Strong atmospheric surface pressure anomalies drive a see-saw in Subantarctic Mode Water formation

2020 ◽  
Author(s):  
Ivana Cerovecki ◽  
Andrew Meijers
Keyword(s):  
Southern Ocean ◽  
Western Indian Ocean ◽  
Air Pressure ◽  
Positive Pressure ◽  
Central Pacific ◽  
Mode Water ◽  
Argo Data ◽  
Subantarctic Mode Water ◽  
The Pacific ◽  
Mixed Layers

<p>The dominant Subantarctic Mode Water (SAMW) formation regions are located in the Indian, and in the Pacific sector of the Southern Ocean. Strong wintertime (Jul-Sep) surface air pressure anomalies with variance maxima at approximately 100°E and 150°W drive a zonal dipole structure in the SAMW formation and thickness, in both the Indian and Pacific sector of the Southern Ocean. This has been documented within gridded Argo data for years 2005-2019. A much weaker surface air pressure anomaly variance maxima is located in the Atlantic Ocean centered at approximately 25°W.</p><p>Anomalously strong positive pressure anomalies result in deepening of the wintertime mixed layers and an increase in the SAMW formation in the eastern part of the Pacific and Indian sector; these effects are due to cold southerly winds, strengthened zonal winds and increased surface ocean heat loss. <br>Anomalously strong negative pressure anomalies result in shoaling of the wintertime mixed layers and a decrease in SAMW formation in these regions, while at the same time deepening the wintertime mixed layers and increasing SAMW formation in the western Indian Ocean and in the central Pacific.</p><p>In years with strong El Nino, the interannual variability of the strength of two surface air pressure anomalies does not co-vary in phase with each other. Strong isopycnal heave in SAMW density range emanates from locations where winter surface air pressure anomalies and mixed layers are most strongly coupled.  </p>

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.

scholarly journals Strong quasi-stationary wintertime atmospheric surface pressure anomalies drive a dipole pattern in the Subantarctic ModeWater formation

2021 ◽  
pp. 1-44
Author(s):  
Ivana Cerovečki ◽  
Andrew J.S. Meijers
Keyword(s):  
Interannual Variability ◽  
Strongly Correlated ◽  
Mode Water ◽  
Sea Level Pressure ◽  
Surface Ocean ◽  
Subantarctic Mode Water ◽  
The Pacific ◽  
Meridional Winds ◽  
Dipole Pattern ◽  
Mixed Layers

AbstractThe deepest wintertime (Jul-Sep) mixed layers associated with Subantarctic Mode Water (SAMW) formation develop in the Indian and Pacific sectors of the Southern Ocean. In these two sectors the dominant interannual variability of both deep wintertime mixed layers and SAMW volume is a east-west dipole pattern in each basin. The variability of these dipoles are strongly correlated with the interannual variability of overlying winter quasi-stationary mean sea level pressure (MSLP) anomalies. Anomalously strong positive MSLP anomalies are found to result in the deepening of the wintertime mixed layers and an increase in the SAMW formation in the eastern parts of the dipoles in the Pacific and Indian sectors. These effects are due to enhanced cold southerly meridional winds, strengthened zonal winds and increased surface ocean heat loss. The opposite occurs in the western parts of the dipoles in these sectors. Conversely, strong negative MSLP anomalies result in shoaling (deepening) of the wintertime mixed layers and a decrease (increase) in SAMW formation in the eastern (western) regions. The MSLP variability of the Pacific and Indian basin anomalies are not always in phase, especially in years with a strong El Niño, resulting in different patterns of SAMW formation in the western vs. eastern parts of the Indian and Pacific sectors. Strong isopycnal depth and thickness anomalies develop in the SAMW density range in years with strong MSLP anomalies. When advected eastward, they act to precondition downstream SAMW formation in the subsequent winter.

scholarly journals The Role of Southern Ocean Surface Forcings and Mixing in the Global Conveyor

