scholarly journals The Effect of Orbital Forcing on the Mean Climate and Variability of the Tropical Pacific

2007 ◽  
Vol 20 (16) ◽  
pp. 4147-4159 ◽  
Author(s):  
A. Timmermann ◽  
S. J. Lorenz ◽  
S-I. An ◽  
A. Clement ◽  
S-P. Xie
Keyword(s):  
Annual Cycle ◽  
General Circulation ◽  
Model Simulation ◽  
Southern Oscillation ◽  
Circulation Model ◽  
Orbital Forcing ◽  
Frequency Entrainment ◽  
Eastern Equatorial Pacific ◽  
The Mean ◽  
Mean Climate

Abstract Using a coupled general circulation model, the responses of the climate mean state, the annual cycle, and the El Niño–Southern Oscillation (ENSO) phenomenon to orbital changes are studied. The authors analyze a 1650-yr-long simulation with accelerated orbital forcing, representing the period from 142 000 yr b.p. (before present) to 22 900 yr a.p. (after present). The model simulation does not include the time-varying boundary conditions due to ice sheet and greenhouse gas forcing. Owing to the mean seasonal cycle of cloudiness in the off-equatorial regions, an annual mean precessional signal of temperatures is generated outside the equator. The resulting meridional SST gradient in the eastern equatorial Pacific modulates the annual mean meridional asymmetry and hence the strength of the equatorial annual cycle. In turn, changes of the equatorial annual cycle trigger abrupt changes of ENSO variability via frequency entrainment, resulting in an anticorrelation between annual cycle strength and ENSO amplitude on precessional time scales.

Improved Climate Simulation by MIROC5: Mean States, Variability, and Climate Sensitivity

2010 ◽  
Vol 23 (23) ◽  
pp. 6312-6335 ◽  
Author(s):  
Masahiro Watanabe ◽  
Tatsuo Suzuki ◽  
Ryouta O’ishi ◽  
Yoshiki Komuro ◽  
Shingo Watanabe ◽  
...  
Keyword(s):  
Climate Sensitivity ◽  
General Circulation ◽  
Southern Oscillation ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Climate Simulation ◽  
Mean Fields ◽  
Equatorial Ocean ◽  
The Mean ◽  
The Difference

Abstract A new version of the atmosphere–ocean general circulation model cooperatively produced by the Japanese research community, known as the Model for Interdisciplinary Research on Climate (MIROC), has recently been developed. A century-long control experiment was performed using the new version (MIROC5) with the standard resolution of the T85 atmosphere and 1° ocean models. The climatological mean state and variability are then compared with observations and those in a previous version (MIROC3.2) with two different resolutions (medres, hires), coarser and finer than the resolution of MIROC5. A few aspects of the mean fields in MIROC5 are similar to or slightly worse than MIROC3.2, but otherwise the climatological features are considerably better. In particular, improvements are found in precipitation, zonal mean atmospheric fields, equatorial ocean subsurface fields, and the simulation of El Niño–Southern Oscillation. The difference between MIROC5 and the previous model is larger than that between the two MIROC3.2 versions, indicating a greater effect of updating parameterization schemes on the model climate than increasing the model resolution. The mean cloud property obtained from the sophisticated prognostic schemes in MIROC5 shows good agreement with satellite measurements. MIROC5 reveals an equilibrium climate sensitivity of 2.6 K, which is lower than that in MIROC3.2 by 1 K. This is probably due to the negative feedback of low clouds to the increasing concentration of CO2, which is opposite to that in MIROC3.2.

scholarly journals Near-Annual SST Variability in the Equatorial Pacific in a Coupled General Circulation Model

2005 ◽  
Vol 18 (21) ◽  
pp. 4454-4473 ◽  
Author(s):  
Renguang Wu ◽  
Ben P. Kirtman
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Equatorial Pacific ◽  
Near Surface ◽  
Boreal Spring ◽  
Eastern Equatorial Pacific ◽  
Zonal Advection ◽  
The Mean ◽  
Annual Mode ◽  
Sst Anomalies

