scholarly journals Effects of Ocean Slow Response under Low Warming Targets

2020 ◽  
Vol 33 (2) ◽  
pp. 477-496 ◽  
Author(s):  
Shang-Min Long ◽  
Shang-Ping Xie ◽  
Yan Du ◽  
Qinyu Liu ◽  
Xiao-Tong Zheng ◽  
...  
Keyword(s):  
Radiative Forcing ◽  
Coupled Model ◽  
Deep Ocean ◽  
Ocean Warming ◽  
Meridional Overturning Circulation ◽  
Slow Response ◽  
Intercomparison Project ◽  
Meridional Overturning ◽  
Near Future

AbstractThe 2015 Paris Agreement proposed targets to limit global-mean surface temperature (GMST) rise well below 2°C relative to preindustrial level by 2100, requiring a cease in the radiative forcing (RF) increase in the near future. In response to changing RF, the deep ocean responds slowly (ocean slow response), in contrast to the fast ocean mixed layer adjustment. The role of the ocean slow response under low warming targets is investigated using representative concentration pathway (RCP) 2.6 simulations from phase 5 of the Coupled Model Intercomparison Project. In RCP2.6, the deep ocean continues to warm while RF decreases after reaching a peak. The deep ocean warming helps to shape the trajectories of GMST and fuels persistent thermosteric sea level rise. A diagnostic method is used to decompose further changes after the RF peak into a slow warming component under constant peak RF and a cooling component due to the decreasing RF. Specifically, the slow warming component amounts to 0.2°C (0.6°C) by 2100 (2300), raising the hurdle for achieving the low warming targets. When RF declines, the deep ocean warming takes place in all basins but is the most pronounced in the Southern Ocean and Atlantic Ocean where surface heat uptake is the largest. The climatology and change of meridional overturning circulation are both important for the deep ocean warming. To keep the GMST rise at a low level, substantial decrease in RF is required to offset the warming effect from the ocean slow response.

scholarly journals Atlantic Meridional Overturning Circulation (AMOC) in CMIP5 Models: RCP and Historical Simulations

2013 ◽  
Vol 26 (18) ◽  
pp. 7187-7197 ◽  
Author(s):  
Wei Cheng ◽  
John C. H. Chiang ◽  
Dongxiao Zhang
Keyword(s):  
North Atlantic ◽  
Coupled Model ◽  
Meridional Overturning Circulation ◽  
First Century ◽  
Model Intercomparison ◽  
The North ◽  
Intercomparison Project ◽  
Meridional Overturning ◽  
The North Atlantic ◽  
Twenty First Century

Abstract The Atlantic meridional overturning circulation (AMOC) simulated by 10 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) for the historical (1850–2005) and future climate is examined. The historical simulations of the AMOC mean state are more closely matched to observations than those of phase 3 of the Coupled Model Intercomparison Project (CMIP3). Similarly to CMIP3, all models predict a weakening of the AMOC in the twenty-first century, though the degree of weakening varies considerably among the models. Under the representative concentration pathway 4.5 (RCP4.5) scenario, the weakening by year 2100 is 5%–40% of the individual model's historical mean state; under RCP8.5, the weakening increases to 15%–60% over the same period. RCP4.5 leads to the stabilization of the AMOC in the second half of the twenty-first century and a slower (then weakening rate) but steady recovery thereafter, while RCP8.5 gives rise to a continuous weakening of the AMOC throughout the twenty-first century. In the CMIP5 historical simulations, all but one model exhibit a weak downward trend [ranging from −0.1 to −1.8 Sverdrup (Sv) century−1; 1 Sv ≡ 106 m3 s−1] over the twentieth century. Additionally, the multimodel ensemble–mean AMOC exhibits multidecadal variability with a ~60-yr periodicity and a peak-to-peak amplitude of ~1 Sv; all individual models project consistently onto this multidecadal mode. This multidecadal variability is significantly correlated with similar variations in the net surface shortwave radiative flux in the North Atlantic and with surface freshwater flux variations in the subpolar latitudes. Potential drivers for the twentieth-century multimodel AMOC variability, including external climate forcing and the North Atlantic Oscillation (NAO), and the implication of these results on the North Atlantic SST variability are discussed.

scholarly journals Antarctic Sea Ice Control on the Depth of North Atlantic Deep Water

2019 ◽  
Vol 32 (9) ◽  
pp. 2537-2551 ◽  
Author(s):  
Louis-Philippe Nadeau ◽  
Raffaele Ferrari ◽  
Malte F. Jansen
Keyword(s):  
Sea Ice ◽  
Deep Water ◽  
Formation Rate ◽  
Deep Ocean ◽  
Meridional Overturning Circulation ◽  
Ice Formation ◽  
Overturning Circulation ◽  
Antarctic Sea Ice ◽  
Meridional Overturning

