07.12.2016: SFB 754 - early career scientists colloquia

8:45 Uhr im Seminarraum, Düsternbrooker Weg 20

Daniel Whitt, Centre for Mathematical Sciences, University of Cambridge, UK: "How winds fuel phytoplankton growth at upper-ocean fronts"

Abstract:

Phytoplankton growth in the sunlit euphotic zone of the open ocean requires nutrients, which are transported to the euphotic zone by unsteady currents and turbulence. I will present results from a process study that builds conceptual understanding of the coupled physical-biogeochemical dynamics that result in upper-ocean phytoplankton distributions, which are beautifully manifest in satellite imagery of chlorophyll fluorescence. The analysis focuses on understanding why phytoplankton are more concentrated at surface fronts, where horizontal density gradients are sharp and currents are swift. In particular, mathematical and numerical models are used to explore: 1) how winds modify vertical velocities and turbulent mixing at fronts and 2) how wind-driven changes in vertical nutrient transport enhance phytoplankton growth at fronts. The first key contribution is a theory for the turbulent mixing layer depth that simultaneously incorporates wind-driven vertical advection and entrainment at a front and thereby concisely characterize these key physical ingredients that promote phytoplankton growth at fronts. The second key contribution is a numerical study of phytoplankton in a front forced by an oscillatory wind, which drives oscillatory vertical velocities that are consistent with Stern’s [1965] generalization of Ekman suction as well as mean vertical velocities that are significantly faster than predicted by Garrett and Loder’s [1981] theory of frictional circulation. The relative biogeochemical significance of different vertical transport processes and timescales at a front forced by realistic unsteady wind and possible applications of these results to observations will be discussed.

SFB 754