Biogeochemical patchiness at the sea surface

Abstract

The surface distributions of many tracers in the ocean are highly correlated in time and space on meso (∼100 km) and smaller scales (Figure 1). However, their characteristic scales of variability differ. Some variables like sea surface chlorophyll (Chl) are very fine‐scaled or patchy, while others like sea surface temperature (SST) are not. We characterize the patchiness of a distribution quantitatively by the dependence of the variance V on the length scale L as V ∼ Lp; smaller p corresponds to greater patchiness. Using scaling and a numerical model we show that patchiness, p, varies with the characteristic response time τ of the tracer to processes that alter its concentration in the upper ocean as p ∼ log τ. This suggests that sea surface Chl is more patchy (has smaller p) than SST at mesoscales because the characteristic time scale of phytoplankton growth in response to the availability of nutrients is less than that for the equilibration of temperature in response to heat fluxes. Similarly, sea surface dissolved oxygen (O2) exhibits more fine‐scaled variability than total dissolved inorganic carbon (TCO2) because O2 equilibrates with the atmosphere much more rapidly than TCO2. Tracers that are more patchy require higher resolution to model and sample; the sampling or model grid spacing required scales as exp(−1/log τ). The quantitative relationship between p and τ can be used to relate various biogeochemical distributions, particularly to those that are remotely sensed, and to deduce biogeochemical response times of various tracers or plankton species from the characteristics of their distributions in space or time.