AMOC and Climate Responses to Dust Reduction and Greening of Sahara during the Mid-Holocene

2021 ◽  
pp. 1-59
Author(s):  
Ming Zhang ◽  
Yonggang Liu ◽  
Jian Zhang ◽  
Qin Wen
Keyword(s):  
Surface Temperature ◽  
North Africa ◽  
General Circulation ◽  
Circulation Model ◽  
Wind Farms ◽  
Ocean General Circulation Model ◽  
Climate Impact ◽  
Meridional Overturning Circulation ◽  
The Impact ◽  
Dust Reduction

AbstractThe North Africa was green during the mid-Holocene (6 ka) and emitting much less dust to the atmosphere than in present day. Here we use a fully coupled atmosphere-ocean general circulation model, CESM1.2.2, to test the impact of dust reduction and greening of Sahara on the Atlantic Meridional Overturning Circulation (AMOC) during this period. Results show that dust removal leads to a decrease of AMOC by 6.2 % while greening of Sahara with 100 % shrub (100 % grass) causes an enhancement of the AMOC by 6.1 % (4.8 %). The AMOC is increased by 5.3 % (2.3 %) when both the dust reduction and green Sahara with 100 % shrub (100 % grass) are considered. The AMOC changes are primarily due to the precipitation change over the west subtropical North Atlantic, from where the salinity anomaly is advected to the deepwater formation region. Global mean surface temperature increases by 0.09 °C and 0.40 °C (0.25 °C) when global dust is removed and when North Africa and Arabian region are covered by shrub (grass), respectively, showing a dominating effect of vegetation over dust. The comparison between modeled and reconstructed sea-surface temperature is improved when the effect of vegetation is considered. The results may have implication for climate impact of future wetting over North Africa, either through global warming or through building of solar farms and wind farms.

Probabilistic Estimates of Transient Climate Sensitivity Subject to Uncertainty in Forcing and Natural Variability

2011 ◽  
Vol 24 (21) ◽  
pp. 5521-5537 ◽  
Author(s):  
Lauren E. Padilla ◽  
Geoffrey K. Vallis ◽  
Clarence W. Rowley
Keyword(s):  
Surface Temperature ◽  
Climate Sensitivity ◽  
General Circulation ◽  
Temperature Response ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Balance Model ◽  
Transient Temperature ◽  
Model Parameters ◽  
The Impact

Abstract In this paper, the authors address the impact of uncertainty on estimates of transient climate sensitivity (TCS) of the globally averaged surface temperature, including both uncertainty in past forcing and internal variability in the climate record. This study provides a range of probabilistic estimates of the TCS that combine these two sources of uncertainty for various underlying assumptions about the nature of the uncertainty. The authors also provide estimates of how quickly the uncertainty in the TCS may be expected to diminish in the future as additional observations become available. These estimates are made using a nonlinear Kalman filter coupled to a stochastic, global energy balance model, using the filter and observations to constrain the model parameters. This study verifies that model and filter are able to emulate the evolution of a comprehensive, state-of-the-art atmosphere–ocean general circulation model and to accurately predict the TCS of the model, and then apply the methodology to observed temperature and forcing records of the twentieth century. For uncertainty assumptions best supported by global surface temperature data up to the present time, this paper finds a most likely present-day estimate of the transient climate sensitivity to be 1.6 K, with 90% confidence the response will fall between 1.3 and 2.6 K, and it is estimated that this interval may be 45% smaller by the year 2030. The authors calculate that emissions levels equivalent to forcing of less than 475 ppmv CO2 concentration are needed to ensure that the transient temperature response will not exceed 2 K with 95% confidence. This is an assessment for the short-to-medium term and not a recommendation for long-term stabilization forcing; the equilibrium temperature response to this level of CO2 may be much greater. The flat temperature trend of the last decade has a detectable but small influence on TCS. This study describes how the results vary if different uncertainty assumptions are made and shows they are robust to variations in the initial prior probability assumptions.

scholarly journals The Rossby radius in the Arctic Ocean

Ocean Science ◽  
2014 ◽  
Vol 10 (6) ◽  
pp. 967-975 ◽  
Author(s):  
A. J. G. Nurser ◽  
S. Bacon
Keyword(s):  
Arctic Ocean ◽  
General Circulation ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
High Arctic ◽  
The Arctic ◽  
Hudson Bay ◽  
Ocean Eddies ◽  
Shelf Seas ◽  
The Impact

