Coupled Atmosphere Ocean General Circulation Research Articles

Subsurface ocean biases in climate models and its implications in the simulated interannual variability: A case study for Indian Ocean

Coupled ocean atmosphere general circulation models (CGCMs) are known to have deep ocean biases when simulated over long term climate timescales. Here, an analysis has been done for investigating the impacts of these deep ocean biases, especially those which occur in temperature and salinity, ensuing biases in ocean dynamics and large scale air-sea interactions in state of the art CGCMs. The outputs from historical runs of 20 Coupled Model Intercomparison Project Phase 5 (CMIP-5) models have been analyzed for Indian Ocean. All candidate models develop internal warm and saline biases approximately between a depth range of 100 and 800 m in long term simulations. These internal biases are found to have implications in large scale ocean dynamics via their linkage through baroclinicity of the ocean. The role of internal biases in the ensuing dynamics is correlated via relations between Brunt Väisälä frequency (N2) and baroclinic wave speeds using both Sturm-Loiuville theorem and WKBJ approximation. The CMIP-5 models analyzed here have a higher baroclinic speed compared to that of observations. Annual propagating modes in the ocean reveal higher speeds in most of the models, and that the phase reversal is taking place at lead or lag by a month or more as compared to observations. The study suggests that faster wave propagation in climate models due to subsurface biases in temperature, salinity, N2 and baroclinicity have potential to impact simulated planetary scale events in terms of their life cycle, periodicity and seasonality as analyzed with Indian Ocean Dipole Zonal Mode. A corollary being a cautionary outlook on climate projections made by coupled models as long as the biases are persistent.

Exploring the MIS M2 glaciation occurring during a warm and high atmospheric CO2 Pliocene background climate

Prior to the Northern Hemisphere glaciation around ∼2.7 Ma, a large global glaciation corresponding to a 20 to 60 m sea-level drop occurred during Marine Isotope Stage (MIS) M2 (3.312–3.264 Ma), interrupted the period of global warmth and high CO2 concentration (350–450 ppmv) of the mid Piacenzian. Unlike the late Quaternary glaciations, the M2 glaciation only lasted 50 kyrs and occurred under uncertain CO2 concentration (220–390 ppmv). The mechanisms causing the onset and termination of the M2 glaciation remain enigmatic, but a recent geological hypothesis suggests that the re-opening and closing of the shallow Central American Seaway (CAS) might have played a key role. In this article, thanks to a series of climate simulations carried out using a fully coupled Atmosphere Ocean General Circulation Model (GCM) and a dynamic ice sheet model, we show that re-opening of the shallow CAS helps precondition the low-latitude oceanic circulation and affects the related northward energy transport, but cannot alone explain the onset of the M2 glaciation. The presence of a shallow open CAS, together with favourable orbital parameters, 220 ppmv of CO2 concentration, and the related vegetation and ice sheet feedback, led to a global ice sheet build-up producing a global sea-level drop in the lowest range of proxy-derived estimates. More importantly, our results show that the simulated closure of the CAS has a negligible impact on the NH ice sheet melt and cannot explain the MIS M2 termination.

Observational constraints indicate risk of drying in the Amazon basin

Climate warming due to human activities will be accompanied by hydrological cycle changes. Economies, societies and ecosystems in South America are vulnerable to such water resource changes. Hence, water resource impact assessments for South America, and corresponding adaptation and mitigation policies, have attracted increased attention. However, substantial uncertainties remain in the current water resource assessments that are based on multiple coupled Atmosphere Ocean General Circulation models. This uncertainty varies from significant wetting to catastrophic drying. By applying a statistical method, we characterized the uncertainty and identified global-scale metrics for measuring the reliability of water resource assessments in South America. Here, we show that, although the ensemble mean assessment suggested wetting across most of South America, the observational constraints indicate a higher probability of drying in the Amazon basin. Thus, over-reliance on the consensus of models can lead to inappropriate decision making.

CMCC-SXF025: A High-Resolution Coupled Atmosphere Ocean General Circulation Climate Model

This technical report summarizes the SINTEX-F025 (CMCC-SXF025) model technical structure. CMCC-SXF025 is an Atmosphere Ocean General Circulation Model (AOGCM) developed at INGV-CMCC to perform global seasonal forecasting.This model is used to run special focus experiments in the framework of MERSEA (Marine EnviRonment and Security for the European Area EU-Project).CMCC-SXF025 is an evolution of SINTEX and SINTEX-F and this report indicates the improvement with respect to these previous INGV AOCM models.The new model includes an ocean model with higher resolution: 0.25 degrees as horizontal resolution and 46 vertical levels. The description of model components, coupling methods, compiling and running environments is a guideline for model users.

