Annual periodicity in ocean color observations across diverse oceanic regions.

The annual periodicity present in spectral remote sensing reflectances (R_rs(λ)) derived from ocean color satellite data processed by NASA's Ocean Biology Processing Group has been shown to diverge from that observed in situ, with the source of the discrepancies remaining unresolved. Here, we investigate such periodicity in reflectances derived from four satellite sensors and seven in situ locations. Periodicity in the time series of R_rs(λ) spectra is examined without prior assumption of the main constituent frequency through application of the floating-mean Lomb-Scargle periodogram. We show that a one-term sinusoidal function specified at the annual frequency with a constant offset reproduces seasonal trends observed at various regional oceanic sites. Although unique regional features in mean, amplitude, and phase are generally well-captured by R_rs(λ) derived from satellite measurements, discrepancies perpetuate the annual frequency into the satellite-to-in situR_rs(λ) matchup difference. Specifying the origin of the discrepancy is challenging because the annual frequency is significantly contained in time series of many physical measurements, corrections, and ancillary data required for R_rs(λ) retrievals. Until differences in amplitude and phase between satellite and in situR_rs(λ) time series are reduced, improved uncertainty estimates should be incorporated into downstream product analyses.