16 March 2018

Timescales of Southern Ocean Upwelling

A comparison of Lagrangian upwelling timescales in a hierarchy of model resolutions.

ABC
Science

In this paper, we use three ocean models with varying resolution to investigate how long it takes for deep ocean waters to upwell to the surface of the Southern Ocean around Antarctica. We find that this journey takes 87 years in a standard coarse-resolution climate model, but only 17 years in a state of the art high-resolution climate model. The difference between the timescales is due to eddies which contribute strongly to the upwelling, but are too small to be represented in standard climate models.

Impact

The model differences suggest that upwelling timescales and interbasin merging of deep waters may be overestimated by coarse-resolution models. This impacts the skill of centennial-scale climate change projections, as it may be possible for heat currently being injected into the deep North Atlantic to reemerge around the Antarctic ice sheet faster than previously thought.

Summary

In this paper, we study upwelling pathways and timescales of Southern Ocean deep waters in a hierarchy of models using a Lagrangian particle tracking method. Lagrangian timescales of upwelling decrease from 87 years to 31 years to 17 years as the ocean resolution is refined from 1° to 0.25° to 0.1°. We attribute some of the differences in timescale to the strength of the eddy fields, as demonstrated by temporally degrading the high-resolution model velocity fields. Consistent with the timescale dependence, we find that an average Lagrangian particle completes 3.2 circumpolar loops around Antarctica in the 1° model in comparison to 0.9 loops in the 0.1° model. These differences suggest that advective timescales and thus interbasin merging of upwelling deep waters may be overestimated by coarse-resolution models, potentially affecting the skill of centennial-scale climate change projections.

Contact
Adele Morrison
Australian National University
Publications
Drake, H, A Morrison, S Griffies, J Sarmiento, W Weijer, and A Gray.  2018.  "Lagrangian Timescales of Southern Ocean Upwelling in a Hierarchy of Model Resolutions."  Geophysical Research Letters 48: 891-898.  https://doi.org/10.1002/2017gl076045.