1) Tidal mixing was likely greater in the glacial ocean during the Last Glacial Maximum (LGM) around 20,000 years ago compared to present day due to different ocean bathymetry from large ice sheets. 2) Increased tidal mixing in climate model simulations of the LGM leads to a stronger Atlantic Meridional Overturning Circulation (AMOC) and more vigorous Antarctic Bottom Water (AABW) formation. 3) The stronger AMOC and AABW in the model simulations with increased LGM tidal mixing improves agreement with sediment core data on carbon and oxygen isotopes.

1. Gordon Research Conference Ocean Mixing June 8, 2022
Tidal Mixing in the Glacial Ocean
Andreas Schmittner

2. Last Glacial
Maximum (LGM)
20,000 years ago
Earth was
- colder (~5K)
- more ice sheets
- sea level lower (120 m)
Tierney et al. (2020)

3. Tidal Energy Dissipation
Present Day
Shallow
~1.5 TW
Deep
~1.5 TW
Jayne & St Laurent (2001)
Egbert & Ray (2003)
Egbert et al. (2004)

4. Tidal Energy Dissipation
Last Glacial Maximum
Shallow
~0 TW
Deep
~3 TW
Jayne & St Laurent (2001)
Egbert & Ray (2003)
Egbert et al. (2004)

5. OCE-1559153
Sophie Wilmes
Bangor University
Questions
• Was vertical mixing di
ff
erent during the LGM?
• Did it a
ff
ect AMOC?
• Do sediment data provide constraints?
(2021)
(2019)

6. Methods
Tide Model Simulations
• OTIS (Oregon State Tidal Inversion
Software)
• M2, S2, O1, K1
• Di
ff
erent resolutions (up to 1/12°)
• Di
ff
erent Internal Tide (IT) Drag
Parameterizations
• Di
ff
erent LGM bathymetries (ICE5G,
ICE6G)
Climate Model Simulations
• Oregon State University version of
University of Victoria model (OSU-UVic)
• Tidal Mixing Parameterization (Jayne, St.
Laurent, Simmons)
• Model of Ocean Biogeochemistry &
Isotopes (MOBI) includes paleo tracers
δ13C, and radiocarbon
• Vary Southern Ocean buoyancy
fl
uxes
• Standard LGM Boundary Conditions (ice
sheets, greenhouse gas concentr., orbit)

7. Zaron &
Egbert
(2006)
Jayne &
St. Laurent
(2001)
IT drag
param.
Tidal Dissipation
Wilmes et al., (2019)
LGM-PD
M2+S2+K1+O1
z > 500m

8. =
Tidal Mixing Parameterization
Diapycnal Di
ff
usivity:
Subgrid-scale bathymetry:
Schmittner & Egbert (2014) Geosc. Mod. Devel.
Dissipation E
ffi
ciency = Fraction
of locally dissipated energy
Considers only changes in
locally dissipated energy, which
is only 1/3 of the total!

9. Reduced
atmospheric
meridional
moisture
fl
ux in
Southern
Hemisphere
Saltier AABW,
shallower &
weaker AMOC
Wilmes et al., (2021)

10. (in the model)
LGM tidal mixing
• Increases kv
• Increases AMOC
• Increases AABW
Atlantic 30S-30N
10
Wilmes et al., (2021)

11. Radiocarbon
Sediment Data Compilation from
Skinner et al. (2017)
~250 measurements from ~120 sites

12. δ13C
Sediment Data Compilation from
Peterson et al. (2017)
~400 sites mostly Atlantic

13. Atlantic Pro
fi
les
Radiocarbon Age δ13C

14. δ13C
Radiocarbon
Age

15. Conclusions
• Increased tidal mixing in LGM is robust result, but quantitatively depends on
reconstructed bathymetry (basin geometry; land ice extent)
• Increased di
ff
usivities increase AMOC & AABW
fl
ow rates
• AMOC strongly a
ff
ects isotopes
• E
ff
ect of increased mixing is more subtle but improves model-data
agreement
• All these results are conservative because they neglect changes in remotely
dissipated energy
• To Do: include remotely dissipated energy in future simulations e.g. by using Eden & Olbers
parameterization (IDEMIX)

16. Conclusions
• Feedback (climate <-> mixing) is probably small
• Even large increase in tidal dissipation (doubling - tripling) leads only to
modest increases in AMOC (1-3 Sv) and AABW (1-2 Sv)
• (Except when AMOC is close to threshold)
• E
ff
ects of changes in surface buoyancy
fl
uxes are much larger
• Paleo simulations should include changes in tidal mixing