Interpreting intraseasonal variability of subsurface tracers observed by a profiling float

Recent developments in autonomous biogeochemical instruments can provide new opportunities for investigating biogeochemical variability at unprecedented temporal resolutions. Early studies indicate the importance of relatively rapid and small-scale processes on biogeochemical variability. This analysis focuses on a profiling float deployed in the eastern subpolar North Pacific, showing significant intraseasonal variability in potential density, oxygen and nitrate. We find substantial variability for all tracers in the main thermocline at a depth of about 150 m, indicating a common mechanism. A strong linear correlation between the intraseasonal variability of isopycnal oxygen and nitrate (on σ θ=26.5 surface) with isopycnal spiciness indicates the role of physical transport and mixing. Power spectrum analysis shows a statistically significant spectral peak of about 1/(18 days) for observed tracers in the main thermocline. This high-frequency variability does not show any significant relationship with independent satellite measures of relevant physical and biological properties. With approximately 5 day sampling periods, this spectral peak could be produced by aliasing of the inertial and tidal frequencies, rather than true intraseasonal variability. The low-frequency component (<30 days) shows the spectral slope of ω^-2 consistent with the stochastic null hypothesis. The growing number of autonomous biogeochemical observations will likely open up considerable opportunities for further research, and the analytical approaches in this paper will be useful for a further analysis of temporal variability of biogeochemical tracers.