Storm tracks are a key component of global atmospheric circulation. Their influence ranges from m... more Storm tracks are a key component of global atmospheric circulation. Their influence ranges from macro- to mesoscale dynamics, from large-scale movement of heat and momentum to extreme weather events. The scale of their impact makes understanding storm track dynamics critical to forecasting and climate projections. In this study, we assess CMIP6 historical experiment fidelity to observations of the Southern Hemisphere storm track. Specifically, storm track climatology, variability, and its interactions with low-frequency variability, with the aim of providing confidence for projections of future climate. We find CMIP6 models replicate results from the ERA-5 reanalysis with high fidelity in some regards; namely, capturing climatology of the 500hPa geopotential height field, the role of large-scale variability, and the baroclinic connection with high-frequency variability. However, models fail to capture the magnitude and variability of the storm track, particularly canonical zonal asy...
For the first time in the latest Assessment Report of the Intergovernmental Panel on Climate Chan... more For the first time in the latest Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), water has been the focus of dedicated chapters in both Working Group 1 (Chapter 8) and 2 (Chapter 4). Nevertheless, we argue here that water has not yet received the full attention it deserves from both scientists and policymakers for several reasons. Firstly, the historical focus on temperature change has been further increased with the use of global warming levels motivated by an aim to be consistent with current policy framings. Secondly, an increasing attention paid to extreme weather has sometimes overshadowed longer time-scale changes such as the aridification of an increasing fraction of arable land and the increasing variability of the water cycle from month to month, season to season, and year to year that also yield cascading impacts on all water use sectors. Thirdly, a stronger focus is needed on understanding the effectiveness of current and future adaptation strat...
In the version of the article initially published, there was an error in Fig. 1d: the third graph... more In the version of the article initially published, there was an error in Fig. 1d: the third graph, now labelled "150 hPa height", incorrectly read "500 hPa height". This has been corrected in the HTML and PDF versions of the article.
Journal Of Geophysical Research: Atmospheres, Jan 4, 2018
Over the past 60 years, the climate of East Antarctica cooled while portions of West Antarctica w... more Over the past 60 years, the climate of East Antarctica cooled while portions of West Antarctica were among the most rapidly warming regions on the planet. The East Antarctic cooling is attributed to a positive trend in the Southern Annular Mode (SAM) and a strengthening of the westerlies, while West Antarctic warming is tied to zonally asymmetric circulation changes forced by the tropics. This study finds recent (post-1979) surface cooling of East Antarctica during austral autumn to also be tied to tropical forcing, namely, an increase in La Niña events. The recent increase in La Niña conditions forces a Rossby wave into the Southern Hemisphere that increases anticyclonic circulation over the South Atlantic. The South Atlantic anticyclone is associated with cold air advection, weakened northerlies, and increased sea ice concentrations across the western East Antarctic coast, which has increased the rate of cooling at Novolazarevskaya and Syowa stations after 1979. This enhanced cooling over western East Antarctica is tied more broadly to a zonally asymmetric temperature trend pattern across East Antarctica during autumn that is consistent with a tropically forced Rossby wave rather than a SAM pattern; the positive SAM pattern is associated with ubiquitous cooling across East Antarctica, which is not seen in temperature observations after 1979. We conclude that El Niño-Southern Oscillation-related circulation anomalies, particularly zonal asymmetries that locally enhance meridional wind, are an important component of East Antarctic climate variability during autumn, and future changes in tropical Pacific climate will likely have implications for East Antarctica.
lobal mean surface air temperature (SAT) has been increasing since at least the 1880s. Approximat... more lobal mean surface air temperature (SAT) has been increasing since at least the 1880s. Approximately 0.6 °C of warming occurred over the twentieth century (average rate of warming around 0.05 °C decade −1), although the rate of warming has accelerated to 0.15-0.20 °C decade −1 since the 1970s 1. However, substantial spatial heterogeneity exists in climate trends, with regional SAT trends sometimes greatly exceeding, or even opposing, the global mean. These departures from the global-mean trend may signify important regional-scale processes, mechanisms and feedbacks. Since the mid-twentieth century, Antarctica has experienced many of the planet's largest regional temperature trends 2,3. For example, over the second half of the twentieth century, West Antarctica and the Antarctic Peninsula warmed more than twice as fast as the global average 4-7 , but over the first two decades of the twenty-first century, temperature trends there strikingly reversed 8-11. Such strong regional changes are often most pronounced in the polar regions due to positive ice-albedo feedbacks that amplify warming 5,12 , while stratospheric ozone depletion and extreme decadal variability can even induce regional cooling 9,13-15. In Antarctica, strong fluctuations in SAT occur, in part, because of the continent's sensitivity to tropical forcing 9,16-21 , the strength and position of the circumpolar westerly winds 14,22-25 , and its exposure to relatively mild and moist oceanic airmass intrusions along its coastline 26-31. Here, we examine recent SAT changes at Earth's southernmost weather observatory, the Amundsen-Scott station located at the South Pole. An analysis in the 'State of the Climate in 2018' 32 report showed that Amundsen-Scott station (and Vostok, the only other Antarctic Plateau station with a similarly long SAT time series) recorded their warmest year on record in 2018, both being +2.4 °C above their 1981-2010 climatological averages. To place Antarctic climate in the context of increasing global SAT from continuing greenhouse gas forcing, a comprehensive diagnosis of the drivers of such extreme fluctuations in temperature is crucial.