Abstract. The first (and second) baroclinic deformation (or Rossby) radii are presented north of ~60° N, focusing on deep basins and shelf seas in the high Arctic Ocean, the Nordic seas, Baffin Bay, Hudson Bay and the Canadian Arctic Archipelago, derived from climatological ocean data. In the high Arctic Ocean, the first Rossby radius increases from ~5 km in the Nansen Basin to ~15 km in the central Canadian Basin. In the shelf seas and elsewhere, values are low (1–7 km), reflecting weak density stratification, shallow water, or both. Seasonality strongly impacts the Rossby radius only in shallow seas, where winter homogenization of the water column can reduce it to below 1 km. Greater detail is seen in the output from an ice–ocean general circulation model, of higher resolution than the climatology. To assess the impact of secular variability, 10 years (2003–2012) of hydrographic stations along 150° W in the Beaufort Gyre are also analysed. The first-mode Rossby radius increases over this period by ~20%. Finally, we review the observed scales of Arctic Ocean eddies.

scholarly journals Modeling the present and future impact of aviation on climate: an AOGCM approach with online coupled chemistry

2013 ◽  
Vol 13 (19) ◽  
pp. 10027-10048 ◽  
Author(s):  
P. Huszar ◽  
H. Teyssèdre ◽  
M. Michou ◽  
A. Voldoire ◽  
D. J. L. Olivié ◽  
...  
Keyword(s):  
Co2 Emissions ◽  
General Circulation ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Climate Impacts ◽  
Ozone Production ◽  
The Arctic ◽  
Near Surface ◽  
Aviation Emissions ◽  
The Impact

Abstract. Our work is among the first that use an atmosphere-ocean general circulation model (AOGCM) with online chemistry to evaluate the impact of future aviation emissions on temperature. Other particularities of our study include non-scaling to the aviation emissions, and the analysis of models' transient response using ensemble simulations. The model we use is the Météo-France CNRM-CM5.1 earth system model extended with the REPROBUS chemistry scheme. The time horizon of our interest is 1940–2100, assuming the A1B SRES scenario. We investigate the present and future impact of aviation emissions of CO2, NOx and H2O on climate, taking into account changes in greenhouse gases, contrails and contrail-induced cirrus (CIC). As in many transport-related impact studies, we distinguish between the climate impacts of CO2 emissions and those of non-CO2 emissions. Aviation-produced aerosol is not considered in the study. Our modeling system simulated a notable sea-ice bias in the Arctic, and therefore results concerning the surface should be viewed with caution. The global averaged near-surface CO2 impact reaches around 0.1 K by the end of the 21st century, while the non-CO2 impact reaches 0.2 K in the second half of the century. The NOx emissions impact is almost negligible in our simulations, as our aviation-induced ozone production is small. As a consequence, the non-CO2 signal is very similar to the CIC signal. The seasonal analysis shows that the strongest warming due to aviation is modeled for the late summer and early autumn. In the stratosphere, a significant cooling is attributed to aviation CO2 emissions (−0.25 K by 2100). A −0.3 K temperature decrease is modeled when considering all the aviation emissions, but no significant signal appears from the CIC or NOx forcings in the stratosphere.

scholarly journals The Brewer–Dobson Circulation in a Changing Climate: Impact of the Model Configuration

2013 ◽  
Vol 70 (5) ◽  
pp. 1437-1455 ◽  
Author(s):  
Felix Bunzel ◽  
Hauke Schmidt
Keyword(s):  
General Circulation ◽  
Climate Models ◽  
Wave Reflection ◽  
Circulation Model ◽  
Climate Impact ◽  
Wave Drag ◽  
Changing Climate ◽  
Model Configuration ◽  
Underlying Mechanisms ◽  
The Impact

Abstract Most climate models simulate a strengthening of the Brewer–Dobson circulation (BDC) under a changing climate. However, the magnitude of the trend as well as the underlying mechanisms varies significantly among the models. In this work the impact of both vertical resolution and vertical extent of a model on the simulated BDC change is investigated by analyzing sensitivity simulations performed with the general circulation model ECHAM6 in three different model configurations for three different climate states. Tropical upwelling velocities and age of stratospheric air are used as measures for the strength of the BDC. Both consistently show a BDC strengthening from the preindustrial to the future climate state for all configurations of the model. However, the amplitude and origin of this change vary between the different setups. Analyses of the tropical upward mass flux indicate that in the model with a lid at 10 hPa the BDC strengthening at 70 hPa is primarily produced by resolved wave drag, while in the model with a higher lid (0.01 hPa) the parameterized wave drag yields the main contribution to the BDC increase. This implies that consistent changes in the BDC originate from different causes when the stratosphere is not sufficiently resolved in a model. Furthermore, the effect of enhancing the horizontal diffusion in the upper model layers to avoid resolved wave reflection at the model lid is quantified, and a possible link to the different behavior of the low-top model with regard to the origin of the BDC change is identified.

