The Driving Mechanisms on Southern Ocean Upwelling Change during the Last Deglaciation

Wei Liu,Zhengyu Liu,Shouwei Li

doi:10.3390/geosciences11070266

Wei Liu, Zhengyu Liu + Show 1 more

Open Access

https://doi.org/10.3390/geosciences11070266

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Abstract

We explore the change in Southern Ocean upwelling during the last deglaciation, based on proxy records and a transient climate model simulation. Our analyses suggest that, beyond a conventional mechanism of the Southern Hemisphere westerlies shift, Southern Ocean upwelling is strongly influenced by surface buoyancy forcing and the local topography. Over the Antarctic Circumpolar Current region, the zonal mean and local upwelled flows exhibited distinct evolution patterns during the last deglaciation, since they are driven by different mechanisms. The zonal mean upwelling is primarily driven by surface wind stress via zonal mean Ekman pumping, whereas local upwelling is driven by both wind and buoyancy forcing, and is tightly coupled to local topography. During the early stage of the last deglaciation, the vertical extension of the upwelled flows increased downstream of submarine ridges but decreased upstream, which led to enhanced and diminished local upwelling, downstream and upstream of the submarine ridges, respectively.

Highlights

Paleo-proxy reconstructions have shown dramatic changes in the Southern Ocean upwelling during the last deglaciation, which potentially contribute to the deglacial rise of atmospheric CO2 [1,2,3,4,5]
Combining paleo-proxy records and a simulation of the Transient Climate of the Last 21,000 Years (TraCE-21ka) [33], we show that, besides the “westerlies shift" hypothesis, several alternative mechanisms, such as freshwater forcing and topography effects, can explain the changes in Southern Ocean upwelling during the last deglaciation
We combined paleo-proxy observations and a fully coupled climate model simulation, TraCE-21ka, to analyze the change in Southern Ocean upwelling during the last deglaciation

Summary

Introduction

Paleo-proxy reconstructions have shown dramatic changes in the Southern Ocean upwelling during the last deglaciation, which potentially contribute to the deglacial rise of atmospheric CO2 [1,2,3,4,5]. During the LGM, Antarctic sea ice, which modulates the meridional and vertical gradients of atmosphere temperature, greatly expanded, counteracting the equatorward tendency of wind shifts in the colder climate and bringing out a slight poleward displacement of the Southern Hemisphere westerlies [28,29]. Combining paleo-proxy records and a simulation of the Transient Climate of the Last 21,000 Years (TraCE-21ka) [33], we show that, besides the “westerlies shift" hypothesis, several alternative mechanisms, such as freshwater forcing and topography effects, can explain the changes in Southern Ocean upwelling during the last deglaciation. Our mechanisms are built on Southern Ocean dynamics [34,35] in which the bottom topography in the Southern Ocean can strongly affect the wind-driven upwelling by undermining the barotropic structure of vertical flows This topographic effect, is largely dependent on deep ocean stratification.

Model and Approaches

Southern Ocean Upwelling Indicated by Proxy Records and in TraCE-21ka

The Mechanism of Local Ocean Upwelling Changes in the Southern Ocean

Conclusions and Discussion