AbstractDuring 1979–2014, eastern tropical Pacific sea surface temperatures significantly cooled,... more AbstractDuring 1979–2014, eastern tropical Pacific sea surface temperatures significantly cooled, which has generally been attributed to the transition of the Pacific Decadal Oscillation to its negative phase after 1999. We find the eastern tropical Pacific cooling to be associated with: (1) an intensified Walker Circulation during austral summer (December–February, DJF) and autumn (March–May, MAM); (2) a weakened South Pacific Hadley cell and subtropical jet during MAM; and (3) a strengthening of the circumpolar westerlies between 50 and 60°S during DJF and MAM. Observed cooling in the eastern tropical Pacific is linearly congruent with 60–80 % of the observed Southern Hemisphere positive zonal-mean zonal wind trend between 50 and 60°S during DJF (~35 % of the interannual variability), and around half of the observed positive zonal-mean zonal wind trend during MAM (~15 % of the interannual variability). Although previous studies have linked the strengthened DJF and MAM circumpolar westerlies to stratospheric ozone depletion and increasing greenhouse gases, we note that the continuation of the positive SAM trends into the twenty-first century is partially associated with eastern tropical Pacific cooling, especially during MAM when zonal wind anomalies associated with eastern tropical Pacific cooling project strongly onto the observed trends. Outside of DJF and MAM, eastern tropical Pacific cooling is associated with opposing zonal wind anomalies over the Pacific and Indian sectors, which we infer is the reason for the absence of significant positive SAM trends outside of DJF and MAM despite significant eastern tropical Pacific cooling seen during all seasons.
Using empirical orthogonal function (EOF) analysis and atmospheric reanalyses, the principal patt... more Using empirical orthogonal function (EOF) analysis and atmospheric reanalyses, the principal patterns of seasonal West Antarctic surface air temperature (SAT) and their connection to sea ice and the Amundsen Sea low (ASL) are examined. During austral summer, the leading EOF (EOF1) explains 35% of West Antarctic SAT variability and consists of a widespread SAT anomaly over the continent linked to persistent sea ice concentration anomalies over the Ross and Amundsen Seas from the previous spring. Outside of summer, EOF1 (explaining ;40%-50% of the variability) consists of an east-west dipole over the continent with SAT anomalies over the Antarctic Peninsula opposite those over western West Antarctica. The dipole is tied to variability in the southern annular mode (SAM) and in-phase El Niño-Southern Oscillation (ENSO)/SAM combinations that influence the depth of the ASL over the central Amundsen Sea (near 1058W). The second EOF (EOF2) during autumn, winter, and spring (explaining ;15%-20% of the variability) consists of a dipole shifted approximately 308 west of EOF1 with a widespread SAT anomaly over the continent. During winter and spring, EOF2 is closely tied to variability in ENSO and a tropically forced wave train that influences the ASL in the western Amundsen/eastern Ross Seas (near 1358W) with an opposite-sign circulation anomaly over the Weddell Sea; the ENSO-related circulation brings anomalous thermal advection deep onto the continent. The authors conclude that the ENSO-only circulation pattern is associated with SAT variability across interior West Antarctica, especially during winter and spring, whereas the SAM circulation pattern is associated with an SAT dipole over the continent.
Significant austral spring trends have previously been observed in West Antarctica and Antarctic ... more Significant austral spring trends have previously been observed in West Antarctica and Antarctic Peninsula temperatures and in atmospheric circulation across the southern Pacific and Atlantic. Here, physical mechanisms for the observed trends are investigated through analysis of monthly circulation and temperatures from the ERA-Interim dataset and outgoing longwave radiation (OLR) data. The negative pressure trend over the South Pacific during spring is strongest in September, while the positive pressure trend over the South Atlantic is strongest in October. Pressure trends in November are generally nonsignificant. The authors demonstrate that a significant September trend toward increased convection (reduced OLR) in the poleward portion of the South Pacific convergence zone (SPCZ) is statistically related to Rossby wave-like circulation changes across the southern oceans. The wave response is strongest over the South Pacific in September and propagates eastward to the South Atlantic in October. OLR-related changes are linearly congruent with around half of the observed total changes in circulation during September and October and are consistent with observed trends in South Pacific sea ice concentration and surface temperature over western West Antarctica and the western Antarctic Peninsula. These results suggest SPCZ variability in early spring, especially on the poleward side of the SPCZ, is an important contributor to circulation and surface temperature trends across the South Pacific/Atlantic and West Antarctica.
