Abstract
Abstract. Phytoplankton patchiness has been investigated with multifractal analysis techniques. We analyzed oceanic chlorophyll maps, measured by the SeaWiFS orbiting sensor, which are considered to be good proxies for phytoplankton. The study area is the Senegalo-Mauritanian upwelling region, because it has a low cloud cover and high chlorophyll concentrations. Multifractal properties are observed, from the sub-mesoscale up to the mesoscale, and are found to be consistent with the Corssin-Obukhov scale law of passive scalars. This result indicates that, in this specific region and within this scale range, turbulent mixing would be the dominant effect leading to the observed variability of phytoplankton fields. Finally, it is shown that multifractal patchiness can be responsible for significant biases in the nonlinear source and sink terms involved in biogeochemical numerical models.
Highlights
It is sometimes argued that turbulent mixing leads to homogeneous fields
Studies remained confined to second-order moments, such as the slope of the power spectrum, whereas more recent research takes into account the intermittent transfer of conservative quantities in scale space, such as energy and scalar variance, which give rise to multifractal statistics through cascade processes (Seuront et al, 1996a, b, 1999; Seuront and Schmitt, 2004, 2005a, b; Lovejoy et al, 2001a, b; Pottier et al, 2008)
The aim of the present study is to analyze oceanic chlorophyll maps obtained through the use of this type of sensor
Summary
It is sometimes argued that turbulent mixing leads to homogeneous fields. Kolmogorov (1941), Obukhov (1949) and Corssin (1951) have shown that, on the contrary, turbulent mixing generates highly irregular structures that are heterogeneous at all scales. In the case where the physical quantity is the concentration of a passive tracer, these authors demonstrated that its variability exhibits fractal properties which can be described statistically using scale laws. This result is often referred to as the theory of passive scalars. Phytoplankton patchiness is strongly related to turbulence. This consequence has led numerous authors to study the scale invariance properties of phytoplankton patches, and to confront experimental data with phenomenological models derived from, or inspired by, the theory of passive scalars. Studies remained confined to second-order moments, such as the slope of the power spectrum (see, e.g., Platt, 1972), whereas more recent research takes into account the intermittent transfer of conservative quantities in scale space, such as energy and scalar variance, which give rise to multifractal statistics through cascade processes (Seuront et al, 1996a, b, 1999; Seuront and Schmitt, 2004, 2005a, b; Lovejoy et al, 2001a, b; Pottier et al, 2008)
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