The scaling of the meridional overturning with the meridional temperature gradient in idealised general circulation models

Simulating the proxy-derived surface warming and reduced meridional temperature gradient of the early Eocene greenhouse climate still represents a challenge for most atmosphere-ocean general circulation models. A profound understanding of uncertainties associated with the respective model results is thereby essential to reliably identify any similarities or misfits to the proxy record. Besides incomplete knowledge of past greenhouse gas concentrations and other boundary conditions, structural and parametric uncertainties are the main factors that determine our confidence in paleoclimate simulation results.The recent publication of coordinated model experiments that apply identical paleogeographic boundary conditions for key time periods of the early Eocene (DeepMIP) allows a systematic analysis of inter-model differences and therefore of structural uncertainties in the simulated surface warming. Here we additionally explore the parametric uncertainty of the early Eocene climatic optimum (EECO) surface warming within one DeepMIP model. For this we performed perturbed parameter ensemble (PPE) simulations with HadCM3B at different atmospheric CO2 concentrations following the DeepMIP protocol. Twenty-one parameter sets based on changes in six atmospheric parameters, a sea-ice parameter and the ocean background diffusivity were branched off from the respective DeepMIP control simulations and integrated for a further 1500 model years. The selected parameter sets are based on previous results demonstrating their ability to simulate a pre-industrial global-mean surface temperature within &#177;2 &#176;C of the standard configuration.Preliminary results indicate a large spread of the simulated low-latitude surface warming in the PPE and therefore significant changes of the large-scale meridional temperature gradient for the EECO. Some ensemble members develop numerical instabilities at CO2 concentrations of 840 ppmv and above, most likely in consequence of high temperatures in the tropical troposphere. We further compare the magnitude of the parametric uncertainty of the HadCM3B perturbation experiments with the structural differences found in the DeepMIP multi-model ensemble and explore the sensitivity of the results to the strength of the applied greenhouse gas forcing. Model skill of the PPE members is tested against the most recent DeepMIP compilations of marine and terrestrial proxy temperatures.

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The Tropospheric Response to Tropical and Subtropical Zonally Asymmetric Torques: Analytical and Idealized Numerical Model Results

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-11-0139.1 ◽

2012 ◽

Vol 69 (1) ◽

pp. 214-235 ◽

Cited By ~ 7

Author(s):

Tiffany A. Shaw ◽

William R. Boos

Keyword(s):

Temperature Gradient ◽

Northern Hemisphere ◽

General Circulation ◽

Large Scale ◽

Critical Line ◽

Circulation Model ◽

General Circulation Models ◽

Kelvin Wave ◽

Meridional Temperature Gradient ◽

Poleward Shift

Abstract The tropospheric response to prescribed tropical and subtropical zonally asymmetric torques, which can be considered as idealizations of vertical momentum transfers by orographic gravity waves or convection, is investigated. The linear analytical Gill model response to westward upper-tropospheric torques is compared to the response to a midtropospheric heating, which is a familiar point of reference. The response to an equatorial torque projects onto a Kelvin wave response to the east that is of opposite sign to the response to the east of the heating at upper levels. In contrast, the torque and heating both produce Rossby gyres of the same sign to the west of the forcing and the zonal-mean streamfunction responses are identical. When the forcings are shifted into the Northern Hemisphere, the streamfunction responses have opposite signs: there is upwelling in the Southern (Northern) Hemisphere in response to the torque (heating). The nonlinear response to westward torques was explored in idealized general circulation model experiments. In the absence of a large-scale meridional temperature gradient, the response to an equatorial torque was confined to the tropics and was qualitatively similar to the linear solutions. When the torque was moved into the subtropics, the vorticity budget response was similar to a downward control–type balance in the zonal mean. In the presence of a meridional temperature gradient, the response to an equatorial torque involved a poleward shift of the midlatitude tropospheric jet and Ferrel cell. The response in midlatitudes was associated with a poleward shift of the regions of horizontal eddy momentum flux convergence, which coincided with a shift in the upper-tropospheric critical line for baroclinic waves. The shift in the critical line was caused (in part) by the zonal wind response to the prescribed torque, suggesting a possible cause of the response in midlatitudes. Overall, this hierarchy of analytical and numerical results highlights robust aspects of the response to tropical and subtropical zonally asymmetric torques and represents the first step toward understanding the response in fully comprehensive general circulation models.

