Variability of the biological front south of Africa from SeaWiFS and a coupled physical-biological model,
Llido, J. Machu,E., Sudre J., Dadou I. and Garçon, V.,
Journal of Marine Research, 62(4), 595-609.
The spatio-temporal variability of the biological front in the Agulhas Current system is investigated by comparing SeaWiFS chlorophyll a data and modelled chlorophyll fields over the October 1997 October 2001 period. The latter fields are simulated using a regional eddy-permitting (1/3° x 1/3°) coupled physical (AGAPE)-biological model forced by the monthly atmospheric NCEP/NCAR reanalysis. The annual cycle of the observed chlorophyll within the Agulhas Current system biogeochemical provinces is quite well reproduced by the model. The modelled phase of the seasonality in the SWSIG (South Western Indian Subtropical Gyre) is opposite to that of the SCZ (Subtropical Convergence Zone encompassing the Agulhas Front -AF, the Subtropical Front- STF and the Subantarctic Front-SAF), in agreement with observations. In the SWSIG, the switch from nitrates limitation to light control for the modelled phytoplankton growth shifts southward from winter to summer. In the SCZ, light availability modulates growth throughout the year. The wavelet average variance of the SeaWiFS data is slightly underestimated by the modelled chlorophyll variance over the 4 years period within the 36°-45°S and 15°-45°E domain. This might originate in the interannual monthly NCEP forcing which does not include the high frequency information of the atmospheric fluxes. The model coarse resolution precludes a proper simulation of vertical motions produced by submesoscale flows thereby underestimating biological variability. Interestingly, the modelled chlorophyll distribution mimicks the strong early retroflection of the Agulhas Current in summer 2001 which induces a southward displacement of the STF/SAF double front.