Abstract Changes in deep-ocean circulation and stratification have been argued to contribute to climatic shifts between glacial and interglacial climates by affecting the atmospheric carbon dioxide concentrations. It has been recently proposed that such changes are associated with variations in Antarctic sea ice through two possible mechanisms: an increased latitudinal extent of Antarctic sea ice and an increased rate of Antarctic sea ice formation. Both mechanisms lead to an upward shift of the Atlantic meridional overturning circulation (AMOC) above depths where diapycnal mixing is strong (above 2000 m), thus decoupling the AMOC from the abyssal overturning circulation. Here, these two hypotheses are tested using a series of idealized two-basin ocean simulations. To investigate independently the effect of an increased latitudinal ice extent from the effect of an increased ice formation rate, sea ice is parameterized as a latitude strip over which the buoyancy flux is negative. The results suggest that both mechanisms can effectively decouple the two cells of the meridional overturning circulation (MOC), and that their effects are additive. To illustrate the role of Antarctic sea ice in decoupling the AMOC and the abyssal overturning cell, the age of deep-water masses is estimated. An increase in both the sea ice extent and its formation rate yields a dramatic “aging” of deep-water masses if the sea ice is thick and acts as a lid, suppressing air–sea fluxes. The key role of vertical mixing is highlighted by comparing results using different profiles of vertical diffusivity. The implications of an increase in water mass ages for storing carbon in the deep ocean are discussed.

scholarly journals Does External Forcing Interfere with the AMOC’s Influence on North Atlantic Sea Surface Temperature?

2015 ◽  
Vol 28 (16) ◽  
pp. 6309-6323 ◽  
Author(s):  
Neil F. Tandon ◽  
Paul J. Kushner
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
North Atlantic ◽  
Coupled Model ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
Coupled Models ◽  
Community Earth System Model ◽  
Intercomparison Project ◽  
Meridional Overturning

Abstract Numerous studies have suggested that variations in the strength of the Atlantic meridional overturning circulation (AMOC) may drive predictable variations in North Atlantic sea surface temperature (NASST). However, two recent studies have presented results suggesting that coupled models disagree on both the sign and the phasing of the correlation between AMOC and NASST indices. These studies analyzed linearly detrended output from twentieth-century historical simulations in phases 3 and 5 of the Coupled Model Intercomparison Project (CMIP3 and CMIP5). The present study argues that the apparent disagreement among models arises from a comingling of two processes: 1) a bottom-up effect in which unforced AMOC changes lead to NASST changes of the same sign and 2) a top-down effect in which forced NASST changes lead to AMOC changes of the opposite sign. Linear detrending is not appropriate for separating these two effects because the time scales of forced and unforced variations are not well separated. After forced variations are properly removed, the models come into much closer agreement with each other. This argument is supported by analysis of CMIP5 historical simulations, as well as preindustrial control simulations and a 29-member ensemble of the Community Earth System Model, version 1, covering the period 1920–2005. Additional analysis is presented suggesting that, even after the data are linearly detrended, a significant portion of observed NASST persistence may be externally forced.

scholarly journals Sensitivity to Glacial Forcing in the CCSM4

2013 ◽  
Vol 26 (6) ◽  
pp. 1901-1925 ◽  
Author(s):  
Esther C. Brady ◽  
Bette L. Otto-Bliesner ◽  
Jennifer E. Kay ◽  
Nan Rosenbloom
Keyword(s):  
Climate Change ◽  
Atmospheric Aerosols ◽  
Radiative Forcing ◽  
Coupled Model ◽  
Meridional Overturning Circulation ◽  
Deep Basin ◽  
Intergovernmental Panel ◽  
Climate System Model ◽  
Orbital Parameters ◽  
Intercomparison Project

