regional representativeness of RICE snow
accumulation data is assessed by correlating the ERAi precipitation time
series, extracted for the RICE location, with the ERAi precipitation grid
data (Fig. 4a). The correlation suggests that precipitation variability at
Roosevelt Island is representative of the observed variability across the
Ross Ice Shelf, the southern Ross Sea, and western West Antarctica. We note
from Fig. 4a that the sites of the Siple Dome ice core (green circle) and the WDC (red circle) are situated
within the positive correlation field, while Talos Dome (purple circle) and
Taylor Dome (orange circle) show no correlation to ERAi precipitation at
RICE. A negative correlation is found with the region of the South Pacific,
Antarctic Peninsula and eastern West Antarctica. In Fig. 4b, the RICE snow
accumulation record is correlated with ERAi precipitation data and shows a
similar pattern. The resemblance of the correlation patterns suggests that
the variability in the RICE snow accumulation data (Fig. 4b) reflects
regional precipitation variability (Fig. 4a) and thus can likely elucidate
regional snow accumulation variability in the past, in particular for array
reconstructions such as Thomas et al. (2017). We also test the correlation
with the optimised age scale derived for the δDo record
and find that for snow accumulation data this adjusted age scale
(Acco) reduces the correlation but remains statistically
significant (Table 2). This result suggests that the optimised age solution
is not superior to the RICE17 age scale, and we note that the sensitivity of
the correlation to those minor adjustments is founded in the brevity of the
common time period. However, there is no significant difference between the
overall pattern and relationships of the two age scale solutions.
Overview of correlation coefficients for annual means of the common
time period 1979–2012 between climate parameters, proxies and indices: the
original RICE (δD) and optimised (δDo) data (this
paper), the original RICE Acc (Winstrup et al., 2017), and data adjusted to
the revised age scale of δDo – Acco, ERAi
SAT and precipitation (ERAi Precip; Dee at el., 2011), Ross–Amundsen Sea
SIEJ (Jones et al., 2016), SAMA (Abram et al., 2014), SOI
(Trenberth and Stepaniak, 2001), Niño 4 Index (Trenberth and Stepaniak,
2001), Niño 3.4 (Emile-Geay et al., 2013), Inter-decadal Pacific
Oscillation (IPO) Index (Henley et al., 2015),
and NB2014. Significance values are adjusted for degree of freedom depending
on the length of the time series. Only correlation coefficients exceeding
95 % (r≥0.34, n=34) are shown; bold italic values exceed 99 %
(r≥0.42, n=34); bold values exceed 99.9 % (r≥0.54, n=34).
SAMA and IPO have been adjusted for a lower degree of freedom
(df = 28) as the reconstructions end in 2007. “Nss” denotes “not
statistically significant”. Correlation between RICE δD and RICE Acc
is r=0.49, p<0.01; correlation between RICE δDo and RICE Acco is r=0.62, p<0.01.
RERAi SATERAi PrecipSIEJSAMASOINiño 4Niño 3.4IPONB2014RICEδD/δDo0.42/0.660.36/0.43–0.49/–0.58nss/-0.40nss/nssnss/nssnss/nssnss/nssnss/nssRICE Acc/Acco0.60/0.390.67/0.42–0.56/–0.44–0.46/nssnss/nssnss/nssnss/nssnss/nssnss/nssERAi SATx0.66-0.38–0.49nssnssnssNssnssERAi Precip0.66x–0.67–0.42–0.490.370.390.44nssSIEJ-0.38–0.67x0.450.55–0.48–0.48–0.58nssInfluence of regional sea ice variability on RICE isotope and snow
accumulation
Sea ice variability has been shown to influence isotope values in
precipitated snow, particularly in coastal locations (Küttel et al.,
2012; Noone and Simmonds, 2004; Thomas and Bracegirdle, 2009) through the
increased contribution of enriched water vapour during times of reduced sea
ice and increased sensible heat flux due to a higher degree of atmospheric
stratification leading to more vigorous moisture transport. Tuohy et al.
(2015) demonstrated that for the period 2006–2012, ∼ 40–60 % of
precipitation arriving at Roosevelt Island came from local sources in the
southern Ross Sea. In addition, Emanuelsson et al. (2018) demonstrated the
important role of blocking events that are associated with over 88 % of
large precipitation events at RICE, on sea ice variability via meridional
wind field anomalies.
Snow accumulation at RICE is negatively correlated with sea ice concentration
(SIC) in the Ross Sea region and northern Amundsen Sea region (Fig. 5a),
which predominately represents sea ice exported from the Ross Sea. We observe
years of increased (decreased) SIC leading to reduced (increased)
accumulation at RICE, confirming the sensitivity of moisture-bearing marine
air mass intrusions to local ocean moisture sources and hence regional SIC.
The correlation between ERAi SIC and the optimised RICE δDo record (Fig. 5b) similarly shows a negative correlation of
SIC in the Ross Sea (perhaps with the exception of the Ross and Terra Nova
polynyas) and the northern Amundsen Sea, suggesting more depleted (enriched)
values during years of increased (reduced) SIC.
