On the Interannual Variations of the Chlorophyll-a Concentration in the South Atlantic driven by the MOC

The advection of mass, heat, freshwater, and nutrients affect the
oceanic primary production in a multiscale fashion. In this study we
investigate the influence of South Atlantic Meridional Overturning
Circulation (MOC) in the distribution of chlorophyll-a concentration
from interannual to decadal scales. The chlorophyll data used in this
study were produced by the GlobColour Project.
This product combines data from different satellite platforms to
compose a database with higher spatial and temporal
resolution. Nutrient data came from the PISCES V2 biogeochemical
model. The MOC volume transport was estimated from the ECCO model
which assimilates measurements from various observational platforms to
provide estimates of physical oceanographic variables. We focused our
attention to the period from 1998 to 2017. A method known as the
Multi-dimensional Empirical Mode Decomposition was used to separate
the time series of the variables into several spectral bands.

We found that at lower frequency scales (interannual to decadal) there
is a significant correlation between the northward MOC volume
transport with the water-column integrated nitrate, and thus with the
chlorophyll concentration in the South Atlantic. As a possible
mechanism to explain this correlation was through the influence of the
Subantarctic Mode Water (SAMW), a feature present in the upper limb of
the South Atlantic MOC. SAMW interacts with regions of strong coastal
upwelling at the eastern boundary of the basin. Therefore, the
nutrient transported by the SAMW is greatly responsible for the
primary production in the tropical region. In our study, we chose the
mean depth of SAMW as the lower limit for the nitrate vertical
integration. It was concluded that variations of low frequency on the
MOC volume transport can alter the spatio-temporal pattern of the
chlorophyll concentration on the South Atlantic.