scholarly journals Enhanced Atlantic subpolar gyre variability through baroclinic threshold in a Coarse Resolution Model

2012 ◽  
Vol 3 (1) ◽  
pp. 259-278 ◽  
Author(s):  
M. Mengel ◽  
A. Levermann ◽  
C.-F. Schleussner ◽  
A. Born
Keyword(s):  
Wind Stress ◽  
North Atlantic ◽  
General Circulation ◽  
Climate Model ◽  
Circulation Model ◽  
Salt Transport ◽  
Subpolar Gyre ◽  
Atmospheric Variability ◽  
Coarse Resolution ◽  
The Impact

Abstract. Direct observations, satellite measurements and paleorecords reveal strong variability in the Atlantic subpolar gyre on various time scales. Here we show that variations of comparable amplitude can only be simulated in a coupled climate model in the proximity of a dynamical threshold. The threshold and the associated dynamic response is due to a positive feedback involving increased salt transport in the subpolar gyre and enhanced deep convection in its center. A series of sensitivity experiments is performed with a coarse resolution ocean general circulation model coupled to a statistical-dynamical atmosphere model which in itself does not produce atmospheric variability. To simulate the impact of atmospheric variability, the model system is perturbed with freshwater forcing of varying but small amplitude and multidecadal to centennial periodicity, and observational variations in wind stress. While both freshwater and wind-stress-forcing have a small direct effect on the strength of the subpolar gyre, the magnitude of the gyre's response is strongly increased in the vicinity of the threshold. Our results thus indicate that baroclinic self-amplification in the North Atlantic ocean can play an important role in presently observed SPG variability and thereby North Atlantic climate variability on multidecadal scales.

scholarly journals Enhanced Atlantic subpolar gyre variability through baroclinic threshold in a coarse resolution model

2012 ◽  
Vol 3 (2) ◽  
pp. 189-197 ◽  
Author(s):  
M. Mengel ◽  
A. Levermann ◽  
C.-F. Schleussner ◽  
A. Born
Keyword(s):  
Wind Stress ◽  
North Atlantic ◽  
General Circulation ◽  
Climate Model ◽  
Circulation Model ◽  
Salt Transport ◽  
Subpolar Gyre ◽  
Atmospheric Variability ◽  
Coarse Resolution ◽  
The Impact

Abstract. Direct observations, satellite measurements and paleo records reveal strong variability in the Atlantic subpolar gyre on various time scales. Here we show that variations of comparable amplitude can only be simulated in a coupled climate model in the proximity of a dynamical threshold. The threshold and the associated dynamic response is due to a positive feedback involving increased salt transport in the subpolar gyre and enhanced deep convection in its centre. A series of sensitivity experiments is performed with a coarse resolution ocean general circulation model coupled to a statistical-dynamical atmosphere model which in itself does not produce atmospheric variability. To simulate the impact of atmospheric variability, the model system is perturbed with freshwater forcing of varying, but small amplitude and multi-decadal to centennial periodicities and observational variations in wind stress. While both freshwater and wind-stress-forcing have a small direct effect on the strength of the subpolar gyre, the magnitude of the gyre's response is strongly increased in the vicinity of the threshold. Our results indicate that baroclinic self-amplification in the North Atlantic ocean can play an important role in presently observed SPG variability and thereby North Atlantic climate variability on multi-decadal scales.

scholarly journals The Effect of a Well-Resolved Stratosphere on Surface Climate: Differences between CMIP5 Simulations with High and Low Top Versions of the Met Office Climate Model

2012 ◽  
Vol 25 (20) ◽  
pp. 7083-7099 ◽  
Author(s):  
S. C. Hardiman ◽  
N. Butchart ◽  
T. J. Hinton ◽  
S. M. Osprey ◽  
L. J. Gray
Keyword(s):  
General Circulation ◽  
Climate Model ◽  
Southern Oscillation ◽  
Coupled Model ◽  
Circulation Model ◽  
Climate Simulation ◽  
Reanalysis Dataset ◽  
Quasi Biennial Oscillation ◽  
Surface Climate ◽  
The Impact