Panels (a) and (b): spatial correlation of ERAi
sea ice concentration (SIC) fields with the time series of (a) RICE
snow accumulation and (b) RICE δDo. Panels
(c) and (d): spatial correlation of the Ross–Amundsen Sea
SIEJ time series (Jones et al., 2016) with (c) ERAi
precipitation and (d) ERAi SAT fields. Only fields exceeding ≥ 95% significance are shown. The correlation has been performed using
http://climatereanalyzer.org/, University of Maine, USA. Coloured dots
indicate locations of WDC (red), Siple Dome (green), RICE (blue),
TALDICE and Talos Dome (purple) and Taylor Dome (orange).
In Fig. 5c and d, the sea ice extent index (SIEJ) for the Ross–Amundsen
Sea, developed by Jones et al. (2016), is correlated with ERAi SAT and
precipitation data. The analysis also identifies the co-variance of sea ice
extent (SIE) and SAT, with increasing (decreasing) sea ice coinciding with
cooler (warmer) SAT. This suggests that during years of increased SIE, the
Ross Ice Shelf and western Marie Byrd Land experience lower temperatures and
less snow accumulation. The correlation between SIEJ and ERAi
precipitation at RICE is r=-0.67, and for SIEJ and RICE snow
accumulation it is r=-0.56 (Table 2). Moreover, the correlation between
SIEJ and ERAi SAT and SIEJ and RICE δDo is also
statistically significant with r=-0.38 and r=-0.58, respectively. The
higher correlation with RICE δDo perhaps suggests that
the influence of SIE in the Ross Sea region affects the RICE δD
record both through direct temperature changes in the region as well as
fractionation processes that are independent of temperature, such as the
lengthened distillation pathway to RICE during periods of more extensive SIE.
The ERAi SAT and ERAi precipitation data at RICE (Table 2) reveal a positive
correlation over large areas of Antarctica with higher correlation
coefficients over the eastern Ross Sea and eastern Weddell Sea (spatial
fields not shown). At the RICE site, the correlation reaches r=0.66(p<0.001). Moreover, the correlation between RICE δD
and RICE snow accumulation data (RICE Acc) (or RICE δDo and RICE Acco) data
is statistically significant, with r=0.49(p<0.01) (or ro=0.62,
po<0.001. This suggests that years with positive isotope
anomalies are frequently characterised by higher snow accumulation rates. In
contrast, precipitation during low snow accumulation years might be dominated
by precipitation from air masses that have travelled further and perhaps
across West Antarctica (Emanuelsson et al., 2018) leading to more depleted
isotope values and lower snow accumulation rates than local air masses from
the Ross Sea region.
Influence of climate drivers on prevailing conditions in the Ross Sea
region
Seasonal biases and the enhancing or compensating effects of the relative
phasing of SAM, ENSO and IPO conditions, on seasonal, annual and decadal
timescales make linear associations of climate conditions and their
relationship with climate drivers in the South Pacific challenging. We use
the Southern Annular Mode Index (SAMA) developed by Abram et al. (2014) to test the
fidelity of the SAM relationship with the climatic conditions in the Ross Sea
over the past millennium (Table 2). The SAMA is highly correlated
(r=0.75) with the SAM record developed by Marshall (2003) for their common
time period (1957–2009). In addition, the Southern Oscillation Index (SOI;
Trenberth and Stepaniak, 2001), Niño 3.4 Index (Rayner et al., 2003b),
Niño 4 Index (Trenberth and Stepaniak, 2001) and the IPO Index (Henley et
al., 2015) are used to investigate the influence of SST variability in the
eastern and central tropical Pacific on annual and decadal timescales (IPO).
In addition, we take advantage of an 850-year reconstruction of the Niño
3.4 index (Emile-Geay et al., 2013) to investigate any long-term influence of
the eastern Pacific SST on environmental conditions in the Ross Sea.
The correlation of ERAi data and modern ice core records covering the
1979–2012 CE period with indices of relevant climate drivers (Table 2)
suggests that SAMA has an enduring statistically significant
relationship with temperature, snow accumulation and SIE in the Ross Sea,
with the positive SAM being associated with cooler temperatures, lower snow
accumulation or precipitation and more extensive SIE. The correlations remain
robust and at comparable levels using a detrended SAMA record. In
contrast, ENSO (SOI, Niño 3.4 and 4) and ENSO-like variability (IPO) have
only linear statistically significant relationships with ERAi precipitation
(but not with RICE snow accumulation) and SIEJ. The dynamic relationship
between the phasing of SAM, ENSO and IPO maybe masking aspects of the
interactions (Fogt and Bromwich, 2006; Marshall and Thompson, 2016). In 2010,
an anomalously cold year is observed. If only the time series from 1979 to
2009 is considered, the correlations between RICE δD and these
considered climate drivers becomes statistically significant: SOI (r=-0.48,
p=0.006), Niño 3.4 (r=0.48, p=0.007), Niño 4 (r=0.57,
p=0.001) and IPO (r=0.44, p=0.014). This further highlights the
vulnerability of this analysis to individual years due to the brevity of the
time series further complicating a linear analysis between individual drivers
and regional responses. Moreover, statistically significant correlations
might also be obtained if seasonal averages could be used for the comparison
as ENSO events usually peak during the austral summer, in particular December
(Turner, 2004). Nonetheless, this analysis confirms previous findings that
the Ross Sea region is sensitive to the cumulative, independent and dependent
influences of SAM, ENSO and the IPO.