Journal Of Geophysical Research: Atmospheres, Aug 22, 2016
Recent warming of the Antarctic Peninsula during austral autumn, winter, and spring has been link... more Recent warming of the Antarctic Peninsula during austral autumn, winter, and spring has been linked to sea surface temperature (SST) trends in the tropical Pacific and tropical Atlantic, while warming of the northeast Peninsula during summer has been linked to a strengthening of westerly winds traversing the Peninsula associated with a positive trend in the Southern Annular Mode (SAM). Here we demonstrate that circulation changes associated with the SAM dominate interannual temperature variability across the entire Antarctic Peninsula during both summer and autumn, while relationships with tropical Pacific SST variability associated with the El Niño-Southern Oscillation (ENSO) are strongest and statistically significant primarily during winter and spring only. We find the ENSO-Peninsula temperature relationship during autumn to be weak on interannual time scales and regional circulation anomalies associated with the SAM more important for interannual temperature variability across the Peninsula during autumn. Consistent with previous studies, western Peninsula temperatures during autumn, winter, and spring are closely tied to changes in the Amundsen Sea Low (ASL) and associated meridional wind anomalies. The interannual variability of ASL depth is most strongly correlated with the SAM index during autumn, while the ENSO relationship is strongest during winter and spring. Investigation of western and northeast Peninsula temperatures separately reveals that interannual variability of northeast Peninsula temperatures is primarily sensitive to zonal wind anomalies crossing the Peninsula and resultant leeside adiabatic warming rather than to meridional wind anomalies, which is closely tied to variability in the zonal portion of the SAM pattern. the preceding autumn and winter seasons [Ding and Steig, 2013; Clem and Fogt, 2015; Clem and Renwick, 2015]. The warming and reduced sea ice along the western Peninsula have had major biological impacts, including an increase in competition for krill-eating predators due to reductions in sea ice [Trivelpiece et al., 2011] and the ability for king crab populations to survive at unprecedented shallow depths along the outer shelf of the western Antarctic Peninsula [Aronson et al., 2015].
Understanding the causes of recent climatic trends and variability in the high-latitude Southern ... more Understanding the causes of recent climatic trends and variability in the high-latitude Southern Hemisphere is hampered by a short instrumental record. Here, we analyse recent atmosphere, surface ocean and sea-ice observations in this region and assess their trends in the context of palaeoclimate records and climate model simulations. Over the 36-year satellite era, significant linear trends in annual mean sea-ice extent, surface temperature and sea-level pressure are superimposed on large interannual to decadal variability. However, most observed trends are not unusual when compared with Antarctic paleoclimate records of the past two centuries. With the exception of the positive trend in the Southern Annular Mode, climate model simulations that include anthropogenic forcing are not compatible with the observed trends. This suggests that natural variability likely overwhelms the forced response in the observations, but the models may not fully represent this natural variability or may overestimate the magnitude of the forced response.
Here we examine the water stable-isotope data from the Roosevelt Island Climate Evolution (RICE) ... more Here we examine the water stable-isotope data from the Roosevelt Island Climate Evolution (RICE) ice core. Roosevelt Island is an independent ice rise located at the northeastern margin of the Ross Ice Shelf. In this study, we use empirical orthogonal function (EOF) analysis to investigate the relationship between RICE ice-core oxygen-18 isotopes (δ 18 O) and Southern Hemisphere atmospheric circulation during the extended austral winter (April-November). The RICE δ 18 O record is correlated with Southern Annular Mode (SAM) and Paci c-South American pattern 1 (PSA1), which both project onto the Amundsen-Bellingshausen Sea (ABS) geopotential height eld. Paci c sector Southern Ocean, eastern Ross Sea, and West Antarctic's atmospheric circulation, sea ice, and surface air temperature (SAT) anomalies, as well as RICE δ 18 O, are strongest when El Niño-Southern Oscillation (ENSO) and SAM are "in-phase". That is when the SAM−/PSA1+ (El Niño) and SAM+/PSA1− (La Niña) phasing prevails. When in-phase, the δ 18 O correlation with the 500-hPa geopotential height (Z500) is strong in regions (e.g., the Amundsen Sea) where their anomalies associated with SAM and PSA1 show the same sign. SAM−/PSA1+ (El Niño) and SAM+/PSA1− (La Niña) is associated with positive and negative δ 18 O anomalies, respectively. RICE δ 18 O can aid in establishing past natural variability of the strength of the SH high-latitude Paci c sector ENSO-SAM connection and associated atmospheric circulation, SIC, and SAT extremes.
Understanding precipitation is essential for quantifying weather and climate-related risks. Chang... more Understanding precipitation is essential for quantifying weather and climate-related risks. Changes in precipitation climatology are typically based on independent analysis of precipitation frequency and intensity. Here we show that where it rains more often, it also rains harder. When grouping global precipitation reanalysis data and observations from the past 40 years into regions of similar wet-day frequency, regardless of geographical separation, there is a strong correlation with wet-day intensity distributions. These wet-day-frequency regions are also more physically coherent than regions based on geographical location. We find the coherent relationship between wet-day frequency and intensity distributions is partially explained by wet-day-frequency regions having similar vertical velocity and convective available potential energy distributions, once polar regions are excluded. These represent dynamic and thermodynamic processes that indicate how conducive wet-day-frequency re...