Abstract Equatorial Pacific sea surface temperature (SST) anomalies in the Center for Ocean–Land–Atmosphere Studies (COLA) interactive ensemble coupled general circulation model show near-annual variability as well as biennial El Niño–Southern Oscillation (ENSO) variability. There are two types of near-annual modes: a westward propagating mode and a stationary mode. For the westward propagating near-annual mode, warm SST anomalies are generated in the eastern equatorial Pacific in boreal spring and propagate westward in boreal summer. Consistent westward propagation is seen in precipitation, surface wind, and ocean current. For the stationary near-annual mode, warm SST anomalies develop near the date line in boreal winter and decay locally in boreal spring. Westward propagation of warm SST anomalies also appears in the developing year of the biennial ENSO mode. However, warm SST anomalies for the westward propagating near-annual mode occur about two months earlier than those for the biennial ENSO mode and are quickly replaced by cold SST anomalies, whereas warm SST anomalies for the biennial ENSO mode only experience moderate weakening. Anomalous zonal advection contributes to the generation and westward propagation of warm SST anomalies for both the westward propagating near-annual mode and the biennial ENSO mode. However, the role of mean upwelling is markedly different. The mean upwelling term contributes to the generation of warm SST anomalies for the biennial ENSO mode, but is mainly a damping term for the westward propagating near-annual mode. The development of warm SST anomalies for the stationary near-annual mode is partially due to anomalous zonal advection and upwelling, similar to the amplification of warm SST anomalies in the equatorial central Pacific for the biennial ENSO mode. The mean upwelling term is negative in the eastern equatorial Pacific for the stationary near-annual mode, which is opposite to the ENSO mode. The development of cold SST anomalies in the aftermath of warm SST anomalies for the westward propagating near-annual mode is coupled to large easterly wind anomalies, which occur between the warm and cold SST anomalies. The easterly anomalies contribute to the cold SST anomalies through anomalous zonal, meridional, and vertical advection and surface evaporation. The cold SST anomalies, in turn, enhance the easterly anomalies through a Rossby-wave-type response. The above processes are most effective during boreal spring when the mean near-surface-layer ocean temperature gradient is the largest. It is suggested that the westward propagating near-annual mode is related to air–sea interaction processes that are limited to the near-surface layers.

scholarly journals The Monash Simple Climate Model Experiments (MSCM-DB v1.0): An interactive database of mean climate, climate change and scenario simulations

2018 ◽  
Author(s):  
Dietmar Dommenget ◽  
Kerry Nice ◽  
Tobias Bayr ◽  
Dieter Kasang ◽  
Christian Stassen ◽  
...  
Keyword(s):  
General Circulation ◽  
Climate Model ◽  
Model Simulation ◽  
Co2 Concentration ◽  
General Circulation Models ◽  
Simple Climate Model ◽  
Model Simulations ◽  
Sensitivity Experiments ◽  
The Mean ◽  
Mean Climate

Abstract. This study introduces the Monash Simple Climate Model (MSCM) experiment database. The model simulations are based on the Globally Resolved Energy Balance (GREB) model. They provide a basis to study three different aspects of climate model simulations: (1) understanding the processes that control the mean climate, (2) the response of the climate to a doubling of the CO2 concentration, and (3) scenarios of external CO2 concentration and solar radiation forcings. A series of sensitivity experiments in which elements of the climate system are turned off in various combinations are used to address (1) and (2). This database currently provides more than 1,300 experiments and has an online web interface for fast analysis of the experiments and for open access to the data. We briefly outline the design of all experiments, give a discussion of some results, and put the findings into the context of previously published results from similar experiments. We briefly discuss the quality and limitations of the MSCM experiments and also give an outlook on possible further developments. The GREB model simulation of the mean climate processes is quite realistic, but does have uncertainties in the order of 20–30 %. The GREB model without flux corrections has a root mean square error in mean state of about 10 °C, which is larger than those of general circulation models (2 °C). However, the MSCM experiments show good agreement to previously published studies. Although GREB is a very simple model, it delivers good first-order estimates, is very fast, highly accessible, and can be used to quickly try many different sensitivity experiments or scenarios.

scholarly journals Diagnosing the Annual Cycle Modes in the Tropical Atlantic Ocean Using a Directly Coupled Atmosphere–Ocean GCM

2006 ◽  
Vol 19 (20) ◽  
pp. 5319-5342 ◽  
Author(s):  
David G. DeWitt ◽  
Edwin K. Schneider
Keyword(s):  
Heat Flux ◽  
Southern Hemisphere ◽  
Vertical Velocity ◽  
Annual Cycle ◽  
General Circulation ◽  
Model Simulation ◽  
Circulation Model ◽  
Surface Fluxes ◽  
Thermocline Depth ◽  
Tropical Atlantic