Temperature and snow accumulation variability over the past 2.7 kyr
Decadally smoothed RICE isotope and snow accumulation records for the past
2.7 kyr (Fig. 6) are compared with published data
from the Ross Sea region (Siple Dome – green), coastal East Antarctica
(Talos Dome/TALDICE – purple; Taylor Dome – orange) and West Antarctica
(WDC – red).
(a) Isotope records for the past 2700 years for RICE, Siple
Dome (Brook et al., 2005), WDC (WAIS Divide Project Members, 2013), TALDICE
(Stenni et al., 2011) and Taylor Dome (Steig et al., 1999, 2000; Sigl et al.,
2014; PAGES2k Consortium, 2017); (b) snow accumulation data for RICE
(Winstrup et al., 2017), WDC (Fudge et al., 2016) and Talos Dome (TD96). No
snow accumulation data are available for Siple Dome or Taylor Dome;
(c) reconstructions of climate indices for SAMA (Abram
et al., 2014) and Niño 3.4 based on HadSST2 (Emile-Geay et al., 2013).
Colour coding identifies above- and below-average values. Grey shaded area
emphasises period of negative SAMA; yellow shaded area emphasises
period of synchronous warming at RICE, Siple Dome, WDC and TALDICE.
(a) Frequency occurrences of decadal temperature variations
(10-year moving averages) as reconstructed from the RICE ice core are shown
for three periods: 660 BCE to 580 CE – dark blue; 581–1477 CE – light
blue; and 1478–2012 CE – cyan. The 10-year average temperature for the
most recent decade contained in the record (2002–2012, -25.78 ∘C)
is shown in pink. (b) Frequency occurrences of decadal RICE snow
accumulation variations: 10-year moving averages for 660 to 623 CE (dark
blue), 624–1736 CE (light blue) and 1737–2012 CE (cyan). The 10-year
average for the most recent decade contained in the record (2002–2012,
0.2 m w.e.) is shown in pink.
Regional temperature variability
We find that the RICE and Siple Dome (Brook et al., 2005) water isotope
records share a long-term isotopic warming trend in the Ross Sea region. In
contrast, WDC isotope (Steig et al., 2013) and borehole temperature data
(Orsi et al., 2012) exhibit a long-term cooling trend for West Antarctica,
while TALDICE (Stenni et al., 2002) and Taylor Dome (Steig et al., 1998,
2000) recorded stable-isotopic conditions for coastal East Antarctica in the
western Ross Sea.
Elevation changes influence water isotope values
(Vinther et al., 2009). The thinning of
Roosevelt Island, inferred from the amplitude of arched isochrones beneath
the crest of the divide and the depth–age relationship from the ice core
(Conway et al., 1999), is less than 2 cm a-1 for the
past 3.5 kyr. Thus, the surface elevation has decreased less than 50 m over
the past 2.7 kyr. Assuming that these elevation changes are sufficient to
influence vertical movement of the precipitating air mass, such an elevation
change could account for an isotopic enrichment of δD =∼ 2 ‰
(Vinther et al., 2009), which is
insufficient to explain the total observed increase of 8 ‰.
Furthermore, the RICE δD trend is
characterised by two step changes at 580 CE ± 27 years and 1477 CE ± 10 years
(grey dotted lines in Fig. 6).
These change points were identified using a minimum threshold
parameterisation to achieve a minimised residual error (Killick et al.,
2012; Lavielle, 2005). At the identified change points, the decadal isotope
values increase by ∼ 3 and
∼ 5 ‰, respectively, which suggests that
elevation changes were not a principal driver. Using the temporal slope of
3.37 ‰ per degree ∘C, these abrupt temperature
transitions could represent an increase in the average decadal temperature
(Fig. 7a) from -28.5 to -27.7 ∘C and from -27.7 to -26.2 ∘C. An underlying influence from long-term
thinning, accounting for a δD shift of 2.4 ‰,
would exaggerate the observed warming of 2.3 ∘C by 0.7 ∘C,
thus suggesting a minimum isotopic temperature warming
of at least 1.6 ∘C. The modern decadal isotope temperature
average (2003–2012) of -25.1 ∘C (pink bar,
Fig. 7a) and ERAi temperature of -26.5 ∘C lie within the 1σ distribution of the natural
decadal temperature variability in the past 500 years. We note that changes
in atmospheric circulation and sea ice extent might also have contributed
significantly to the change in the observed isotopic shift, and we offer some
suggestions in the following sections.
