Can variations of the moon's orbit around the earth affect climate? Numerous studies have associated bidecadal variability in observations of physical climate and biogeochemical variables with the LNC. Particularly in and around the North Pacific, where diurnal tide constituents are strong, these observations point towards a significant influence of the LNC on temperatures, oceanic and atmospheric circulation and biogeochemical cycles. Model simulations without LNC forcing show strong internal modes of decadal variability in the North Pacific such as the Pacific Decadal Oscillation. LNC forcing could modulate these internal modes by (1) shifting variability into the bidecadal frequency range, (2) generate resonance, or (3) synchronize their phases. This project will test hypotheses relating LNC oscillations in tides to bidecadal variability in climate via modulations of ocean mixing and attendant effects on temperatures and modes of decadal climate variability.
A globally consistent parameterization of tidal mixing, that has already been tested in a climate model of intermediate complexity and shown to lead to climatically significant sea surface temperature anomalies, will be implemented in a comprehensive global climate model. This parameterization includes a new scheme of sub-grid scale bathymetry and separately considers four tidal constituents (M2, S2, K1 and O1) and their temporal variations due to the LNC. The model will be used to simulate the effects of LNC oscillations of tidal mixing on diapycnal diffusivities, temperatures, oceanic and atmospheric circulation, and modes of decadal variability in the North Pacific. Model results will be analyzed and compared to existing observations. Satellite altimeter data, now available over one full LNC, will be analyzed with the goal to improve tidal energy dissipation estimates, tidal mixing parameterizations, and the understanding of mechanisms of barotropic energy loss.
Broader Impacts: Understanding forcing mechanisms of climate change is critically important for decadal climate predictability. In recent decades much progress has been made in identifying and incorporating effects of aerosols, solar and volcanic variability in climate models. However, despite a considerable literature describing observations of the 18.6-year lunar nodal cycle in various climate variables its effect is not included in climate models. This project has the potential to improve climate predictions on decadal time scales particularly in the North Pacific region. Implementing the effect of the lunar nodal cycle on ocean mixing in CCSM4, which is one of the models used by the IPCC assessment reports, will contribute to these international reports of significant policy relevance. The model code will be made available to the large user community. Decadal variability in the North Pacific affects ecosystems and societies. Improved predictability of hypoxic events on the continental shelf of the west coast of North America, for example, may benefit society as these events affect living systems in the California Current Large Marine Ecosystem and many commercially important fisheries in the region. Collaboration between a team of three PIs with different expertise, background (global climate modeling, tides, coupled physical- biological regional ocean modeling) and gender diversity will be fostered. A postdoctoral researcher will be trained in climate modeling and data analysis and given teaching opportunities. An undergraduate student will participate through a summer internship. A session on decadal variability in the North Pacific will be organized at the 2014 Fall Meeting of the American Geophysical Union.