Atmospheric data derived from ice cores Temperature proxies

The Antarctic presents a complex picture of temperature change in recent decades (Turner et al., 2005a). While the Antarctic Peninsula and some coastal portions of the Antarctic Ice Sheets have experienced pronounced warming over the last 50 years, most continental stations have shown little or no significant surface warming over this same period. Despite this, significant warming trends have been identified in the mid-troposphere in winter (Turner et al., 2006). (See also Steig et al, 2009)

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Figure 2.5 The weighting functions for channels 4-14 of the AMSU-A instrument.

High-resolution (sub-annual) proxy temperature data are derived from stable water isotope measurements on ice cores, and records of such data presently range from several centuries up to around 1,000 years (e.g. Graf et al., 2002; Morgan and van Ommen, 1997;

Steig et al., 2005). The largest-scale proxy reconstruction presently available in Antarctica (Schneider et al., 2006) extends the record of temperature changes over the continent (excluding the Peninsula) to 200 years, using ice cores from 5 sites. The proxy data show a long-term increase in temperature of around 0.2°C since the late nineteenth century. This change is in phase with the Southern Hemisphere mean until around 1975, when this primarily interior Antarctic temperature record diverges, showing a very slight cooling trend.

The proxy reconstruction and observations reveal significant decadal-scale variability in temperatures associated with variability in the Southern Annular Mode (SAM). This suggests that this surface cooling may, at least in part, reflect recent changes in the SAM (Thompson and Solomon, 2002), a connection also noted by Turner et al. (2005a).

Atmospheric circulation proxies using ice cores

Advances in the sampling and analysis of chemical ion concentrations and stable isotope composition of snow and firn have enabled the collection of multivariate data sets from firn and ice cores, which have increasing application as proxy climate data. Glaciochemical time series in conjunction with accumulation rate time series have been used to unravel the likely moisture sources and atmospheric circulation history for many global locations in the last decade. For a review of glaciochemistry and an introduction to its early use as climate proxy data see Legrand and Mayewski (1997).

Coupled to the advancements in geochemical analyses is the availability of global climate reanalysis data, and instrumental station climate data that have enabled the calibration

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of ice core proxy climate data, particularly pressure. The two main climate reanalysis data sets are: the US National Centers for Environmental Prediction (NCEP) - National Center for Atmospheric Research (NCAR) climate reanalysis data set that is referred to as the NNR data set (Kalnay et al., 1996; Kistler et al., 1985) available from the US National Oceanic &

Atmospheric Administration (NOAA); and ERA-40 climate reanalysis data available from the ECMWF. These data sets contain gridded 6 hourly, daily and monthly data for the full range of climate parameters, from the surface through the atmosphere. The monthly data are of most relevance to the calibration of annual to sub-annually resolved ice core data. Sea-salt aerosols, calcium, nitrate and MS aerosols are the most widely used proxies for atmospheric circulation in the Antarctic region. Examples of the use of these proxies to reconstruct atmospheric circulation and precipitation are given below.

Ice core nitrate as a proxy for atmospheric ridging and the SAM. The Queen Maud and Wilkes Land regions of East Antarctica lie entirely within the strong katabatic wind zone where erosional surface winds from east-southeast to south-southeast drain cold air from the ice sheet's interior down-slope to the coast. Snow precipitation accompanied by an east to east-southeast wind occurs during synoptic-scale maritime cyclonic incursions over the katabatic slope, at least to the 2,000 m elevation. Cullather et al. (1998) established that the strength of the ridging over Wilkes Land influenced the circumpolar storm tracks, resulting in cyclones being steered into Wilkes Land. Murphy and Simmonds (1993) and Bromwich et al.

(1993) have established that tropospheric ridging has a significant influence on the surface windfield. Katabatic winds were found to intensify in response to stronger than average surface temperature inversions produced by an enhanced high surface pressure and at the 500 hPa level over the East Antarctic interior. These high pressure anomalies over East Antarctica are associated with a blocking anticyclone to the south east of New Zealand (170 to 180°E), and approaching low pressure systems in the circumpolar trough at 100-120°E. Strong wind events are the product of enhanced katabatic wind flow down slope together with strong geostrophic wind flow across the Wilkes Land slopes (Murphy and Simmonds, 1993).

