Late-Holocene Climate Variability in Southern New Zealand: A reconstruction of regional climate from an annually laminated sediment sequence from Lake Ohau

<p>This research aims to improve understanding of synoptic climate systems influencing southern New Zealand and document changes in the intensity and frequency of these systems beyond the historical record by analyzing a 1,350-year annually laminated sediment sequence recovered from Lake Ohau, South Island, New Zealand (44.234°S, 169.854°E). Climatological patterns originating in both the tropics (El-Niño-Southern Oscillation (ENSO), Interdecadal Pacific Oscillation (IPO)) and in the Antarctic (Southern Annular Mode (SAM)) influence year-to-year variability in New Zealand’s climate (e.g. temperature and precipitation). However, the range of natural variability of these systems in the southwest Pacific over time is poorly known because the instrumental record is short (~100 years). The high-resolution record from Lake Ohau offers a unique opportunity to investigate changes in regional hydrology and climate, and to also explore connections to large-scale climate patterns over the last millennium. Hydrodynamic and hydroclimatic processes that influence and control the production, transport, and deposition of sediment within the Lake Ohau catchment are examined and constrained in order to develop a robust climate record. A key aim is to determine the role that meteorology and climate play in controlling sediment flux. The physical properties and facies of a 5.5-meter-long Lake Ohau sediment core are analyzed using thin-sections, high-resolution X-radiographs scans, and particle-size analyses. Time-series analysis is used to establish links between varve facies, hydroclimate variability and regional synoptic climate types over the instrumental record. Utilizing this climate-proxy relationship, inflow conditions are reconstructed over the last 1,350 years and compared with regional temperature reconstructions to generate a Western South Island paleo-atmospheric circulation index. Relationship between this paleocirculation index and other proxy reconstructions show significant variability in the relative forcing of tropical (ENSO) and Southern Hemisphere highlatitude (SAM) synoptic climate drivers on New Zealand and southwest Pacific climate. Overall, this work demonstrates that: a) the laminated sediments from Lake Ohau are varves and the formation of the annual stratigraphy is strongly controlled by lake hydrodynamics, in particular, thermal lake stratification; b) sediment stratigraphy reflects changes in austral warm period (December-May) inflow, enabling a highresolution reconstruction of hydroclimate over the last 1,350 years and; c) the generation of a paleocirculation index for the Western South Island points to significant changes between northerly or southerly dominated atmospheric conditions in southern New Zealand, particularly over the ‘Little Ice Age’ (1385-1710 AD). During this time, the strength of tropical teleconnections weakened and a strong negative phase SAM persisted. Comparison with high-resolution regional proxy records from Antarctica and the Central Pacific point to significant regional coherence with a strong negative phase SAM acting as a primary driver of the onset of Little Ice Age conditions across the South Pacific.</p>

Late-Holocene Climate Variability in Southern New Zealand: A reconstruction of regional climate from an annually laminated sediment sequence from Lake Ohau

<p>This research aims to improve understanding of synoptic climate systems influencing southern New Zealand and document changes in the intensity and frequency of these systems beyond the historical record by analyzing a 1,350-year annually laminated sediment sequence recovered from Lake Ohau, South Island, New Zealand (44.234°S, 169.854°E). Climatological patterns originating in both the tropics (El-Niño-Southern Oscillation (ENSO), Interdecadal Pacific Oscillation (IPO)) and in the Antarctic (Southern Annular Mode (SAM)) influence year-to-year variability in New Zealand’s climate (e.g. temperature and precipitation). However, the range of natural variability of these systems in the southwest Pacific over time is poorly known because the instrumental record is short (~100 years). The high-resolution record from Lake Ohau offers a unique opportunity to investigate changes in regional hydrology and climate, and to also explore connections to large-scale climate patterns over the last millennium. Hydrodynamic and hydroclimatic processes that influence and control the production, transport, and deposition of sediment within the Lake Ohau catchment are examined and constrained in order to develop a robust climate record. A key aim is to determine the role that meteorology and climate play in controlling sediment flux. The physical properties and facies of a 5.5-meter-long Lake Ohau sediment core are analyzed using thin-sections, high-resolution X-radiographs scans, and particle-size analyses. Time-series analysis is used to establish links between varve facies, hydroclimate variability and regional synoptic climate types over the instrumental record. Utilizing this climate-proxy relationship, inflow conditions are reconstructed over the last 1,350 years and compared with regional temperature reconstructions to generate a Western South Island paleo-atmospheric circulation index. Relationship between this paleocirculation index and other proxy reconstructions show significant variability in the relative forcing of tropical (ENSO) and Southern Hemisphere highlatitude (SAM) synoptic climate drivers on New Zealand and southwest Pacific climate. Overall, this work demonstrates that: a) the laminated sediments from Lake Ohau are varves and the formation of the annual stratigraphy is strongly controlled by lake hydrodynamics, in particular, thermal lake stratification; b) sediment stratigraphy reflects changes in austral warm period (December-May) inflow, enabling a highresolution reconstruction of hydroclimate over the last 1,350 years and; c) the generation of a paleocirculation index for the Western South Island points to significant changes between northerly or southerly dominated atmospheric conditions in southern New Zealand, particularly over the ‘Little Ice Age’ (1385-1710 AD). During this time, the strength of tropical teleconnections weakened and a strong negative phase SAM persisted. Comparison with high-resolution regional proxy records from Antarctica and the Central Pacific point to significant regional coherence with a strong negative phase SAM acting as a primary driver of the onset of Little Ice Age conditions across the South Pacific.</p>

