scholarly journals Imprint of Southern Ocean mesoscale eddies on chlorophyll

2018 ◽  
Vol 15 (15) ◽  
pp. 4781-4798 ◽  
Author(s):  
Ivy Frenger ◽  
Matthias Münnich ◽  
Nicolas Gruber
Keyword(s):  
Southern Ocean ◽  
Light Exposure ◽  
Mixed Layer Depth ◽  
Mesoscale Eddies ◽  
Mode Water ◽  
Upper Echelon ◽  
Nutrient Contents ◽  
Mesoscale Ocean Eddies ◽  
Circumpolar Current ◽  
Lateral Advection

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.

scholarly journals Imprint of Southern Ocean eddies on chlorophyll

2018 ◽  
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 ±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.

scholarly journals Simulation of Subantarctic Mode and Antarctic Intermediate Waters in Climate Models

2007 ◽  
Vol 20 (20) ◽  
pp. 5061-5080 ◽  
Author(s):  
Bernadette M. Sloyan ◽  
Igor V. Kamenkovich
Keyword(s):  
Southern Ocean ◽  
Climate Models ◽  
Intermediate Water ◽  
Mixing Processes ◽  
Intergovernmental Panel ◽  
Mode Water ◽  
Buoyancy Forcing ◽  
The North ◽  
Circumpolar Current ◽  
Ipcc Ar4

Abstract The Southern Ocean’s Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) are two globally significant upper-ocean water masses that circulate in all Southern Hemisphere subtropical gyres and cross the equator to enter the North Pacific and North Atlantic Oceans. Simulations of SAMW and AAIW for the twentieth century in eight climate models [GFDL-CM2.1, CCSM3, CNRM-CM3, MIROC3.2(medres), MIROC3.2(hires), MRI-CGCM2.3.2, CSIRO-Mk3.0, and UKMO-HadCM3] that provided their output in support of the Intergovernmental Panel on Climate Change’s Fourth Assessment Report (IPCC AR4) have been compared to the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Atlas of Regional Seas. The climate models, except for UKMO-HadCM3, CSIRO-Mk3.0, and MRI-CGCM2.3.2, provide a reasonable simulation of SAMW and AAIW isopycnal temperature and salinity in the Southern Ocean. Many models simulate the potential vorticity minimum layer and salinity minimum layer of SAMW and AAIW, respectively. However, the simulated SAMW layer is generally thinner and at lighter densities than observed. All climate models display a limited equatorward extension of SAMW and AAIW north of the Antarctic Circumpolar Current. Errors in the simulation of SAMW and AAIW property characteristics are likely to be due to a combination of many errors in the climate models, including simulation of wind and buoyancy forcing, inadequate representation of subgrid-scale mixing processes in the Southern Ocean, and midlatitude diapycnal mixing parameterizations.

scholarly journals The Southern Ocean and Its Climate in CCSM4

2012 ◽  
Vol 25 (8) ◽  
pp. 2652-2675 ◽  
Author(s):  
Wilbert Weijer ◽  
Bernadette M. Sloyan ◽  
Mathew E. Maltrud ◽  
Nicole Jeffery ◽  
Matthew W. Hecht ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Climate System ◽  
Intermediate Water ◽  
Mode Water ◽  
Ice Dynamics ◽  
Climate System Model ◽  
Interbasin Exchange ◽  
Circumpolar Current ◽  
Earth’S Climate ◽  
New Community

Abstract The new Community Climate System Model, version 4 (CCSM4), provides a powerful tool to understand and predict the earth’s climate system. Several aspects of the Southern Ocean in the CCSM4 are explored, including the surface climatology and interannual variability, simulation of key climate water masses (Antarctic Bottom Water, Subantarctic Mode Water, and Antarctic Intermediate Water), the transport and structure of the Antarctic Circumpolar Current, and interbasin exchange via the Agulhas and Tasman leakages and at the Brazil–Malvinas Confluence. It is found that the CCSM4 has varying degrees of accuracy in the simulation of the climate of the Southern Ocean when compared with observations. This study has identified aspects of the model that warrant further analysis that will result in a more comprehensive understanding of ocean–atmosphere–ice dynamics and interactions that control the earth’s climate and its variability.

scholarly journals Characterization of distinct bloom phenology regimes in the Southern Ocean

2015 ◽  
Vol 72 (6) ◽  
pp. 1985-1998 ◽  
Author(s):  
Jean-Baptiste Sallée ◽  
J. Llort ◽  
A. Tagliabue ◽  
M. Lévy
Keyword(s):  
Southern Ocean ◽  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Encounter Rate ◽  
Surface Cooling ◽  
Winter Solstice ◽  
Surface Warming ◽  
Colour Measurements ◽  
Circumpolar Current ◽  
Biomass Increase