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Influence of the SST Rise on Baroclinic Instability Wave Activity under an Aquaplanet Condition

Journal of the Atmospheric Sciences ◽

10.1175/2009jas2964.1 ◽

2009 ◽

Vol 66 (8) ◽

pp. 2272-2287 ◽

Cited By ~ 17

Author(s):

Chihiro Kodama ◽

Toshiki Iwasaki

Keyword(s):

Global Warming ◽

Temperature Gradient ◽

Wave Energy ◽

General Circulation ◽

Baroclinic Instability ◽

Lower Troposphere ◽

Wave Activity ◽

Instability Wave ◽

Meridional Temperature Gradient ◽

Poleward Shift

Abstract The influence of the sea surface temperature (SST) rise on extratropical baroclinic instability wave activity is investigated using an aquaplanet general circulation model (GCM). Two types of runs were performed: the High+3 run, in which the SST is increased by 3 K only at high latitudes, and the All+3 run, in which the SST is increased uniformly by 3 K all over the globe. These SST rises were intended to reproduce essential changes of the surface air temperature due to global warming. Wave activity changes are analyzed and discussed from the viewpoint of the energetics. In the High+3 run, midlatitude meridional temperature gradient is decreased in the lower troposphere and the wave energy is suppressed in the extratropics. In the All+3 run, although the large tropical latent heat release greatly enhances the midlatitude meridional temperature gradient in the upper troposphere, global mean wave energy does not change significantly. These results suggest that the low-level baroclinicity is much more important for baroclinic instability wave activity than upper-level baroclinicity. A poleward shift of wave energy, seen in global warming simulations, is evident in the All+3 run. Wave energy generation analysis suggests that the poleward shift of wave activity may be caused by the enhanced and poleward-shifted baroclinicity in the higher latitudes and the increased static stability in the lower latitudes. Poleward expansion of the high-baroclinicity region is still an open question.

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The Leading, Interdecadal Eigenmode of the Atlantic Meridional Overturning Circulation in a Realistic Ocean Model

Journal of Climate ◽

10.1175/jcli-d-11-00023.1 ◽

2013 ◽

Vol 26 (7) ◽

pp. 2160-2183 ◽

Cited By ~ 47

Author(s):

Florian Sévellec ◽

Alexey V. Fedorov

Keyword(s):

Temperature Gradient ◽

North Atlantic ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Meridional Temperature Gradient ◽

Overturning Circulation ◽

Temperature Anomalies ◽

The North ◽

Meridional Overturning ◽

The North Atlantic

Abstract Variations in the strength of the Atlantic meridional overturning circulation (AMOC) are a major potential source of decadal and longer climate variability in the Atlantic. This study analyzes continuous integrations of tangent linear and adjoint versions of an ocean general circulation model [Océan Parallélisé (OPA)] and rigorously shows the existence of a weakly damped oscillatory eigenmode of the AMOC centered in the North Atlantic Ocean and controlled solely by linearized ocean dynamics. In this particular GCM, the mode period is roughly 24 years, its e-folding decay time scale is 40 years, and it is the least-damped oscillatory mode in the system. Its mechanism is related to the westward propagation of large-scale temperature anomalies in the northern Atlantic in the latitudinal band between 30° and 60°N. The westward propagation results from a competition among mean eastward zonal advection, equivalent anomalous westward advection caused by the mean meridional temperature gradient, and westward propagation typical of long baroclinic Rossby waves. The zonal structure of temperature anomalies alternates between a dipole (corresponding to an anomalous AMOC) and anomalies of one sign (yielding no changes in the AMOC). Further, it is shown that the system is nonnormal, which implies that the structure of the least-damped eigenmode of the tangent linear model is different from that of the adjoint model. The “adjoint” mode describes the sensitivity of the system (i.e., it gives the most efficient patterns for exciting the leading eigenmode). An idealized model is formulated to highlight the role of the background meridional temperature gradient in the North Atlantic for the mode mechanism and the system nonnormality.

