Abstract. Although mesoscale ocean eddies are ubiquitous in the Southern Ocean, their
average regional and seasonal association with phytoplankton has not been
quantified systematically yet. To this end, we identify over 100 000
mesoscale eddies with diameters of 50 km and more in the Southern Ocean and
determine the associated phytoplankton biomass anomalies using
satellite-based chlorophyll-a (Chl) as a proxy. The mean Chl anomalies,
δChl, associated with these eddies, comprising the upper echelon of
the oceanic mesoscale, exceed ±10 % over wide regions. The structure of
these anomalies is largely zonal, with cyclonic, thermocline lifted, eddies
having positive anomalies in the subtropical waters north of the Antarctic
Circumpolar Current (ACC) and negative anomalies along its main flow path.
The pattern is similar, but reversed for anticyclonic, thermocline deepened
eddies. The seasonality of δChl is weak in subtropical waters, but
pronounced along the ACC, featuring a seasonal sign switch. The spatial
structure and seasonality of the mesoscale δChl can be explained
largely by lateral advection, especially local eddy-stirring. A
prominent exception is the ACC region in winter, where δChl is
consistent with a modulation of phytoplankton light exposure caused by an
eddy-induced modification of the mixed layer depth. The clear impact of
mesoscale eddies on phytoplankton may implicate a downstream effect on
Southern Ocean biogeochemical properties, such as mode water nutrient
contents.
The Biogeochemical Structure of Southern Ocean Mesoscale Eddies