Orbital signals in the diurnal cycle of radiation

Abstract

Time of day plays a critical role in determining the dominance of the Earth's orbital parameters involving eccentricity, precession, and obliquity. Extension of orbital radiation data to the diurnal cycle permits the recognition of well‐defined, temporally separate signals corresponding to all three major orbital parameters and to combinations of these parameters. Past research involving computed radiation changes at the daily and seasonal scales has not shown significant eccentricity cycles. Results of this study show: (1) at the solstices, a marked change in dominance from precession (for the solar elevation classes associated with early morning/late afternoon) to obliquity (for the elevation class containing solar noon) occurs in solar radiation; (2) at the equinoxes, cycles corresponding to those associated with eccentricity are evident in the radiation data at all individual time intervals of the day. Past studies have shown that the daily radiative total reflects only the influence of precession. A third set of cycles at the equinoxes centered near 11 kyr corresponds to a combination of orbital signals. These results imply that paleoclimatic proxy data displaying strong spectral peaks corresponding to eccentricity (100 kyr) and obliquity (41 kyr), such as records of global ice volume, may be related to radiation changes occurring in the solar elevation class containing solar noon, while paleoclimatic proxy data displaying strong spectral peaks corresponding to precession, for example, sea‐surface temperatures, appear to be showing the dominance of precession on the daily total radiation.