Impact of atmospheric and oceanic feedbacks on the stability of the meridional overturning circulation

The fundamental climatic importance of the Atlantic Meridional Overturning Circulation (AMOC) lies in the northward heat transport in the Atlantic Ocean associated with this component of the circulation. Despite its importance, the understanding of the AMOC dynamics, and in particular of its stability properties, is fragmentary at best. Paleoclimatic data and numerical models suggest that the AMOC may undergo abrupt, irreversible collapses if appropriately perturbed, in particular by freshwater anomalies in the northern North Atlantic. However, it is unclear whether such an abrupt transition is possible in the real ocean, and some state of the art coupled climate models show in fact no such collapse. This different behaviour has generally been attributed to deficiencies of simpler numerical models, but it has also been suggested that biases of freshwater transport in coupled climate models may prevent any irreversible collapse of the AMOC therein. In this thesis, a minimal atmospheric model is derived from a coarse resolution numerical model using linear regressions of surface fluxes. Combining an ocean general circulation model and this minimal atmospheric model, a Hybrid Coupled Model (HCM) was implemented and tested. The HCM was then used for studying the sensitivty of the AMOC stability to changes in the freshwater budget of the Atlantic Ocean. The numerical simulations performed indicate that the zonal salinity gradient at the southern end of the Atlantic Ocean plays a key role in controlling the sensitivity of the AMOC to freshwater perturbations. These results show that the AMOC response to external perturbations is strongly affected by the freshwater budget of the Atlantic Ocean, and by the biases that affect its representation in climate models. These results also stress the importance of the freshwater transport by the overturning circulation for the stability of the overturning circulation itself. These findings were confirmed and extended in a different numerical model, and further studied in the highly idealised framework of a box model. This simple model includes a basic representation of the Atlantic basin with a periodic channel at its southern end. In particular, it is suggested that an overturning rate scaling with the meridional density gradient is an essential element. Furthermore, it is shown under which conditions the freshwater transport by the overturning circulation can measure the AMOC stability in the model. The relationship between the meridional density gradient and the overturning circulation rate is further studied in an idealised numerical model which includes a basin spanning two hemispheres and a periodic channel at its southern end. It is shown that, even if the AMOC is in geostrophic balance, the meridional density gradient is highly correlated with the overturning rate. This is connected to the stratification induced by the presence of a periodic channel in the south. The AMOC strength is determined not only by the dense water formation in the north, but also by the water properties at the southern end of the Atlantic Ocea

Impact of atmospheric and oceanic feedbacks on the stability of the meridional overturning circulation

The fundamental climatic importance of the Atlantic Meridional Overturning Circulation (AMOC) lies in the northward heat transport in the Atlantic Ocean associated with this component of the circulation. Despite its importance, the understanding of the AMOC dynamics, and in particular of its stability properties, is fragmentary at best. Paleoclimatic data and numerical models suggest that the AMOC may undergo abrupt, irreversible collapses if appropriately perturbed, in particular by freshwater anomalies in the northern North Atlantic. However, it is unclear whether such an abrupt transition is possible in the real ocean, and some state of the art coupled climate models show in fact no such collapse. This different behaviour has generally been attributed to deficiencies of simpler numerical models, but it has also been suggested that biases of freshwater transport in coupled climate models may prevent any irreversible collapse of the AMOC therein. In this thesis, a minimal atmospheric model is derived from a coarse resolution numerical model using linear regressions of surface fluxes. Combining an ocean general circulation model and this minimal atmospheric model, a Hybrid Coupled Model (HCM) was implemented and tested. The HCM was then used for studying the sensitivty of the AMOC stability to changes in the freshwater budget of the Atlantic Ocean. The numerical simulations performed indicate that the zonal salinity gradient at the southern end of the Atlantic Ocean plays a key role in controlling the sensitivity of the AMOC to freshwater perturbations. These results show that the AMOC response to external perturbations is strongly affected by the freshwater budget of the Atlantic Ocean, and by the biases that affect its representation in climate models. These results also stress the importance of the freshwater transport by the overturning circulation for the stability of the overturning circulation itself. These findings were confirmed and extended in a different numerical model, and further studied in the highly idealised framework of a box model. This simple model includes a basic representation of the Atlantic basin with a periodic channel at its southern end. In particular, it is suggested that an overturning rate scaling with the meridional density gradient is an essential element. Furthermore, it is shown under which conditions the freshwater transport by the overturning circulation can measure the AMOC stability in the model. The relationship between the meridional density gradient and the overturning circulation rate is further studied in an idealised numerical model which includes a basin spanning two hemispheres and a periodic channel at its southern end. It is shown that, even if the AMOC is in geostrophic balance, the meridional density gradient is highly correlated with the overturning rate. This is connected to the stratification induced by the presence of a periodic channel in the south. The AMOC strength is determined not only by the dense water formation in the north, but also by the water properties at the southern end of the Atlantic Ocea

