Hydrologic Change in New Zealand During the Last Deglaciation Linked to Reorganization of the Southern Hemisphere Westerly Winds

Abstract The Southern Hemisphere westerly winds strongly influence deep ocean circulation and carbon storage1. While the westerlies are hypothesised to play a key role in regulating atmospheric CO2 over glacial-interglacial cycles2–4, past changes in their position and strength remain poorly constrained5–7. Here, we use a compilation of planktic foraminiferal δ18O from across the Southern Ocean and constraints from an ensemble of climate models to reconstruct changes in the westerlies over the last deglaciation. We find a 4.7° (2.9-6.9°, 95% confidence interval) equatorward shift and about a 25% weakening of the westerlies during the Last Glacial Maximum (about 20,000 years ago) relative to the mid-Holocene (about 6,000 years ago). Our reconstruction shows that the poleward shift in the westerlies over deglaciation closely mirrors the rise in atmospheric CO2. Experiments with a 0.25° resolution ocean-sea-ice-carbon model demonstrate that shifting the westerlies equatorward substantially reduces the overturning rate of the abyssal ocean, leading to a suppression of CO2 outgassing from the Southern Ocean. Our results establish a central role for the westerly winds in driving the deglacial CO2 rise, and suggest natural CO2 outgassing from the Southern Ocean is likely to increase as the westerlies shift poleward due to anthropogenic warming8–10.

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A palaeoenvironmental record of the Southern Hemisphere last glacial maximum from the Mount Cass loess section, North Canterbury, Aotearoa/New Zealand

Quaternary Research ◽

10.1017/qua.2020.95 ◽

2020 ◽

pp. 1-15

Author(s):

Peter C. Almond ◽

Sándor Gulyás ◽

Pál Sümegi ◽

Balázs P. Sümegi ◽

Stephen Covey-Crump ◽

...

Keyword(s):

New Zealand ◽

Southern Hemisphere ◽

Last Glacial Maximum ◽

Climatic Variability ◽

Glacial Maximum ◽

Pollen Record ◽

Good Correspondence ◽

Last Glacial ◽

Event Stratigraphy ◽

Westerly Winds

Abstract Calcareous loess in North Canterbury, eastern South Island, New Zealand (NZ), preserves subfossil bird bone, terrestrial gastropods, and eggshell, whose abundances and radiocarbon ages allowed us to reconstruct aspects of palaeoenvironment at high resolution through 25 to 21 cal ka BP. This interval includes millennial-scale climatic variability during the extended last glacial maximum (30–18 ka) of Australasia. Our loess palaeoclimatic record shows good correspondence with stadial and interstadial climate events of the NZ Climate Event Stratigraphy, which were defined from a pollen record on the western side of South Island. An interstade from 25.4 to 24 cal ka BP was warm but also relatively humid on eastern South Island, and loess grain size may indicate reduced vigour of the Southern Hemisphere westerly winds. The subsequent stade (24–22.6 cal ka BP) was drier, colder, and probably windier. The next interstade remained relatively dry on eastern South Island, and westerly winds remained vigorous. The 25.4–24 ka interstade is synchronous with Heinrich stade 2, which may have driven a southward migration of the subtropical front, leading to warming and wetting of northern and central South Island and retreat of Southern Alps glaciers at ca. 26.5 ka.

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Hydrographic shifts south of Australia over the last deglaciation and possible interhemispheric linkages

Quaternary Research ◽

10.1017/qua.2021.12 ◽

2021 ◽

pp. 1-12

Author(s):

Matthias Moros ◽

Patrick De Deckker ◽

Kerstin Perner ◽

Ulysses S. Ninnemann ◽

Lukas Wacker ◽

...

Keyword(s):

Southern Ocean ◽

Southern Hemisphere ◽

Critical Role ◽

Temperature Response ◽

South Australia ◽

Last Deglaciation ◽

Tight Coupling ◽

Oceanic Fronts ◽

The Last Deglaciation ◽

Circumpolar Current

Abstract Northern and southern hemispheric influences—particularly changes in Southern Hemisphere westerly winds (SSW) and Southern Ocean ventilation—triggered the stepwise atmospheric CO2 increase that accompanied the last deglaciation. One approach for gaining potential insights into past changes in SWW/CO2 upwelling is to reconstruct the positions of the northern oceanic fronts associated with the Antarctic Circumpolar Current. Using two deep-sea cores located ~600 km apart off the southern coast of Australia, we detail oceanic changes from ~23 to 6 ka using foraminifer faunal and biomarker alkenone records. Our results indicate a tight coupling between hydrographic and related frontal displacements offshore South Australia (and by analogy, possibly the entire Southern Ocean) and Northern Hemisphere (NH) climate that may help confirm previous hypotheses that the westerlies play a critical role in modulating CO2 uptake and release from the Southern Ocean on millennial and potentially even centennial timescales. The intensity and extent of the northward displacements of the Subtropical Front following well-known NH cold events seem to decrease with progressing NH ice sheet deglaciation and parallel a weakening NH temperature response and amplitude of Intertropical Convergence Zone shifts. In addition, an exceptional poleward shift of Southern Hemisphere fronts occurs during the NH Heinrich Stadial 1. This event was likely facilitated by the NH ice maximum and acted as a coup-de-grâce for glacial ocean stratification and its high CO2 capacitance. Thus, through its influence on the global atmosphere and on ocean mixing, “excessive” NH glaciation could have triggered its own demise by facilitating the destratification of the glacial ocean CO2 state.

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A 250-year periodicity in Southern Hemisphere westerly winds over the last 2600 years

Climate of the Past ◽

10.5194/cp-12-189-2016 ◽

2016 ◽

Vol 12 (2) ◽

pp. 189-200 ◽

Cited By ~ 22

Author(s):

C. S. M. Turney ◽

R. T. Jones ◽

C. Fogwill ◽

J. Hatton ◽

A. N. Williams ◽

...

Keyword(s):

Southern Hemisphere ◽

Global Climate Change ◽

Late Holocene ◽

Global Climate ◽

Falkland Islands ◽

The Core ◽

Atmosphere System ◽

Westerly Winds ◽

Ocean Atmosphere ◽

Southern Hemisphere Westerly Winds

Abstract. Southern Hemisphere westerly airflow has a significant influence on the ocean–atmosphere system of the mid- to high latitudes with potentially global climate implications. Unfortunately, historic observations only extend back to the late 19th century, limiting our understanding of multi-decadal to centennial change. Here we present a highly resolved (30-year) record of past westerly wind strength from a Falkland Islands peat sequence spanning the last 2600 years. Situated within the core latitude of Southern Hemisphere westerly airflow (the so-called furious fifties), we identify highly variable changes in exotic pollen and charcoal derived from South America which can be used to inform on past westerly air strength. We find a period of high charcoal content between 2000 and 1000 cal. years BP, associated with increased burning in Patagonia, most probably as a result of higher temperatures and stronger westerly airflow. Spectral analysis of the charcoal record identifies a pervasive ca. 250-year periodicity that is coherent with radiocarbon production rates, suggesting that solar variability has a modulating influence on Southern Hemisphere westerly airflow. Our results have important implications for understanding global climate change through the late Holocene.

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