scholarly journals Characteristics and robustness of Agulhas Leakage estimates: an inter-comparison study of Lagrangian methods

2021 ◽  
Author(s):  
Christina Schmidt ◽  
Franziska Schwarzkopf ◽  
Siren Rühs ◽  
Arne Biastoch
Keyword(s):  
General Circulation ◽  
General Circulation Models ◽  
Horizontal Resolution ◽  
Meridional Overturning Circulation ◽  
Clear Trend ◽  
Agulhas Current ◽  
Ocean General Circulation Models ◽  
Cape Basin ◽  
Meridional Overturning ◽  
The Indian Ocean

<p>The exchange of water between the Indian Ocean and South Atlantic with their different thermohaline properties via Agulhas Leakage is important for the meridional overturning circulation. Agulhas Leakage as well as the output of ocean general circulation models in general can be analysed using a Lagrangian approach with a variety of different tools available. Here, Agulhas Leakage is estimated with both the newly developed tool Parcels and the well established tool Ariane, and different designs of the Lagrangian experiment are analysed. In a hindcast simulation with the eddy-rich ocean sea-ice model INALT20 (1/20° horizontal resolution) under the new JRA55-do forcing, Agulhas Leakage increases from the early 1960s to mid 1980s, but there is no clear trend afterwards, which is in contrast to earlier studies using hindcast simulations under the CORE forcing. During the transit from the Agulhas Current at 32°S to the Cape Basin, a cooling and freshening of Agulhas Leakage waters occurs especially in the western part of the Retroflection, resulting in a density increase as the thermal effect dominates. The average transport, its variability, trend and the transit time from the Agulhas Current to the Cape Basin of Agulhas Leakage is simulated equally with the Lagrangian tools Ariane and Parcels, emphasising the robustness of our method.</p>

scholarly journals Submesoscale flows impact Agulhas leakage in ocean simulations

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
René Schubert ◽  
Jonathan Gula ◽  
Arne Biastoch
Keyword(s):  
Indian Ocean ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Ocean Water ◽  
Agulhas Current ◽  
Cape Basin ◽  
Northern Edge ◽  
Meridional Overturning ◽  
The Indian Ocean ◽  
Shear Edge

AbstractAgulhas leakage, the warm and salty inflow of Indian Ocean water into the Atlantic Ocean, is of importance for the climate-relevant Atlantic Meridional Overturning Circulation. South of Africa, the eastward turning Agulhas Current sheds Agulhas rings, cyclones and filaments of order 100 km that carry the Indian Ocean water into the Cape Basin and further into the Atlantic. Here, we show that the resolution of submesoscale flows of order 10 km in an ocean model leads to 40 % more Agulhas leakage and more realistic Cape Basin water-masses compared to a parallel non-submesoscale resolving simulation. Moreover, we show that submesoscale flows strengthen shear-edge eddies and in consequence lee cyclones at the northern edge of the Agulhas Current, as well as the leakage pathway in the region of the filaments that takes place outside of mesoscale eddies. This indicates that the increase in leakage can be attributed to stronger Agulhas filaments, when submesoscale flows are resolved.

scholarly journals Characteristics and robustness of Agulhas leakage estimates: an inter-comparison study of Lagrangian methods

Ocean Science ◽  
2021 ◽  
Vol 17 (4) ◽  
pp. 1067-1080
Author(s):  
Christina Schmidt ◽  
Franziska U. Schwarzkopf ◽  
Siren Rühs ◽  
Arne Biastoch
Keyword(s):  
Global Climate ◽  
Horizontal Resolution ◽  
South Atlantic ◽  
Comparison Study ◽  
The South ◽  
Agulhas Current ◽  
Cape Basin ◽  
Total Transport ◽  
The Indian Ocean ◽  
Ice Model

Abstract. The inflow of relatively warm and salty water from the Indian Ocean into the South Atlantic via Agulhas leakage is important for the global overturning circulation and the global climate. In this study, we analyse the robustness of Agulhas leakage estimates as well as the thermohaline property modifications of Agulhas leakage south of Africa. Lagrangian experiments with both the newly developed tool Parcels and the well established tool Ariane were performed to simulate Agulhas leakage in the eddy-rich ocean–sea-ice model INALT20 (1/20∘ horizontal resolution) forced by the JRA55-do atmospheric boundary conditions. The average transport, its variability, trend and the transit time from the Agulhas Current to the Cape Basin of Agulhas leakage is simulated comparably with both Lagrangian tools, emphasizing the robustness of our method. Different designs of the Lagrangian experiment alter in particular the total transport of Agulhas leakage by up to 2 Sv, but the variability and trend of the transport are similar across these estimates. During the transit from the Agulhas Current at 32∘ S to the Cape Basin, a cooling and freshening of Agulhas leakage waters occurs especially at the location of the Agulhas Retroflection, resulting in a density increase as the thermal effect dominates. Beyond the strong air–sea exchange around South Africa, Agulhas leakage warms and salinifies the water masses below the thermocline in the South Atlantic.

scholarly journals High-latitude obliquity as a dominant forcing in the Agulhas current system