The Siple Dome ice core δ18O record exhibits a similar isotope
history to the RICE δD record. Siple Dome isotope data reveal an
abrupt warming or isotopic enrichment at 605 CE, some 25 years later than in
RICE but within the cumulative age uncertainty of the two records. After
605 CE, Siple Dome isotope temperatures remain stable, although recording
somewhat warmer isotope temperatures from about 1875 CE. Late-Holocene
elevation changes (thinning) at Siple Dome have been reported to be
negligible (Price et al., 2007) and are unlikely to have caused the
observed abrupt isotopic shift at 605 CE. In contrast to records from the
western Ross Sea (Bertler et al., 2011; Rhodes et al., 2012; Stenni et al.,
2002) and West Antarctica (Orsi et al., 2012), RICE and Siple Dome do
not show an isotopic warming or cooling associated with the Medieval Warm
Period (MWP) or the Little Ice Age (LIA), respectively.
The WDC δ18O record suggests a long-term isotope cooling of West
Antarctica, confirmed by borehole temperature reconstructions (Orsi et al.,
2012). This trend is consistent with warmer than average temperatures during
the MWP and cooler conditions during the LIA but may also reflect changes in
elevation and decreasing insolation (Steig et al., 2013). The cooling trend
is followed by an increase in temperature in recent decades (Orsi et al.,
2012; Steig et al., 2009) consistent with an increase in marine air mass
intrusions (Steig et al., 2013). We note that within 200 years of the onset
of the isotopic warming at RICE (at 580 CE ± 27 years), the WDC
borehole temperature and isotope data start to record a temperature decline,
in line with the observed anti-phased relationship of WDC with RICE and Siple
Dome. No notable change is observed in WDC water stable-isotope temperature
data in the late 15th century. In contrast, WDC borehole temperature
suggests the onset of a warming trend within the last 100 years, marking the
modern divergence between WDC isotope and borehole records. The TALDICE and
Taylor Dome water stable-isotope temperatures do not exhibit a long-term
trend over the past 2.7 kyr. Yet colder water stable-isotope temperature
anomalies have been associated with the LIA period at both sites (Stenni et
al., 2002), which coincide with cooler conditions at WDC and the intensified
increase in isotope temperature at RICE.
The similarity between the RICE and Siple Dome isotope records suggests that
the eastern Ross Sea was dominated by regionally coherent climate drivers
over the past 2.7 kyr, perhaps receiving precipitation via similar air mass
trajectories. Overall this comparison shows that isotope temperature trends
in the eastern Ross Sea (isotopic warming at RICE and Siple Dome) and West
Antarctica (WDC cooling) were anti-phased for over 2 kyr (660 BCE to
∼ 1500 CE), while the western Ross Sea (TALDICE) remained
stable, forming a distinct Ross Sea Dipole pattern. From the 17th
century onwards this relationship changes. While WDC water stable-isotope
temperatures continue to cool, from the 17th century, the WDC borehole
temperature records a warming. At the same time, RICE and Siple Dome
experience warmer isotope temperatures while TALDICE recovers from its
coldest recorded isotope temperature during the study period.
Regional snow accumulation variability
Investigating long-term trends in snow accumulation records (Fig. 6b), the
decadally smoothed RICE shows a discernible positive trend from about 600 CE
of about 0.2±0.1 cm w.e. per century (Winstrup et al., 2017) for the
eastern Ross Sea, while WDC (Fudge et al., 2016) displays a decreasing trend
for central West Antarctica. The RICE snow accumulation data reach a maximum
in the 13th century, with a trend towards lower values from the late 17th
century onwards of about -0.9±0.6 cm w.e. per century (Winstrup et
al., 2017). Since 1950 CE, this trend has steepened to -6.6 cm w.e. per century (Winstrup et al., 2017). Based on
the negative correlation between RICE snow accumulation and SIC in recent
decades, we interpret the long-term increase in snow accumulation as
representing a long-term reduction in SIC in the Ross Sea, consistent with a
long-term increase in RICE isotope temperature. The recent rapid decline in
snow accumulation rates could be related to increases in sea ice conditions
in the eastern Ross Sea, perhaps marking the modern onset of local sea ice
expansion. The modern decadal average (2002–2012) of 20 cm w.e. a-1
lies within the 2σ variability in decadal RICE ice core snow
accumulation rates (Fig. 7). Talos Dome records its maximum snow accumulation
rates at the end of the 13th century and reduced snow accumulation during the
Little Ice Age (Stenni et al., 2002). Until the 15th century, RICE and WDC
snow accumulation and isotope data show an expected positive correlation
between their respective isotope and snow accumulation records. Regionally,
RICE and WDC isotope and snow accumulation are anti-phased. From the 16th
century onwards, this relationship reverses and RICE and WDC snow
accumulation are now in phase, suggesting below-average snow accumulation in
the eastern Ross Sea and West Antarctica. We note that from the 16th century
onwards, snow accumulation at RICE and WDC displays a negative correlation
with water stable isotope (RICE) and borehole temperature (WDC)
reconstructions. At RICE this relationship is again positive for 1979–2012.
