Anomalous mid-twentieth century atmospheric circulation change over the South Atlantic compared to the last 6000 years

Meteorological observations on the Falkland Islands commenced at Cape Pembroke Lighthouse around CE 1850 and ended in 1947, with parallel (but discontinuous) measurements made at Stanley from 1923 until 1986 (Lister and Jones 2015). During the Falkland Islands Conflict (1982) observations were disrupted and the meteorological station relocated to the airport at Mount Pleasant, where measurements were first made in 1986. Recent work by Lister and Jones report an updated climate dataset from the Falkland Islands, combining the above series into one continuous record, commencing from CE 1895 (Brooks 1920, Lister and Jones 2015). In parallel to the Falkland Islands record, South Georgia temperature observation started with whaling operations on the island from 1905 but experienced significant disruption as a result of the 1982 conflict, with continuous observations not resuming until 2001. Both datasets are available at http://hadobs.metoffice.com/crutem4/data/download.html. Mean monthly sea level pressure data from Stanley were obtained from sub-daily values (Cram et al 2015, Jones et al 2016), available online at http://cru.uea.ac.uk/cru/data/tpi/ and from the International Surface Pressure Databank on request (https://reanalyses.org/observations/international-surface-pressure-databank), the latter hosted by the National Oceanic and Atmospheric Administration (NOAA) National Centers for Environmental Information (NCEI). The surface pressure data has been gathered through international cooperation facilitated by the international Atmospheric Circulation Reconstructions over the Earth (ACRE) initiative and other contributing organisations, assembled under the auspices of the Global Climate Observing System (GCOS) Working Group on Surface Pressure and the World Climate Research Programme/GCOS Working Group on Observational Data Sets for Reanalysis by NOAA Earth System Research Laboratory (ESRL), NCEI, and the University of Colorado's Cooperative Institute for Research in Environmental Sciences (CIRES). The sub-daily pressure series was checked for gross errors by looking at pressure differences between adjacent readings. If pressure differences were large, they were compared to adjacent readings and any other available weather information. Individual readings were removed where the indications pointed towards an error.

Three peat sequences were cored on two South Atlantic islands: an Ericaceous-grass dominated peatland on Canopus Hill, above Port Stanley Airport, Falkland Islands (a 1.6 m sequence at 51.691°S, 57.785°W, approximately 30 m above sea level), and moss species Polytrichum strictum and Chorisodontium aciphyllum-dominated peat in both King Edward Cove, Cumberland Bay, South Georgia (a 0.8 m sequence at 54.293°S, 36.494°W, approximately 5 m above sea level) and Junction Valley, South Georgia (a 0.70 m sequence at 54.298°S, 36.524°W, approximately 80 m above sea level) (figure 1). The two South Georgian sites are particularly susceptible to föhn winds that can exaggerate surface warming (Bannister and King 2015). Each sequence was contiguously sampled (every centimetre) for total organic carbon (%TOC) and nitrogen, determined using a LECO TruSpec CN analyser at the University of New South Wales Analytical Centre following standard techniques (Harris et al 2001). Comprehensive dating of the sequences (see details below) allowed us to reconstruct changes in the carbon flux (the amount of carbon sequestered) in each of the peat sequences using a method that takes into account the bulk density through each profile (Cannell et al 1993).

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To provide a robust geochronological framework we undertook a multidating programme, using a combination of radiocarbon dating of terrestrial plant macrofossils and ¹³⁷Cs in the peat. Terrestrial plant macrofossils (seeds and leaves) were extracted from the peat sequences and given an acid-base-acid pretreatment and then combusted and graphitized in the University of Waikato AMS laboratory, with ¹⁴C/¹²C measurement by the University of California at Irvine (UCI) on a NEC compact (1.5SDH) AMS system. The pretreated samples were converted to CO₂ by combustion in sealed pre-baked quartz tubes, containing Cu and Ag wire. The CO₂ was then converted to graphite using H₂ and a Fe catalyst, and loaded into aluminum target holders for measurement at UCI. This was supplemented by ¹³⁷Cs measurements down the profile to detect the onset of nuclear tests. ¹³⁷Cs analysis was undertaken following standard techniques with measurements made using an ORTEC high-resolution, low-background coaxial germanium detector at the University of Exeter (Mitchell et al 1992, MacKenzie et al 1997). Detectable measurements of ¹³⁷Cs were used for further age control (Leslie and Hancock 2008).