2008 ◽  
Vol 38 (7) ◽  
pp. 1377-1400 ◽  
Author(s):  
Daniele Iudicone ◽  
Gurvan Madec ◽  
Bruno Blanke ◽  
Sabrina Speich
Keyword(s):  
Southern Ocean ◽  
North Atlantic ◽  
Deep Water ◽  
Thermohaline Circulation ◽  
North Atlantic Deep Water ◽  
Surface Fluxes ◽  
Weddell Sea ◽  
Mode Water ◽  
Circumpolar Deep Water ◽  
Subantarctic Mode Water

Abstract Despite the renewed interest in the Southern Ocean, there are yet many unknowns because of the scarcity of measurements and the complexity of the thermohaline circulation. Hence the authors present here the analysis of the thermohaline circulation of the Southern Ocean of a steady-state simulation of a coupled ice–ocean model. The study aims to clarify the roles of surface fluxes and internal mixing, with focus on the mechanisms of the upper branch of the overturning. A quantitative dynamical analysis of the water-mass transformation has been performed using a new method. Surface fluxes, including the effect of the penetrative solar radiation, produce almost 40 Sv (1 Sv ≡ 106 m3 s−1) of Subantarctic Mode Water while about 5 Sv of the densest water masses (γ > 28.2) are formed by brine rejection on the shelves of Antarctica and in the Weddell Sea. Mixing transforms one-half of the Subantarctic Mode Water into intermediate water and Upper Circumpolar Deep Water while bottom water is produced by Lower Circumpolar Deep Water and North Atlantic Deep Water mixing with shelf water. The upwelling of part of the North Atlantic Deep Water inflow is due to internal processes, mainly downward propagation of the surface freshwater excess via vertical mixing at the base of the mixed layer. A complementary Lagrangian analysis of the thermohaline circulation will be presented in a companion paper.

A New Algorithm for Finding Mixed Layer Depths with Applications to Argo Data and Subantarctic Mode Water Formation*

2009 ◽  
Vol 26 (9) ◽  
pp. 1920-1939 ◽  
Author(s):  
James Holte ◽  
Lynne Talley
Keyword(s):  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Gradient Methods ◽  
Temperature Threshold ◽  
Intermediate Water ◽  
Mode Water ◽  
Southwest Atlantic ◽  
Argo Data ◽  
Southeastern Pacific ◽  
Subantarctic Mode Water

Abstract A new hybrid method for finding the mixed layer depth (MLD) of individual ocean profiles models the general shape of each profile, searches for physical features in the profile, and calculates threshold and gradient MLDs to assemble a suite of possible MLD values. It then analyzes the patterns in the suite to select a final MLD estimate. The new algorithm is provided in online supplemental materials. Developed using profiles from all oceans, the algorithm is compared to threshold methods that use the C. de Boyer Montégut et al. criteria and to gradient methods using 13 601 Argo profiles from the southeast Pacific and southwest Atlantic Oceans. In general, the threshold methods find deeper MLDs than the new algorithm and the gradient methods produce more anomalous MLDs than the new algorithm. When constrained to using only temperature profiles, the algorithm offers a clear improvement over the temperature threshold and gradient methods; the new temperature algorithm MLDs more closely approximate the density algorithm MLDs than the temperature threshold and gradient MLDs. The algorithm is applied to profiles from a formation region of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). The density algorithm finds that the deepest MLDs in this region routinely reach 500 dbar and occur north of the A. H. Orsi et al. mean Subantarctic Front in the southeastern Pacific Ocean. The deepest MLDs typically occur in August and September and are congruent with the subsurface salinity minimum, a signature of AAIW.

scholarly journals Deglacial intermediate water reorganization: new evidence from the Indian Ocean

2014 ◽  
Vol 10 (1) ◽  
pp. 293-303 ◽  
Author(s):  
S. Romahn ◽  
A. Mackensen ◽  
J. Groeneveld ◽  
J. Pätzold
Keyword(s):  
Indian Ocean ◽  
Southern Ocean ◽  
Ocean Circulation ◽  
Equatorial Indian Ocean ◽  
Western Indian Ocean ◽  
Intermediate Water ◽  
Mode Water ◽  
Climate Anomalies ◽  
Tunnel Mechanism ◽  
Intermediate Waters