During the 20th century, Talos Dome records an increase in snow accumulation
of ∼ 11% in the western Ross Sea (Stenni et al., 2002), in contrast
to the observed snow accumulation decrease at RICE and WDC. However, the
values are not unique within the variability in the 800 years captured in the
Talos Dome record.
Drivers and patterns of decadal to centennial climate variability
Palaeo-reconstructions of the SAMA (Abram et al., 2014) and
Niño 3.4 (Emile-Geay et al., 2013) indices (Fig. 6) provide important
opportunities to investigate the influence of dominant drivers of regional
climate conditions over the past millennium. In particular, the Niño 3.4
index captures the ENSO signal originating in the central-eastern tropical
Pacific associated with the PSA1 pattern. Emile-Geay et al. (2013) note that
while the reconstructions based on three model outputs agree well on decadal
to multi-decadal time periods, they show different sensitivities at
centennial resolution (but not different signs). The SAM index (Thompson and Wallace, 2000; Thompson and
Solomon, 2002a) was developed during the late 20th century, at a time when
SAM was characterised by a strong positive trend. The SAMA
reconstruction showed that the modern SAM is now at its most positive state
in all of the past millennium (Abram et al., 2014). As a consequence, the
reconstructed SAMA is mainly negative. To investigate the
influence of positive and negative anomalies of the SAMA relative
to its average state over the past 1 kyr, we plot the SAMA
reconstruction by indicating values as being above (light purple) or below
(purple) the long-term average of -1.3 rather than 0. The traditional positive SAMA values (above
0, dark purple) are also shown for reference. Assessing the relationship
between RICE, Siple Dome, WDC and TALDICE, we identify three major time
periods of change: 660 BCE to 1367 CE (long-term baseline), 1368 to
1683 CE (negative SAMA) and 1684 to 2012 CE (onset of the
positive SAMA).
Long-term baseline 660 BCE to 1367 CE
We find that for over 2 kyr – from 660±44 BCE to 1367 ±12 CE – the eastern Ross Sea (RICE and Siple Dome) shows an enduring
anti-phase relationship with West Antarctica (WDC), while coastal East
Antarctica in the western Ross Sea (TALDICE, Taylor Dome) remains neutral
(Fig. 6). Moreover, with some minor exceptions,
isotope and snow accumulation records at RICE, WDC and TALDICE are positively correlated.
Negative SAMA – 1368 to 1683 CE
The SAMA reconstruction suggests that, over the past millennium, the
SAM was at its most negative (Abram et al., 2014) from 1368±12 years CE
to 1683±8 CE years. As noted by Abram et al. (2014), the
SAMA and Niño 3.4 reconstructions (Emile-Geay et al.,
2013) are anti-phased on multi-decadal to centennial timescales with the
Niño 3.4 index recording some of the warmest SSTs over the past 850 years
during this period of negative SAMA.
During the negative SAMA, RICE shows a distinct and sudden increase in
isotope temperature, while TALDICE records its coldest conditions over the
past 2.7 kyr. The Taylor Dome record also shows prolonged cold isotope
temperature anomalies during this time period. Previously published shorter
records from the western Ross Sea from Victoria Lower Glacier in the McMurdo
Dry Valleys (Bertler et al., 2011) and Mt Erebus Saddle
(Rhodes et al., 2012) also suggest colder
conditions in the western Ross Sea during this period, with more extensive
sea ice and stronger katabatic flow. We observe that WDC and TALDICE show
below-average snow accumulation values, while RICE snow accumulation changes
from a long-term positive to a negative trend. Such trends are consistent
with the reported increased SIE in the western Ross Sea (colder SAT and lower
snow accumulation at TALDICE; cooler conditions with more extensive sea
ice and stronger katabatic winds at Victoria Lower Glacier and Mt Erebus)
and Bellingshausen Sea (less snow accumulation and colder SAT at WDC). In
contrast, RICE records warmer isotope temperatures along with less and more
variable snow accumulation, displaying a distinct Ross Sea Dipole.
Coinciding with the sudden increase in RICE δD in 1492 CE is the
decoupling of the local isotope temperature from snow accumulation, evident
from the diversion of the RICE snow accumulation and δD trends. The
reduction in snow accumulation might be linked to a negative SAM-induced
weakening of the ASL, perhaps leading to the development of fewer blocking
events in the eastern Ross Sea. Alternatively, the abrupt change to warmer
isotope temperatures at RICE might also point towards the development of the
Roosevelt Island polynya. In recent decades, a Roosevelt Island polynya has
been observed and merges at times with the much larger Ross Sea polynya
(Morales Maqueda et al., 2004). In contrast to the Ross Sea
Polynya (Sinclair et al., 2010), a local polynya could
provide a potent source of isotopically enriched vapour for precipitation at
RICE, perhaps exaggerating the actual warming of the area as interpreted
from water stable-isotope data. We expect the influence of a Roosevelt
Island polynya to have a reduced effect on the more distant Siple Dome,
which is consistent with our observations.