The radiocarbon and ¹³⁷Cs ages were used to develop an age model using a P_sequence deposition model in OxCal 4.2 (Bronk Ramsey 2008); with variable atmospheric ¹⁴C over the Southern Ocean (Turney et al 2016b) we undertook RScaled Outlier analysis detection (probability = 0.05) (Bronk Ramsey 2009). The ¹⁴C ages were calibrated against the Southern Hemisphere (SHCal13) and Bomb04SH calibration datasets (Hua and Barbetti 2004, Hogg et al 2013) with the prior U(1952, 2011) used to capture the range of dates for the onset of ¹³⁷Cs deposition in each sequence (Hancock et al 2011). Using Bayes' theorem, the algorithms employed sample possible solutions with a probability that is the product of the prior and likelihood probabilities. Taking into account the deposition model and the actual age measurements, the posterior probability densities quantify the most likely age distributions; the outlier option was used to detect ages that fall outside the calibration model for each group, and if necessary, down-weight their contribution to the final age estimates. Modelled ages reported in the text are described as thousands of calendar years BP or cal BP (table 1).

To investigate whether the changes in peat growth may reflect some natural long-term forcing we undertook tipping point analysis. This technique is based on the fact that abrupt climate changes, if characterised by long-term forcing prior to reaching a tipping point in the system dynamics, can be mathematically detected by looking at the pattern of fluctuations in the short-term trends of the data before the shift takes place (Dakos et al 2008). This is based on the concept of 'critical slowing down', where on the approach to such an abrupt shift, the equilibrium state of the system takes increasingly longer to recover from small perturbations (Held and Kleinen 2004, Dakos et al 2012). This increased recovery time is detected as a short-term increase in the lag-1 autocorrelation or 'memory' of the time series (Ives 1995). An increasing trend in variance is also often found due to the ability of the system to travel farther from its equilibrium point as the basin of attraction (which describes the stability of the system) shallows and widens (Lenton et al 2012). Some data pre-processing can be undertaken, including detrending to remove non-stationarities, and interpolation to make the data equidistant. The analysis was carried out with and without this pre-processing, as when there is a relative sparseness of data points, and/or if there is no obvious long-term trend, interpolation and detrending can bias the analysis (Dakos et al 2012). Autocorrelation at lag-1 and variance were then measured over a sliding window of 50% of the length of the dataset, using the R functions ar.ols() and var() respectively. The Kendall tau rank correlation coefficient is used to provide a quantitative measure of the trend (Kendall 1948) by assessing the predominance of concordant pairs, providing an objective evaluation of the statistical evidence for the trend.

The South Atlantic climate series from South Georgia and the Falklands provides a valuable record of climate change in the mid-latitudes of the Southern Hemisphere (figure 2). While spring–summer (September–February) temperature trend records a relatively modest long-term increase on both islands (figure 3), the complete annual Falklands record captures a highly statistically significant warming of 0.5 °C/century from CE 1910–2010 (p = 0.002) (Lister and Jones 2015). Where there is overlapping observational data (1905–2011), the deseasonalised and detrended temperature trends on the Falkland Islands and South Georgia are statistically similar (r = 0.506, p < 0.0001), demonstrating a coherent change across the region. Importantly, while there does appear to be a long-term warming trend, climate regime analysis of the precipitation dataset (Rodionov 2004) identifies a significant (90% confidence) and substantial step-change to sustained wetter conditions in the mid-1940s (representing a 23% increase). The observed change to wetter conditions followed a short-lived period of higher temperatures observed across South America and the Antarctic Peninsula (Turner et al 2005, Schneider and Steig 2008) (figure 3).

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The increases in South Atlantic temperature and precipitation do not appear to have been accompanied by any long-term change in local mean sea level pressure in spite of the regional (Visbeck 2009) and hemispheric-wide (Marshall 2003) atmospheric circulation changes represented by the recent positive SAM index values (figure 3). Crucially, although the multi-decadal trend to more positive SAM and the deepening of the Amundsen Sea Low (ASL) has taken place since the mid-20th century (Fogt et al 2012, Hosking et al 2013), increasing northerly airflow (and temperature) over the region (Abram et al 2014, Lister and Jones 2015) and reanalysis of late 20th century atmospheric circulation in both the ERA-Interim (Dee et al 2011) and the 20CR 2c (Compo et al 2011) suggests wetter conditions have the opposite relationship (figures 4(A)–(F)). Here we find high rainfall over the subantarctic islands is associated with conditions that comprise both relatively low pressure in the South Atlantic and high pressure over the Bellingshausen and Amundsen seas (rather than a weakening); a configuration that enhances meridional airflow across the region, delivering moist southerly air masses over the islands, with westerly winds displaced to the north.

