A Climatology of Chlorophyll Fluorescence in the California Current Region

The California Underwater Glider Network (CUGN) has been monitoring the coast of California since 2006 with Spray underwater gliders that typically dive down to 500 m, resulting in profiles approximately 3 km apart every 3 hours. The fluorometer onboard Spray emits blue light with wavelengths of approximately 470 nm and measures wavelengths of 685 nm, which allows for estimating chlorophyll fluorescence. However, differences between sensors from mission to mission pose a challenge in producing continuous and consistent long-term measurements of fluorescence from underwater gliders. In order to address this issue, here we propose a calibration model consisting of an offset and a gain to correct the differences between remotely sensed chlorophyll estimates from MODIS-Aqua and the Spray glider chlorophyll fluorescence. Our calibration model is applied to over 100,000 profiles from the CUGN along three recurrent California Cooperative Oceanic Fisheries Investigations (CalCOFI) lines from 2006 to the present. Then, we construct a climatology of chlorophyll fluorescence that is consistent with MODIS-Aqua, which may provide insight into the vertical structure of chlorophyll, complementing the near-surface satellite estimates. Because the method proposed here uses widely available satellite data, this approach may be generalized for other autonomous profiling devices.