Impact of Accurate Geoid Fields on Estimates of the Ocean Circulation

2007 ◽  
Vol 24 (8) ◽  
pp. 1464-1478 ◽  
Author(s):  
Detlef Stammer ◽  
Armin Köhl ◽  
Carl Wunsch
Keyword(s):  
Ocean Circulation ◽  
General Circulation ◽  
Circulation Model ◽  
Formal Proof ◽  
Meridional Overturning Circulation ◽  
Dynamic Topography ◽  
Model Errors ◽  
Geoid Model ◽  
Meridional Heat Transport ◽  
The Impact

Abstract The impact of new geoid height models on estimates of the ocean circulation, now available from the Gravity Recovery and Climate Experiment (GRACE) spacecraft, is assessed, and the implications of far more accurate geoids, anticipated from the European Space Agency’s (ESA) Gravity and Ocean Circulation Explorer (GOCE) mission, are explored. The study is based on several circulation estimates obtained over the period 1992–2002 by combining most of the available ocean datasets with a global general circulation model on a 1° horizontal grid and by exchanging only the EGM96 geoid model with two different geoid models available from GRACE. As compared to the EGM96-based solution, the GRACE geoid leads to an estimate of the ocean circulation that is more consistent with the Levitus temperature and salinity climatology. While not a formal proof, this finding supports the inference of a substantially improved GRACE geoid skill. However, oceanographic implications of the GRACE model are only modest compared to what can be obtained from ocean observations alone. To understand the extent to which this is merely a consequence of a not-optimally converged solution or if a much more accurate geoid field could in principle play a profound role in the ocean estimation procedure, an additional experiment was performed in which the geoid error was artificially reduced relative to all other datasets. Adjustments occur then in all elements of the ocean circulation, including 10% changes in the meridional overturning circulation and the corresponding meridional heat transport in the Atlantic. For an optimal use of new geoid fields, improved error information is required. The error budget of existing time-mean dynamic topography estimates may now be dominated by residual errors in time-mean altimetric corrections. Both these and the model errors need to be better understood before improved geoid estimates can be fully exploited. As is commonly found, the Southern Ocean is of particular concern.

scholarly journals Simulating the vegetation response to abrupt climate changes under glacial conditions with the ORCHIDEE/IPSL models

2012 ◽  
Vol 9 (9) ◽  
pp. 12895-12950
Author(s):  
M.-N. Woillez ◽  
M. Kageyama ◽  
N. Combourieu-Nebout ◽  
G. Krinner
Keyword(s):  
General Circulation ◽  
Western Europe ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Strong Impact ◽  
The North ◽  
Freshwater Fluxes ◽  
Global Vegetation ◽  
The Impact ◽  
Pollen Records

Abstract. The last glacial period has been punctuated by two types of abrupt climatic events, the Dansgaard-Oeschger (DO) and Heinrich (HE) events. These events, recorded in Greenland ice and in marine sediments, involved changes in the Atlantic Meridional Overturning Circulation (AMOC) and led to major changes in the terrestrial biosphere. Here we use the dynamical global vegetation model ORCHIDEE to simulate the response of vegetation to abrupt changes in the AMOC strength. To do so, we force ORCHIDEE off-line with outputs from the IPSL_CM4 general circulation model, in which we have forced the AMOC to change by adding freshwater fluxes in the North Atlantic. We investigate the impact of a collapse and recovery of the AMOC, at different rates, and focus on Western Europe, where many pollen records are available to compare with. The impact of an AMOC collapse on the European mean temperatures and precipitations simulated by the GCM is relatively small but sufficient to drive an important regression of forests and expansion of grasses in ORCHIDEE, in qualitative agreement with pollen data for an HE event. On the contrary, a run with a rapid shift of the AMOC to an hyperactive state of 30 Sv, mimicking the warming phase of a DO event, does not exhibit a strong impact on the European vegetation compared to the glacial control state. For our model, simulating the impact of an HE event thus appears easier than simulating the abrupt transition towards the interstadial phase of a DO. For both a collapse or a recovery of the AMOC the vegetation starts to respond to climatic changes immediately but reaches equilibrium about 200 yr after the climate equilibrates, suggesting a possible bias in the climatic reconstructions based on pollen records, which assume equilibrium between climate and vegetation. However, our study does not take into account vegetation feedbacks on the atmosphere.