Ensemble, water isotope–enabled, coupled general circulation modeling insights into the 8.2 ka event

Freshwater forcing has long been postulated as a catalyst for abrupt climate change because of its potential to interfere with thermohaline circulation (THC). The most recent example may have occurred about 8.2 ka ago with the sudden drainage of glacial lakes Agassiz and Ojibway into the Hudson Bay. We perform an ensemble of simulations for this freshwater release using the fully coupled atmosphere ocean general circulation model, Goddard Institute for Space Studies ModelE‐R. In all cases, simulated effects include reduced ocean heat transport and enhanced atmospheric heat transport in the Atlantic, increased surface albedo (through greater low cloud and sea ice cover), and local cooling of up to 3°C. Our suite of ensemble experiments allows us to examine the importance of the initial ocean state, in particular the presence or absence of Labrador Sea Water, in controlling the magnitude and length of the climate response. Water isotope tracers included in this model provide an improved means for direct comparisons of the modeled tracer response to water isotope–based, climate proxy data. Comparison of model simulations to data implies that there was an abrupt approximate halving of Atlantic THC, hence providing strong support for the hypothesis that the 8.2 ka event was caused by an abrupt release of fresh water into the North Atlantic.

The INGV-CMCC IPCC Scenario Simulations

This technical report describes the climate scenario simulations provided by the INGV-CMCC to the Intergovernmental Panel on Climate Change (IPCC). The model used to run these simulations is SINTEX-G (INGV-SXG), a fully coupled Atmosphere Ocean General Circulation Model (AOGCM) able to run experiments driven by radiative forcing (Ozone, Sulfate Aerosols, Greenhouse Gases) as specified in the protocol for the IPCC standard experiments.Six experiments have been performed: preindustrial simulation, XX century simulaton, XXI century simulatons under different IPCC radiative forcing conditions (IPCC scenarios A1B and A2), CO2 increase simulations to doubling and quadrupling.Scenario Simulations settings and model data availability are fully explained in the text.

On the robustness of changes in extreme precipitation over Europe from two high resolution climate change simulations

AbstractTwo Regional Climate Model (RCM) projections of changes in extreme precipitation over Europe are assessed and compared. This provides insight into the importance of RCM formulation in representing changes in climate extremes at high spatial resolution. The models concerned are two recent Hadley Centre RCMs, HadRM2 and HadRM3, and are applied at a horizontal resolution of approximately 50 km over Europe, nested within the Hadley Centre coupled Atmosphere Ocean General Circulation Model (AOGCM), HadCM2. The simulation periods are thirty years with fixed concentrations of greenhouse gases representing the climate of 1961–1990 and twenty years representing transient climate change for 2080–2100. The use of common boundary conditions to drive the two RCMs allows us to determine whether their different formulations significantly alter the downscaled projections.The RCM simulations of precipitation extremes are compared with observations from a dense rain‐gauge network over Great Britain, aggregated to the grid used by the RCMs. Both RCMs simulate realistically extreme precipitation occurring over timescales of one to thirty days and for return periods of two to twenty years. In particular, relative errors in the magnitude of extreme precipitation are generally no larger than those in the mean. The two regional models show different patterns of errors for daily precipitation extremes, with the main difference in the western and upland areas of Great Britain where they are underestimated in HadRM2 and overestimated in HadRM3. Change in extremes over all land areas in the domain show increases in intensity everywhere (except for the Iberian peninsula and Mediterranean coast) with most of these significant at the 5% level. Projected increases are greatest for those extremes which are the rarest and shortest duration (i.e. the most intense), both in relative and thus absolute terms. The large‐scale patterns of these changes are very similar in the two RCMs implying they are generally robust to the RCM formulation changes. Given the demonstrated quality of the models this enhances our confidence in the projected changes and suggests that they are mainly conditioned by the large‐scale response in the driving GCM. Copyright Crown Copyright 2007. Reproduced with the permission of the Controller of HMSO. Published by John Wiley & Sons, Ltd

An AOGCM simulation of the climate response to a volcanic super-eruption

Volcanic ‘super-eruptions’ have been suggested to have significantly influenced the Earth’s climate, perhaps causing glaciations and impacting on the human population. Climatic changes following a hypothetical ‘super-eruption’ are simulated using a coupled atmosphere ocean general circulation model, incorporating scaled volcanic stratospheric aerosols. Assumptions are made about the stratospheric sulphate aerosol loading, size distribution, lifetime, chemical make up and spatial distribution. As this study is concentrating on the physical climatological impacts over long timescales, microphysics and chemical interactive processes are not simulated. Near-surface temperatures fall by as much as 10 K globally for a few months and a considerable deviation from normal temperatures continues for several decades. A warming pattern is evident over northern land masses during the winter due to increased longwave forcing and a positive AO mode. The overturning rate of the North Atlantic thermohaline circulation doubles in intensity. Snow and ice increases in extent to a maximum coverage of 35% of the Earth. Despite these and other impacts longer term climatic changes that could lead to a transition to a glaciation do not occur, for present day boundary conditions and one possible plausible aerosol loading.