Reconciling the north–south density difference scaling for the Meridional Overturning Circulation strength with geostrophy

Since the formulation of the Stommel two-box model for the meridional overturning circulation (MOC), various theoretical and conceptual models for the MOC emerged based on scaling the MOC strength with the north south density difference. At the same time the MOC should obey geostrophic balance with an east-west density difference. Scaling with the north south density gradient seems to violate the common assumption of geostrophic balance for the large-scale circulation, which implies that the pressure gradient is orthogonal to the flow. In this brief report, we report on the results of a series of numerical simulations in an idealized ocean basin (with a zonally periodic channel at its southern end). The simulations performed with different surface forcing conditions indicate that the meridional and zonal density gradients, important for the MOC strength, are in fact related to each other through the stratification located at the northern end of the periodic channel. The results suggest that the water properties at the northern end of the periodic channel play a crucial role in setting the MOC strength, possibly explaining the sensitivity of climate models to the conditions in this area

Meridional overturning circulation: stability and ocean feedbacks in a box model

A box model of the inter-hemispheric Atlantic meridional overturning circulation is developed, including a variable pycnocline depth for the tropical and subtropical regions. The circulation is forced by winds over a periodic channel in the south and by freshwater forcing at the surface. The model is aimed at investigating the ocean feedbacks related to perturbations in freshwater forcing from the atmosphere, and to changes in freshwater transport in the ocean. These feedbacks are closely connected with the stability properties of the meridional overturning circulation, in particular in response to freshwater perturbations. A separate box is used for representing the region north of the Antarctic circumpolar current in the Atlantic sector. The density difference between this region and the north of the basin is then used for scaling the downwelling in the north. These choices are essential for reproducing the sensitivity of the meridional overturning circulation observed in general circulation models, and therefore suggest that the southernmost part of the Atlantic Ocean north of the Drake Passage is of fundamental importance for the stability of the meridional overturning circulation. With this configuration, the magnitude of the freshwater transport by the southern subtropical gyre strongly affects the response of the meridional overturning circulation to external forcing. The role of the freshwater transport by the overturning circulation (Mov) as a stability indicator is discussed. It is investigated under which conditions its sign at the latitude of the southern tip of Africa can provide information on the existence of a second, permanently shut down, state of the overturning circulation in the box model. Mov will be an adequate indicator of the existence of multiple equilibria only if salt-advection feedback dominates over other processes in determining the response of the circulation to freshwater anomalies. Mov is a perfect indicator if feedbacks other than salt-advection are negligible