Abstract In this study, we document the regional variations of bloom phenology in the Southern Ocean, based on a 13-year product of ocean colour measurements co-located with observation-based estimates of the mixed-layer depth. One key aspect of our work is to discriminate between mixed-layer integrated blooms and surface blooms. By segregating blooms that occur before or after the winter solstice and blooms where integrated and surface biomass increase together or display a lag, we define three dominating Southern Ocean bloom regimes. While the regime definitions are solely based on bloom timing characteristics, the three regimes organize coherently in geographical space, and are associated with distinct dynamical regions of the Southern Ocean: the subtropics, the subantarctic, and the Antarctic Circumpolar Current region. All regimes have their mixed-layer integrated onset between autumn and winter, when the daylength is short and the mixed layer actively mixes and deepens. We discuss how these autumn–winter bloom onsets are controlled by either nutrient entrainment and/or reduction in prey-grazer encounter rate. In addition to the autumn–winter biomass increase, the subantarctic regime has a significant spring biomass growth associated with the shutdown of turbulence when air–sea heat flux switches from surface cooling to surface warming.

scholarly journals Role of the Seasonal Cycle in the Subduction Rates of Upper–Southern Ocean Waters

2013 ◽  
Vol 43 (6) ◽  
pp. 1096-1113 ◽  
Author(s):  
Eun Young Kwon ◽  
Stephanie M. Downes ◽  
Jorge L. Sarmiento ◽  
Riccardo Farneti ◽  
Curtis Deutsch
Keyword(s):  
Exchange Rate ◽  
Southern Ocean ◽  
Layer Thickness ◽  
Mixed Layer ◽  
Temporal Change ◽  
Mixed Layer Depth ◽  
Layer Depth ◽  
Mode Water ◽  
Annual Cycles ◽  
Mode Waters

Abstract A kinematic approach is used to diagnose the subduction rates of upper–Southern Ocean waters across seasonally migrating density outcrops at the base of the mixed layer. From an Eulerian viewpoint, the term representing the temporal change in the mixed layer depth (which is labeled as the temporal induction in this study; i.e., Stemp = ∂h/∂t where h is the mixed layer thickness, and t is time) vanishes over several annual cycles. Following seasonally migrating density outcrops, however, the temporal induction is attributed partly to the temporal change in the mixed layer thickness averaged over a density outcrop following its seasonally varying position and partly to the lateral movement of the outcrop position intersecting the sloping mixed layer base. Neither the temporal induction following an outcrop nor its integral over the outcrop area vanishes over several annual cycles. Instead, the seasonal eddy subduction, which arises primarily because of the subannual correlations between the seasonal cycles of the mixed layer depth and the outcrop area, explains the key mechanism by which mode waters are transferred from the mixed layer to the underlying pycnocline. The time-mean exchange rate of waters across the base of the mixed layer is substantially different from the exchange rate of waters across the fixed winter mixed layer base in mode water density classes. Nearly 40% of the newly formed Southern Ocean mode waters appear to be diapycnally transformed within the seasonal pycnocline before either being subducted into the main pycnocline or entrained back to the mixed layer through lighter density classes.

scholarly journals Hydrographic shifts south of Australia over the last deglaciation and possible interhemispheric linkages

2021 ◽  
pp. 1-12
Author(s):  
Matthias Moros ◽  
Patrick De Deckker ◽  
Kerstin Perner ◽  
Ulysses S. Ninnemann ◽  
Lukas Wacker ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Southern Hemisphere ◽  
Critical Role ◽  
Temperature Response ◽  
South Australia ◽  
Last Deglaciation ◽  
Tight Coupling ◽  
Oceanic Fronts ◽  
The Last Deglaciation ◽  
Circumpolar Current

Abstract Northern and southern hemispheric influences—particularly changes in Southern Hemisphere westerly winds (SSW) and Southern Ocean ventilation—triggered the stepwise atmospheric CO2 increase that accompanied the last deglaciation. One approach for gaining potential insights into past changes in SWW/CO2 upwelling is to reconstruct the positions of the northern oceanic fronts associated with the Antarctic Circumpolar Current. Using two deep-sea cores located ~600 km apart off the southern coast of Australia, we detail oceanic changes from ~23 to 6 ka using foraminifer faunal and biomarker alkenone records. Our results indicate a tight coupling between hydrographic and related frontal displacements offshore South Australia (and by analogy, possibly the entire Southern Ocean) and Northern Hemisphere (NH) climate that may help confirm previous hypotheses that the westerlies play a critical role in modulating CO2 uptake and release from the Southern Ocean on millennial and potentially even centennial timescales. The intensity and extent of the northward displacements of the Subtropical Front following well-known NH cold events seem to decrease with progressing NH ice sheet deglaciation and parallel a weakening NH temperature response and amplitude of Intertropical Convergence Zone shifts. In addition, an exceptional poleward shift of Southern Hemisphere fronts occurs during the NH Heinrich Stadial 1. This event was likely facilitated by the NH ice maximum and acted as a coup-de-grâce for glacial ocean stratification and its high CO2 capacitance. Thus, through its influence on the global atmosphere and on ocean mixing, “excessive” NH glaciation could have triggered its own demise by facilitating the destratification of the glacial ocean CO2 state.