Adjusting to policy expectations in climate change modeling - An interdisciplinary study of flux adjustments in coupled atmosphere-ocean general circulation models

This paper surveys and interprets the attitudes of scientists to the use of flux adjustments in climate projections with coupled Atmosphere Ocean General Circulation Models. The survey is based largely on the responses of 19 climate modellers to several questions and a discussion document circulated in 1995. We interpret the responses in terms of the following factors: the implicit assumptions which scientists hold about how the environmental policy process deals with scientific uncertainty over human-related global warming; the different scientific styles that exist in climate research; and the influence of organisations, institutions, and policy upon research agendas. We find evidence that scientists' perceptions of the policy process do play a role in shaping their scientific practices. In particular, many of our respondents expressed a preference for keeping discussion of the issue of flux adjustments within the climate modeling community, apparently fearing that climate contrarians would exploit the issue in the public domain. While this may be true, we point to the risk that such an approach may backfire. We also identify assumptions and cultural commitments lying at a deeper level which play at least as important a role as perceptions of the policy process in shaping scientific practices. This leads us to identify two groups of scientists, ‘pragmatists’ and ‘purists’, who have different implicit standards for model adequacy, and correspondingly are or are not willing to use flux adjustments.

A flexible climate model for use in integrated assessments

Because of significant uncertainty in the behavior of the climate system, evaluations of the possible impact of an increase in greenhouse gas concentrations in the atmosphere require a large number of long-term climate simulations. Studies of this kind are impossible to carry out with coupled atmosphere ocean general circulation models (AOGCMs) because of their tremendous computer resource requirements. Here we describe a two dimensional (zonally averaged) atmospheric model coupled with a diffusive ocean model developed for use in the integrated framework of the Massachusetts Institute of Technology (MIT) Joint Program on the Science and Policy of Global Change. The 2-D model has been developed from the Goddard Institute for Space Studies (GISS) GCM and includes parametrizations of all the main physical processes. This allows it to reproduce many of the nonlinear interactions occurring in simulations with GCMs. Comparisons of the results of present-day climate simulations with observations show that the model reasonably reproduces the main features of the zonally averaged atmospheric structure and circulation. The model’s sensitivity can be varied by changing the magnitude of an inserted additional cloud feedback. Equilibrium responses of different versions of the 2-D model to an instantaneous doubling of atmospheric CO2 are compared with results of similar simulations with different AGCMs. It is shown that the additional cloud feedback does not lead to any physically inconsistent results. On the contrary, changes in climate variables such as precipitation and evaporation, and their dependencies on surface warming produced by different versions of the MIT 2-D model are similar to those shown by GCMs. By choosing appropriate values of the deep ocean diffusion coefficients, the transient behavior of different AOGCMs can be matched in simulations with the 2-D model, with a unique choice of diffusion coefficients allowing one to match the performance of a given AOGCM for a variety of transient forcing scenarios. Both surface warming and sea level rise due to thermal expansion of the deep ocean in response to a gradually increasing forcing are reasonably reproduced on time scales of 100–150 y. However a wide range of diffusion coefficients is needed to match the behavior of different AOGCMs. We use results of simulations with the 2-D model to show that the impact on climate change of the implied uncertainty in the rate of heat penetration into the deep ocean is comparable with that of other significant uncertainties.

A non‐flux corrected transient CO2 experiment using the BMRC Coupled Atmosphere/Ocean GCM

A transient CO2 experiment has been performed with the BMRC coupled atmosphere ocean General Circulation Model (GCM). No flux corrections were used in the experiment. A roughly linear temporal increase in global surface temperature is found in response to the CO2 increase. The rate of increase is consistent with the (relatively low) equilibrium response found previously using the Atmospheric GCM and a simple ocean. Ocean surface temperatures increase more at mid latitudes than at high northern or southern latitudes where heat is sequestered into the deep ocean. Despite the secular climate drift which occurs in the model, the major patterns of atmospheric and oceanic temperature change are similar to changes noted elsewhere (from flux corrected models). This adds further support to the main conclusions drawn from transient CO2 experiments performed elsewhere with coupled GCMs.