Vertical mixing derived from surface chlorophyll-a concentrations of the North Atlantic Ocean

Abstract

Vertical mixing is thought to play an essential role in phytoplankton blooms, yet measurements of mixing properties are very sparse. This paper presents a methodology to estimate profiles of the upper-ocean vertical mixing from satellite color observations, using a coupled turbulence-phytoplankton model and data assimilation-based calibration techniques. The method is tested at a location in the eastern North Atlantic for which an integrated set of observations (vertical mixing, phytoplankton, nutrients) is available. Results of identical twin experiments show that the method is very robust and achieves accurate turbulence model parameter calibrations even with noisy or sparsely sampled observations. The application of surface chlorophyll-a (Chl a) concentration to MODIS Aqua satellite observations leads two independent cases (data for the years 2009 and 2011) to a calibration of the model parameterization that produces weaker winter mixing compared to the standard configuration. As a consequence of the weaker mixing, the timing and intensity of increased surface Chl a satellite observations in spring and summer was reproduced by the model. Moreover, the weaker mixing resembles the in situ observations of vertical mixing better than the stronger mixing based on the standard configuration. This shows that the new calibration indeed improves the performance of the turbulence model.