The Unusual Surface Chlorophyll Signatures of Southern Ocean Eddies

The response of the major ocean currents to changes in wind stress forcing is investigated with a series of idealized, but eddy-permitting, model simulations. Previously, ostensibly similar models have shown considerable variation in the oceanic response to changing wind stress forcing. Here, it is shown that a major reason for these differences in model sensitivity is subtle modification of the idealized bathymetry. The key bathymetric parameter is the extent to which the strong eddy field generated in the circumpolar current can interact with the bottom water formation process. The addition of an embayment, which insulates bottom water formation from meridional eddy fluxes, acts to stabilize the deep ocean density and enhances the sensitivity of the circumpolar current. The degree of interaction between Southern Ocean eddies and Antarctic shelf processes may thereby control the sensitivity of the Southern Ocean to change.

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Review of MS No.: bg-2018-70, Title: Imprint of Southern Ocean eddies on chlorophyll, Author(s): Ivy Frenger et al.

10.5194/bg-2018-70-rc2 ◽

2018 ◽

Author(s):

Volker Strass

Keyword(s):

Southern Ocean ◽

Ocean Eddies

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Mechanism for potential strengthening of Atlantic overturning prior to collapse

Earth System Dynamics Discussions ◽

10.5194/esdd-5-29-2014 ◽

2014 ◽

Vol 5 (1) ◽

pp. 29-62

Author(s):

D. Ehlert ◽

A. Levermann

Keyword(s):

Southern Ocean ◽

North Atlantic ◽

Climate Models ◽

Meridional Overturning Circulation ◽

Freshwater Flux ◽

Overturning Circulation ◽

Ocean Eddies ◽

The North ◽

Meridional Overturning ◽

The North Atlantic

Abstract. The Atlantic meridional overturning circulation (AMOC) carries large amounts of heat into the North Atlantic influencing climate regionally as well as globally. Paleorecords and simulations with comprehensive climate models suggest that the positive salt-advection feedback may yield a threshold behaviour of the system. That is to say that beyond a certain amount of freshwater flux into the North Atlantic, no meridional overturning circulation can be sustained. Concepts of monitoring the AMOC and identifying its vicinity to the threshold rely on the fact that the volume flux defining the AMOC will be reduced when approaching the threshold. Here we advance conceptual models that have been used in a paradigmatic way to understand the AMOC, by introducing a density-dependent parameterization for the Southern Ocean eddies. This additional degree of freedom uncovers a mechanism by which the AMOC can increase with additional freshwater flux into the North Atlantic, before it reaches the threshold and collapses: an AMOC that is mainly wind-driven will have a constant upwelling as long as the Southern Ocean winds do not change significantly. The downward transport of tracers occurs either in the northern sinking regions or through Southern Ocean eddies. If freshwater is transported, either atmospherically or via horizontal gyres, from the low- to high-latitudes, this would reduce the eddy transport and by continuity increase the northern sinking which defines the AMOC until a threshold is reached at which the AMOC cannot be sustained. If dominant in the real ocean this mechanism would have significant consequences for monitoring the AMOC.

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The Simulated Biological Response to Southern Ocean Eddies via Biological Rate Modification and Physical Transport

Global Biogeochemical Cycles ◽

10.1029/2019gb006385 ◽

2020 ◽

Vol 34 (6) ◽

Cited By ~ 1

Author(s):

Tyler Rohr ◽

Cheryl Harrison ◽

Matthew C. Long ◽

Peter Gaube ◽

Scott C. Doney

Keyword(s):

Southern Ocean ◽

Biological Response ◽

Ocean Eddies

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Mechanism for potential strengthening of Atlantic overturning prior to collapse

Earth System Dynamics ◽

10.5194/esd-5-383-2014 ◽

2014 ◽

Vol 5 (2) ◽

pp. 383-397

Author(s):

D. Ehlert ◽

A. Levermann

Keyword(s):

Southern Ocean ◽

North Atlantic ◽

Climate Models ◽

Meridional Overturning Circulation ◽

Freshwater Flux ◽

Overturning Circulation ◽

Ocean Eddies ◽

The North ◽

Meridional Overturning ◽

The North Atlantic

Abstract. The Atlantic meridional overturning circulation (AMOC) carries large amounts of heat into the North Atlantic influencing climate regionally as well as globally. Palaeo-records and simulations with comprehensive climate models suggest that the positive salt-advection feedback may yield a threshold behaviour of the system. That is to say that beyond a certain amount of freshwater flux into the North Atlantic, no meridional overturning circulation can be sustained. Concepts of monitoring the AMOC and identifying its vicinity to the threshold rely on the fact that the volume flux defining the AMOC will be reduced when approaching the threshold. Here we advance conceptual models that have been used in a paradigmatic way to understand the AMOC, by introducing a density-dependent parameterization for the Southern Ocean eddies. This additional degree of freedom uncovers a mechanism by which the AMOC can increase with additional freshwater flux into the North Atlantic, before it reaches the threshold and collapses: an AMOC that is mainly wind-driven will have a constant upwelling as long as the Southern Ocean winds do not change significantly. The downward transport of tracers occurs either in the northern sinking regions or through Southern Ocean eddies. If freshwater is transported, either atmospherically or via horizontal gyres, from the low to high latitudes, this would reduce the eddy transport and by continuity increase the northern sinking which defines the AMOC until a threshold is reached at which the AMOC cannot be sustained. If dominant in the real ocean this mechanism would have significant consequences for monitoring the AMOC.

