Impact of deep ocean mixing on the climatic mean state in the Southern Ocean

Hiroaki Tatebe,Yoshiki Komuro,Yuki Tanaka,Hiroyasu Hasumi

doi:10.1038/s41598-018-32768-6

Abstract

The Southern Ocean is of great importance for the global stratification and biological carbon storage because it is connected to the global ocean conveyor by which atmospheric information absorbed in the Southern Ocean is redistributed globally and buffered over centuries. Therefore, understanding what controls the Southern Ocean climate, the global ocean conveyor, and links between them is a key to quantifying uncertainties in future climate projections. Based on a set of climate model experiments, here we show that the tide-induced micro-scale mixing in the Pacific deep ocean has significant impacts on the wintertime Southern Ocean climate through basin-scale reorganization of ocean stratification and resultant response of the global ocean conveyor. Specifically, Pacific deep water, which is modified by the deep ocean mixing while travelling south, reinforces the subsurface stratification and suppresses deep convection in the Southern Ocean. Resultant increase of the Ross Sea sea-ice leads to decrease of incoming shortwave radiation and strengthening of the westerly and storms. Because the Southern Ocean could regulate the global warming progress through its role as heat and carbon sink, our study implies that better representation of deep ocean mixing in climate models contributes to reliability improvement in regional-to-global climate projections.

Highlights

The Southern Ocean dominates the global ocean heat and carbon uptake through wintertime mixed layer deepening and the resultant subduction of surface waters to the thermocline and intermediate layers[1,2,3]
The Southern Ocean dominates the global ocean heat and carbon uptake; a better understanding for the formation processes of the Southern Ocean climate is essential for reducing uncertainty in transient climate response (TCR) to increasing greenhouse gas emissions[42], and the global warming projections[43,44]
Many of current climate models are suffering from persistent overestimation of incoming solar radiation (ISR) and associated underestimation of sea-ice area in the Southern Ocean

Summary

Introduction

The Southern Ocean dominates the global ocean heat and carbon uptake through wintertime mixed layer deepening and the resultant subduction of surface waters to the thermocline and intermediate layers[1,2,3]. Poor representations of mixed layer depths and open ocean deep convection are well-known issues in global ocean modeling, and they are due to lack of mesoscale processes in the Antarctic Circumpolar Current[14,15], uncertainties in surface fluxes[16], and other missing physics in the ocean Another issue is overestimation of incoming solar radiation (ISR) mainly due to cloud radiative processes in atmospheric modeling[17,18]. In the Pacific, this tide-induced mixing causes buoyancy-forced basin-wide upwelling of Circumpolar Deep Water (CDW)[22,23] This upwelling and associated southward flow directed to the Southern Ocean, which compose the Pacific MOC and constitute a part of the global ocean conveyor. A special emphasis is laid on the representation of deep ocean mixing in climate models, which can be a crucial factor for quantifying uncertainty in the driving processes of the Southern Ocean climate

Methods

Results

Conclusion