Determining Upper Ocean Vertical Mixing from Surface Chlorophyll-a Concentrations
Determining Upper Ocean Vertical Mixing from Surface Chlorophyll-a Concentrations
Abstract:
Upper ocean vertical mixing is a key driver for the exchange of e.g. heat and greenhouse gases between the ocean and the atmosphere. Numerical turbulence models are used to study and analyse processes involved in the vertical mixing. However, to close the differential equations within these models, a number of parameters have to be determined empirically based on observations or laboratory experiments. Observations are sparse, since they need to be derived from microscale in-situ measurements, and laboratory experiments may not represent all processes relevant for upper ocean vertical mixing.
In this study, remotely sensed ocean surface observations are used to calibrate parameters of the one-dimensional General Ocean Turbulence Model (GOTM) to specific locations in the North Atlantic. For the calibration, a time-series of MODIS Aqua surface chlorophyll-a is assimilated with GOTM coupled to a Nutrient-Phytoplankton model (GOTM-NP). This new approach takes advantage of the strong link between upper ocean stratification and biogeochemical processes leading to rapid changes in surface chlorophyll-a concentrations, so-called phytoplankton blooms. The timing of the bloom is controlled by the shoaling of the mixed layer, while its intensity depends on the resupply of nutrients to the surface layer by deep winter mixing prior to the bloom.
The presentation focusses on the potentials and limitations of using surface chlorophyll-a observations to calibrate parameters in the GOTM-NP. Model results based on a calibration to North Atlantic MODIS Aqua data are evaluated with in-situ observations of upper ocean vertical mixing.
In this study, remotely sensed ocean surface observations are used to calibrate parameters of the one-dimensional General Ocean Turbulence Model (GOTM) to specific locations in the North Atlantic. For the calibration, a time-series of MODIS Aqua surface chlorophyll-a is assimilated with GOTM coupled to a Nutrient-Phytoplankton model (GOTM-NP). This new approach takes advantage of the strong link between upper ocean stratification and biogeochemical processes leading to rapid changes in surface chlorophyll-a concentrations, so-called phytoplankton blooms. The timing of the bloom is controlled by the shoaling of the mixed layer, while its intensity depends on the resupply of nutrients to the surface layer by deep winter mixing prior to the bloom.
The presentation focusses on the potentials and limitations of using surface chlorophyll-a observations to calibrate parameters in the GOTM-NP. Model results based on a calibration to North Atlantic MODIS Aqua data are evaluated with in-situ observations of upper ocean vertical mixing.