Abstract The annual cycle of sea surface temperature (SST) in the tropical Atlantic of a directly coupled atmosphere–ocean general circulation model (CGCM) is decomposed into the parts forced by different surface fluxes (denoted as modes) for the two extreme months of March and August using forced ocean experiments. Almost all previous diagnostic work of the forcing of the SST annual cycle in the Atlantic has concentrated on the near-equatorial region. Here, the annual cycle is examined within the latitude range of 25°S–25°N to facilitate comparison with the interannual variability. The structure of the response to the different surface flux forcings bears some resemblance to the interannual SST modes in the tropical Atlantic, which are diagnosed using rotated empirical orthogonal function (REOF) analysis. Diagnosis of the forcing of the annual cycle modes and the interannual modes shows that they do not always have a common cause. Hence, the simple interpretation that the leading interannual modes are perturbations to the annual cycle is not always valid. In particular, the equatorial SST annual cycle mode is primarily driven by variations in vertical velocity while the equatorial interannual mode is associated with eastward-propagating thermocline anomalies and is forced by both thermocline anomalies and vertical velocity anomalies. As for the interannual modes, there exist off-equatorial annual cycle modes in both the Northern and Southern Hemispheres. The annual cycle off-equatorial mode in both hemispheres is shown to be primarily driven by heat flux variations. The Southern Hemisphere interannual mode is primarily driven by heat flux variations while the Northern Hemisphere interannual mode shows a strong influence of thermocline depth anomalies. In addition, the Southern Hemisphere interannual mode is centered about 10° south of the annual cycle mode. An interannual mode that has maximum variability along the South American coast south of the equator is shown to be associated with thermocline depth anomalies. This interannual mode has no analog in the annual cycle modes. The coupled model simulation of the annual cycle is found to be fairly realistic so that the results presented here could have applicability to the observed Atlantic.

scholarly journals Ocean Circulation and Tropical Variability in the Coupled Model ECHAM5/MPI-OM

2006 ◽  
Vol 19 (16) ◽  
pp. 3952-3972 ◽  
Author(s):  
J. H. Jungclaus ◽  
N. Keenlyside ◽  
M. Botzet ◽  
H. Haak ◽  
J.-J. Luo ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
General Circulation ◽  
Southern Oscillation ◽  
Surface Wind ◽  
Coupled Model ◽  
Circulation Model ◽  
Tropical Variability ◽  
Sst Variability ◽  
The Mean ◽  
Mean State

Abstract This paper describes the mean ocean circulation and the tropical variability simulated by the Max Planck Institute for Meteorology (MPI-M) coupled atmosphere–ocean general circulation model (AOGCM). Results are presented from a version of the coupled model that served as a prototype for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) simulations. The model does not require flux adjustment to maintain a stable climate. A control simulation with present-day greenhouse gases is analyzed, and the simulation of key oceanic features, such as sea surface temperatures (SSTs), large-scale circulation, meridional heat and freshwater transports, and sea ice are compared with observations. A parameterization that accounts for the effect of ocean currents on surface wind stress is implemented in the model. The largest impact of this parameterization is in the tropical Pacific, where the mean state is significantly improved: the strength of the trade winds and the associated equatorial upwelling weaken, and there is a reduction of the model’s equatorial cold SST bias by more than 1 K. Equatorial SST variability also becomes more realistic. The strength of the variability is reduced by about 30% in the eastern equatorial Pacific and the extension of SST variability into the warm pool is significantly reduced. The dominant El Niño–Southern Oscillation (ENSO) period shifts from 3 to 4 yr. Without the parameterization an unrealistically strong westward propagation of SST anomalies is simulated. The reasons for the changes in variability are linked to changes in both the mean state and to a reduction in atmospheric sensitivity to SST changes and oceanic sensitivity to wind anomalies.

Multiscale Impacts of Variable Heating in Climate

2007 ◽  
Vol 20 (23) ◽  
pp. 5677-5695 ◽  
Author(s):  
Prashant D. Sardeshmukh ◽  
Philip Sura
Keyword(s):  
General Circulation ◽  
Climate Models ◽  
Climate Model ◽  
Circulation Model ◽  
General Circulation Models ◽  
Heat Fluxes ◽  
Winter Climate ◽  
The Mean ◽  
Mean Climate ◽  
Diabatic Forcing

