We explore the representation of the Atlantic Meridional Overturning Circulation (AMOC) in 27 models from the CMIP6 multimodel ensemble. Comparison with RAPID and SAMBA observations suggests that the ...ensemble mean represents the AMOC strength and vertical profile reasonably well. Linear trends over the entire historical period (1850–2014) are generally neutral, but many models exhibit an AMOC peak around the 1980s. Ensemble mean AMOC decline in future (SSP) scenarios is stronger in CMIP6 than CMIP5 models. In fact, AMOC decline in CMIP6 is surprisingly insensitive to the scenario at least up to 2060. We find an emergent relationship among a majority of models between AMOC strength and 21st century AMOC decline. Constraining this relationship with RAPID observations suggests that the AMOC might decline between 6 and 8 Sv (34–45%) by 2100. A smaller group of models projects much less AMOC weakening of only up to 30%.
Plain Language Summary
The Atlantic Meridional Overturning Circulation (AMOC) is a circulation pattern in the Atlantic Ocean that is an important component of the climate system, due to its ability to redistribute and sequester heat and carbon. An accurate representation of the AMOC is a critical test for climate models and essential for building confidence in their projections. Here we investigate the AMOC in 27 climate models that contributed simulations to the Coupled Model Intercomparison Project Phase 6 (CMIP6). We find that many models reproduce the observed AMOC quite well, but there are still several models in which the AMOC is too weak or too strong. Most models suggest a slight upward trend in the AMOC from 1850 to the 1980s. Simulations representing different scenarios for future socioeconomic development suggest a stronger AMOC decline compared to previous assessments. Using direct measurements of the AMOC since 2004 and an emerging across‐model relationship between AMOC decline in the 21st century and their present‐day mean state, we find that the majority of CMIP6 models point to an end of century AMOC weakening of 34–45% of its present‐day strength. A smaller group of models projects much less weakening of only up to 30% of its present state.
Key Points
AMOC mean strength is well reproduced by the CMIP6 multimodel mean, but large model spread persists
Projected AMOC decline by the end of the 21st century shows weak dependence on the SSP scenarios
An emergent constraint between AMOC strength and projected decline suggests possible AMOC decline between 34% and 45% by 2100
The notion that the Atlantic Meridional Overturning Circulation (AMOC) can have more than one stable equilibrium emerged in the 1980s as a powerful hypothesis to explain rapid climate variability ...during the Pleistocene. Ever since, the idea that a temporary perturbation of the AMOC—or a permanent change in its forcing—could trigger an irreversible collapse has remained a reason for concern. Here we review literature on the equilibrium stability of the AMOC and present a synthesis that puts our understanding of past and future AMOC behavior in a unifying framework. This framework is based on concepts from Dynamical Systems Theory, which has proven to be an important tool in interpreting a wide range of model behavior. We conclude that it cannot be ruled out that the AMOC in our current climate is in, or close to, a regime of multiple equilibria. But there is considerable uncertainty in the location of stability thresholds with respect to our current climate state, so we have no credible indications of where our present‐day AMOC is located with respect to thresholds. We conclude by identifying gaps in our knowledge and proposing possible ways forward to address these gaps.
Key Points
It cannot be ruled out that the current or future AMOC is in a regime of multiple equilibria
Determining the proximity of the AMOC to stability thresholds is a main research challenge
Further research is needed to identify observable metrics that distinguish between an AMOC slowdown and a collapse
The sensitivity of the Atlantic Meridional Overturning Circulation (AMOC) to high‐latitude freshwater input is one of the key uncertainties in the climate system. Considering the importance of the ...AMOC for global heat transports, and the vulnerability of the Greenland Ice Sheet (GrIS) to global warming, assessing this sensitivity is critical for climate change projections. Here we present a unique set of computational experiments to investigate the adjustment of the AMOC to enhanced melt water from the GrIS under present‐day conditions. For the first time, the response in a global, strongly‐eddying ocean model is systematically compared to that of an ocean model typical of IPCC‐class climate models. We find that the overall decline of the AMOC on decadal time scales is quantitatively similar (<10%) in the two configurations. Nonetheless, the transient response is significantly different, as the AMOC decline and reduction in wintertime convection is markedly more gradual and persistent in the strongly‐eddying configuration.
Key Points
Mesoscale features impact the response of the AMOC to GrIS meltwater input
Overall AMOC decline on decadal time scales robust
Transient response is more gradual and more persistent when resolving eddies
Interocean exchange of heat and salt around South Africa is thought to be a key link in the maintenance of the global overturning circulation of the ocean. It takes place at the Agulhas ...Retroflection, largely by the intermittent shedding of enormous rings that penetrate into the South Atlantic Ocean. This makes it extremely hard to estimate the inter ocean fluxes. Estimates of direct Agulhas leakage from hydrographic and tracer data range between 2 and 10 Sv (1 Sv = 106 m3 s−1). The average ring shedding frequency, determined from satellite information, is approximately six rings per year. Their associated interocean volume transport is between 0.5 and 1.5 Sv per ring. A number of Agulhas rings have been observed to cross the South Atlantic. They decay exponentially to less than half their initial size (measured by their available potential energy) within 1000 km from the shedding region. Consequently, most of their properties mix into the surroundings of the Benguela region, probably feeding directly into the upper (warm) limb of the global thermohaline circulation. The most recent observations suggest that in the present situation Agulhas water and Antarctic Intermediate Water are about equally important sources for the Benguela Current. Variations in the strength of these may lead to anomalous stratification and stability of the Atlantic at decadal and longer timescales. Modeling studies suggest that the Indian‐Atlantic interocean exchange is strongly related to the structure of the wind field over the South Indian Ocean. This leads in the mean to a subtropical supergyre wrapping around the subtropical gyres of the South Indian and Atlantic Oceans. However, local dynamical processes in the highly nonlinear regime around South Africa play a crucial role in inhibiting the connection between the two oceans. The regional bottom topography also seems to play an important role in locking the Agulhas Currents' retroflection. State‐of‐the‐art global and regional “eddy‐permitting” models show a reasonably realistic representation of the mean Agulhas system; but the mesoscale variability and the local geometrical and topographic features that determine largely the interocean fluxes still need considerable improvement. In this article we present a review of the above mentioned aspects of the interocean exchange around South Africa: the estimation of the fluxes into the South Atlantic from different types of observations, our present level of understanding of the exchanges dynamics and forcing, its representation in state‐of‐the‐art models, and, finally, the impact of the Indian‐Atlantic fluxes on regional and global scale both within the Atlantic Ocean and in interaction with the overlying atmosphere.