Megadroughts and pluvials in southwest Australia: 1350–2017 CE

Declining winter rainfall coupled with recent prolonged drought poses significant risks to water resources and agriculture across southern Australia. While rainfall declines over recent decades are largely consistent with modelled climate change scenarios, particularly for southwest Australia, the significance of these declines is yet to be assessed within the context of long-term hydroclimatic variability. Here, we present a new 668-year (1350–2017 CE) tree-ring reconstruction of autumn–winter rainfall over inland southwest Australia. This record reveals that a recent decline in rainfall over inland southwest Australia (since 2000 CE) is not unusual in terms of either magnitude or duration relative to rainfall variability over the last seven centuries. Drought periods of greater magnitude and duration than those in the instrumental record occurred prior to 1900 CE, including two ‘megadroughts’ of > 30 years duration in the eighteenth and nineteenth centuries. By contrast, the wettest > decadal periods of the last seven centuries occurred after 1900 CE, making the twentieth century the wettest of the last seven centuries. We conclude that the instrumental rainfall record (since ~ 1900 CE) does not capture the full scale of natural hydroclimatic variability for inland southwest Australia and that the risk of prolonged droughts in the region is likely much higher than currently estimated.

Interpreting erosion frequency and magnitude from luminescence profiles in boulders

&lt;p&gt;Exposed bedrock is ubiquitous on terrestrial and planetary landscapes, yet little is known&lt;br&gt;about the rate of bedrock erosion at a granular scale on timescales longer than the&lt;br&gt;instrumental record. As recently suggested, using the bleaching depth of luminescence&lt;br&gt;signals as a measure of bedrock erosion may fit these scales. Yet this approach assumes&lt;br&gt;constant erosion through time, a condition likely violated by the stochastic nature of erosional&lt;br&gt;events. Here we simulate bleaching in response to power-law distributions of removal&lt;br&gt;lengths and hiatus durations. We compare simulation results with previously measured&lt;br&gt;luminescence profiles from boulder surfaces to illustrate that prolonged hiatuses are unlikely&lt;br&gt;and that typical erosion scales are sub-granular with occasional loss at mm scales,&lt;br&gt;consistent with ideas about microflaws governing bedrock detachment. For a wide range of&lt;br&gt;erosion rates, measurements are integrated over many removal events, producing&lt;br&gt;reasonably accurate estimates despite the stochastic nature of the simulated process. We&lt;br&gt;hypothesize that the greater or equal erosion rates atop large boulders compared to rates at&lt;br&gt;ground level suggest that subcritical cracking may be more influential than aeolian abrasion&lt;br&gt;for boulder degradation in the Eastern Pamirs, China.&lt;/p&gt;

Oxford Weather and Climate since 1767

Oxford Weather and Climate since 1767 provides a detailed description and analysis of the weather records made at the Radcliffe Observatory in Oxford, the longest continuous series of single-site weather records in Britain and one of the longest in the world. The earliest records date from 1767, and daily records are unbroken since November 1813. The records allow the reconstruction of 200-year temperature and rainfall series and places the Oxford records in the context of long-term climate change. In this, the first full publication of the entire dataset, the long Oxford record is both celebrated and described. Detailed commentaries on weather by month and by season are provided, including numerous contemporary documentary and photographic evidence of past weather events. Drought and flood feature prominently, but so too do fog, frost, ice and snow. Some long-term changes are obvious, such as the increase in air temperature over the period of the instrumental record, but the impact on the growing season and the ability to grow grapes commercially near Oxford are less well known.

The Relationship between Cool and Warm Season Moisture over the Central United States, 1685–2015

Land surface feedbacks impart a significant degree of persistence between cool and warm season moisture availability in the central United States. However, the degree of correlation between these two variables is subject to major changes that appear to occur on decadal to multidecadal time scales, even in the relatively short 115-yr instrumental record. Tree-ring reconstructions have extended the limited observational record of long-term soil moisture levels, but such reconstructions do not resolve the seasonal differences in moisture conditions. We present two separate 331-yr-long seasonal moisture reconstructions for the central United States, based on sensitive subannual and annual tree-ring chronologies that have strong and separate seasonal moisture signals: an estimate of the long-term May soil moisture balance and a second estimate of the short-term June–August atmospheric moisture balance. The predictors used in each seasonal reconstruction are not significantly correlated with the alternate season target. Both reconstructions capture over 70% of the interannual variance in the instrumental data for the calibration period and also share significant decadal and multidecadal variability with the instrumental record in both the calibration and validation periods. The instrumental and reconstructed moisture levels are both positively correlated between spring and summer strongly enough to have potential value in seasonal prediction. However, the relationship between spring and summer moisture exhibits major decadal changes in strength and even sign that appear to be related to large-scale ocean–atmosphere dynamics associated with the Atlantic multidecadal oscillation.