Abstract While it is obvious that the mean diabatic forcing of the atmosphere is crucial for maintaining the mean climate, the importance of diabatic forcing fluctuations is less evident in this regard. Such fluctuations do not appear directly in the equations of the mean climate but affect the mean indirectly through their effects on the time-mean transient-eddy fluxes of heat, momentum, and moisture. How large are these effects? What are the effects of tropical phenomena associated with substantial heating variations such as ENSO and the MJO? To what extent do variations of the extratropical surface heat fluxes and precipitation affect the mean climate? What are the effects of the rapid “stochastic” components of the heating fluctuations? Most current climate models misrepresent ENSO and the MJO and ignore stochastic forcing; they therefore also misrepresent their mean effects. To what extent does this contribute to climate model biases and to projections of climate change? This paper provides an assessment of such impacts by comparing with observations a long simulation of the northern winter climate by a dry adiabatic general circulation model forced only with the observed time-mean diabatic forcing as a constant forcing. Remarkably, despite the total neglect of all forcing variations, the model reproduces most features of the observed circulation variability and the mean climate, with biases similar to those of some state-of-the-art general circulation models. In particular, the spatial structures of the circulation variability are remarkably well reproduced. Their amplitudes, however, are progressively underestimated from the synoptic to the subseasonal to interannual and longer time scales. This underestimation is attributed to the neglect of the variable forcing. The model also excites significant tropical variability from the extratropics on interannual scales, which is overwhelmed in reality by the response to tropical heating variability. It is argued that the results of this study suggest a role for the stochastic, and not only the coherent, components of transient diabatic forcing in the dynamics of climate variability and the mean climate.

scholarly journals Influence of orbital forcing and solar activity on water isotopes in precipitation during the mid and late Holocene

2012 ◽  
Vol 8 (4) ◽  
pp. 3791-3829
Author(s):  
S. Dietrich ◽  
M. Werner ◽  
T. Spangehl ◽  
G. Lohmann
Keyword(s):  
Solar Activity ◽  
General Circulation ◽  
Late Holocene ◽  
Circulation Model ◽  
Oxygen Isotopic Composition ◽  
Orbital Forcing ◽  
Solar Forcing ◽  
Surface Temperatures ◽  
Mean Climate ◽  
The Impact

Abstract. In this study we investigate the impact of mid and late Holocene orbital forcing and solar activity on variations of the oxygen isotopic composition in precipitation. Our study is based on a set of novel climate simulations performed with the atmosphere general circulation model ECHAM5-wiso enhanced by explicit water isotope diagnostics. From the performed model experiments we derive the following major results: (1) the response of both orbital and solar forcing lead to changes in surface temperatures and δ18O in precipitation with similar magnitudes during the mid and late Holocene. (2) Past δ18O anomalies correspond to changing temperatures in the orbital driven simulations. This does not hold true if an additional solar forcing is added. (3) Two orbital driven mid Holocene experiments, simulating the mean climate state approximately 5000 and 6000 yr ago, yield very similar results. However, if an identical additional solar activity-induced forcing is added, the simulated changes of surface temperatures as well as δ18O between both periods differ. From our findings we conclude that the Holocene variability of δ18O in precipitation, as stored in many paleoclimate archives, is rather complex to understand since the combined effect of different external forcings on δ18O in precipitation is non-linear.

scholarly journals Pacific Climate Change and ENSO Activity in the Mid-Holocene

2009 ◽  
Vol 22 (4) ◽  
pp. 923-939 ◽  
Author(s):  
J. C. H. Chiang ◽  
Y. Fang ◽  
P. Chang
Keyword(s):  
Annual Cycle ◽  
Climate Changes ◽  
Southern Oscillation ◽  
Asian Summer Monsoon ◽  
Coupled Model ◽  
Tropical Pacific ◽  
Orbital Forcing ◽  
Frequency Entrainment ◽  
Asian Summer ◽  
The Tropical Pacific

Abstract The authors argue that a reduction to the stochastic forcing of the El Niño–Southern Oscillation (ENSO) wrought by Pacific-wide climate changes in response to mid-Holocene (6000 BP) orbital forcing is a viable hypothesis for the observed reduction of ENSO activity during that time. This conclusion is based on comprehensive analysis of an intermediate coupled model that achieves significant reduction to ENSO variance in response to mid-Holocene orbital forcing. The model’s excellent simulation of the tropical Pacific interannual variability lends credibility to the results. Idealized simulations demonstrate that the mid-Holocene influence is communicated to the tropical Pacific largely via climate changes outside of the tropical Pacific, rather than from insolation changes directly on the tropical Pacific. This is particularly true for changes to the ENSO, but also with changes to the cold tongue annual cycle. Previously proposed mechanisms for teleconnected mid-Holocene ENSO changes, including forcing of ENSO by a strengthened Asian summer monsoon and an increase in the annual cycle forcing on the tropical Pacific leading to a reduction in ENSO activity by frequency entrainment, do not appear to occur in these simulations. Rather, the authors show that the modeled mid-Holocene climate exhibits a pronounced reduction in Pacific meridional mode activity that has been recently shown to be a forcing on ENSO, though the reasons for this reduction are still to be explained. The contrasting nature of the results compared to previous studies highlights the effect of the prevailing ENSO paradigm on this problem. By showing that an externally forced ENSO model is equally capable of explaining mid-Holocene ENSO reduction as its nonlinear, weakly chaotic counterpart, it is demonstrated that the mid-Holocene ENSO data point cannot yet discriminate between these two paradigms of ENSO.

