A South Atlantic island record uncovers shifts in westerlies and hydroclimate during the last glacial

Svante Björck,Hui Jiang,Jayne E Rattray,Jesper Sjolte,Thomas F Stocker,Sofia Holmgren,Malin E Kylander,Yamoah K K Afrifa,Raimund Muscheler,Karl Ljung,Florian Adolphi,Roger Flower,Nathalie Van Der Putten,Rienk H Smittenberg

doi:10.5194/cp-15-1939-2019

Abstract

Abstract. Changes in the latitudinal position and strength of the Southern Hemisphere westerlies (SHW) are thought to be tightly coupled to important climate processes, such as cross-equatorial heat fluxes, Atlantic Meridional Overturning Circulation (AMOC), the bipolar seesaw, Southern Ocean ventilation and atmospheric CO2 levels. However, many uncertainties regarding magnitude, direction, and causes and effects of past SHW shifts still exist due to lack of suitable sites and scarcity of information on SHW dynamics, especially from the last glacial. Here we present a detailed hydroclimate multiproxy record from a 36.4–18.6 kyr old lake sediment sequence on Nightingale Island (NI). It is strategically located at 37∘ S in the central South Atlantic (SA) within the SHW belt and situated just north of the marine Subtropical Front (SF). This has enabled us to assess hydroclimate changes and their link to the regional climate development as well as to large-scale climate events in polar ice cores. The NI record exhibits a continuous impact of the SHW, recording shifts in both position and strength, and between 36 and 31 ka the westerlies show high latitudinal and strength-wise variability possibly linked to the bipolar seesaw. This was followed by 4 kyr of slightly falling temperatures, decreasing humidity and fairly southerly westerlies. After 27 ka temperatures decreased 3–4 ∘C, marking the largest hydroclimate change with drier conditions and a variable SHW position. We note that periods with more intense and southerly-positioned SHW seem to be related to periods of increased CO2 outgassing from the ocean, while changes in the cross-equatorial gradient during large northern temperature changes appear as the driving mechanism for the SHW shifts. Together with coeval shifts of the South Pacific westerlies, our results show that most of the Southern Hemisphere experienced simultaneous atmospheric circulation changes during the latter part of the last glacial. Finally we can conclude that multiproxy lake records from oceanic islands have the potential to record atmospheric variability coupled to large-scale climate shifts over vast oceanic areas.

Highlights

IntroductionThe lack of climate proxy records directly reflecting atmospheric conditions in the central South Atlantic means that such information at these latitudes during the glacial are primarily based on remote proxy records or climate model simulations
The Southern Hemisphere westerlies (SHW) are a major determinant of hydroclimate in the Southern Hemisphere (SH)
The lower 162 cm consists of a dark-brown slightly silty gyttja, overlain by a gray-brown silty clay gyttja, all deposited under anaerobic conditions

Summary

Introduction

The lack of climate proxy records directly reflecting atmospheric conditions in the central South Atlantic means that such information at these latitudes during the glacial are primarily based on remote proxy records or climate model simulations. This results in a largely unconstrained understanding of glacial conditions over vast parts of the mid-South Atlantic, especially between 20 and 50◦ S where archives reflecting atmospheric processes are absent. We explore links between past SHW strength and atmospheric CO2 For these questions the Tristan da Cunha archipelago is uniquely situated in the South Atlantic (Fig. 1a)

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