Research by Goodwin et al. (2003) has demonstrated that the nitrate time series from Wilkes Land sites, located in convergent windfields, is a proxy for the strength of the surface windfield. They established from the annual sea salt and nitrate concentration time series that anomalously high accumulation rate and nitrate concentrations at these sites are the product of the accumulation of blown snow, plus increased surface wind speed and/or wind pumping efficiency, rather than synoptic precipitation. Goodwin et al. (2003) have shown that the annual mean nitrate concentration in eastern Wilkes Land snow has a strong statistical relationship with winter (June, July and August) meridional mean sea level pressure (MSLP) indices (Macquarie Island - Scott Base, and Kerguelen Island - Casey MSLP) that describe the oscillations in the SAM (Thompson and Solomon, 2002). The decreasing trend in annual mean nitrate concentration in snow after 1964 suggests a reduced incidence of mid-latitude atmospheric ridging into Wilkes Land during winter (Figure 2.6). This is in agreement with the observed deepening of the circumpolar trough since the mid 1960s (Allan and Haylock, 1993) and the trend towards the high index phase of the SAM (Thompson and Solomon, 2002).

Ice core sea-salt as a proxy for mid-latitude and circum-Antarctic sea-level pressure and the SAM. Goodwin et al. (2004) have derived a 700 year proxy record (at monthly resolution) for winter (May, June, July (MJJ)) MSLP variability over the Southern Ocean, by analysing sea-salt (sodium) aerosol concentrations in the DSS ice core from Law Dome in East Antarctica. The relationship between modern patterns of mid-latitude and sub-Antarctic atmospheric circulation and variations in sodium (Na) delivery to Law Dome ice was identified by analysing co-variations between Na concentrations, MSLP and wind field data.

The observed relationship was then used to hindcast a proxy record of early winter MSLP anomalies and the SAM. The proxy mid-latitude MSLP time series is most correlated to

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atmospheric variability in the region southeast of Australia and south of the Tasman Sea (Macquarie Island and Campbell Island). This is a function of the Rossby Wave number 3 pattern, the periodic atmospheric blocking in this region, and interaction with the SAM. The MSLP patterns (MSLP anomalies that are associated with high and low sea salt aerosol deposition in the DSS (Dome Summit South, Law Dome) core) are shown in Figure 2.7 (Goodwin et al., 2004). The full 700 year record of early winter (MJJ) sea salt sodium and MSLP variability is shown in Figure 2.8 (Goodwin et al., 2004).

Figure 2.6 Mean annual nitrate concentration at GD09 (eastern Wilkes Land, East Antarctica) plotted against the average mean sea level pressure (MSLP) gradient between Macquarie Island and Scott Base for June, July and August (JJA). The MSLP index represents the difference in MSLP across the circumpolar trough to the east of Wilkes Land, where ridging is prevalent. Low values of the MSLP index are indicative of ridging (meridional conditions) across the circumpolar trough in the 130 to 160° E longitudes, whilst high values represent strong zonal conditions.

Ice core sea-salt as a potential proxy for mid-latitude winter rainfall variability. Recent work by Goodwin (in prep.) has focused on the application of the proxy mid-latitude MSLP and SAM index time series, to investigate methods for hindcasting southern Australian rainfall over the past millennium. Proxy May - July MSLP and SAM data have been cross-correlated against May - July total rainfall data for stations located in southwest and southern Western Australia. Preliminary results, indicate that the highest correlation between the data was calculated for stations close to the coastal escarpment in southwest Western Australia, indicating the strong relationship between winter rainfall, the phase of the SAM and the circumpolar longwave circulation pattern, particularly the meridional location of the Rossby wave number 3 troughs and ridges in the Indo-Australian region. Interdecadal winter rainfall variability across coastal Southern Australia appears to be strongly associated with the time-varying behaviour of the longwave pattern and the SAM. The work in progress indicates that southwest Western Australia experienced periods of higher mean winter rainfall, with high interdecadal variability during 1300 to 1600 AD, followed by lower mean but less variable winter rainfall from 1600 to 1900 AD, which is similar to the past 50 years (Goodwin, in prep.). These preliminary results, illustrate the potential for using high-resolution (monthly)

