Satellite observations of chlorophyll, phytoplankton biomass, and Ekman pumping in nonlinear mesoscale eddies

Abstract

[1] Nonlinear mesoscale eddies can influence biogeochemical cycles in the upper ocean through vertical and horizontal advection of nutrients and marine organisms. The relative importance of these two processes depends on the polarity of an eddy (cyclones versus anticyclones) and the initial biological conditions of the fluid trapped in the core of the eddy at the time of formation. Eddies originating in the eastern South Indian Ocean are unique in that anticyclones, typically associated with downwelling, contain elevated levels of chlorophyll-a, enhanced primary production and phytoplankton communities generally associated with nutrient-replete environments. From analysis of 9 years of concurrent satellite measurements of sea surface height, chlorophyll, phytoplankton carbon, and surface stress, we present observations that suggest eddy-induced Ekman upwelling as a mechanism that is at least partly responsible for sustaining positive phytoplankton anomalies in anticyclones of the South Indian Ocean. The biological response to this eddyinduced Ekman upwelling is evident only during the Austral winter. During the Austral summer, the biological response to eddy-induced Ekman pumping occurs deep in the euphotic zone, beyond the reach of satellite observations of ocean color.

Imprint of Southern Ocean eddies on chlorophyll

“…In contrast, seasonal variations have been reported to be relatively weak globally (Gaube et al, 2014), except for the eastern Indian Ocean and the South China Sea (Gaube et al, 2013;Guo et al, 2017). And seasonal changes in the sign of δChl in a particular region have to our knowledge not been reported before.…”

“…Further, it has been suggested 5 that advection of Chl by eddies via trapping, i.e., the enclosing and dragging along of waters, causes δChl (Gaube et al, 2014), particularly in boundary current regions characterized by steep zonal Chl gradients. Numerous other potential mechanisms through which eddies affect phytoplankton have been identified (e.g., McGillicuddy et al, 2007;D'Ovidio et al, 2010;Siegel et al, 2011;Gaube et al, 2013Gaube et al, , 2014Dufois et al, 2016;Gruber et al, 2011), including modifications of mixed layer depth, vertical mixing, thermocline lifting, and providing of spatial niches. These mechanisms modulate the phytoplankton's light 10 exposure, their nutrient availability or their grazing pressure, i.e., they affect their net balance between growth and decay.…”

Monitoring the mesoscale eddies of the Lofoten Basin: importance, progress, and challenges

“…Thus, similar to other regions of the world oceans (e.g. Falkowski et al 1991;Raj, Peter, and Pushpadas 2010;Gaube et al 2013), mesoscale eddies are important to the biology and fisheries of the Lofoten Basin. The above-mentioned importance of mesoscale eddies in the Lofoten Basin underlines the importance of monitoring it.…”

Phytoplankton Size Structure in Association with Mesoscale Eddies off Central-Southern Chile: The Satellite Application of a Phytoplankton Size-Class Model

“…Other mechanisms involve a strong vertical exchange of waters, such as eddy pumping during eddy formation, generating a positive (negative) Chl-a monopole centered on the core of cyclonic (anticyclonic) eddies [10,20]. An opposite pattern is observed after the development phase, mainly in association with eddy-Ekman pumping due to the relative movement of the wind curl and eddy current velocity field [4,8,[21][22][23]. Other important vertical exchanges of waters can be attributed to eddy-eddy interactions, which may also lead to frontogenesis, resulting in a convergent front between eddies and in an enhancement of Chl-a in the warm (anticyclonic) side and isopycnal subduction of Chl-a in the cold (cyclonic) side [24][25][26][27].…”