Abstract The importance of using a general circulation model that includes a well-resolved stratosphere for climate simulations, and particularly the influence this has on surface climate, is investigated. High top model simulations are run with the Met Office Unified Model for the Coupled Model Intercomparison Project Phase 5 (CMIP5). These simulations are compared to equivalent simulations run using a low top model differing only in vertical extent and vertical resolution above 15 km. The period 1960–2002 is analyzed and compared to observations and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis dataset. Long-term climatology, variability, and trends in surface temperature and sea ice, along with the variability of the annular mode index, are found to be insensitive to the addition of a well-resolved stratosphere. The inclusion of a well-resolved stratosphere, however, does improve the impact of atmospheric teleconnections on surface climate, in particular the response to El Niño–Southern Oscillation, the quasi-biennial oscillation, and midwinter stratospheric sudden warmings (i.e., zonal mean wind reversals in the middle stratosphere). Thus, including a well-represented stratosphere could improve climate simulation on intraseasonal to interannual time scales.

scholarly journals The 1986–1989 ENSO cycle in a chemical climate model

2006 ◽  
Vol 6 (12) ◽  
pp. 4669-4685 ◽  
Author(s):  
S. Brönnimann ◽  
M. Schraner ◽  
B. Müller ◽  
A. Fischer ◽  
D. Brunner ◽  
...  
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Climate Models ◽  
Climate Model ◽  
Circulation Model ◽  
Enso Cycle ◽  
Atlantic Sector ◽  
Ensemble Simulations ◽  
Chemical Climate ◽  
Middle Stratosphere

Abstract. A pronounced ENSO cycle occurred from 1986 to 1989, accompanied by distinct dynamical and chemical anomalies in the global troposphere and stratosphere. Reproducing these effects with current climate models not only provides a model test but also contributes to our still limited understanding of ENSO's effect on stratosphere-troposphere coupling. We performed several sets of ensemble simulations with a chemical climate model (SOCOL) forced with global sea surface temperatures. Results were compared with observations and with large-ensemble simulations performed with an atmospheric general circulation model (MRF9). We focus our analysis on the extratropical stratosphere and its coupling with the troposphere. In this context, the circulation over the North Atlantic sector is particularly important. Relative to the La Niña winter 1989, observations for the El Niño winter 1987 show a negative North Atlantic Oscillation index with corresponding changes in temperature and precipitation patterns, a weak polar vortex, a warm Arctic middle stratosphere, negative and positive total ozone anomalies in the tropics and at middle to high latitudes, respectively, as well as anomalous upward and poleward Eliassen-Palm (EP) flux in the midlatitude lower stratosphere. Most of the tropospheric features are well reproduced in the ensemble means in both models, though the amplitudes are underestimated. In the stratosphere, the SOCOL simulations compare well with observations with respect to zonal wind, temperature, EP flux, meridional mass streamfunction, and ozone, but magnitudes are underestimated in the middle stratosphere. With respect to the mechanisms relating ENSO to stratospheric circulation, the results suggest that both, upward and poleward components of anomalous EP flux are important for obtaining the stratospheric signal and that an increase in strength of the Brewer-Dobson circulation is part of that signal.

scholarly journals Arctic/Atlantic Exchanges via the Subpolar Gyre*

2012 ◽  
Vol 25 (7) ◽  
pp. 2421-2439 ◽  
Author(s):  
Helene R. Langehaug ◽  
Iselin Medhaug ◽  
Tor Eldevik ◽  
Odd Helge Otterå
Keyword(s):  
North Atlantic ◽  
Climate Model ◽  
Decadal Variability ◽  
Sea Water ◽  
Similar Response ◽  
Subpolar Gyre ◽  
North Atlantic Current ◽  
Atmospheric Variability ◽  
The North ◽  
The North Atlantic

Abstract In the present study the decadal variability in the strength and shape of the subpolar gyre (SPG) in a 600-yr preindustrial simulation using the Bergen Climate Model is investigated. The atmospheric influence on the SPG strength is reflected in the variability of Labrador Sea Water (LSW), which is largely controlled by the North Atlantic Oscillation, the first mode of the North Atlantic atmospheric variability. A combination of the amount of LSW, the overflows from the Nordic seas, and the second mode of atmospheric variability, the East Atlantic Pattern, explains 44% of the modeled decadal variability in the SPG strength. A prior increase in these components leads to an intensified SPG in the western subpolar region. Typically, an increase of one standard deviation (std dev) of the total overflow (1 std dev = 0.2 Sv; 1 Sv ≡ 106 m3 s−1) corresponds to an intensification of about one-half std dev of the SPG strength (1 std dev = 2 Sv). A similar response is found for an increase of one std dev in the amount of LSW, and simultaneously the strength of the North Atlantic Current increases by one-half std dev (1 std dev = 0.9 Sv).

