Ocean tidal effects on paleo-climate

Ocean bathymetry is changed on geological timescales by tectonic and volcanic activity. The modification of ocean basin geometry alters resonance conditions of ocean tides. We study the effect of changing ocean tides on the ocean general circulation and the atmosphere by forcing the coupled atmosphere-ocean model ECHAM5/MPIOM with the complete lunisolar tidal potential. We performed simulations for five tectonic and climatological important time-slices: the Early Albian (ca. 110 million years ago, Ma), the Cenomanian-Turonian Boundary (ca. 93 Ma, CTB), the Early Eocene (ca. 55 Ma), the Early Pliocene (ca. 3.5 Ma), and a pre-industrial period (ca. 1850 CE).

The largest global mean tidal potential energy is obtained for the Early Eocene, when it was almost three times larger than at the CTB. Ocean velocities are tripled in 10% of the Early Eocene deep ocean due to ocean tides. The influence of tidal forcing on the general ocean circulation is smaller in the other time-slices, but still amounts to changes of more than 20% in over 50% of the deep ocean (Weber and Thomas, 2017a). Although in the Early Eocene horizontal velocities are altered by tides and the meridional overturning circulation is strengthened, the meridional heat transport is not changed significantly (Weber and Thomas, 2017b).

Tidally enhanced vertical mixing and modified transports also affect the ocean-atmosphere energy exchange. This is most pronounced in the Pliocene and the pre-industrial simulations because a positive feedback loop develops between sea ice concentration, heat transfer between atmosphere and ocean, and atmospheric near surface temperatures. In the pre-industrial simulation sea ice concentration is reduced by up to 30% in the Weddell Sea and atmospheric 2m temperatures are increased by up to 4°C. 

Uncertainties in the reconstruction of bathymetry over millions of years limit the accuracy of our quantitative results. However, the qualitative conclusions suggest impacts of ocean tides on ocean dynamics in an order of magnitude that requires its consideration as additional uncertainty in the interpretation of paleoceanographic studies. 

The project has been funded for the period 2012 - 2016 by Deutsche Forschungsgemeinschaft (DFG) within the priority program SPP 1375 "South Atlantic Margin Processes and Links with onshore Evolution" (SAMPLE), project: "Reconstruction of atmosphere-ocean circulation patterns for geological time slices by self consistent coupled model simulations (RECOM)".

Publications

  • Uenzelmann-Neben, G., Weber, T., Gruetzner, J. & Thomas, M. Transition from the Cretaceous ocean to Cenozoic circulation in the western South Atlantic — A twofold reconstruction. Tectonophysics 716, 225–240 (2017).
  • Weber, T. Impact of ocean tides on the climate system during the pre-industrial period, the early Eocene, and the Albian. PhD thesis, Department of Earth Sciences, Freie Universität Berlin (2016). 
  • Weber, T. & Thomas, M. Tidal dynamics and their influence on the climate system from the Cretaceous to present day. Glob. Planet. Change 158, 173–183 (2017a).
  • Weber, T. & Thomas, M. Influence of ocean tides on the general ocean circulation in the early Eocene. Paleoceanography 32, 553-570 (2017b).