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Ocean eddies strongly affect global mean sea-level projections

Science Advances ◽

10.1126/sciadv.abf1674 ◽

2021 ◽

Vol 7 (15) ◽

pp. eabf1674

Author(s):

René M. van Westen ◽

Henk A. Dijkstra

Keyword(s):

Southern Ocean ◽

Sea Level ◽

Large Scale ◽

Scale Effects ◽

Small Scale ◽

Ocean Temperature ◽

Mean Sea Level ◽

Ocean Eddies ◽

Global Mean Sea Level ◽

Global Mean

Current sea-level projections are based on climate models in which the effects of ocean eddies are parameterized. Here, we investigate the effect of ocean eddies on global mean sea-level rise (GMSLR) projections, using climate model simulations. Explicitly resolving ocean eddies leads to a more realistic Southern Ocean temperature distribution and volume transport. These quantities control the rate of basal melt, which eventually results in Antarctic mass loss. In a model with resolved ocean eddies, the Southern Ocean temperature changes lead to a smaller Antarctic GMSLR contribution compared to the same model in which eddies are parameterized. As a result, the projected GMSLR is about 25% lower at the end of this century in the eddying model. Relatively small-scale ocean eddies can hence have profound large-scale effects and consequently affect GMSLR projections.

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Imprint of Southern Ocean eddies on chlorophyll

10.5194/bg-2018-70 ◽

2018 ◽

Cited By ~ 2

Author(s):

Ivy Frenger ◽

Matthias Münnich ◽

Nicolas Gruber

Keyword(s):

Southern Ocean ◽

Light Exposure ◽

Mixed Layer Depth ◽

Mode Water ◽

Nutrient Contents ◽

Ocean Eddies ◽

Mesoscale Ocean Eddies ◽

Circumpolar Current ◽

Pm 10 ◽

Lateral Advection

Abstract. Although mesoscale ocean eddies are ubiquitous in the Southern Ocean, their spatial and seasonal association with phytoplankton has to date not been quantified in detail. To this end, we identify over 100,000 eddies in the Southern Ocean and determine the associated phytoplankton biomass anomalies using satellite-based chlorophyll-a (Chl) as a proxy. The mean eddy associated Chl anomalies (𝛿Chl) exceed &pm;10 % over wide regions. The structure of these anomalies is largely zonal, with cyclonic, thermocline lifting, eddies having positive anomalies in the subtropical waters north of the Antarctic Circumpolar Current (ACC) and negative anomalies along the ACC. The pattern is similar, but reversed for anticyclonic, thermocline deepening 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 𝛿Chl can be explained largely by lateral advection, especially 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 eddies on phytoplankton may implicate a downstream effect on Southern Ocean biogeochemical properties, such as mode water nutrient contents.

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In situ measurement of the biogeochemical properties of Southern Ocean mesoscale eddies in the Southwest Indian Ocean, April 2014

Earth System Science Data Discussions ◽

10.5194/essdd-8-809-2015 ◽

2015 ◽

Vol 8 (2) ◽

pp. 809-825

Author(s):

S. de Villiers ◽

K. Siswana ◽

K. Vena

Keyword(s):

Southern Ocean ◽

Polar Front ◽

Open Ocean ◽

Southwest Indian Ridge ◽

Cyclonic Eddy ◽

Southwest Indian Ocean ◽

Data Archiving ◽

Chl A ◽

Ocean Eddies ◽

Warm Core

Abstract. Several open-ocean mesoscale features, a "young" warm-core (anti-cyclonic) eddy at 52° S, an "older" warm-core eddy at 57.5° S, as well as an adjacent cold-core (cyclonic) eddy at 56° S, were surveyed during a M/V S.A. Agulhas II cruise in April 2014. The main aim of the survey was to obtain hydrographical and biogeochemical profile data for contrasting open-ocean eddies in the Southern Ocean, that will be suitable for comparison and modelling of their heat, salt and nutrient characteristics, and the changes that occur in these properties as warm-core eddies migrate from the polar front southwards into the Southern Ocean. Results show that the older warm-core eddy at 57.5° S is, at its core, 2.7 °C colder than a younger eddy at 52° S, while its dissolved silicate levels are almost 500 % higher and accompanied by chl a levels that are more than 200 % higher than that in the younger eddy. A total of 18 CTD stations were occupied in a sector south of the Southwest Indian Ridge, along three transects crossing several mesoscale features identified from satellite altimetry data prior to the cruise. The CTD data, as well as chl a and dissolved nutrient data (for NO3−, NO2−, PO43− and SiO2) have been processed, quality-controlled and made available via the PANGAEA Data Archiving and Publication database at doi:10.1594/PANGAEA.848875.

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