Meridional overturning circulation: stability and ocean feedbacks in a box model

A box model of the inter-hemispheric Atlantic meridional overturning circulation is developed, including a variable pycnocline depth for the tropical and subtropical regions. The circulation is forced by winds over a periodic channel in the south and by freshwater forcing at the surface. The model is aimed at investigating the ocean feedbacks related to perturbations in freshwater forcing from the atmosphere, and to changes in freshwater transport in the ocean. These feedbacks are closely connected with the stability properties of the meridional overturning circulation, in particular in response to freshwater perturbations. A separate box is used for representing the region north of the Antarctic circumpolar current in the Atlantic sector. The density difference between this region and the north of the basin is then used for scaling the downwelling in the north. These choices are essential for reproducing the sensitivity of the meridional overturning circulation observed in general circulation models, and therefore suggest that the southernmost part of the Atlantic Ocean north of the Drake Passage is of fundamental importance for the stability of the meridional overturning circulation. With this configuration, the magnitude of the freshwater transport by the southern subtropical gyre strongly affects the response of the meridional overturning circulation to external forcing. The role of the freshwater transport by the overturning circulation (Mov) as a stability indicator is discussed. It is investigated under which conditions its sign at the latitude of the southern tip of Africa can provide information on the existence of a second, permanently shut down, state of the overturning circulation in the box model. Mov will be an adequate indicator of the existence of multiple equilibria only if salt-advection feedback dominates over other processes in determining the response of the circulation to freshwater anomalies. Mov is a perfect indicator if feedbacks other than salt-advection are negligible

Reconciling the north–south density difference scaling for the Meridional Overturning Circulation strength with geostrophy

Since the formulation of the Stommel two-box model for the meridional overturning circulation (MOC), various theoretical and conceptual models for the MOC emerged based on scaling the MOC strength with the north south density difference. At the same time the MOC should obey geostrophic balance with an east-west density difference. Scaling with the north south density gradient seems to violate the common assumption of geostrophic balance for the large-scale circulation, which implies that the pressure gradient is orthogonal to the flow. In this brief report, we report on the results of a series of numerical simulations in an idealized ocean basin (with a zonally periodic channel at its southern end). The simulations performed with different surface forcing conditions indicate that the meridional and zonal density gradients, important for the MOC strength, are in fact related to each other through the stratification located at the northern end of the periodic channel. The results suggest that the water properties at the northern end of the periodic channel play a crucial role in setting the MOC strength, possibly explaining the sensitivity of climate models to the conditions in this area

The Importance of Systematic Spatial Variability in the Surface Heat Flux of a Large Lake: A Multiannual Analysis for Lake Geneva

The spatiotemporal surface heat flux (SurHF) distribution over Lake Geneva, the largest lake in Western Europe, was estimated for a 7‐year period (2008–2014). Data sources included hourly maps of over‐the‐lake assimilated meteorological data from a validated numerical weather model and lake surface water temperature (LSWT) from satellite imagery. A set of bulk algorithms, previously optimized and calibrated at two locations in Lake Geneva, was used. Results indicate a systematic long‐term average spatial range of >40 Wm‐2 between different parts of the lake and little year‐to‐year variability. This variability is mainly due to topographically induced wind sheltering over parts of the lake, which in turn produces spatial variability in the sensible and latent heat fluxes. These results are supported by a systematic spatial heat content variability obtained from long‐term temperature profile measurements in the lake. During spring, a lower SurHF spatial range was evident. Unlike other seasons, the spring spatial variability of air‐water temperature differences and, to a lesser extent, the global radiation variability resulting from sheltering by the mountainous topography were the main drivers of the SurHF spatial variability. Analysis of the atmospheric thermal boundary layer showed stable conditions from March to early June and unstable conditions for the rest of the year. This regime change can explain the low SurHF spatial variability observed during spring. The results emphasize that spatial variability in meteorological and LSWT patterns, and consequently in the spatiotemporal SurHF data, should be considered when assessing the time evolution of the heat budget of large lakes

Effect of atmospheric feedbacks on the stability of the Atlantic meridional overturning circulation