scholarly journals Evaluation of CLaMS, KASIMA and ECHAM5/MESSy1 simulations in the lower stratosphere using observations of Odin/SMR and ILAS/ILAS-II

2009 ◽  
Vol 9 (1) ◽  
pp. 1977-2020
Author(s):  
F. Khosrawi ◽  
R. Müller ◽  
M. H. Proffitt ◽  
R. Ruhnke ◽  
O. Kirner ◽  
...  
Keyword(s):  
Atmospheric Chemistry ◽  
General Circulation ◽  
Lower Stratosphere ◽  
Climate Model ◽  
Middle Atmosphere ◽  
Circulation Model ◽  
Lagrangian Model ◽  
Data Sets ◽  
Data Set ◽  
The Impact

Abstract. 1-year data sets of monthly averaged nitrous oxide (N2O) and ozone (O3) derived from satellite measurements were used as a tool for the evaluation of atmospheric photochemical models. Two 1-year data sets, one derived from the Improved Limb Atmospheric Spectrometer (ILAS and ILAS-II) and one from the Odin Sub-Millimetre Radiometer (Odin/SMR) were employed. Here, these data sets are used for the evaluation of two Chemical Transport Models (CTMs), the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA) and the Chemical Lagrangian Model of the Stratosphere (CLaMS) as well as for one Chemistry-Climate Model (CCM), the atmospheric chemistry general circulation model ECHAM5/MESSy1 (E5M1) in the lower stratosphere with focus on the Northern Hemisphere. Since the Odin/SMR measurements cover the entire hemisphere, the evaluation is performed for the entire hemisphere as well as for the low latitudes, midlatitudes and high latitudes using the Odin/SMR 1-year data set as reference. To assess the impact of using different data sets for such an evaluation study we repeat the evaluation for the polar lower stratosphere using the ILAS/ILAS-II data set. Only small differences were found using ILAS/ILAS-II instead of Odin/SMR as a reference, thus, showing that the results are not influenced by the particular satellite data set used for the evaluation. The evaluation of CLaMS, KASIMA and E5M1 shows that all models are in good agreement with Odin/SMR and ILAS/ILAS-II. Differences are generally in the range of ±20%. Larger differences (up to −40%) are found in all models at 500±25 K for N2O mixing ratios greater than 200 ppb. Generally, the largest differences were found for the tropics and the lowest for the polar regions. However, an underestimation of polar winter ozone loss was found both in KASIMA and E5M1 both in the Northern and Southern Hemisphere.

scholarly journals Intergyre Salt Transport in the Climate Warming Response

2020 ◽  
Vol 50 (1) ◽  
pp. 255-268
Author(s):  
Samuel J. Levang ◽  
Raymond W. Schmitt
Keyword(s):  
Ocean Circulation ◽  
Climate Warming ◽  
Climate Model ◽  
Water Cycle ◽  
Global Climate ◽  
Salt Transport ◽  
Ocean Dynamics ◽  
Subpolar Gyre ◽  
Salinity Response ◽  
The Impact

ABSTRACTRegional connectivity is important to the global climate salinity response, particularly because salinity anomalies do not have a damping feedback with atmospheric freshwater fluxes and may therefore be advected over long distances by ocean circulation, resulting in nonlocal influences. Climate model intercomparison experiments such as CMIP5 exhibit large uncertainty in some aspects of the salinity response, hypothesized here to be a result of ocean dynamics. We use two types of Lagrangian particle tracking experiments to investigate pathways of exchange for salinity anomalies. The first uses forward trajectories to estimate average transport time scales between water cycle regimes. The second uses reverse trajectories and a freshwater accumulation method to quantitatively identify remote influences in the salinity response. Additionally, we compare velocity fields with both resolved and parameterized eddies to understand the impact of eddy stirring on intergyre exchange. These experiments show that surface anomalies are readily exchanged within the ocean gyres by the mean circulation, but intergyre exchange is slower and largely eddy driven. These dynamics are used to analyze the North Atlantic salinity response to climate warming and water cycle intensification, where the system is broadly forced with fresh surface anomalies in the subpolar gyre and salty surface anomalies in the subtropical gyres. Under these competing forcings, strong intergyre eddy fluxes carry anomalously salty subtropical water into the subpolar gyre which balances out much of the local freshwater input.