Onset of the positive SAM – 1684 to 2012 CE
At 1684 CE ±7 years, the SAMA increases and remains above
its long-term average until modern times while the Niño 3.4 index
suggests a change to the prevalence of strong La Niña-like conditions,
conditions conducive to a strengthening of the ASL. RICE δD suggest
the continuation of warm, isotopically enriched conditions, while snow
accumulation drops below the long-term average, with the RICE snow
accumulation trend now in phase with the negative trend at WDC (Fig. 6). At
the same time, Talos Dome records highly variable snow accumulation rates
with an 11 % increase in snow accumulation rates from the 20th century
onwards. The change to above-average SAMA (or even positive
SAMA)
values coincides with the onset of the diversion of WDC water isotope and
borehole temperature reconstructions and the decoupling of the RICE δD and snow accumulation trends. Coincident positive SAM (purple
SAMA values in Fig. 6c) and La Niña events have been linked
in recent decades to increases in SIE in the western Ross Sea and decreases
in the Bellingshausen Sea (Stammerjohn et al., 2008). This is consistent with
the notable reduction in snow accumulation at RICE and the trend towards
warmer conditions and increased marine air mass intrusions into West
Antarctica as inferred from WDC isotope data (Steig et al., 2013) but is
inconsistent with the reduction in snow precipitation at WDC and the trend to
warmer isotope temperatures at RICE. We interpret the continuation of warm
RICE isotope temperatures to reflect the persistence of the Roosevelt Island
polynya.
Dipole pattern on decadal to centennial timescales
To investigate the drivers of decadal to centennial variability, we compare
the linearly detrended time series of RICE water stable-isotope records with
those from (i) Siple Dome and WDC (West Antarctica; Fig. 8a) and (ii) TALDICE
(East Antarctica, western Ross Sea; Fig. 8b). The analysis
suggests that from about 500 CE onwards, RICE and Siple Dome variability are
in phase with West Antarctic climate variability (WDC). In contrast, RICE
and TALDICE isotope variability alternates between spatial pattern of
in-phase (purple) and anti-phase (grey, Ross Sea Dipole) relationships.
During the negative phase of the SAMA (Fig. 8),
the anti-correlation between RICE and TALDICE is particularly strong.
Phasing of multi-decadal and centennial climate temperature
variability at RICE, Siple Dome, WDC and TALDICE using detrended, normalised
isotope records, smoothed with a 200-year moving average. RICE is compared
with (a) Siple Dome and WDC and (b) TALDICE to investigate phase relationships
of climate variability in the eastern Ross Sea (RICE, Siple Dome) with West
(WDC) and East Antarctica (TALDICE). WA: West Antarctica; EA: East
Antarctica. The red shading indicates periods of synchroneity of RICE and
Siple Dome records with WA. Grey shading indicates time periods where RICE
(eastern Ross Sea) shows an anti-phase relationship (a Ross Sea Dipole) with
TALDICE (western Ross Sea). Purple shading identifies times when RICE and EA
are in phase. The normalised SAMA and Niño 3.4 records, smoothed
with a 200-year moving average, are shown for comparison.
Wavelet coherence and cross spectrum analysis of (a) RICE δD and Siple Dome δ18O, (b) RICE δD and WDC δ18O, and (c) RICE δD and TALDICE δ18O. The analysis
was conducted on decadally averaged, detrended data, smoothed with a 200-year
moving average. The coherence is computed using the Morlet wavelet and
is expressed as magnitude-squared coherence (msc). The phase of the wavelet
cross-spectrum is provide for values over 0.6 msc using Welch's overlapped
averaged periodogram method (Kay, 1988; Rabiner et al., 1978). Arrows to
the right indicate RICE is leading; arrows to the left indicate RICE is
lagging. An upward or downward arrow represents quarter cycle
difference.
To assess the correlation of cyclicities apparent in the RICE, Siple Dome,
WDC and TALDICE isotope records, wavelet coherence spectrum analyses were
conducted (Fig. 9) on the time series shown in
Fig. 8. The analysis of RICE and Siple Dome
(Fig. 9a) suggests that the two records
positively correlate on a broad spectrum of frequencies with cyclicities
between 200 and 500 years. The correlation is weakest during 660–100 BCE. A
strong anti-phased coherence is also observed for the 30–70-year periodicity
from 100 to 800 CE. The high coherency suggests that RICE and Siple Dome
respond to similar forcings. The coherence analysis between RICE and WDC
shows an enduring in-phase correlation from ∼ 1000 CE to
today for the bandwidth of 200–700 years. An anti-phase coherence is found
from 0 to 500 CE. The coherence analysis between RICE and TALDICE identifies
strong relationships predominantly for the early part of the records, from
660 BCE to ∼ 800 CE, and a weak coherence from about 800 to 1700 CE,
when RICE leads by ∼ 75–100 years. The analysis suggests
that for the past 2.7 kyr the eastern Ross Sea (RICE, Siple Dome) and western
West Antarctica (WDC) are climatologically closely linked in their response
to forcings on decadal to centennial timescales and are positively
correlated for the past 1.6 kyr. In contrast, the relationship between the
western (TALDICE) and eastern (RICE, Siple Dome) Ross Sea is more variable.