scholarly journals Ocean-sea-ice coupling in a global ocean general circulation model

1997 ◽  
Vol 25 ◽  
pp. 116-120 ◽  
Author(s):  
S. Legutke ◽  
E. Maier-Reimkr ◽  
A. Stössel ◽  
A. Hellbach
Keyword(s):  
Sea Ice ◽  
General Circulation Model ◽  
Ocean Circulation ◽  
General Circulation ◽  
Circulation Model ◽  
Ice Cover ◽  
Ocean General Circulation Model ◽  
Global Ocean ◽  
Ocean General Circulation ◽  
The Impact

A global ocean general circulation model has been coupled with a dynamic thermodynamic sea-ice model. This model has been spun-up in a 1000 year integration using daily atmosphere model data. Main water masses and currents are reproduced as well as the seasonal characteristics of the ice cover of the Northern and Southern Hemispheres. Model results for the Southern Ocean, however, show the ice cover as too thin, and there are large permanent polynyas in the Weddell and Ross Seas. These polynyas are due to a large upward oceanic heat flux caused by haline rejection during the freezing of sea ice. Sensitivity studies were performed to test several ways of treating the sea-surface salinity and the rejected brine. The impact on the ice cover, water-mass characteristics, and ocean circulation are described.

Impact of Orbital Parameters and Greenhouse Gas on the Climate of MIS 7 and MIS 5 Glacial Inceptions

2014 ◽  
Vol 27 (23) ◽  
pp. 8918-8933 ◽  
Author(s):  
Florence Colleoni ◽  
Simona Masina ◽  
Annalisa Cherchi ◽  
Doroteaciro Iovino
Keyword(s):  
Greenhouse Gas ◽  
General Circulation ◽  
Regional Scale ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Cold Climate ◽  
The Arctic ◽  
Orbital Parameters ◽  
The Impact ◽  
Mis 5

Abstract This work explores the impact of orbital parameters and greenhouse gas concentrations on the climate of marine isotope stage (MIS) 7 glacial inception and compares it to that of MIS 5. The authors use a coupled atmosphere–ocean general circulation model to simulate the mean climate state of six time slices at 115, 122, 125, 229, 236, and 239 kyr, representative of a climate evolution from interglacial to glacial inception conditions. The simulations are designed to separate the effects of orbital parameters from those of greenhouse gas (GHG). Their results show that, in all the time slices considered, MIS 7 boreal lands mean annual climate is colder than the MIS 5 one. This difference is explained at 70% by the impact of the MIS 7 GHG. While the impact of GHG over Northern Hemisphere is homogeneous, the difference in temperature between MIS 7 and MIS 5 due to orbital parameters differs regionally and is linked with the Arctic Oscillation. The perennial snow cover is larger in all the MIS 7 experiments compared to MIS 5, as a result of MIS 7 orbital parameters, strengthened by GHG. At regional scale, Eurasia exhibits the strongest response to MIS 7 cold climate with a perennial snow area 3 times larger than in MIS 5 experiments. This suggests that MIS 7 glacial inception is more favorable over this area than over North America. Furthermore, at 239 kyr, the perennial snow covers an area equivalent to that of MIS 5 glacial inception (115 kyr). The authors suggest that MIS 7 glacial inception is more extensive than MIS 5 glacial inception over the high latitudes.

scholarly journals Using Temperature–Salinity Relations in a Global Ocean Implementation of a Multivariate Data Assimilation Scheme

2007 ◽  
Vol 135 (11) ◽  
pp. 3785-3807 ◽  
Author(s):  
A. Bellucci ◽  
S. Masina ◽  
P. DiPietro ◽  
A. Navarra
Keyword(s):  
Data Assimilation ◽  
General Circulation ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Global Ocean ◽  
Special Focus ◽  
Background Error ◽  
Background Error Covariance ◽  
Error Covariance ◽  
The Impact

Abstract In this paper results from the application of an ocean data assimilation (ODA) system, combining a multivariate reduced-order optimal interpolator (OI) scheme with a global ocean general circulation model (OGCM), are described. The present ODA system, designed to assimilate in situ temperature and salinity observations, has been used to produce ocean reanalyses for the 1962–2001 period. The impact of assimilating observed hydrographic data on the ocean mean state and temporal variability is evaluated. A special focus of this work is on the ODA system skill in reproducing a realistic ocean salinity state. Results from a hierarchy of different salinity reanalyses, using varying combinations of assimilated data and background error covariance structures, are described. The impact of the space and time resolution of the background error covariance parameterization on salinity is addressed.

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