Anthropogenic climate change is affecting rivers worldwide, threatening water availability and al... more Anthropogenic climate change is affecting rivers worldwide, threatening water availability and altering the risk of natural hazards. Understanding the pattern of regional streamflow trends can help to inform region-specific policies to mitigate and adapt to any negative impacts on society and the environment. We present a benchmark dataset of long, near-natural streamflow records across Aotearoa New Zealand (NZ) and the first nationwide analysis of observed spatiotemporal streamflow trends. Individual records rarely have significant trends, but when aggregated within homogenous hydrologic regions (determined through cluster analyses), significant regional trends emerge. A multitemporal approach that uses all available data for each region and considers trend significance over time reveals the influence of decadal variability in some seasons and regions, and consistent trends in others. Over the last 50+ years, winter streamflow has significantly increased in the west South Island an...
Journal of Geophysical Research: Atmospheres, 2021
Antarctica’s sea ice cover is an important component of the global climate system, yet the driver... more Antarctica’s sea ice cover is an important component of the global climate system, yet the drivers of sea ice variability are not well understood. Here we investigated the effects of climate variability on sea ice concentration (SIC) around East Antarctica by correlating the 40‐years (1979–2018) satellite sea ice record and ERA5 reanalysis data. We found that summer and autumn SIC around Dronning Maud Land (DML) between 10 and 70°E exhibited a statistically significant negative correlation with the Niño 3.4 index. Sea ice in DML was also correlated with sea surface temperature (SST) anomalies in the tropical Pacific, and to an atmospheric wave train pattern extending from the South Pacific to DML. We suggest that a southward‐propagating atmospheric wave train triggered by SST anomalies in the tropical Pacific extends into DML and alters sea ice concentration by encouraging meridional airflow. Our results showed that shifts in meridional flow in DML affected sea ice thermodynamically...
During austral summers (DJF) 1934/35, 2017/18 and 2018/19, the New Zealand (NZ) region (approxima... more During austral summers (DJF) 1934/35, 2017/18 and 2018/19, the New Zealand (NZ) region (approximately 4 million km 2) experienced the most intense coupled oceanatmosphere heatwaves on record. Average air temperature anomalies over land were + 1.7 to 2.1°C while sea surface temperatures (SST) were 1.2 to 1.9°C above average. All three heatwaves exhibited maximum SST anomalies west of the South Island of NZ. Atmospheric circulation anomalies showed a pattern of blocking centred over the Tasman Sea extending southeast of NZ, accompanied by strongly positive Southern Annular Mode conditions, and reduced trough activity over NZ. Rapid melt of seasonal snow occurred in all three cases. For the two most recent events, combined ice loss in the Southern Alps was estimated at 8.9 km 3 (22% of the 2017 volume). Sauvignon blanc and Pinot noir wine grapes had above average berry number and bunch mass in 2018 but were below average in 2019. Summerfruit harvest (cherries and apricots) was 14 and 2 days ahead of normal in 2017/18 and 2018/19 respectively. Spring wheat simulations suggested earlier flowering and lower grain yields compared to average, and below-average yield and tuber quality in potatoes crops occurred. Major species disruption occurred in marine ecosystems. Hindcasts indicate that the heatwaves were either atmospherically driven or arose from combinations of atmospheric surface warming and oceanic heat advection.
Hazard analysis at caldera volcanoes is challenging due to the wide range of eruptive and environ... more Hazard analysis at caldera volcanoes is challenging due to the wide range of eruptive and environmental conditions that can plausibly occur during renewed activity. Taupo volcano, New Zealand, is a frequently active and productive rhyolitic caldera volcano that has hosted the world's youngest known supereruption and numerous smaller explosive events. To assess ashfall hazard from future eruptions, we have simulated atmospheric ash dispersal using the Ash3d model. We consider five eruption scenarios spanning magma volumes of 0.1–500 km3 and investigate the main factors governing ash dispersal in modern atmospheric conditions. Our results are examined in the context of regional synoptic weather patterns (Kidson types) that provide a framework for assessing the variability of ashfall distribution in different wind fields. For the smallest eruptions (~0.1‐km3 magma), ashfall thicknesses >1 cm are largely confined to the central North Island, with dispersal controlled by day‐to‐da...
Persistent positive 500-hPa geopotential height anomalies from the ECMWF ERA-Interim reanalysis a... more Persistent positive 500-hPa geopotential height anomalies from the ECMWF ERA-Interim reanalysis are used to quantify Amundsen-Bellingshausen Sea (ABS) anticyclonic event occurrences associated with precipitation in West Antarctica (WA). We demonstrate that multi-day (minimum 3-day duration) anticyclones play a key role in the ABS by dynamically inducing meridional transport, which is associated with heat and moisture advection into WA. This affects surface climate variability and trends, precipitation rates and thus WA ice sheet surface mass balance. We show that the snow accumulation record from the Roosevelt Island Climate Evolution (RICE) ice core reflects interannual variability of blocking and geopotential height conditions in the ABS/Ross Sea region. Furthermore, our analysis shows that larger precipitation events are related to enhanced anticyclonic circulation and meridional winds, which cause pronounced dipole patterns in air temperature anomalies and sea ice concentrations between the eastern Ross Sea and the Bellingshausen Sea/Weddell Sea, as well as between the eastern and western Ross Sea.
The record high temperature for regions south of 60°S latitude is a balmy 19.8°C (67.6°F), record... more The record high temperature for regions south of 60°S latitude is a balmy 19.8°C (67.6°F), recorded 30 January 1982 at a research station on Signy Island.