2011 ◽  
Vol 7 (4) ◽  
pp. 1285-1296 ◽  
Author(s):  
T. Caley ◽  
J.-H. Kim ◽  
B. Malaizé ◽  
J. Giraudeau ◽  
T. Laepple ◽  
...  
Keyword(s):  
High Latitude ◽  
Global Climate ◽  
Current System ◽  
Meridional Overturning Circulation ◽  
Agulhas Current ◽  
Meridional Overturning ◽  
The Indian Ocean ◽  
Forcing Mechanisms ◽  
Glacial Periods

Abstract. The Agulhas Current transport of heat and salt from the Indian Ocean into the South Atlantic around South Africa (Agulhas leakage), can affect the Atlantic meridional overturning circulation (AMOC) and, thus, influence global climate. However, efforts to elucidate forcing mechanisms connecting the Agulhas leakage with the upstream dynamics of the current have been hampered by a lack of climate records extracted from the area where the Agulhas current originates. We determine 800-kyr sea surface temperature (SST) and salinity (SSS) records from the "precursor" region of the Agulhas current and show that these records contain strong 100-kyr and 41-kyr cycles. This latter obliquity-driven cycle is nearly in phase with changes in the annual mean insolation and air temperature at high southern latitudes. In contrast, our SST and SSS records did not reveal precession-driven cycles, which is surprising given the low-latitude location of the upstream Agulhas current. Together, this indicates that the dynamics of the Agulhas current system is mainly controlled by high latitude obliquity through its influence on the position of the Southern Hemisphere subtropical front (STF) and its associated westerlies. Our study demonstrates that obliquity may drive an important part of the 100 kyr cycles observed in the system rather than precession. Our results also suggest that a stronger Agulhas current, associated with a northward shift of the wind system during glacial periods, leads to reduced leakage, in accordance with the theory. We argue that during terminations, stronger Agulhas leakage of heat and salt was triggered by increased obliquity exerting a positive feedback on the global climate system through modulating long-term AMOC variations.

scholarly journals Characteristics and robustness of Agulhas leakage estimates: an inter-comparison study of Lagrangian methods

2021 ◽  
Author(s):  
Christina Schmidt ◽  
Franziska U. Schwarzkopf ◽  
Siren Rühs ◽  
Arne Biastoch
Keyword(s):  
Global Climate ◽  
Horizontal Resolution ◽  
South Atlantic ◽  
Comparison Study ◽  
The South ◽  
Agulhas Current ◽  
Cape Basin ◽  
Total Transport ◽  
The Indian Ocean ◽  
Ice Model

Abstract. The inflow of relatively warm and salty water from the Indian Ocean into the South Atlantic via Agulhas leakage is important for the global overturning circulation and the global climate. In this study, we analyse the robustness of Agulhas leakage estimates as well as the thermohaline property modifications of Agulhas leakage south of Africa. Lagrangian experiments with both the newly developed tool Parcels and the well established tool Ariane were performed to simulate Agulhas leakage in the eddy-rich ocean-sea ice model INALT20 (1/20° horizontal resolution) forced by the JRA55-do atmospheric boundary conditions. The average transport, its variability, trend and the transit time from the Agulhas Current to the Cape Basin of Agulhas leakage is simulated comparably with both Lagrangian tools, emphasising the robustness of our method. Different designs of the Lagrangian experiment alter in particular the total transport of Agulhas leakage by up to 2 Sv, but the variability and trend of the transport is similar across these estimates. During the transit from the Agulhas Current at 32° S to the Cape Basin, a cooling and freshening of Agulhas leakage waters occurs especially at the location of the Agulhas Retroflection, resulting in a density increase as the thermal effect dominates. Beyond the strong air-sea exchange around South Africa, Agulhas leakage warms and salinifies the water masses below the thermocline in the South Atlantic.

Why is the AMOC Monostable in Coupled General Circulation Models?

2014 ◽  
Vol 27 (6) ◽  
pp. 2427-2443 ◽  
Author(s):  
Wei Liu ◽  
Zhengyu Liu ◽  
Esther C. Brady
Keyword(s):  
General Circulation ◽  
General Circulation Models ◽  
Meridional Overturning Circulation ◽  
Surface Salinity ◽  
Sensitivity Experiment ◽  
Climate System Model ◽  
Control Simulation ◽  
Coupled General Circulation Models ◽  
The Stability ◽  
Flux Adjustment

Abstract This paper is concerned with the question: why do coupled general circulation models (CGCM) seem to be biased toward a monostable Atlantic meridional overturning circulation (AMOC)? In particular, the authors investigate whether the monostable behavior of the CGCMs is caused by a bias of model surface climatology. First observational literature is reviewed, and it is suggested that the AMOC is likely to be bistable in the real world in the past and present. Then the stability of the AMOC in the NCAR Community Climate System Model, version 3 (CCSM3) is studied by comparing the present-day control simulation (without flux adjustment) with a sensitivity experiment with flux adjustment. It is found that the monostable AMOC in the control simulation is altered to a bistable AMOC in the flux-adjustment experiment because a reduction of the surface salinity biases in the tropical and northern North Atlantic leads to a reduction of the bias of freshwater transport in the Atlantic. In particular, the tropical bias associated with the double ITCZ reduces salinity in the upper South Atlantic Ocean and, in turn, the AMOC freshwater export, which tends to overstabilize the AMOC and therefore biases the AMOC from bistable toward monostable state. This conclusion is consistent with a further analysis of the stability indicator of two groups of IPCC Fourth Assessment Report (AR4) CGCMs: one without and the other with flux adjustment. Because the tropical bias is a common feature among all CGCMs without flux adjustment, the authors propose that the surface climate bias, notably the tropical bias in the Atlantic, may contribute significantly to the monostability of AMOC behavior in current CGCMs.

