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Oceanography Seminar - Lisa Hahn-Woernle

TITLE:Estimation of Upper Ocean Vertical Mixing from Surface Ocean Colour Observations

ABSTRACT: The growth of marine phytoplankton is tightly connected to the strength of upper ocean vertical mixing and the depth of the mixed layer. Deep vertical mixing supplies e.g. nutrients from deeper ocean water to the ocean surface. Weaker mixing and a shallow mixed layer loosen the light-limitation while the nutrient supply is reduced. Thanks to the chlorophyll-a (Chl a) pigments in the phytoplankton cells, changes in the phytoplankton concentration can be remotely tracked with satellite imagery. As central topic of my PhD thesis, the potentials of using ocean surface Chl a observations to estimate upper ocean vertical mixing for the Northern Atlantic are presented. A central tool for the estimation is a one-dimensional nutrient-phytoplankton (NP) model. To guarantee that the NP model represents the characteristic growth of the Northern Atlantic, biological model parameters are first calibrated to in situ observations of vertical Chl a and nutrient profiles. Next, the NP model is coupled to the k − ε turbulence model of the General Ocean Turbulence Model (GOTM) framework. The k − ε model is a common choice to simulate upper ocean turbulent processes. Yet, a comparison of in situ and satellite observations with modelling results shows that the turbulence model overestimates the vertical mixing. To improve the model results, the turbulence parameters of the k − ε model need to be calibrated. As a novel approach, the calibration of the turbulence parameters with surface Chl a concentrations will be presented. Measuring vertical mixing and Chl a concentrations in the open ocean is very time and cost intensive. As an alternative, a measurement campaign was organized at Lake Garda in Spring 2014 to explore the potentials of deep lakes for the study of daily to inter-seasonal and inter-annual variability caused by environmental factors like vertical mixing. The preliminary measurement results will be presented.

Event Sponsor
Dept of Oceanography, Mānoa Campus

More Information
Chris Measures, (808) 956-7633, ocean@soest.hawaii.edu,