Storm tracks are a key component of global atmospheric circulation. Their influence ranges from m... more Storm tracks are a key component of global atmospheric circulation. Their influence ranges from macro- to mesoscale dynamics, from large-scale movement of heat and momentum to extreme weather events. The scale of their impact makes understanding storm track dynamics critical to forecasting and climate projections. In this study, we assess CMIP6 historical experiment fidelity to observations of the Southern Hemisphere storm track. Specifically, storm track climatology, variability, and its interactions with low-frequency variability, with the aim of providing confidence for projections of future climate. We find CMIP6 models replicate results from the ERA-5 reanalysis with high fidelity in some regards; namely, capturing climatology of the 500hPa geopotential height field, the role of large-scale variability, and the baroclinic connection with high-frequency variability. However, models fail to capture the magnitude and variability of the storm track, particularly canonical zonal asy...
For the first time in the latest Assessment Report of the Intergovernmental Panel on Climate Chan... more For the first time in the latest Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), water has been the focus of dedicated chapters in both Working Group 1 (Chapter 8) and 2 (Chapter 4). Nevertheless, we argue here that water has not yet received the full attention it deserves from both scientists and policymakers for several reasons. Firstly, the historical focus on temperature change has been further increased with the use of global warming levels motivated by an aim to be consistent with current policy framings. Secondly, an increasing attention paid to extreme weather has sometimes overshadowed longer time-scale changes such as the aridification of an increasing fraction of arable land and the increasing variability of the water cycle from month to month, season to season, and year to year that also yield cascading impacts on all water use sectors. Thirdly, a stronger focus is needed on understanding the effectiveness of current and future adaptation strat...
In the version of the article initially published, there was an error in Fig. 1d: the third graph... more In the version of the article initially published, there was an error in Fig. 1d: the third graph, now labelled "150 hPa height", incorrectly read "500 hPa height". This has been corrected in the HTML and PDF versions of the article.
Journal Of Geophysical Research: Atmospheres, Jan 4, 2018
Over the past 60 years, the climate of East Antarctica cooled while portions of West Antarctica w... more Over the past 60 years, the climate of East Antarctica cooled while portions of West Antarctica were among the most rapidly warming regions on the planet. The East Antarctic cooling is attributed to a positive trend in the Southern Annular Mode (SAM) and a strengthening of the westerlies, while West Antarctic warming is tied to zonally asymmetric circulation changes forced by the tropics. This study finds recent (post-1979) surface cooling of East Antarctica during austral autumn to also be tied to tropical forcing, namely, an increase in La Niña events. The recent increase in La Niña conditions forces a Rossby wave into the Southern Hemisphere that increases anticyclonic circulation over the South Atlantic. The South Atlantic anticyclone is associated with cold air advection, weakened northerlies, and increased sea ice concentrations across the western East Antarctic coast, which has increased the rate of cooling at Novolazarevskaya and Syowa stations after 1979. This enhanced cooling over western East Antarctica is tied more broadly to a zonally asymmetric temperature trend pattern across East Antarctica during autumn that is consistent with a tropically forced Rossby wave rather than a SAM pattern; the positive SAM pattern is associated with ubiquitous cooling across East Antarctica, which is not seen in temperature observations after 1979. We conclude that El Niño-Southern Oscillation-related circulation anomalies, particularly zonal asymmetries that locally enhance meridional wind, are an important component of East Antarctic climate variability during autumn, and future changes in tropical Pacific climate will likely have implications for East Antarctica.
lobal mean surface air temperature (SAT) has been increasing since at least the 1880s. Approximat... more lobal mean surface air temperature (SAT) has been increasing since at least the 1880s. Approximately 0.6 °C of warming occurred over the twentieth century (average rate of warming around 0.05 °C decade −1), although the rate of warming has accelerated to 0.15-0.20 °C decade −1 since the 1970s 1. However, substantial spatial heterogeneity exists in climate trends, with regional SAT trends sometimes greatly exceeding, or even opposing, the global mean. These departures from the global-mean trend may signify important regional-scale processes, mechanisms and feedbacks. Since the mid-twentieth century, Antarctica has experienced many of the planet's largest regional temperature trends 2,3. For example, over the second half of the twentieth century, West Antarctica and the Antarctic Peninsula warmed more than twice as fast as the global average 4-7 , but over the first two decades of the twenty-first century, temperature trends there strikingly reversed 8-11. Such strong regional changes are often most pronounced in the polar regions due to positive ice-albedo feedbacks that amplify warming 5,12 , while stratospheric ozone depletion and extreme decadal variability can even induce regional cooling 9,13-15. In Antarctica, strong fluctuations in SAT occur, in part, because of the continent's sensitivity to tropical forcing 9,16-21 , the strength and position of the circumpolar westerly winds 14,22-25 , and its exposure to relatively mild and moist oceanic airmass intrusions along its coastline 26-31. Here, we examine recent SAT changes at Earth's southernmost weather observatory, the Amundsen-Scott station located at the South Pole. An analysis in the 'State of the Climate in 2018' 32 report showed that Amundsen-Scott station (and Vostok, the only other Antarctic Plateau station with a similarly long SAT time series) recorded their warmest year on record in 2018, both being +2.4 °C above their 1981-2010 climatological averages. To place Antarctic climate in the context of increasing global SAT from continuing greenhouse gas forcing, a comprehensive diagnosis of the drivers of such extreme fluctuations in temperature is crucial.