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Whilst the ASL is generally considered quasi-stationary because of the large number of low pressure systems in this sector of the circumpolar trough, they are by no means permanent (Hosking et al 2013). With large variability in pressure systems across the region on daily to seasonal timescales, our analysis suggests that when both low pressure is present over the South Atlantic and high pressure is found west of the Peninsula, more rainfall is delivered to the subantarctic islands and across the wider southern South America (figures 4(A)–(F)).

Reconciling the 1940s stepped increase in Falkland Islands' precipitation with the above association of pressure systems across the South Atlantic and Peninsula is not immediately apparent. To investigate the synoptic conditions for the first half of the Port Stanley record (CE 1904–1940) we interrogated the 20CR 2c (figures 4(G)–(I)) (Compo et al 2011, Cram et al 2015). Whilst the observational record that contributes to the high-latitudes of the Southern Hemisphere is relatively sparse during the early 20th century (Jones et al 2016), we find Falkland Islands precipitation was similarly driven by southerly airflow (figure 4(I)). In marked contrast to the last seventy years, however, the synoptic configuration appears to have been different. Crucially, during the first half of the 20th century there appears to have been no dynamic link with pressure over the Amundsen and Bellingshausen Seas. Instead, high precipitation before the mid-1940s was solely influenced by the South Atlantic pressure system centred over South Georgia (figure 4(G)) with return northerly airflow strengthened out to the east (figure 4(I)). The ASL is a semi-permanent feature, a consequence of the Antarctic Peninsula and regional topography that dynamically influences atmospheric flow in the southeast Pacific sector of the Southern Ocean (Fogt et al 2012), suggesting a fundamental change in atmospheric circulation took place across the 1940s that brought the South Atlantic into the sphere of influence of this dynamic feature. The 'absence' of a role for pressure changes over the Amundsen and Bellingshausen Seas prior to the 1940s appears to have limited precipitation over the islands.

Several physical mechanisms need to be considered to understand the circulation changes. One possible cause may be related to the release of CFCs that started in the 1920s, though measurable ozone depletion was not observed until the 1980s (Thompson et al 2011). Alternatively, the changes may be linked to expansion of the tropical belt and Hadley circulation (Seidel et al 2008) which have recently been reported to have shifted south since the 1940s (Brönnimann et al 2015, Nguyen et al 2015), potentially affecting circulation across the mid-latitudes. Additionally, during at least the last four decades the Antarctic Peninsula region is known to be sensitive to tropical Pacific temperatures that generate an atmospheric Rossby wave response (Yuan 2004, Compo and Sardeshmukh 2010, Gordon et al 2010, Steig et al 2013, Turney et al 2015), causing anomalously high pressure over the Amundsen and Bellingshausen Seas, and enhancing southerly (colder) airflow across the Weddell Sea and South Atlantic (Abram et al 2014). To investigate a possible role for the tropical Pacific in atmospheric circulation changes over the South Atlantic, we regressed the Port Stanley precipitation record against the post-1950 HadISST sea surface temperature dataset (Rayner et al 2003) and find a highly significant relationship with the tropical Pacific (figure 5). Specifically, high precipitation is associated with warmer temperatures in the central and eastern Pacific (Nino 3.4 and 3 regions), with zero lag. Our results imply the step change to wetter conditions across the 1940s marked a threshold after which low-latitude climate variability was more strongly projected on the South Atlantic, consistent with previous studies suggesting disproportionate remote impacts of tropical Pacific temperature changes (Sardeshmukh et al 2000).

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Although further work is needed to increase the density of early observations in reanalysis products, our results are consistent with the stepped change to wetter conditions from the mid-1940s (figure 3) and suggests this came about from a change in circulation across the mid-latitudes of the Southern Hemisphere (possibly linked to increasing sea surface temperatures in the tropical Pacific) that allowed the influence of synoptic conditions in the Amundsen and Bellingshausen seas to align with those over the South Atlantic.

Comprehensive radiocarbon and ¹³⁷Cs dating of the subantarctic sequences (table 1) provides a robust geochronological framework for reconstructing change across the region. The Junction Valley sequence preserves a record of more than 6000 years, with Canopus Hill extending over 4000 years (figure 6). The youngest record is the relatively low-lying King Edward Cove that captures changes in peat growth during the past 1200 years. Fortunately, the rapid and dramatic increase in atmospheric ¹⁴C as a consequence of nuclear tests (the so-called 'bomb' spike) (Hua and Barbetti 2004) and the associated increase in ¹³⁷Cs (with a half-life of 30 years) (Leslie and Hancock 2008) provide two independent time-parallel marker horizons in each sequence (figure 6). The %TOC values for the three sites fluctuate around 40%–50%, which is typical of cold climate Holocene peats (Vitt et al 2000, Beilman et al 2009, Nakatsubo et al 2015). The carbon:nitrogen (C:N) profiles are also broadly comparable across the three sites with values ranging from ∼30 to 40. Subtle down core fluctuations in the TOC% and C:N records most likely reflect past hydrological change impacting humification rates (Kuhry and Vitt 1996, Anderson 2002). The relative complacency of both the TOC% and C:N records down profile across the three sites highlights the long-term geomorphic stability of each bog catchment, with no evidence for significant periods of inwash.