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ice core glaciochemical data to reconstruct and predict atmospheric circulation and rainfall distribution patterns across Southern Australia at interannual to interdecadal resolution. Since there is a strong correlation between mid-latitude MSLP, South Indian Ocean sea surface temperatures (SST) and rainfall over southwest Western Australia (Smith I. et al., 2000) ice core sites in Queen Mary Land, East Antarctica appear to have the best potential for reconstructing MSLP and rainfall variability over southwest Western Australia for the past few hundred years. This long record would be of enormous economic benefit to all water users in Western Australia.

Figur e 2.7 The spatial pattern of ‘early winter’ May - July NNR MSLP anomalies calculated for: a) the 6 highest (1990, 1977, 1992, 1970, 1986 and 1975) Na concentration years at DSS between 1970 to 1995; b) the 6 lowest (1982, 1981, 1995, 1985, 1993 and 1971) Na concentration years; and, c) from the difference between the 6 highest and 6 lowest Na concentration years. The difference plot accentuates the circulation changes between SAM phases in the mid-latitude trough in the southwest Indian and Pacific Ocean sectors, together with the enhanced ridging in the circumpolar trough centred on 110° E during high Na concentration years at DSS.

Figur e 2.8 Sodium May – July time-series from 1300-1995 in µEq/L are shown as anomalies from the 1950-1995 mean (dashed line). Data are low filter. Right (inverted) axis shows equivalent MSLP anomalies for the southwest Pacific region (Campbell Is. and Macquarie Is.) using the observed calibration slope of -0.65 hPa/µEq/L derived from the instrumental period (see text). Negative (positive), states of the SAM correspond to negative (positive) MSLP anomalies at mid latitudes.

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Although full-scale reconstructions of past climate over Antarctica have yet to be finalized, SCAR’s International Trans-Antarctic Scientific Expedition (ITASE) (Mayewski et al., 2005a) has, as noted above, pioneered calibration tools and reconstruction of climate indices and evidence for climate forcing using single-sites through to multiple arrays of sites.

Initial syntheses of combined ITASE and deep ice core records demonstrate that inclusion of instrumentally calibrated ITASE ice core records allows previously unavailable reconstruction of past regional to continental scale variability in atmospheric circulation and temperature (Mayewski et al., 2005a). Emerging results demonstrate the utilization of ITASE records in testing meteorological reanalysis products (e.g. Genthon et al., 2005). Connections have also been made between ITASE climate proxies and global scale climate indices such as ENSO (Meyerson et al., 2002; Bertler et al., 2004) and to major atmospheric circulation features over the Southern Hemisphere - such as the Amundsen Sea Low, East Antarctic High, Southern Hemisphere westerlies and SAM (Kreutz et al., 2000a; Souney et al., 2002a;

Goodwin et al., 2004; Proposito et al., 2002; Shulmeister et al., 2004; Kaspari et al., 2005;

Yan et al., 2006). ITASE research is also focused on understanding the factors that control natural climate variability over Antarctica and the Southern Ocean, through, for example, documentation of the impact of solar forcing via UV induced changes in stratospheric ozone concentration on the strength of the zonal westerlies at the edge of the polar vortex (Mayewski et al., 2005b).

ITASE proxy circulation reconstructions provide an estimate for the state of modern Antarctic climate. Evidence is emerging for inland penetration of marine tropospheric air masses over the past few decades in summer into portions of coastal West Antarctica near the Amundsen Sea (Dixon et al., 2006) and the Antarctic Peninsula. However, ice core proxy reconstructions for the Amundsen Sea Low suggest that this system is still within the range of variability established over the last 1,200 years (Mayewski and Maasch, 2006), while proxy reconstruction of the Southern Hemisphere westerlies reveals intensification in the 1980s (Dixon et al., in review) consistent with the impact of human-induced changes in stratospheric ozone on the strength of the westerlies around the edge of the polar vortex (Thompson and Solomon, 2002).