AbstractDuring 1979–2014, eastern tropical Pacific sea surface temperatures significantly cooled,... more AbstractDuring 1979–2014, eastern tropical Pacific sea surface temperatures significantly cooled, which has generally been attributed to the transition of the Pacific Decadal Oscillation to its negative phase after 1999. We find the eastern tropical Pacific cooling to be associated with: (1) an intensified Walker Circulation during austral summer (December–February, DJF) and autumn (March–May, MAM); (2) a weakened South Pacific Hadley cell and subtropical jet during MAM; and (3) a strengthening of the circumpolar westerlies between 50 and 60°S during DJF and MAM. Observed cooling in the eastern tropical Pacific is linearly congruent with 60–80 % of the observed Southern Hemisphere positive zonal-mean zonal wind trend between 50 and 60°S during DJF (~35 % of the interannual variability), and around half of the observed positive zonal-mean zonal wind trend during MAM (~15 % of the interannual variability). Although previous studies have linked the strengthened DJF and MAM circumpolar westerlies to stratospheric ozone depletion and increasing greenhouse gases, we note that the continuation of the positive SAM trends into the twenty-first century is partially associated with eastern tropical Pacific cooling, especially during MAM when zonal wind anomalies associated with eastern tropical Pacific cooling project strongly onto the observed trends. Outside of DJF and MAM, eastern tropical Pacific cooling is associated with opposing zonal wind anomalies over the Pacific and Indian sectors, which we infer is the reason for the absence of significant positive SAM trends outside of DJF and MAM despite significant eastern tropical Pacific cooling seen during all seasons.
Using empirical orthogonal function (EOF) analysis and atmospheric reanalyses, the principal patt... more Using empirical orthogonal function (EOF) analysis and atmospheric reanalyses, the principal patterns of seasonal West Antarctic surface air temperature (SAT) and their connection to sea ice and the Amundsen Sea low (ASL) are examined. During austral summer, the leading EOF (EOF1) explains 35% of West Antarctic SAT variability and consists of a widespread SAT anomaly over the continent linked to persistent sea ice concentration anomalies over the Ross and Amundsen Seas from the previous spring. Outside of summer, EOF1 (explaining ;40%-50% of the variability) consists of an east-west dipole over the continent with SAT anomalies over the Antarctic Peninsula opposite those over western West Antarctica. The dipole is tied to variability in the southern annular mode (SAM) and in-phase El Niño-Southern Oscillation (ENSO)/SAM combinations that influence the depth of the ASL over the central Amundsen Sea (near 1058W). The second EOF (EOF2) during autumn, winter, and spring (explaining ;15%-20% of the variability) consists of a dipole shifted approximately 308 west of EOF1 with a widespread SAT anomaly over the continent. During winter and spring, EOF2 is closely tied to variability in ENSO and a tropically forced wave train that influences the ASL in the western Amundsen/eastern Ross Seas (near 1358W) with an opposite-sign circulation anomaly over the Weddell Sea; the ENSO-related circulation brings anomalous thermal advection deep onto the continent. The authors conclude that the ENSO-only circulation pattern is associated with SAT variability across interior West Antarctica, especially during winter and spring, whereas the SAM circulation pattern is associated with an SAT dipole over the continent.
Significant austral spring trends have previously been observed in West Antarctica and Antarctic ... more Significant austral spring trends have previously been observed in West Antarctica and Antarctic Peninsula temperatures and in atmospheric circulation across the southern Pacific and Atlantic. Here, physical mechanisms for the observed trends are investigated through analysis of monthly circulation and temperatures from the ERA-Interim dataset and outgoing longwave radiation (OLR) data. The negative pressure trend over the South Pacific during spring is strongest in September, while the positive pressure trend over the South Atlantic is strongest in October. Pressure trends in November are generally nonsignificant. The authors demonstrate that a significant September trend toward increased convection (reduced OLR) in the poleward portion of the South Pacific convergence zone (SPCZ) is statistically related to Rossby wave-like circulation changes across the southern oceans. The wave response is strongest over the South Pacific in September and propagates eastward to the South Atlantic in October. OLR-related changes are linearly congruent with around half of the observed total changes in circulation during September and October and are consistent with observed trends in South Pacific sea ice concentration and surface temperature over western West Antarctica and the western Antarctic Peninsula. These results suggest SPCZ variability in early spring, especially on the poleward side of the SPCZ, is an important contributor to circulation and surface temperature trends across the South Pacific/Atlantic and West Antarctica.