Abstract. The importance of intermediate water masses in climate change and ocean circulation has been emphasized recently. In particular, Southern Ocean Intermediate Waters (SOIW), such as Antarctic Intermediate Water and Subantarctic Mode Water, are thought to have acted as active interhemispheric transmitter of climate anomalies. Here we reconstruct changes in SOIW signature and spatial and temporal evolution based on a 40 kyr time series of oxygen and carbon isotopes as well as planktic Mg/Ca based thermometry from Site GeoB12615-4 in the western Indian Ocean. Our data suggest that SOIW transmitted Antarctic temperature trends to the equatorial Indian Ocean via the "oceanic tunnel" mechanism. Moreover, our results reveal that deglacial SOIW carried a signature of aged Southern Ocean deep water. We find no evidence of increased formation of intermediate waters during the deglaciation.

New insights into concurrent air-sea heat flux forcing of Subantarctic Mode Water formation from mooring observations in the Southeast Indian and Southeast Pacific sectors of the Southern Ocean

2020 ◽  
Author(s):  
Simon Josey ◽  
Veronica Tamsitt ◽  
Ivana Cerovecki ◽  
Sarah Gille ◽  
Eric Schulz
Keyword(s):  
Heat Flux ◽  
Southern Ocean ◽  
Heat Loss ◽  
Indirect Pathway ◽  
The South ◽  
Mode Water ◽  
Cold Air ◽  
Subantarctic Mode Water ◽  
Southeast Pacific ◽  
Indian Sector

<p>Wintertime surface ocean heat loss is the key driver of Subantarctic Mode Water (SAMW) formation. However, until now there have been very few direct observations of fluxes, particularly during winter. Here, we present results from the first concurrent (2015-17 with gaps), air-sea flux mooring deployments in two key SAMW formation regions: the Southern Ocean Flux Site (SOFS) in the Southeast Indian sector and the Ocean Observatories Initiative (OOI) mooring in the Southeast Pacific sector. Gridded Argo and ERA5 reanalysis provide temporal and spatial context for the mooring observations. Turbulent ocean heat loss is found to be on average 1.5 times larger at the Southeast Indian than Southeast Pacific sites with stronger extreme heat flux events in the Southeast Indian leading to larger cumulative winter heat loss. For the first time, we show that turbulent heat loss events in the Southeast Indian sector occur in two atmospheric regimes (a direct cold air pathway from the south and an indirect pathway circulating dry Antarctic air via the north). In contrast, heat loss events in the Southeast Pacific sector occur in a single atmospheric regime (cold air from the south). On interannual timescales, wintertime anomalies in net heat flux and mixed layer depth (MLD) are often correlated at the two sites, particularly when wintertime MLDs are anomalously deep. Using ERA5, we show that this is part of a larger zonal dipole in heat flux and MLD anomalies present in both the Indian and Pacific SAMW formation regions, associated with anomalous meridional atmospheric circulation. These recent results will be placed in the context of multidecadal variability in the SAMW formation region dominant heat flux patterns over the past 40 years over all 3 sectors of the Southern Ocean (Pacific, Indian and Atlantic).</p>

scholarly journals Mooring Observations of Air–Sea Heat Fluxes in Two Subantarctic Mode Water Formation Regions

2020 ◽  
Vol 33 (7) ◽  
pp. 2757-2777 ◽  
Author(s):  
Veronica Tamsitt ◽  
Ivana Cerovečki ◽  
Simon A. Josey ◽  
Sarah T. Gille ◽  
Eric Schulz
Keyword(s):  
Heat Flux ◽  
Southern Ocean ◽  
Heat Loss ◽  
Heat Fluxes ◽  
The South ◽  
Mode Water ◽  
Cold Air ◽  
Subantarctic Mode Water ◽  
Basin Scale ◽  
Southeast Pacific

