Gateway-driven weakening of ocean gyres leads to Southern Ocean cooling

Isabel Sauermilch,David R Munday,Katharina Hochmuth,Peter K Bijl,Joseph H Lacasce,Andreas Klocker,Joanne M Whittaker

doi:10.1038/s41467-021-26658-1

Abstract

Declining atmospheric CO2 concentrations are considered the primary driver for the Cenozoic Greenhouse-Icehouse transition, ~34 million years ago. A role for tectonically opening Southern Ocean gateways, initiating the onset of a thermally isolating Antarctic Circumpolar Current, has been disputed as ocean models have not reproduced expected heat transport to the Antarctic coast. Here we use high-resolution ocean simulations with detailed paleobathymetry to demonstrate that tectonics did play a fundamental role in reorganising Southern Ocean circulation patterns and heat transport, consistent with available proxy data. When at least one gateway (Tasmanian or Drake) is shallow (300 m), gyres transport warm waters towards Antarctica. When the second gateway subsides below 300 m, these gyres weaken and cause a dramatic cooling (average of 2–4 °C, up to 5 °C) of Antarctic surface waters whilst the ACC remains weak. Our results demonstrate that tectonic changes are crucial for Southern Ocean climate change and should be carefully considered in constraining long-term climate sensitivity to CO2.

Highlights

Declining atmospheric CO2 concentrations are considered the primary driver for the Cenozoic Greenhouse-Icehouse transition, ~34 million years ago
During the Early Cenozoic, the Earth underwent one of the most fundamental global climate changes known in geological history, from hot Greenhouse conditions (~52–34 million years ago, Ma) to cold Icehouse conditions (
In addition to the main proposed driving processes, other processes are thought to provide crucial pre-conditions, or contribute to the global cooling, including: Antarctic glacial expansion leading to enhanced westerlies, Southern Ocean deep-water formation and consequent benthic cooling[9,10]; and the initiation and/or strengthening of the Atlantic Meridional Overturning Circulation leading to deepwater formation and CO2 drawdown[11,12]

Summary

Introduction

Declining atmospheric CO2 concentrations are considered the primary driver for the Cenozoic Greenhouse-Icehouse transition, ~34 million years ago. A role for tectonically opening Southern Ocean gateways, initiating the onset of a thermally isolating Antarctic Circumpolar Current, has been disputed as ocean models have not reproduced expected heat transport to the Antarctic coast. A long, gradual cooling from ~52 Ma culminated in a dramatic decline of global mean surface[2] and deep[1] ocean temperatures, the expansion of continent-wide glaciers in Antarctica[3,4], and the start of a circum-Antarctic sea ice ecosystem[5], around 34–33 Ma. Understanding the key mechanisms driving this transition remains difficult, as several potential triggering events occurred around this time period, and only sparse geological records exist, for the Eocene–Oligocene transition (EOT, ~34 Ma). In addition to the main proposed driving processes, other processes are thought to provide crucial pre-conditions, or contribute to the global cooling, including: Antarctic glacial expansion leading to enhanced westerlies, Southern Ocean deep-water formation and consequent benthic cooling[9,10]; and the initiation and/or strengthening of the Atlantic Meridional Overturning Circulation leading to deepwater formation and CO2 drawdown[11,12]

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Results