The heat and salt input from the Indian to Atlantic Oceans by Agulhas Leakage is found to influence the Atlantic overturning circulation in a low-resolution Ocean General Circulation Model (OGCM). ...The model used is the Hamburg Large-Scale Geostrophic (LSG) model, which is forced by mixed boundary conditions. Agulhas Leakage is parameterized by sources of heat and salt in the upper South Atlantic Ocean, which extend well into the intermediate layers.
It is shown that the model's overturning circulation is sensitive to the applied sources of heat and salt. The response of the overturning strength to changes in the source amplitudes is mainly linear, interrupted once by a stepwise change. The South Atlantic buoyancy sources influence the Atlantic overturning strength by modifying the basin-scale meridional density and pressure gradients. The non-linear, stepwise response is caused by abrupt changes in the convective activity in the northern North Atlantic.
Two additional experiments illustrate the adjustment of the overturning circulation upon sudden introduction of heat and salt sources in the South Atlantic. The North Atlantic overturning circulation responds within a few years after the sources are switched on. This is the time it takes for barotropic and baroclinic Kelvin waves to reach the northern North Atlantic in this model. The advection of the anomalies takes three decades to reach the northern North Atlantic.
The model results give support to the hypothesis that the re-opening of the Agulhas Gap at the end of the last ice-age, as indicated by palaeoclimatological data, may have stimulated the coincident strengthening of the Atlantic overturning circulation.
In this paper we introduce a new software framework for the offline calculation of tracer transport in the ocean. The Fast Equilibration of Ocean Tracers Software (FEOTS) is an end-to-end set of ...tools to efficiently calculate tracer distributions on a global or regional sub-domain using transport operators diagnosed from a comprehensive ocean model. To the best of our knowledge, this is the first application of a transport matrix model to an eddying ocean state. While a Newton–Krylov-based equilibration capability is still under development and not presented here, we demonstrate in this paper the transient modeling capabilities of FEOTS in an application focused on the Argentine Basin, where intense eddy activity and the Zapiola Anticyclone lead to strong mixing of water masses. The demonstration illustrates progress in developing offline passive tracer simulation capabilities, while highlighting the challenges of the impulse response functions approach in capturing tracer transports by a non-linear advection scheme. Our future work will focus on improving the computational efficiency of the code to reduce time-to-solution, using different basis functions to better represent non-linear advection operators, applying FEOTS to a parent model with unstructured grids (Ocean Model for Prediction Across Scales, MPAS-Ocean), and fully implementing a Newton–Krylov steady-state solver.
Normal modes of the Mascarene Basin Weijer, Wilbert
Deep-sea research. Part I, Oceanographic research papers,
2008, 2008-1-00, 20080101, Volume:
55, Issue:
1
Journal Article
Peer reviewed
In this paper the origin of the bi-monthly variability in the Mascarene Basin is reconsidered. Free oscillatory modes of the Mascarene Basin are determined by performing normal mode analysis on the ...motionless solution in a barotropic shallow-water model with realistic bathymetry. Several modes are identified with monthly to bi-monthly time scales. The mode that agrees best with recent current meter observations can be interpreted as a barotropic Rossby basin mode, confined to the tilted geometry of the Mascarene Basin.
Objectives To compare the incidence of vaginal spotting bleeding events and breast pain between therapy with tibolone 2.5 mg and continuous combined transdermal estradiol (E2) norethisterone acetate ...(NETA) 50 μg 140 μg after 24 weeks of treatment.
Methods A double-blind, double-dummy, randomized, controlled trial was performed and assessments were performed at baseline, week 12 and week 24. Bleeding spotting events were recorded in a daily diary. Breast signs and symptoms were collected as adverse events.
Results A total of 403 women (mean age 56 years) were randomized. Bleeding spotting events during weeks 1-12 with tibolone and E2 NETA were experienced by 16% and 56% of women, respectively (p < 0.001). The corresponding percentages during weeks 13-24 were 12% and 51%, respectively (p < 0.001). E2 NETA was significantly more likely than tibolone to be associated with vaginal hemorrhage (11% vs. 0%; p < 0.001) and breast signs and symptoms (11% vs. 4%; p = 0.015). Early discontinuations resulting from adverse events were significantly more common in the E2 NETA group than in the tibolone group (20% vs. 12%), primarily related to withdrawal due to vaginal hemorrhage (8% vs. 0%).
Conclusions Tibolone has a significantly better tolerability profile than transdermal E2 NETA as measured by vaginal bleeding, breast pain and treatment continuation.
The role played by interocean fluxes of buoyancy in stabilizing the present-day overturning circulation of the Atlantic Ocean is examined. A 2D model of the Atlantic overturning circulation is used ...in which the interocean fluxes of heat and salt are represented by sources and sinks.
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