AbstractWintertime surface ocean heat loss is the key process driving the formation of Subantarctic Mode Water (SAMW), but there are few direct observations of heat fluxes, particularly during winter. The Ocean Observatories Initiative (OOI) Southern Ocean mooring in the southeast Pacific Ocean and the Southern Ocean Flux Station (SOFS) in the southeast Indian Ocean provide the first concurrent, multiyear time series of air–sea fluxes in the Southern Ocean from two key SAMW formation regions. In this work we compare drivers of wintertime heat loss and SAMW formation by comparing air–sea fluxes and mixed layers at these two mooring locations. A gridded Argo product and the ERA5 reanalysis product provide temporal and spatial context for the mooring observations. Turbulent ocean heat loss is on average 1.5 times larger in the southeast Indian (SOFS) than in the southeast Pacific (OOI), with stronger extreme heat flux events in the southeast Indian leading to larger cumulative winter ocean heat loss. Turbulent heat loss events in the southeast Indian (SOFS) occur in two atmospheric regimes (cold air from the south or dry air circulating via the north), while heat loss events in the southeast Pacific (OOI) occur in a single atmospheric regime (cold air from the south). On interannual time scales, wintertime anomalies in net heat flux and mixed layer depth (MLD) are often correlated at the two sites, particularly when wintertime MLDs are anomalously deep. This relationship is part of a larger basin-scale zonal dipole in heat flux and MLD anomalies present in both the Indian and Pacific basins, associated with anomalous meridional atmospheric circulation.

scholarly journals Southern Ocean Thermocline Ventilation

2010 ◽  
Vol 40 (3) ◽  
pp. 509-529 ◽  
Author(s):  
Jean-Baptiste Sallée ◽  
Kevin Speer ◽  
Steve Rintoul ◽  
S. Wijffels
Keyword(s):  
Southern Ocean ◽  
Cell Structure ◽  
Surface Fluxes ◽  
Ekman Transport ◽  
Intermediate Water ◽  
Ekman Pumping ◽  
Regional Variability ◽  
Mode Water ◽  
Argo Data ◽  
Thermocline Ventilation

Abstract An approximate mass (volume) budget in the surface layer of the Southern Ocean is used to investigate the intensity and regional variability of the ventilation process, discussed here in terms of subduction and upwelling. Ventilation resulting from Ekman pumping is estimated from satellite winds, the geostrophic mean component is assessed from a climatology strengthened with Argo data, and the eddy-induced advection is included via the parameterization of Gent and McWilliams, together with eddy mixing estimates. All three components contribute significantly to ventilation. Finally, the seasonal cycle of the upper ocean is resolved using Argo data. The circumpolar-averaged circulation shows an upwelling in the Antarctic Intermediate Water (AAIW) density classes, which is carried north into a zone of dense Subantarctic Mode Water (SAMW) subduction. Although no consistent net production is found in the light SAMW density classes, a large subduction of Subtropical Mode Water (STMW) is observed. The STMW area is fed by convergence of a southward and a northward residual meridional circulation. The eddy-induced contribution is important for the water mass transport in the vicinity of the Antartic Circumpolar Current. It balances the horizontal northward Ekman transport as well as the vertical Ekman pumping. While the circumpolar-averaged upper cell structure is consistent with the average surface fluxes, it hides strong longitudinal regional variations and does not represent any local regime. Subduction shows strong regional variability with bathymetrically constrained hotspots of large subduction. These hotspots are consistent with the interior potential vorticity structure and circulation in the thermocline. Pools of SAMW and AAIW of different densities are found along the circumpolar belt in association with the regional pattern of subduction and interior circulation.

Equatorial Pacific bulk 𝛿15N supports a secular increase in subantarctic zone nitrate utilization after the mid-Pleistocene Transition

2021 ◽  
Author(s):  
Jonathan Lambert ◽  
Kelly Gibson ◽  
Braddock Linsley ◽  
Samantha Bova ◽  
Yair Rosenthal ◽  
...  
Keyword(s):  
New Guinea ◽  
Isotopic Signature ◽  
Polar Front ◽  
Nitrogen Dynamics ◽  
Equatorial Pacific ◽  
Glacial Cycles ◽  
Mode Water ◽  
Subantarctic Mode Water ◽  
The Pacific ◽  
Basin Scale