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To investigate the role of temperature and precipitation on peat growth (MacDonald et al 2006, Dise 2009, Loisel and Yu 2013), we interrogated the highest resolution record with the most complete climate dataset (figure 7). The carbon accumulation rate from Canopus Hill (Falkland Islands) was binned at decadal resolution across the 20th century and log transformed. Whilst higher mean spring–summer (September–February) temperatures are associated with increased accumulation (r² = 0.26), the strongest relationship is observed with cumulative spring–summer precipitation (r² = 0.66), suggesting the delivery of moisture across the growing season is the primary driver of growth. The implication is following recharge of water table levels by moisture-laden southerly airmasses, warmer spring/summer conditions encourages further growth.

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Crucially, all three sequences record a dramatic increase in peat growth and carbon sequestration that commenced immediately prior to the 1950s onset of nuclear tests (figure 8). The same trend is observed in all three sequences using the method reported by (Yu et al 2003) (data not shown). The Canopus Hill sequence on the Falklands records the greatest increase with a tenfold increase in accumulation across the 20th century, with a pronounced peak commencing in the 1940s. Similar increases in accumulation are also recorded in the South Georgia sequences, with a threefold increase in King Edward Cove and a sixfold increase in Junction Valley. The increase in peat growth is consistent with the observational record showing a stepped increase to sustained higher precipitation during the 1940s (figure 3). The greatest accumulation of peat at this time supports precipitation as a key driver of growth on the islands, supported by warmer temperatures (figure 7). At no other time over the past 6000 years do we observe a comparable rate of peat growth as the mid- to late-20th century (figure 8).

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To investigate whether changes in the peat accumulation might reflect long-term forcing we undertook tipping point analysis (figure 9). In a long-term forcing scenario, the leading early warning indicators of critical slowing down (autocorrelation and variance) would be expected to increase, as the basin of attraction of the system widens and shallows. If, on the other hand, there is no underlying long-term forcing, these indicators would show no trend. Our tipping point analysis on the three peat sequences from the Falklands and South Georgia show no consistent parallel increase in autocorrelation or variance, under all pre-processing scenarios (figure 9), arguing against a long-term forcing that led to a shift in the climate system. These results are consistent regardless of the sampling resolution of the record; the Canopus Hill sequence, with the highest resolution averaging 30 years, shows negative trends for autocorrelation and variance. The abrupt acceleration in peat growth is therefore more likely to be the result of a short-term or noise-induced change, consistent with the observed relatively abrupt nature of wetter conditions across this region.

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Our results identify a shift in atmospheric circulation across the mid latitudes of the Southern Hemisphere in the 1940s that appears anomalous in the context of the last six millennia. This finding is consistent with other records spanning the mid to late Holocene. Recent work using proxy data has argued that the trend to positive SAM since the 1940s is unprecedented over the past millennia (figure 8(B)) (Villalba et al 2012, Abram et al 2014), providing warmer air masses over the South Atlantic subantarctic islands that would have helped drive peat growth (figure 7). Curiously, whilst the James Ross Island ice core record on the Antarctic Peninsula suggests an exceptional rate of warming in the 20th century (Mulvaney et al 2012) it does not preserve the highest absolute temperatures across this period (figure 8(F)) suggesting absolute temperatures may have been offset by enhanced southerly airflow as implied by the South Atlantic subantarctic islands (figures 8(C)–(E)). This interpretation is supported by a multi-proxy reconstruction of Nino 3.4 temperatures for the last millennium (Emile-Geay et al 2013) which shows unprecedented warming in the central tropical Pacific since the mid-20th century (figures 8(A)), consistent with higher pressure in the Amundsen and Bellingshausen seas and enhanced southerly airflow over the region (Yuan 2004, Gordon et al 2010, Abram et al 2014). The dramatic increase in precipitation and associated peat growth therefore implies a threshold in the global climate system was passed during the 1940s that is without parallel in the last 6000 years. If correct, the south Atlantic region may indeed be one of the first to have experienced the effects of anthropogenic climate change (King et al 2015).