Journal Of Geophysical Research: Atmospheres, Aug 22, 2016
Recent warming of the Antarctic Peninsula during austral autumn, winter, and spring has been link... more Recent warming of the Antarctic Peninsula during austral autumn, winter, and spring has been linked to sea surface temperature (SST) trends in the tropical Pacific and tropical Atlantic, while warming of the northeast Peninsula during summer has been linked to a strengthening of westerly winds traversing the Peninsula associated with a positive trend in the Southern Annular Mode (SAM). Here we demonstrate that circulation changes associated with the SAM dominate interannual temperature variability across the entire Antarctic Peninsula during both summer and autumn, while relationships with tropical Pacific SST variability associated with the El Niño-Southern Oscillation (ENSO) are strongest and statistically significant primarily during winter and spring only. We find the ENSO-Peninsula temperature relationship during autumn to be weak on interannual time scales and regional circulation anomalies associated with the SAM more important for interannual temperature variability across the Peninsula during autumn. Consistent with previous studies, western Peninsula temperatures during autumn, winter, and spring are closely tied to changes in the Amundsen Sea Low (ASL) and associated meridional wind anomalies. The interannual variability of ASL depth is most strongly correlated with the SAM index during autumn, while the ENSO relationship is strongest during winter and spring. Investigation of western and northeast Peninsula temperatures separately reveals that interannual variability of northeast Peninsula temperatures is primarily sensitive to zonal wind anomalies crossing the Peninsula and resultant leeside adiabatic warming rather than to meridional wind anomalies, which is closely tied to variability in the zonal portion of the SAM pattern. the preceding autumn and winter seasons [Ding and Steig, 2013; Clem and Fogt, 2015; Clem and Renwick, 2015]. The warming and reduced sea ice along the western Peninsula have had major biological impacts, including an increase in competition for krill-eating predators due to reductions in sea ice [Trivelpiece et al., 2011] and the ability for king crab populations to survive at unprecedented shallow depths along the outer shelf of the western Antarctic Peninsula [Aronson et al., 2015].
Understanding the causes of recent climatic trends and variability in the high-latitude Southern ... more Understanding the causes of recent climatic trends and variability in the high-latitude Southern Hemisphere is hampered by a short instrumental record. Here, we analyse recent atmosphere, surface ocean and sea-ice observations in this region and assess their trends in the context of palaeoclimate records and climate model simulations. Over the 36-year satellite era, significant linear trends in annual mean sea-ice extent, surface temperature and sea-level pressure are superimposed on large interannual to decadal variability. However, most observed trends are not unusual when compared with Antarctic paleoclimate records of the past two centuries. With the exception of the positive trend in the Southern Annular Mode, climate model simulations that include anthropogenic forcing are not compatible with the observed trends. This suggests that natural variability likely overwhelms the forced response in the observations, but the models may not fully represent this natural variability or may overestimate the magnitude of the forced response.
Here we examine the water stable-isotope data from the Roosevelt Island Climate Evolution (RICE) ... more Here we examine the water stable-isotope data from the Roosevelt Island Climate Evolution (RICE) ice core. Roosevelt Island is an independent ice rise located at the northeastern margin of the Ross Ice Shelf. In this study, we use empirical orthogonal function (EOF) analysis to investigate the relationship between RICE ice-core oxygen-18 isotopes (δ 18 O) and Southern Hemisphere atmospheric circulation during the extended austral winter (April-November). The RICE δ 18 O record is correlated with Southern Annular Mode (SAM) and Paci c-South American pattern 1 (PSA1), which both project onto the Amundsen-Bellingshausen Sea (ABS) geopotential height eld. Paci c sector Southern Ocean, eastern Ross Sea, and West Antarctic's atmospheric circulation, sea ice, and surface air temperature (SAT) anomalies, as well as RICE δ 18 O, are strongest when El Niño-Southern Oscillation (ENSO) and SAM are "in-phase". That is when the SAM−/PSA1+ (El Niño) and SAM+/PSA1− (La Niña) phasing prevails. When in-phase, the δ 18 O correlation with the 500-hPa geopotential height (Z500) is strong in regions (e.g., the Amundsen Sea) where their anomalies associated with SAM and PSA1 show the same sign. SAM−/PSA1+ (El Niño) and SAM+/PSA1− (La Niña) is associated with positive and negative δ 18 O anomalies, respectively. RICE δ 18 O can aid in establishing past natural variability of the strength of the SH high-latitude Paci c sector ENSO-SAM connection and associated atmospheric circulation, SIC, and SAT extremes.
Understanding precipitation is essential for quantifying weather and climate-related risks. Chang... more Understanding precipitation is essential for quantifying weather and climate-related risks. Changes in precipitation climatology are typically based on independent analysis of precipitation frequency and intensity. Here we show that where it rains more often, it also rains harder. When grouping global precipitation reanalysis data and observations from the past 40 years into regions of similar wet-day frequency, regardless of geographical separation, there is a strong correlation with wet-day intensity distributions. These wet-day-frequency regions are also more physically coherent than regions based on geographical location. We find the coherent relationship between wet-day frequency and intensity distributions is partially explained by wet-day-frequency regions having similar vertical velocity and convective available potential energy distributions, once polar regions are excluded. These represent dynamic and thermodynamic processes that indicate how conducive wet-day-frequency re...