scholarly journals Sea-air CO2fluxes and carbon transport: A comparison of three ocean general circulation models

2000 ◽  
Vol 14 (4) ◽  
pp. 1267-1281 ◽  
Author(s):  
J. L. Sarmiento ◽  
P. Monfray ◽  
E. Maier-Reimer ◽  
O. Aumont ◽  
R. J. Murnane ◽  
...  
Keyword(s):  
General Circulation ◽  
General Circulation Models ◽  
Ocean General Circulation Models ◽  
Ocean General Circulation ◽  
Carbon Transport

scholarly journals The sloshing and diapycnal meridional overturning circulations in the Indian Ocean

2020 ◽  
Author(s):  
Lei Han
Keyword(s):  
Indian Ocean ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Western Coast ◽  
Ekman Pumping ◽  
Meridional Heat Transport ◽  
Vertical Movements ◽  
Sloshing Mode ◽  
Meridional Overturning ◽  
The Indian Ocean

AbstractThe meridional overturning circulation (MOC) seasonality in the Indian Ocean is investigated with the ocean state estimate product, ECCO v4r3. The vertical movements of water parcels are predominantly due to the heaving of the isopycnals all over the basin except off the western coast. Aided by the linear propagation equation of long baroclinic Rossby waves, the driving factor determining the strength of the seasonal MOC in the Indian Ocean is identified as the zonally-integrated Ekman pumping anomaly, rather than the Ekman transport concluded in earlier studies. A new concept of sloshing MOC is proposed, and its difference with the classic Eulerian MOC leads to the so-called diapycnal MOC. The striking resemblance of the Eulerian and sloshing MOCs implies the seasonal variation of the Eulerian MOC in the Indian Ocean is a sloshing mode. The shallow overturning cells manifest themselves in the diapycnal MOC as the most remarkable structure. New perspectives on the upwelling branch of the shallow overturn in the Indian Ocean are offered based on diapycnal vertical velocity. The discrepancy among the observation-based estimates on the bottom inflow across 32°S of the basin is interpreted with the seasonal sloshing mode. Consequently, the “missing mixing” in the deep Indian Ocean is attributed to the overestimated diapycnal volume fluxes. Decomposition of meridional heat transport (MHT) into sloshing and diapycnal components clearly shows the dominant mechanism of MHT in the Indian Ocean in various seasons.

scholarly journals Variability of the Atlantic Meridional Overturning Circulation in CCSM4

2012 ◽  
Vol 25 (15) ◽  
pp. 5153-5172 ◽  
Author(s):  
Gokhan Danabasoglu ◽  
Steve G. Yeager ◽  
Young-Oh Kwon ◽  
Joseph J. Tribbia ◽  
Adam S. Phillips ◽  
...  
Keyword(s):  
Low Frequency ◽  
Surface Flux ◽  
Atlantic Meridional Overturning Circulation ◽  
Horizontal Resolution ◽  
Meridional Overturning Circulation ◽  
Positive Density ◽  
Overturning Circulation ◽  
Climate System Model ◽  
Control Simulation ◽  
Meridional Overturning

Abstract Atlantic meridional overturning circulation (AMOC) variability is documented in the Community Climate System Model, version 4 (CCSM4) preindustrial control simulation that uses nominal 1° horizontal resolution in all its components. AMOC shows a broad spectrum of low-frequency variability covering the 50–200-yr range, contrasting sharply with the multidecadal variability seen in the T85 × 1 resolution CCSM3 present-day control simulation. Furthermore, the amplitude of variability is much reduced in CCSM4 compared to that of CCSM3. Similarities as well as differences in AMOC variability mechanisms between CCSM3 and CCSM4 are discussed. As in CCSM3, the CCSM4 AMOC variability is primarily driven by the positive density anomalies at the Labrador Sea (LS) deep-water formation site, peaking 2 yr prior to an AMOC maximum. All processes, including parameterized mesoscale and submesoscale eddies, play a role in the creation of salinity anomalies that dominate these density anomalies. High Nordic Sea densities do not necessarily lead to increased overflow transports because the overflow physics is governed by source and interior region density differences. Increased overflow transports do not lead to a higher AMOC either but instead appear to be a precursor to lower AMOC transports through enhanced stratification in LS. This has important implications for decadal prediction studies. The North Atlantic Oscillation (NAO) is significantly correlated with the positive boundary layer depth and density anomalies prior to an AMOC maximum. This suggests a role for NAO through setting the surface flux anomalies in LS and affecting the subpolar gyre circulation strength.

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