Concluding remarks
The recent change to a strongly negative SAM (Marshall Index -3.12) in
November 2016 coincided with a significant reduction in Antarctic SIE,
including the Ross Sea, during the 2016/17 summer (Turner et
al., 2017). Longer observations are necessary to assess whether this recent
trend continues and indeed forces the reduced SIE, but it fuels questions on
the potential acceleration of future environmental change in the Antarctic/Southern
Ocean region. To improve projections for the coming decades, an
improved understanding of the interplay of teleconnections and local
feedbacks is needed.
The Ross Sea region is a climatologically sensitive region that is exposed to
tropical and midlatitude climate drivers. The ASL has been shown to deepen
during coinciding positive SAM and La Niña events and to weaken during
negative SAM and El Niño events. Such interactions have far-reaching
implications for the regional atmospheric and ocean circulations and sea ice
(Turner et al., 2013; Raphael et al., 2016). Additionally, a negative
(positive) IPO leads to cooler (warmer) SSTs in the Ross, Amundsen and
Bellingshausen seas and has the potential to strengthen (in phase) or weaken
(out of phase) the ENSO teleconnection (Henley et al., 2015). Furthermore,
the phasing and strength of ENSO events and SAM have been shown to be time
dependent (Fogt and Bromwich, 2006).
Our data suggest that changes in these dynamically linked climate patterns
coincide with significant and abrupt changes in the past with implications
for regional interpretations of trends, including temperature, mass balance
and SIE. For over 2 kyr, from 660 BCE to the late 14th century, climate
trends in the eastern Ross Sea (RICE and Siple Dome, trend to warmer isotope
temperatures and higher precipitation) are anti-correlated with conditions
in western West Antarctica (WDC, isotopic cooling with reduced
precipitation), while coastal East Antarctica in the western Ross Sea
appeared decoupled (there was no trend in isotope
temperature or precipitation at TALDICE/Taylor Dome and Talos Dome, respectively). This regional pattern was reorganised during a period with strongly negative SAM conditions
(SAMA) accompanied by exceptionally warm tropical SST (Niño 3.4
index). In modern times, such conditions cause a weakening of the ASL, which
in turn can lead to a reduction in marine air mass intrusions into West
Antarctica. Indeed, we observe that western West Antarctica (WDC borehole
and isotope temperature) and the western Ross Sea (TALDICE) show cold
temperatures during this time period that coincides with the Little Ice Age,
while the eastern Ross Sea (RICE, Siple Dome) shows warmer or stable-isotope
temperatures. In the late 17th century, the SAMA
changes to above-average values, concurrent with a change to strong La
Niña-like conditions, a framework conducive to a deepening of the ASL.
Now, West Antarctica (WDC borehole temperature), the eastern Ross Sea (RICE,
Siple Dome) and the western Ross Sea (TALDICE) all experience warmer isotope
temperatures. At the same time, however, we observe reduced snow
accumulation at RICE and WDC and an increase at Talos Dome. We interpret
this pattern to reflect an increase in SIE in the eastern Ross Sea with
perhaps the establishment of the modern Roosevelt Island polynya as a local
moisture source for RICE. The continued improvements of array
reconstructions (Stenni et al., 2017; Thomas et al., 2017) and the
assessment of isotope-enabled climate models are an exciting development to
further our knowledge of the drivers and effects of past change and their
implications for future projections.
Data availability
The following new RICE data are made available in this paper:
RICE water stable isotopes (δD) have been archived at the PANGAEA
data base: 10.1594/PANGAEA.880396 (Bertler et al., 2017).
GPS and radar data have been archived at the U.S. Antarctic Program Data
Center:
https://gcmd.gsfc.nasa.gov/search/Metadata.do?entry=USAP-0944307&subset=GCMD (Kingslake et al., 2014; Matsuoka et al., 2015).
The following published RICE data used in this paper are accessible
in the following way:
RICE17 age scale has been archived at the PANGAEA data base:
10.1594/PANGAEA.882202 (Winstrup and Vallelonga, 2017).
RICE snow accumulation data have been archived at the PANGAEA data base:
10.1594/PANGAEA.882202 (Winstrup and Vallelonga, 2017).
Sources of published ice core data used in this paper are as follows:
WDC water stable isotopes (δ18O) were accessed via
https://www.nature.com/articles/nature12376#supplementary-information (WAIS Divide Project Members, 2013).
WDC snow accumulation data were accessed via
http://www.usap-dc.org/view/dataset/601004 (Fudge et al., 2016).
WDC borehole temperature was accessed via
https://nsidc.org/data/NSIDC-0638/versions/1 (Orsi et al., 2012).
Talos Dome snow accumulation data were accessed via
https://www.ncdc.noaa.gov/paleo/study/22712 (Stenni et al., 2002).