<p>Pacific-wide measurements of nitrate and its isotopic composition have furthered our understanding of modern subsurface circulation and have revealed basin-scale connections between oceanographic and nitrogen cycle processes. From the Eastern Tropical Pacific (ETP), the isotopic signature of denitrification is spread zonally and meridionally via subsurface currents. From the Pacific sector of the Southern Ocean, Subantarctic Mode Water (SAMW) penetrates to the low latitudes, delivering nitrate (and likely its isotopic signature) to equatorial surface waters via upwelling. These two regional processes combine to inform much of the thermocline nitrogen dynamics of the Pacific. Here, we compare a new 1.4-Myr bulk sediment 𝛿<sup>15</sup>N record from the New Guinea margin (IODP Site U1486) to other Pacific 𝛿<sup>15</sup>N records to track Pleistocene changes in denitrification and SAMW properties. Our results highlight a dramatic increasing 𝛿<sup>15</sup>N trend after the mid-Pleistocene Transition (MPT) at equatorial sites that is not observed at the New Guinea and California margin sites. Strong 41-ky forcing at equatorial sites and little detectable influence from denitrification (counter to larger denitrification signals at margin sites) suggests increasing 𝛿<sup>15</sup>N within upwelled SAMW. Because the New Guinea and California margin sites are not below equatorial upwelling, thermocline nitrate is less influenced by SAMW, but rather tracks denitrification in the ETP.</p><p>As equatorial Pacific nitrate utilization has not dramatically increased in the late Pleistocene, an increase in subantarctic zone nitrate utilization is proposed. Initiation of increased nitrate utilization appears to commence near the end of the MPT and accelerate near the Mid-Brunhes Event (~430 ka). The observed southward shift of the polar front at this time (associated with increased sea surface temperature), combined with elevated dust/iron flux, may have contributed to greater nitrate utilization and a more efficient biological pump in the subantarctic zone. Through the production (via denitrification) and sequestration (via nitrate utilization) of greenhouse gases, these biogeochemical processes potentially participated in feedbacks associated with both the MPT and the Mid-Brunhes Event. Until reconstructions of subantarctic zone nitrate are extended beyond the last two glacial cycles, this reconstruction of SAMW properties via equatorial Pacific bulk 𝛿<sup>15</sup>N may provide the best record of long-term changes in nitrogen dynamics in the subantarctic zone.</p>

The Origin and Fate of Subantarctic Mode Water in the Southern Ocean

2021 ◽  
Author(s):  
Zhi Li ◽  
Matthew H. England ◽  
Sjoerd Groeskamp ◽  
Ivana Cerovečki ◽  
Yiyong Luo
Keyword(s):  
Mixed Layer ◽  
Formation Rate ◽  
Ekman Transport ◽  
The South ◽  
Regional Variability ◽  
Mode Water ◽  
South Indian ◽  
Subantarctic Mode Water ◽  
Circumpolar Current ◽  
Mixed Layers

AbstractSubantarctic Mode Water (SAMW) forms in deep mixed layers just north of the Antarctic Circumpolar Current in winter, playing a fundamental role in the ocean uptake of heat and carbon. Using a gridded Argo product and the ERA-Interim reanalysis for years 2004-2018, the seasonal evolution of the SAMW volume is analyzed using both a kinematic estimate of the subduction rate and a thermodynamic estimate of the air-sea formation rate. The seasonal SAMW volume changes are separately estimated within the monthly mixed layer and in the interior below it. We find that the variability of SAMW volume is dominated by changes in SAMW volume in the mixed layer. The seasonal variability of SAMW volume in the mixed layer is governed by formation due to air-sea buoyancy fluxes (45%, lasting from July to August), entrainment (35%), and northward Ekman transport across the Subantarctic Front (10%). The interior SAMW formation is entirely controlled by exchanges between the mixed layer and the interior (i.e. instantaneous subduction), which occurs mainly during August-October. The annual mean subduction estimate from a Lagrangian approach shows strong regional variability with hotspots of large SAMW subduction. The SAMW subduction hotspots are consistent with the distribution and export pathways of SAMW over the central and eastern parts of the south Indian and Pacific Oceans. Hotspots in the south Indian Ocean produce strong subduction of 8 and 9 Sv for the light and southeast Indian SAMW, respectively, while SAMW subduction of 6 and 4 Sv occurs for the central and southeast Pacific SAMW, respectively.

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