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Impact of the Warming Pattern on Global Energetics

Journal of Climate ◽

10.1175/jcli-d-11-00468.1 ◽

2012 ◽

Vol 25 (15) ◽

pp. 5223-5240 ◽

Cited By ~ 3

Author(s):

Daniel Hernández-Deckers ◽

Jin-Song von Storch

Keyword(s):

Temperature Gradient ◽

General Circulation ◽

Circulation Model ◽

Ocean General Circulation Model ◽

Static Stability ◽

Meridional Temperature Gradient ◽

Surface Warming ◽

Warming Pattern ◽

Atmospheric Energetics ◽

The Mean

Abstract The warming pattern due to higher greenhouse gas concentrations is expected to affect the global atmospheric energetics mainly via changes in the (i) meridional temperature gradient and (ii) mean static stability. Changes in surface meridional temperature gradients have been previously regarded as the determining feature for the energetics response, but recent studies suggest that changes in mean static stability may be more relevant than previously thought. This study aims to determine the relative importance of these two effects by comparing the energetics responses due to different warming patterns using a fully coupled atmosphere–ocean general circulation model. By means of an additional diabatic forcing, experiments with different warming patterns are obtained: one with a 2xCO2-like pattern that validates the method, one with only the tropical upper-tropospheric warming, and one with only the high-latitude surface warming. The study’s findings suggest that the dominant aspect of the warming pattern that alters the global atmospheric energetics is not its associated meridional temperature gradient changes, but the mean static stability changes. The tropical upper warming weakens the energetics by increasing the mean static stability, whereas the surface warming strengthens it by reducing the mean static stability. The combined 2xCO2-like response is dominated by the tropical upper-tropospheric warming effect, hence the weaker energetic activity. Eddy kinetic energy changes consistently, but the two opposite responses nearly cancel each other in the 2xCO2 case. Therefore, estimates of future changes in storminess may be particularly sensitive to the relative magnitude of the main features of the simulated warming pattern.

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Characteristics and robustness of Agulhas Leakage estimates: an inter-comparison study of Lagrangian methods

10.5194/egusphere-egu21-12107 ◽

2021 ◽

Author(s):

Christina Schmidt ◽

Franziska Schwarzkopf ◽

Siren Rühs ◽

Arne Biastoch

Keyword(s):

General Circulation ◽

General Circulation Models ◽

Horizontal Resolution ◽

Meridional Overturning Circulation ◽

Clear Trend ◽

Agulhas Current ◽

Ocean General Circulation Models ◽

Cape Basin ◽

Meridional Overturning ◽

The Indian Ocean

The exchange of water between the Indian Ocean and South Atlantic with their different thermohaline properties via Agulhas Leakage is important for the meridional overturning circulation. Agulhas Leakage as well as the output of ocean general circulation models in general can be analysed using a Lagrangian approach with a variety of different tools available. Here, Agulhas Leakage is estimated with both the newly developed tool Parcels and the well established tool Ariane, and different designs of the Lagrangian experiment are analysed. In a hindcast simulation with the eddy-rich ocean sea-ice model INALT20 (1/20&#176; horizontal resolution) under the new JRA55-do forcing, Agulhas Leakage increases from the early 1960s to mid 1980s, but there is no clear trend afterwards, which is in contrast to earlier studies using hindcast simulations under the CORE forcing. During the transit from the Agulhas Current at 32&#176;S to the Cape Basin, a cooling and freshening of Agulhas Leakage waters occurs especially in the western part of the Retroflection, resulting in a density increase as the thermal effect dominates. The average transport, its variability, trend and the transit time from the Agulhas Current to the Cape Basin of Agulhas Leakage is simulated equally with the Lagrangian tools Ariane and Parcels, emphasising the robustness of our method.