TALDICE water stable-isotope data (δ18O) were accessed
via
https://www1.ncdc.noaa.gov/pub/data/paleo/pages2k/stenni2017antarctica/ (Stenni et al., 2017).
Siple Dome water stable isotopes (δ18O) were accessed via
https://www.nature.com/articles/nature12376#supplementary-information (WAIS Divide Project Members, 2013).
Taylor Dome water stable isotopes (δ18O) were accessed
via
https://www1.ncdc.noaa.gov/pub/data/paleo/pages2k/pages2k-temperature-v2-2017/data-current-version/Ant-TaylorDome.Steig.2000.txt (Steig et al., 2000).
Sources of meteorological data and climate indices used in this paper are as follows:
SAMA developed by Abram et al. (2014) was accessed via
ftp://ftp.ncdc.noaa.gov/pub/data/paleo/contributions_by_author/abram2014/abram2014sam.txt.
The Niño 3.4 Index developed by Emile-Geay et al. (2013) was accessed
via
ftp://ftp.ncdc.noaa.gov/pub/data/paleo/contributions_by_author/emile-geay2012/emile-geay2012.xls.
The Niño 3.4 Index (Rayner et al., 2003a) was
accessed via http://www.esrl.noaa.gov/psd/gcos_wgsp/Timeseries/Nino34/.
The Niño 4 Index (Rayner et al., 2003a) was
accessed via https://www.esrl.noaa.gov/psd/gcos_wgsp/Timeseries/Nino4/.
SOI (Allan et al., 1991) was accessed via
http://www.cru.uea.ac.uk/cru/data/soi/.
The Ross–Amundsen Sea SIEJ developed by Jones et al. (2016)
was accessed via
http://www.nature.com/articles/nclimate3103#supplementary-information.
Byrd Station meteorological data (Bromwich et al., 2013) were accessed
via the Byrd Polar Research Centre, Polar Meteorological Group, Ohio State
University: http://www.polarmet.osu.edu/datasets/Byrd_recon/.
Meteorological data for Ferrell, Gill and Margaret AWS were accessed
via the Antarctic Meteorological Research Center and Automatic Weather Station
Project https://amrc.ssec.wisc.edu.
Data for McMurdo Station and Scott Base are accessed via the
MET-READER: https://legacy.bas.ac.uk/met/READER/data.html (Turner et al., 2004).
The IPO Index (Henley et al., 2015) was accessed via
http://www.esrl.noaa.gov/psd/data/timeseries/IPOTPI/.
NB2014 were accessed via
http://polarmet.osu.edu/datasets/Antarctic_recon/ (Nicolas and Bromwich, 2014).
The supplement related to this article is available online at: https://doi.org/10.5194/cp-14-193-2018-supplement.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
Funding for this project was provided by the New Zealand Ministry of
Business, Innovation, and Employment Grants through Victoria University of
Wellington (RDF-VUW-1103, 15-VUW-131) and GNS Science (540GCT32, 540GCT12),
and Antarctica New Zealand (K049), the US National Science Foundation (US
NSF ANT-0944021, ANT-0944307, ANT-1443472), British Antarctic Survey Funding
(BAS PSPE), the Center of Ice and Climate at the Niels Bohr Institute
through the Carlsberg Foundation's “North-South Climate Connection”
project grant, and the Major State Basic Research Development Program of
China (Grant No. 2013CBA01804). The authors appreciate the support of the University
of Wisconsin-Madison Automatic Weather Station Program for the data set,
data display and information (NSF grant number ANT-1543305). We are indebted to Hedley Berge, Jeff
Rawson, Margie Grant, Lou Albershardt, and Antarctica New Zealand staff at
Scott Base and in Christchurch for their support of the RICE field seasons.
We are grateful for the support by US 109th New York Air National Guard
(NYANG) LC-130 Hercules and Canadian Kenn Borek aircraft crews for their
excellent support into and out of Roosevelt Island. Furthermore, we would
like to thank Stephen Mawdesley, Grant Kellett, Ryan Davidson, Ed
Hutchinson, Bruce Crothers and John Futter of the Mechanical and Electronic
Workshops of GNS Science for technical support for the international RICE
core progressing campaigns. We would like to thank Beaudette Ross for
conducting gas isotope measurements at the Scripps Institution of
Oceanography, University of California, San Diego. We are grateful to Diane
Bradshaw and Bevan Hunter for their assistance in naming the New Zealand ice
core drill. We would like to thank Barbara Stenni for advice and discussions
on the Talos Dome and TALDICE records. We thank two anonymous reviewers to
help us make important improvements to this paper. This work is a
contribution to the Roosevelt Island Climate Evolution (RICE) Program,
funded by national contributions from New Zealand, Australia, Denmark,
Germany, Italy, the People's Republic of China, Sweden, the UK and the USA.
Logistics support was provided by Antarctica New Zealand (K049) and the US
Antarctic Program.
Edited by: Hugues Goosse
Reviewed by: two anonymous referees
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