Abstract Results are presented from the Community Climate System Model, version 4 (CCSM4), simulation of the Last Glacial Maximum (LGM) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) at the standard 1° resolution, the same resolution as the majority of the CCSM4 CMIP5 long-term simulations for the historical and future projection scenarios. The forcings and boundary conditions for this simulation follow the protocols of the Paleoclimate Modeling Intercomparison Project, version 3 (PMIP3). Two additional CCSM4 CO2 sensitivity simulations, in which the concentrations are abruptly changed at the start of the simulation to the low 185 ppm LGM concentrations (LGMCO2) and to a quadrupling of the preindustrial concentration (4×CO2), are also analyzed. For the full LGM simulation, the estimated equilibrium cooling of the global mean annual surface temperature is 5.5°C with an estimated radiative forcing of −6.2 W m−2. The radiative forcing includes the effects of the reduced LGM greenhouse gases, ice sheets, continental distribution with sea level lowered by approximately 120 m from the present, and orbital parameters, but not changes to atmospheric aerosols or vegetation biogeography. The LGM simulation has an equilibrium climate sensitivity (ECS) of 3.1(±0.3)°C, comparable to the CCSM4 4×CO2 result. The LGMCO2 simulation shows a greater ECS of 4.2°C. Other responses found at the LGM in CCSM4 include a global precipitation rate decrease at a rate of ~2% °C−1, similar to climate change simulations in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4); a strengthening of the Atlantic meridional overturning circulation (AMOC) with a shoaling of North Atlantic Deep Water and a filling of the deep basin up to sill depth with Antarctic Bottom Water; and an enhanced seasonal cycle accompanied by reduced ENSO variability in the eastern Pacific Ocean’s SSTs.

scholarly journals Aerosol-forced AMOC changes in CMIP6 historical simulations.

2021 ◽  
Author(s):  
Matthew Menary ◽  
Richard Allan ◽  
Jon Robson ◽  
Ben Booth ◽  
Christophe Cassou ◽  
...  
Keyword(s):  
Radiative Forcing ◽  
Coupled Model ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
Anthropogenic Aerosol ◽  
Considerable Uncertainty ◽  
Aerosol Forcing ◽  
Key Factor ◽  
Meridional Overturning ◽  
Aerosol Cloud Interactions

<div> <p>The Atlantic Meridional Overturning Circulation (AMOC) has been, and will continue to be, a key factor in the modulation of climate change both locally and globally. However, there remains considerable uncertainty in recent AMOC evolution. Here, we show that the multi-model mean AMOC strengthened by approximately 10% from 1850-1985 in new simulations from the 6th Coupled Model Inter-comparison Project (CMIP6), a larger change than was seen in CMIP5. Across the models, the strength of the AMOC trend up to 1985 is related to a proxy for the strength of the aerosol forcing. Therefore, the multi-model difference is a result of stronger anthropogenic aerosol forcing on average in CMIP6 than CMIP5, which is primarily due to more models including aerosol-cloud interactions. However, observational constraints - including a historical sea surface temperature fingerprint and shortwave radiative forcing in recent decades - suggest that anthropogenic forcing and/or the AMOC response may be overestimated.</p> </div>

scholarly journals Future Changes in Simulated Evapotranspiration across Continental Africa Based on CMIP6 CNRM-CM6

2021 ◽  
Vol 18 (13) ◽  
pp. 6760
Author(s):  
Isaac Kwesi Nooni ◽  
Daniel Fiifi T. Hagan ◽  
Guojie Wang ◽  
Waheed Ullah ◽  
Jiao Lu ◽  
...  
Keyword(s):  
Extreme Events ◽  
Coupled Model ◽  
Baseline Period ◽  
Model Intercomparison ◽  
Additional Information ◽  
Intercomparison Project ◽  
Key Drivers ◽  
Model Ensembles ◽  
Projected Changes ◽  
Near Future

The main goal of this study was to assess the interannual variations and spatial patterns of projected changes in simulated evapotranspiration (ET) in the 21st century over continental Africa based on the latest Shared Socioeconomic Pathways and the Representative Concentration Pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5) provided by the France Centre National de Recherches Météorologiques (CNRM-CM) model in the Sixth Phase of Coupled Model Intercomparison Project (CMIP6) framework. The projected spatial and temporal changes were computed for three time slices: 2020–2039 (near future), 2040–2069 (mid-century), and 2080–2099 (end-of-the-century), relative to the baseline period (1995–2014). The results show that the spatial pattern of the projected ET was not uniform and varied across the climate region and under the SSP-RCPs scenarios. Although the trends varied, they were statistically significant for all SSP-RCPs. The SSP5-8.5 and SSP3-7.0 projected higher ET seasonality than SSP1-2.6 and SSP2-4.5. In general, we suggest the need for modelers and forecasters to pay more attention to changes in the simulated ET and their impact on extreme events. The findings provide useful information for water resources managers to develop specific measures to mitigate extreme events in the regions most affected by possible changes in the region’s climate. However, readers are advised to treat the results with caution as they are based on a single GCM model. Further research on multi-model ensembles (as more models’ outputs become available) and possible key drivers may provide additional information on CMIP6 ET projections in the region.

