Numerical Reevaluation of the Tide-induced 18.6-year Climate Variability over the Pacific using a Climate Model with an Eddy-permitting OGCM

Hiroaki Tatebe, JAMSTEC Japan Agency for Marine-Earth Science and Technology, Research Center for Environmental Modeling and Application, Yokohama, Japan, Takao Kawasaki, Atmosphere and Ocean Research Institute, the University of Tokyo, Kashiwa, Chiba, Japan, Yuki Tanaka, The University of Tokyo, Graduate School of Science, Tokyo, Japan, Ichiro Yasuda, AORI, University of Tokyo, Kashiwa, Japan and Hiroyasu Hasumi, Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Japan
Tidal forcing amplitudes of the major diurnal and semidiurnal constituents are modulated by the 18.6-year periodicity due to the lunar orbital fluctuation around the earth, which probably induce the associated modulation of tide-induced oceanic mixing due to internal wave breaking. It has been hypothesized that the 18.6-year oscillation of the tide-induced mixing might influence spatio-temporal characteristics of basin-scale climate variability. Previous observational studies reported several-hundred-year-long proxy data collected in coastal regions of the Pacific basin show significant periodic 18.6-year oscillation. As for climate modeling approach, a few studies where the tide-induced 18.6-year oscillation of ocean mixing is employed represented significant tidal regulation of the Pacific climate variability. However, whether the tidal signal in proxy data reflects basin-scale climate variability is equivocal, and there is no consensus on mechanisms linking the tide and climate variations in numerical experiments because of uncertainties in parameterizing tidal mixing as well as climate modeling, for example, lack of oceanic eddies and fronts in the mid-latitudes. Furthermore, significant bidecadal variability in basin-wide instrumental data during the 20th century, which overlaps the 18.6-year periodicity, has been long recognized and it is considered to occur intrinsically even without the tidal oscillations. In the present study, using a state-of-the-art climate model with an eddy-permitting OGCM, possible relationship between the 18.6-year tidal oscillation and Pacific climate variability is reevaluated in order to address the questions described above. Currently, about 200-year-long numerical simulations are finished, and different representation of Pacific climate variability is obtained in the case with or without the tidal oscillation. Detailed analysis will be presented in our talk.