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Abstract:
The atmosphere is strongly dependent on the boundary forcing from the ocean. The ocean circulation is affected by lunisolar tidal potential that induces long-term tidal residual mean currents and thereby influences the general ocean circulation. Through this process the advection of heat is altered in the ocean and, thus, the oceanic temperature distribution is modified. This alters mean sea surface conditions and thus the lower boundary conditions for sea ice and atmosphere. This means that both sea ice dynamics and the lower atmosphere are modified by oceanic tidal dynamics on climatological time-scales. Until now, most climate studies and all paleo-climate reconstructions have neglected the influence of tidal forcing on ocean and atmosphere dynamics.
In this thesis, the coupled atmosphere-ocean model ECHAM5/MPIOM is employed to simulate the climate of a pre-industrial period, the early Eocene, and the Albian. Furthermore, lunisolar tidal forcing is included in the ocean model MPIOM in order to study, for the first time, the effects of tidal dynamics on the ocean and the atmosphere in a paleo-climatic context.
The simulations of the early Eocene and the Albian reveal feedback mechanisms between sea ice and the heat flux from the ocean to the atmosphere that decrease the meridional temperature gradient. The modeled tidal system does not only depend on the lunisolar forcing, but is also influenced by geometric resonance conditions in the ocean that change on geological time-scales. The tidal system and the impact of tides on the ocean are therefore different for the pre-industrial period, the early Eocene, and the Albian. Impacts of tidal forcing include a local increase of multi-annual mean 2 m temperature by up to 4°C in the pre-industrial Weddell Sea, a doubling of velocities in most of the deep ocean during the early Eocene and a modification by more than 20% of the transport by the global meridional overturning circulation in the early Eocene and the Albian.
Tides influence multi-decadal mean ocean currents in the intermediate and deep ocean, the oceanic temperature distribution, the meridional overturning circulation, and the heat fluxes from the ocean to the atmosphere locally in the range of 10% to 200%. The impact of ocean tides on the climate system is therefore relevant for climate simulations of the past, present and future. Future research in climate modeling should hence further consider influences of ocean tides on the climate system.