The impact of atmospheric feedbacks on the multiple equilibria (ME) regime of the Atlantic meridional overturning circulation (MOC) is investigated using a fully implicit hybrid coupled model (HCM). The HCM consists of a global ocean model coupled to an empirical atmosphere model that is based on linear regressions of the heat, net evaporative, and momentum fluxes generated by a fully coupled climate model onto local as well as Northern Hemisphere averaged sea surface temperatures. Using numerical continuation techniques, bifurcation diagrams are constructed for the HCM with the strength of an anomalous freshwater flux as the bifurcation parameter, which allows for an efficient first-order estimation of the effect of interactive surface fluxes on the MOC stability. The different components of the atmospheric fluxes are first considered individually and then combined. Heat feedbacks act to destabilize the present-day state of the MOC and to stabilize the collapsed state, thus leaving the size of the ME regime almost unaffected. In contrast, interactive freshwater fluxes cause a destabilization of both the present-day and collapsed states of theMOC. Wind feedbacks are found to have a minor impact. The joint effect of the three interactive fluxes is to narrow the range of ME. The shift of the saddle-node bifurcation that terminates the present-day state of the ocean is further investigated by adjoint sensitivity analysis of the overturning rate to surface fluxes. It is found that heat feedbacks primarily affect theMOC stability when they change the heat fluxes over the North Atlantic subpolar gyre, whereas interactive freshwater fluxes have an effect everywhere in the Atlantic basin

Dansgaard-Oeschger events: tipping points in the climate system

The largest variability in temperature over the last sixty thousand years is connected with Dansgaard-Oeschger events. Various prototype models have been proposed to explain these rapid climate fluctuations, but until now no observational constraint has been forwarded to choose between different theories. We assess the bimodality of the system reconstructing the topology of the multi-dimensional attractor over which the climate system evolves. Furthermore, we show that Dansgaard-Oeschger events are compatible with the crossing of a tipping point in the climate system. We use high-resolution ice core isotope data to investigate the statistical properties of the climate fluctuations in the period before the onset of the abrupt change. We find that the statistics are consistent with the switches between two different climate equilibrium states in response to a changing external forcing

Historical and idealized climate model experiments: An intercomparison of Earth system models of intermediate complexity

Both historical and idealized climate model experiments are performed with a variety of Earth system models of intermediate complexity (EMICs) as part of a community contribution to the Intergovernmental Panel on Climate Change Fifth Assessment Report. Historical simulations start at 850 CE and continue through to 2005. The standard simulations include changes in forcing from solar luminosity, Earth's orbital configuration, CO2, additional greenhouse gases, land use, and sulphate and volcanic aerosols. In spite of very different modelled pre-industrial global surface air temperatures, overall 20th century trends in surface air temperature and carbon uptake are reasonably well simulated when compared to observed trends. Land carbon fluxes show much more variation between models than ocean carbon fluxes, and recent land fluxes appear to be slightly underestimated. It is possible that recent modelled climate trends or climate–carbon feedbacks are overestimated resulting in too much land carbon loss or that carbon uptake due to CO2 and/or nitrogen fertilization is underestimated. Several one thousand year long, idealized, 2 × and 4 × CO2 experiments are used to quantify standard model characteristics, including transient and equilibrium climate sensitivities, and climate–carbon feedbacks. The values from EMICs generally fall within the range given by general circulation models. Seven additional historical simulations, each including a single specified forcing, are used to assess the contributions of different climate forcings to the overall climate and carbon cycle response. The response of surface air temperature is the linear sum of the individual forcings, while the carbon cycle response shows a non-linear interaction between land-use change and CO2 forcings for some models. Finally, the preindustrial portions of the last millennium simulations are used to assess historical model carbon-climate feedbacks. Given the specified forcing, there is a tendency for the EMICs to underestimate the drop in surface air temperature and CO2 between the Medieval Climate Anomaly and the Little Ice Age estimated from palaeoclimate reconstructions. This in turn could be a result of unforced variability within the climate system, uncertainty in the reconstructions of temperature and CO2, errors in the reconstructions of forcing used to drive the models, or the incomplete representation of certain processes within the models. Given the forcing datasets used in this study, the models calculate significant land-use emissions over the pre-industrial period. This implies that land-use emissions might need to be taken into account, when making estimates of climate–carbon feedbacks from palaeoclimate reconstructions