Anthropogenic climate change is affecting rivers worldwide, threatening water availability and al... more Anthropogenic climate change is affecting rivers worldwide, threatening water availability and altering the risk of natural hazards. Understanding the pattern of regional streamflow trends can help to inform region-specific policies to mitigate and adapt to any negative impacts on society and the environment. We present a benchmark dataset of long, near-natural streamflow records across Aotearoa New Zealand (NZ) and the first nationwide analysis of observed spatiotemporal streamflow trends. Individual records rarely have significant trends, but when aggregated within homogenous hydrologic regions (determined through cluster analyses), significant regional trends emerge. A multitemporal approach that uses all available data for each region and considers trend significance over time reveals the influence of decadal variability in some seasons and regions, and consistent trends in others. Over the last 50+ years, winter streamflow has significantly increased in the west South Island an...
Journal of Geophysical Research: Atmospheres, 2021
Antarctica’s sea ice cover is an important component of the global climate system, yet the driver... more Antarctica’s sea ice cover is an important component of the global climate system, yet the drivers of sea ice variability are not well understood. Here we investigated the effects of climate variability on sea ice concentration (SIC) around East Antarctica by correlating the 40‐years (1979–2018) satellite sea ice record and ERA5 reanalysis data. We found that summer and autumn SIC around Dronning Maud Land (DML) between 10 and 70°E exhibited a statistically significant negative correlation with the Niño 3.4 index. Sea ice in DML was also correlated with sea surface temperature (SST) anomalies in the tropical Pacific, and to an atmospheric wave train pattern extending from the South Pacific to DML. We suggest that a southward‐propagating atmospheric wave train triggered by SST anomalies in the tropical Pacific extends into DML and alters sea ice concentration by encouraging meridional airflow. Our results showed that shifts in meridional flow in DML affected sea ice thermodynamically...
During austral summers (DJF) 1934/35, 2017/18 and 2018/19, the New Zealand (NZ) region (approxima... more During austral summers (DJF) 1934/35, 2017/18 and 2018/19, the New Zealand (NZ) region (approximately 4 million km 2) experienced the most intense coupled oceanatmosphere heatwaves on record. Average air temperature anomalies over land were + 1.7 to 2.1°C while sea surface temperatures (SST) were 1.2 to 1.9°C above average. All three heatwaves exhibited maximum SST anomalies west of the South Island of NZ. Atmospheric circulation anomalies showed a pattern of blocking centred over the Tasman Sea extending southeast of NZ, accompanied by strongly positive Southern Annular Mode conditions, and reduced trough activity over NZ. Rapid melt of seasonal snow occurred in all three cases. For the two most recent events, combined ice loss in the Southern Alps was estimated at 8.9 km 3 (22% of the 2017 volume). Sauvignon blanc and Pinot noir wine grapes had above average berry number and bunch mass in 2018 but were below average in 2019. Summerfruit harvest (cherries and apricots) was 14 and 2 days ahead of normal in 2017/18 and 2018/19 respectively. Spring wheat simulations suggested earlier flowering and lower grain yields compared to average, and below-average yield and tuber quality in potatoes crops occurred. Major species disruption occurred in marine ecosystems. Hindcasts indicate that the heatwaves were either atmospherically driven or arose from combinations of atmospheric surface warming and oceanic heat advection.
Hazard analysis at caldera volcanoes is challenging due to the wide range of eruptive and environ... more Hazard analysis at caldera volcanoes is challenging due to the wide range of eruptive and environmental conditions that can plausibly occur during renewed activity. Taupo volcano, New Zealand, is a frequently active and productive rhyolitic caldera volcano that has hosted the world's youngest known supereruption and numerous smaller explosive events. To assess ashfall hazard from future eruptions, we have simulated atmospheric ash dispersal using the Ash3d model. We consider five eruption scenarios spanning magma volumes of 0.1–500 km3 and investigate the main factors governing ash dispersal in modern atmospheric conditions. Our results are examined in the context of regional synoptic weather patterns (Kidson types) that provide a framework for assessing the variability of ashfall distribution in different wind fields. For the smallest eruptions (~0.1‐km3 magma), ashfall thicknesses >1 cm are largely confined to the central North Island, with dispersal controlled by day‐to‐da...
Persistent positive 500-hPa geopotential height anomalies from the ECMWF ERA-Interim reanalysis a... more Persistent positive 500-hPa geopotential height anomalies from the ECMWF ERA-Interim reanalysis are used to quantify Amundsen-Bellingshausen Sea (ABS) anticyclonic event occurrences associated with precipitation in West Antarctica (WA). We demonstrate that multi-day (minimum 3-day duration) anticyclones play a key role in the ABS by dynamically inducing meridional transport, which is associated with heat and moisture advection into WA. This affects surface climate variability and trends, precipitation rates and thus WA ice sheet surface mass balance. We show that the snow accumulation record from the Roosevelt Island Climate Evolution (RICE) ice core reflects interannual variability of blocking and geopotential height conditions in the ABS/Ross Sea region. Furthermore, our analysis shows that larger precipitation events are related to enhanced anticyclonic circulation and meridional winds, which cause pronounced dipole patterns in air temperature anomalies and sea ice concentrations between the eastern Ross Sea and the Bellingshausen Sea/Weddell Sea, as well as between the eastern and western Ross Sea.
The record high temperature for regions south of 60°S latitude is a balmy 19.8°C (67.6°F), record... more The record high temperature for regions south of 60°S latitude is a balmy 19.8°C (67.6°F), recorded 30 January 1982 at a research station on Signy Island.
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Papers by James Renwick