Decomposing the time-mean Atlantic Meridional Overturning Circulation and its variability with latitude.

2021 ◽  
Author(s):  
Tomas Jonathan ◽  
Mike Bell ◽  
Helen Johnson ◽  
David Marshall
Keyword(s):  
Global Climate ◽  
Dominant Role ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Relative Role ◽  
Overturning Circulation ◽  
Near Surface ◽  
Boundary Density ◽  
Meridional Overturning

<p>The Atlantic Meridional Overturning Circulations (AMOC) is crucial to our global climate, transporting heat and nutrients around the globe. Detecting  potential climate change signals first requires a careful characterisation of inherent natural AMOC variability. Using a hierarchy of global coupled model  control runs (HadGEM-GC3.1, HighResMIP) we decompose the overturning circulation as the sum of (near surface) Ekman, (depth-dependent) bottom velocity, eastern and western boundary density components, as a function of latitude. This decomposition proves a useful low-dimensional characterisation of the full 3-D overturning circulation. In particular, the decomposition provides a means to investigate and quantify the constraints which boundary information imposes on the overturning, and the relative role of eastern versus western contributions on different timescales. </p><p>The basin-wide time-mean contribution of each boundary component to the expected streamfunction is investigated as a function of depth, latitude and spatial resolution. Regression modelling supplemented by Correlation Adjusted coRrelation (CAR) score diagnostics provide a natural ranking of the contributions of the various components in explaining the variability of the total streamfunction. Results reveal the dominant role of the bottom component, western boundary and Ekman components at short time-scales, and of boundary density components at decadal and longer timescales.</p>

scholarly journals Projections of Cold Air Outbreaks In CMIP6 Earth System Models

2021 ◽  
Author(s):  
Erik T. Smith ◽  
Scott Sheridan
Keyword(s):  
North Atlantic ◽  
Climate Models ◽  
Coupled Model ◽  
Meridional Overturning Circulation ◽  
Cold Air ◽  
Complex Processes ◽  
The North ◽  
Meridional Overturning ◽  
Earth System Models ◽  
Cold Air Outbreaks

Abstract Historical and future simulated temperature data from five climate models in the Coupled Model Intercomparing Project Phase 6 (CMIP6) are used to understand how climate change might alter cold air outbreaks (CAOs) in the future. Three different Shared Socioeconomic Pathways (SSPs), SSP 1 – 2.6, SSP 2 – 4.5, and SSP 5 – 8.5 are examined to identify potential fluctuations in CAOs across the globe between 2015 and 2054. Though CAOs may remain persistent or even increase in some regions through 2040, all five climate models show CAOs disappearing by 2054 based on current climate percentiles. Climate models were able to accurately simulate the spatial distribution and trends of historical CAOs, but there were large errors in the simulated interannual frequency of CAOs in the North Atlantic and North Pacific. Fluctuations in complex processes, such as Atlantic Meridional Overturning Circulation, may be contributing to each model’s inability to simulate historical CAOs in these regions.

scholarly journals Climatological Drought Forecasting Using Bias Corrected CMIP6 Climate Data: A Case Study for India

Forecasting ◽  
2020 ◽  
Vol 2 (2) ◽  
pp. 59-84 ◽  
Author(s):  
Alen Shrestha ◽  
Md Mafuzur Rahaman ◽  
Ajay Kalra ◽  
Rohit Jogineedi ◽  
Pankaj Maheshwari
Keyword(s):  
Coupled Model ◽  
Severity Index ◽  
Extreme Drought ◽  
Drought Severity ◽  
Climate Data ◽  
Drought Forecasting ◽  
Drought Duration ◽  
Intercomparison Project ◽  
Near Future

This study forecasts and assesses drought situations in various regions of India (the Araveli region, the Bundelkhand region, and the Kansabati river basin) based on seven simulated climates in the near future (2015–2044). The self-calibrating Palmer Drought Severity Index (scPDSI) was used based on its fairness in identifying drought conditions that account for the temperature as well. Gridded temperature and rainfall data of spatial resolution of 1 km were used to bias correct the multi-model ensemble mean of the Global Climatic Models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) project. Equidistant quantile-based mapping was adopted to remove the bias in the rainfall and temperature data, which were corrected on a monthly scale. The outcome of the forecast suggests multiple severe-to-extreme drought events of appreciable durations, mostly after the 2030s, under most climate scenarios in all the three study areas. The severe-to-extreme drought duration was found to last at least 20 to 30 months in the near future in all three study areas. A high-resolution drought index was developed and proven to be a key to assessing the drought situation.

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