Temporal scales of variability in the Mediterranean Sea ecosystem: Insight from a coupled model

We implement a 3D state-of-the-art hydrodynamic-biogeochemical model that is based on MITgcm-BFM online coupling to describe the temporal scales of the variability of the multi-decadal marine biogeochemistry in the Mediterranean Sea. This model is characterised by a horizontal resolution of 1/12 degrees and a multi-nutrient and multi-plankton framework and can reproduce the main physical and biogeochemical processes from the seasonal (and its inter-annual modulation) to the sub-weekly scales. The corroboration shows that the MITgcm-BFM hindcast exhibits good and consistent performance when simulating the basin-wide gradients of temperature, salinity and nutrients and the seasonal cycles of physical and phytoplankton dynamics, including the deep chlorophyll maximum and the primary production at the basin and sub-basin scales for the period of 1994-2012. Novel statistical metrics show that the seasonal variability equals approximately 50% of the spatio-temporal sub-domain mean for the surface temperature and is higher than 150% for nutrients and chlorophyll. Nevertheless, the high-frequency (sub-weekly) variability is not negligible and is approximately equal to the inter-annual modulation of the seasonal variability, i.e., 3% of the spatio-temporal mean for the temperature and 30% or greater for nutrients and chlorophyll. Moreover, the high-frequency variability exhibits a decreasing southward gradient in the Mediterranean basin, similar to the seasonal variability. A further analysis based on the wavelet method and performed at different Mediterranean sites highlights the significant power that is associated with sub-weekly periods, which adds to the annual and semi-annual patterns, confirming the importance of the high frequency scale of variability in the biogeochemistry of the basin.