scholarly journals Stratospheric Communication of El Niño Teleconnections to European Winter

2009 ◽  
Vol 22 (15) ◽  
pp. 4083-4096 ◽  
Author(s):  
C. J. Bell ◽  
L. J. Gray ◽  
A. J. Charlton-Perez ◽  
M. M. Joshi ◽  
A. A. Scaife
Keyword(s):  
El Niño ◽  
General Circulation ◽  
El Nino ◽  
Southern Oscillation ◽  
Polar Vortex ◽  
Circulation Model ◽  
Active Role ◽  
Late Winter ◽  
The Mean ◽  
Mean State

Abstract The stratospheric role in the European winter surface climate response to El Niño–Southern Oscillation sea surface temperature forcing is investigated using an intermediate general circulation model with a well-resolved stratosphere. Under El Niño conditions, both the modeled tropospheric and stratospheric mean-state circulation changes correspond well to the observed “canonical” responses of a late winter negative North Atlantic Oscillation and a strongly weakened polar vortex, respectively. The variability of the polar vortex is modulated by an increase in frequency of stratospheric sudden warming events throughout all winter months. The potential role of this stratospheric response in the tropical Pacific–European teleconnection is investigated by sensitivity experiments in which the mean state and variability of the stratosphere are degraded. As a result, the observed stratospheric response to El Niño is suppressed and the mean sea level pressure response fails to resemble the temporal and spatial evolution of the observations. The results suggest that the stratosphere plays an active role in the European response to El Niño. A saturation mechanism whereby for the strongest El Niño events tropospheric forcing dominates the European response is suggested. This is examined by means of a sensitivity test and it is shown that under large El Niño forcing the European response is insensitive to stratospheric representation.

A Regional Ocean–Atmosphere Model for Eastern Pacific Climate: Toward Reducing Tropical Biases*

2007 ◽  
Vol 20 (8) ◽  
pp. 1504-1522 ◽  
Author(s):  
Shang-Ping Xie ◽  
Toru Miyama ◽  
Yuqing Wang ◽  
Haiming Xu ◽  
Simon P. de Szoeke ◽  
...  
Keyword(s):  
Annual Cycle ◽  
General Circulation ◽  
Southern Oscillation ◽  
Circulation Model ◽  
Tropical Pacific ◽  
Eastern Pacific ◽  
Surface Mixed Layer ◽  
Ocean Atmosphere ◽  
Low Cloud ◽  
Atmosphere Model

Abstract The tropical Pacific Ocean is a climatically important region, home to El Niño and the Southern Oscillation. The simulation of its climate remains a challenge for global coupled ocean–atmosphere models, which suffer large biases especially in reproducing the observed meridional asymmetry across the equator in sea surface temperature (SST) and rainfall. A basin ocean general circulation model is coupled with a full-physics regional atmospheric model to study eastern Pacific climate processes. The regional ocean–atmosphere model (ROAM) reproduces salient features of eastern Pacific climate, including a northward-displaced intertropical convergence zone (ITCZ) collocated with a zonal band of high SST, a low-cloud deck in the southeastern tropical Pacific, the equatorial cold tongue, and its annual cycle. The simulated low-cloud deck experiences significant seasonal variations in vertical structure and cloudiness; cloud becomes decoupled and separated from the surface mixed layer by a stable layer in March when the ocean warms up, leading to a reduction in cloudiness. The interaction of low cloud and SST is an important internal feedback for the climatic asymmetry between the Northern and Southern Hemispheres. In an experiment where the cloud radiative effect is turned off, this climatic asymmetry weakens substantially, with the ITCZ migrating back and forth across the equator following the sun. In another experiment where tropical North Atlantic SST is lowered by 2°C—say, in response to a slow-down of the Atlantic thermohaline circulation as during the Younger Dryas—the equatorial Pacific SST decreases by up to 3°C in January–April but changes much less in other seasons, resulting in a weakened equatorial annual cycle. The relatively high resolution (0.5°) of the ROAM enables it to capture mesoscale features, such as tropical instability waves, Central American gap winds, and a thermocline dome off Costa Rica. The implications for tropical biases and paleoclimate research are discussed.

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