scholarly journals The Biogeochemical Structure of Southern Ocean Mesoscale Eddies

2020 ◽  
Vol 125 (8) ◽  
Author(s):  
Ramkrushnbhai S. Patel ◽  
Joan Llort ◽  
Peter G. Strutton ◽  
Helen E. Phillips ◽  
Sebastien Moreau ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Mesoscale Eddies

scholarly journals Subantarctic Mode Water Formation, Destruction, and Export in the Eddy-Permitting Southern Ocean State Estimate

2013 ◽  
Vol 43 (7) ◽  
pp. 1485-1511 ◽  
Author(s):  
Ivana Cerovečki ◽  
Lynne D. Talley ◽  
Matthew R. Mazloff ◽  
Guillaume Maze
Keyword(s):  
Southern Ocean ◽  
Buoyancy Flux ◽  
Differential Heat ◽  
Diapycnal Mixing ◽  
Mode Water ◽  
State Estimate ◽  
Surface Formation ◽  
Subantarctic Mode Water ◽  
The Pacific ◽  
The Difference

Abstract Subantarctic Mode Water (SAMW) is examined using the data-assimilating, eddy-permitting Southern Ocean State Estimate, for 2005 and 2006. Surface formation due to air–sea buoyancy flux is estimated using Walin analysis, and diapycnal mixing is diagnosed as the difference between surface formation and transport across 30°S, accounting for volume change with time. Water in the density range 26.5 < σθ < 27.1 kg m−3 that includes SAMW is exported northward in all three ocean sectors, with a net transport of (18.2, 17.1) Sv (1 Sv ≡ 106 m3 s−1; for years 2005, 2006); air–sea buoyancy fluxes form (13.2, 6.8) Sv, diapycnal mixing removes (−14.5, −12.6) Sv, and there is a volume loss of (−19.3, −22.9) Sv mostly occurring in the strongest SAMW formation locations. The most vigorous SAMW formation is in the Indian Ocean by air–sea buoyancy flux (9.4, 10.9) Sv, where it is partially destroyed by diapycnal mixing (−6.6, −3.1) Sv. There is strong export to the Pacific, where SAMW is destroyed both by air–sea buoyancy flux (−1.1, −4.6) Sv and diapycnal mixing (−5.6, −8.4) Sv. In the South Atlantic, SAMW is formed by air–sea buoyancy flux (5.0, 0.5) Sv and is destroyed by diapycnal mixing (−2.3, −1.1) Sv. Peaks in air–sea flux formation occur at the Southeast Indian and Southeast Pacific SAMWs (SEISAMWs, SEPSAMWs) densities. Formation over the broad SAMW circumpolar outcrop windows is largely from denser water, driven by differential freshwater gain, augmented or decreased by heating or cooling. In the SEISAMW and SEPSAMW source regions, however, formation is from lighter water, driven by differential heat loss.

Spiciness Anomalies of Subantarctic Mode Water in the South Indian Ocean

2021 ◽  
Vol 34 (10) ◽  
pp. 3927-3953
Author(s):  
Motoki Nagura
Keyword(s):  
Indian Ocean ◽  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Surface Mixed Layer ◽  
The South ◽  
South Indian Ocean ◽  
Mode Water ◽  
Horizontal Advection ◽  
South Indian ◽  
Subantarctic Mode Water

AbstractThis study investigates spreading and generation of spiciness anomalies of the Subantarctic Mode Water (SAMW) located on 26.6 to 26.8 σθ in the south Indian Ocean, using in situ hydrographic observations, satellite measurements, reanalysis datasets, and numerical model output. The amplitude of spiciness anomalies is about 0.03 psu or 0.13°C and tends to be large along the streamline of the subtropical gyre, whose upstream end is the outcrop region south of Australia. The speed of spreading is comparable to that of the mean current, and it takes about a decade for a spiciness anomaly in the outcrop region to spread into the interior up to Madagascar. In the outcrop region, interannual variability in mixed layer temperature and salinity tends to be density compensating, which indicates that Eulerian temperature or salinity changes account for the generation of isopycnal spiciness anomalies. It is known that wintertime temperature and salinity in the surface mixed layer determine the temperature and salinity relationship of a subducted water mass. Considering this, the mixed layer heat budget in the outcrop region is estimated based on the concept of effective mixed layer depth, the result of which shows the primary contribution from horizontal advection. The contributions from Ekman and geostrophic currents are comparable. Ekman flow advection is caused by zonal wind stress anomalies and the resulting meridional Ekman current anomalies, as is pointed out by a previous study. Geostrophic velocity is decomposed into large-scale and mesoscale variability, both of which significantly contribute to horizontal advection.

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