scholarly journals European Climate Extremes and the North Atlantic Oscillation

2008 ◽  
Vol 21 (1) ◽  
pp. 72-83 ◽  
Author(s):  
Adam A. Scaife ◽  
Chris K. Folland ◽  
Lisa V. Alexander ◽  
Anders Moberg ◽  
Jeff R. Knight
Keyword(s):  
North Atlantic Oscillation ◽  
North Atlantic ◽  
General Circulation ◽  
Climate Model ◽  
Circulation Model ◽  
Extreme Weather Events ◽  
The North ◽  
Weather Events ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

Abstract The authors estimate the change in extreme winter weather events over Europe that is due to a long-term change in the North Atlantic Oscillation (NAO) such as that observed between the 1960s and 1990s. Using ensembles of simulations from a general circulation model, large changes in the frequency of 10th percentile temperature and 90th percentile precipitation events over Europe are found from changes in the NAO. In some cases, these changes are comparable to the expected change in the frequency of events due to anthropogenic forcing over the twenty-first century. Although the results presented here do not affect anthropogenic interpretation of global and annual mean changes in observed extremes, they do show that great care is needed to assess changes due to modes of climate variability when interpreting extreme events on regional and seasonal scales. How changes in natural modes of variability, such as the NAO, could radically alter current climate model predictions of changes in extreme weather events on multidecadal time scales is also discussed.

scholarly journals Hydroclimate variability in Scandinavia over the last millennium – insights from a climate model–proxy data comparison

2017 ◽  
Vol 13 (12) ◽  
pp. 1831-1850 ◽  
Author(s):  
Kristina Seftigen ◽  
Hugues Goosse ◽  
Francois Klein ◽  
Deliang Chen
Keyword(s):  
General Circulation ◽  
Climate Models ◽  
Climate Model ◽  
Model Performance ◽  
Circulation Model ◽  
Last Millennium ◽  
Atmospheric Variability ◽  
Out Of Sample ◽  
Hydroclimate Variability ◽  
Transient Simulations

Abstract. The integration of climate proxy information with general circulation model (GCM) results offers considerable potential for deriving greater understanding of the mechanisms underlying climate variability, as well as unique opportunities for out-of-sample evaluations of model performance. In this study, we combine insights from a new tree-ring hydroclimate reconstruction from Scandinavia with projections from a suite of forced transient simulations of the last millennium and historical intervals from the CMIP5 and PMIP3 archives. Model simulations and proxy reconstruction data are found to broadly agree on the modes of atmospheric variability that produce droughts–pluvials in the region. Despite these dynamical similarities, large differences between simulated and reconstructed hydroclimate time series remain. We find that the GCM-simulated multi-decadal and/or longer hydroclimate variability is systematically smaller than the proxy-based estimates, whereas the dominance of GCM-simulated high-frequency components of variability is not reflected in the proxy record. Furthermore, the paleoclimate evidence indicates in-phase coherencies between regional hydroclimate and temperature on decadal timescales, i.e., sustained wet periods have often been concurrent with warm periods and vice versa. The CMIP5–PMIP3 archive suggests, however, out-of-phase coherencies between the two variables in the last millennium. The lack of adequate understanding of mechanisms linking temperature and moisture supply on longer timescales has serious implications for attribution and prediction of regional hydroclimate changes. Our findings stress the need for further paleoclimate data–model intercomparison efforts to expand our understanding of the dynamics of hydroclimate variability and change, to enhance our ability to evaluate climate models, and to provide a more comprehensive view of future drought and pluvial risks.

scholarly journals Hydroclimate variability in Scandinavia over the last millennium – insights from a climate model-proxy data comparison

