The Unusual Surface Chlorophyll Signatures of Southern Ocean Eddies

Abstract

Southern Ocean mesoscale eddies play an important role in ocean circulation and biogeochemical cycling, but their biological characteristics have not been well quantified at the basin scale. To address this, we combined a 15-year tracked eddy data set with satellite observations of ocean color, sea surface temperature, and autonomous profiling floats to quantify the surface and subsurface properties of eddies. Anomalies of surface temperature and chlorophyll were examined in eddy-centric composite averages constructed from thousands of eddies. Normalized surface chlorophyll anomalies (chl norm ) vary seasonally and geographically. Cyclones typically show positive chl norm , while anticyclones have negative chl norm . The sign of chl norm reverses during late summer and autumn for eddies between the Subtropical and Polar Fronts. The reversal is most obvious in the Indian sector, and we attribute this to a combination of eddy stirring (deformation of surface gradients by the rotational velocity of an eddy) and deeper winter mixing in anticyclones. Both chl norm and sea surface temperature anomalies transition from dipole structures north of the Subtropical Front to monopole structures south of the Subantarctic Front. Sea surface temperature and chl norm composites provide evidence for eddy trapping (transporting of anomalies) and eddy stirring. This research provides a basin-scale study of surface chlorophyll in Southern Ocean eddies and reveals counterintuitive biogeochemical signals.

Eddy Surface Characteristics and Vertical Structure in the Gulf of Mexico from Satellite Observations and Model Simulations

“…SST is from the National Oceanographic and Atmospheric Association National Climatic Data Center's Advanced Very High Resolution Radiometer optimally interpolated product available daily with a 0.25° resolution (Reynolds et al, ; http://www.ncdc.noaa.gov/oisst/data-access). This optimally interpolated product is preferred due to its lack of data gaps (both spatially and temporally) and has been used in eddy studies such as Dawson et al (). We chose this product because the 0.25° spatial resolution is adequate for resolving mesoscale eddy features examined here and also matches the spatial resolution of the ADT and SSS data sets, removing a potential source of error by comparing product with different grid spacings.…”

Mesoscale Eddies Modulate Mixed Layer Depth Globally

“…In addition, primary production in the ocean is modulated by fluxes of nutrients and phytoplankton through the base of the mixed layer and the availability of light (Dawson et al, 2018;Frenger et al, 2018;Song et al, 2018). In addition, primary production in the ocean is modulated by fluxes of nutrients and phytoplankton through the base of the mixed layer and the availability of light (Dawson et al, 2018;Frenger et al, 2018;Song et al, 2018).…”

“…The near-surface mixed layer is the conduit by which the atmosphere influences the ocean interior, and conversely, the ocean modulates fluxes into the atmosphere. In addition, primary production in the ocean is modulated by fluxes of nutrients and phytoplankton through the base of the mixed layer and the availability of light (Dawson et al, 2018;Frenger et al, 2018;Song et al, 2018). The fact that mesoscale eddies modulate the spatial and temporal evolution of the mixed layer has been known for decades (e.g., Klein et al, 1998).…”

“…Furthermore, the work of Hausmann et al (2017) revealed that the magnitude of eddy-mediated MLD perturbations were largest at the center of Southern Ocean eddies and decayed toward the eddy periphery. In the Southern Ocean this eddy-mediated MLD variability has been shown to generate enhanced iron flux in anticyclonic eddies (Song et al, 2018) that leads to elevated near-surface chlorophyll during the austral summer (Dawson et al, 2018;Frenger et al, 2018;Song et al, 2018). Using a global eddy-resolving simulation, 10.1029/2018GL080006 Dufois et al (2016) showed that during the winter, nutrients from below the mixed layer were entrained into the interiors of anticyclones at higher rates when compared to cyclones and the areas outside of the influence of eddies, leading the authors to suggest eddy-mediated MLD perturbations as a mechanism by which elevated near-surface chlorophyll concentrations could be sustained in anticyclones.…”

Distinct iron cycling in a Southern Ocean eddy

“…Cold-core eddies in the Southern Ocean are defined by strong clockwise rotation, cooler temperatures and negative sea-surface height anomalies 7,8 . These eddies can have closed circulation thus trapping 9 the biogeochemical properties of these features such that nutrient, chlorophyll and particle concentrations can be distinct relative to those in the surrounding waters 2,[9][10][11] . They can transport these biogeochemical properties vast distances 12 thus they are important from an oceanographic point of view, especially if they cross water mass boundaries such as the polar front or the subantarctic front 7,11,13 .…”