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Sea Surface Temperature Biases under the Stratus Cloud Deck in the Southeast Pacific Ocean in 19 IPCC AR4 Coupled General Circulation Models

Journal of Climate ◽

10.1175/2011jcli4172.1 ◽

2011 ◽

Vol 24 (15) ◽

pp. 4139-4164 ◽

Cited By ~ 67

Author(s):

Yangxing Zheng ◽

Toshiaki Shinoda ◽

Jia-Lin Lin ◽

George N. Kiladis

Keyword(s):

Temperature Gradient ◽

General Circulation ◽

General Circulation Models ◽

Surface Heat ◽

Heat Fluxes ◽

Sea Surface ◽

Stratus Cloud ◽

Surface Heat Fluxes ◽

Southeast Pacific ◽

Coupled General Circulation Models

Abstract This study examines systematic biases in sea surface temperature (SST) under the stratus cloud deck in the southeast Pacific Ocean and upper-ocean processes relevant to the SST biases in 19 coupled general circulation models (CGCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The 20 years of simulations from each model are analyzed. Pronounced warm SST biases in a large portion of the southeast Pacific stratus region are found in all models. Processes that could contribute to the SST biases are examined in detail based on the computation of major terms in the upper-ocean heat budget. Negative biases in net surface heat fluxes are evident in most of the models, suggesting that the cause of the warm SST biases in models is not explained by errors in net surface heat fluxes. Biases in heat transport by Ekman currents largely contribute to the warm SST biases both near the coast and the open ocean. In the coastal area, southwestward Ekman currents and upwelling in most models are much weaker than observed owing to weaker alongshore winds, resulting in insufficient advection of cold water from the coast. In the open ocean, warm advection due to Ekman currents is overestimated in models because of the larger meridional temperature gradient, the smaller zonal temperature gradient, and overly weaker Ekman currents.

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Leading interannual variability modes of East Asian winter precipitation in CMIP5 general circulation models

Climate Research ◽

10.3354/cr01533 ◽

2018 ◽

Vol 76 (2) ◽

pp. 177-189

Author(s):

C Jin ◽

Y Yang ◽

W Guo

Keyword(s):

Interannual Variability ◽

General Circulation ◽

East Asian ◽

General Circulation Models ◽

Winter Precipitation

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Zonally symmetric global general circulation models with and without the hydrologic cycle

Tellus ◽

10.3402/tellusa.v25i4.9668 ◽

1973 ◽

Vol 25 (4) ◽

pp. 337-354 ◽

Cited By ~ 7

Author(s):

B. G. Hunt

Keyword(s):

General Circulation ◽

General Circulation Models ◽

Hydrologic Cycle

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Successive Modulation of ENSO to the Future Greenhouse Warming

Journal of Climate ◽

10.1175/2007jcli1500.1 ◽

2008 ◽

Vol 21 (1) ◽

pp. 3-21 ◽

Cited By ~ 55

Author(s):

Soon-Il An ◽

Jong-Seong Kug ◽

Yoo-Geun Ham ◽

In-Sik Kang

Keyword(s):

General Circulation ◽

Southern Oscillation ◽

General Circulation Models ◽

Tropical Pacific ◽

Greenhouse Warming ◽

Delayed Response ◽

Subsurface Temperature ◽

Climate System Model ◽

The Mean ◽

The Tropical Pacific

Abstract The multidecadal modulation of the El Niño–Southern Oscillation (ENSO) due to greenhouse warming has been analyzed herein by means of diagnostics of Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) coupled general circulation models (CGCMs) and the eigenanalysis of a simplified version of an intermediate ENSO model. The response of the global-mean troposphere temperature to increasing greenhouse gases is more likely linear, while the amplitude and period of ENSO fluctuates in a multidecadal time scale. The climate system model outputs suggest that the multidecadal modulation of ENSO is related to the delayed response of the subsurface temperature in the tropical Pacific compared to the response time of the sea surface temperature (SST), which would lead a modulation of the vertical temperature gradient. Furthermore, an eigenanalysis considering only two parameters, the changes in the zonal contrast of the mean background SST and the changes in the vertical contrast between the mean surface and subsurface temperatures in the tropical Pacific, exhibits a good agreement with the CGCM outputs in terms of the multidecadal modulations of the ENSO amplitude and period. In particular, the change in the vertical contrast, that is, change in difference between the subsurface temperature and SST, turns out to be more influential on the ENSO modulation than changes in the mean SST itself.

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