2017 ◽  
Author(s):  
Kristina Seftigen ◽  
Hugues Goosse ◽  
Francois Klein ◽  
Deliang Chen
Keyword(s):  
General Circulation ◽  
Climate Models ◽  
Climate Model ◽  
Global Climate ◽  
Model Performance ◽  
Circulation Model ◽  
Global Climate Models ◽  
Last Millennium ◽  
Atmospheric Variability ◽  
Out Of Sample

Abstract. The integration of climate proxy information with General Circulation Model (GCM) results offers considerable potential for deriving greater understanding of the mechanisms underlying climate variability, as well as unique opportunities for out-of-sample evaluations of model performance. In this study, we combine insights from a new tree-ring hydroclimate reconstruction from Scandinavian with projections from a suite of forced transient simulations of the last millennium and historical intervals from the CMIP5 and PMIP3 archives. Model simulations and proxy reconstruction data are found to broadly agree on the modes of atmospheric variability that produces droughts/pluvials in the region. But despite these dynamical similarities, large differences between simulated and reconstructed hydroclimate time series remain. We find simulated interannual components of variability to be overestimated, while the multidecadal/longer timescale components generally are too weak. Specifically, summertime moisture variability and temperature are weakly negatively associated at inter-annual timescales but positively correlated at decadal timescales, revealed from observational and proxy evidences. On this background, the CMIP5/PMIP3 simulated timescale dependent relationship between regional precipitation and temperature is considerably biased, because the short-term negative association is overestimated, and the long-term relationship is significantly underestimated. The lack of adequate understanding for mechanisms linking temperature and moisture supply on longer timescales has important implication for future projections. Weak multidecadal variability in models also implies that inference about future persistent droughts and pluvials based on the latest generation global climate models will likely underestimate the true risk of these events.

scholarly journals Large-basin hydrological response to climate model outputs: uncertainty caused by the internal atmospheric variability

2015 ◽  
Vol 12 (2) ◽  
pp. 2305-2348 ◽  
Author(s):  
A. Gelfan ◽  
V. A. Semenov ◽  
E. Gusev ◽  
Y. Motovilov ◽  
O. Nasonova ◽  
...  
Keyword(s):  
General Circulation ◽  
Climate Model ◽  
Initial Conditions ◽  
Lower Boundary ◽  
Circulation Model ◽  
The Arctic ◽  
Hydrological Models ◽  
Atmospheric Variability ◽  
Ensemble Simulations ◽  
The Mean

Abstract. An approach is proposed to assess hydrological simulation uncertainty originating from internal atmospheric variability. The latter is one of three major factors contributing to the uncertainty of simulated climate change projections (along with so-called "forcing" and "climate model" uncertainties). Importantly, the role of the internal atmospheric variability is the most visible over the spatial–temporal scales of water management in large river basins. The internal atmospheric variability is represented by large ensemble simulations (45 members) with the ECHAM5 atmospheric general circulation model. The ensemble simulations are performed using identical prescribed lower boundary conditions (observed sea surface temperature, SST, and sea ice concentration, SIC, for 1979–2012) and constant external forcing parameters but different initial conditions of the atmosphere. The ensemble of the bias-corrected ECHAM5-outputs as well as ensemble averaged ECHAM5-output are used as the distributed input for ECOMAG and SWAP hydrological models. The corresponding ensembles of runoff hydrographs are calculated for two large rivers of the Arctic basin: the Lena and the Northern Dvina rivers. A number of runoff statistics including the mean and the SD of the annual, monthly and daily runoff, as well as the annual runoff trend are assessed. The uncertainties of runoff statistics caused by the internal atmospheric variability are estimated. It is found that the uncertainty of the mean and SD of the runoff has a distinguished seasonal dependence with maximum during the periods of spring-summer snowmelt and summer-autumn rainfall floods. A noticeable non-linearity of the hydrological models' response to the ensemble ECHAM5 output is found most strongly expressed for the Northern Dvine River basin. It is shown that the averaging over ensemble members effectively filters stochastic term related to internal atmospheric variability. The simulated trends are close to normally distributed around ensemble mean value that indicates that a considerable portion of the observed trend can be externally driven.

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