Climate models have continued to be developed and improved since the AR4, and many models have been extended into Earth System models by including the representation of biogeochemical cycles important to climate change. These models allow for policy-relevant calculations such as the carbon dioxide (CO2) emissions compatible with a specified climate stabilization target. In addition, the range of climate variables and processes that have been evaluated has greatly expanded, and differences between models and observations are increasingly quantified using ‘performance metrics’. In this chapter, model evaluation covers simulation of the mean climate, of historical climate change, of variability on multiple time scales and of regional modes of variability. This evaluation is based on recent internationally coordinated model experiments, including simulations of historic and paleo climate, specialized experiments designed to provide insight into key climate processes and feedbacks and regional climate downscaling. Figure 9.44 provides an overview of model capabilities as assessed in this chapter, including improvements, or lack thereof, relative to models assessed in the AR4. The chapter concludes with an assessment of recent work connecting model performance to the detection and attribution of climate change as well as to future projections.

/pdf/evaluation-of-climate-models-21sy7e310w.pdf

Evaluation of climate models

We present the global general circulation model IPSL-CM5 developed to study the long-term response of the climate system to natural and anthropogenic forcings as part of the 5th Phase of the Coupled Model Intercomparison Project (CMIP5). This model includes an interactive carbon cycle, a representation of tropospheric and stratospheric chemistry, and a comprehensive representation of aerosols. As it represents the principal dynamical, physical, and bio-geochemical processes relevant to the climate system, it may be referred to as an Earth System Model. However, the IPSL-CM5 model may be used in a multitude of configurations associated with different boundary conditions and with a range of complexities in terms of processes and interactions. This paper presents an overview of the different model components and explains how they were coupled and used to simulate historical climate changes over the past 150 years and different scenarios of future climate change. A single version of the IPSL-CM5 model (IPSL-CM5A-LR) was used to provide climate projections associated with different socio-economic scenarios, including the different Representative Concentration Pathways considered by CMIP5 and several scenarios from the Special Report on Emission Scenarios considered by CMIP3. Results suggest that the magnitude of global warming projections primarily depends on the socio-economic scenario considered, that there is potential for an aggressive mitigation policy to limit global warming to about two degrees, and that the behavior of some components of the climate system such as the Arctic sea ice and the Atlantic Meridional Overturning Circulation may change drastically by the end of the twenty-first century in the case of a no climate policy scenario. Although the magnitude of regional temperature and precipitation changes depends fairly linearly on the magnitude of the projected global warming (and thus on the scenario considered), the geographical pattern of these changes is strikingly similar for the different scenarios. The representation of atmospheric physical processes in the model is shown to strongly influence the simulated climate variability and both the magnitude and pattern of the projected climate changes.

/pdf/climate-change-projections-using-the-ipsl-cm5-earth-system-2gm93qrdy6.pdf

Climate change projections using the IPSL-CM5 Earth System Model: From CMIP3 to CMIP5

A new version of the general circulation model CNRM-CM has been developed jointly by CNRM-GAME (Centre National de Recherches Meteorologiques—Groupe d’etudes de l’Atmosphere Meteorologique) and Cerfacs (Centre Europeen de Recherche et de Formation Avancee) in order to contribute to phase 5 of the Coupled Model Intercomparison Project (CMIP5). The purpose of the study is to describe its main features and to provide a preliminary assessment of its mean climatology. CNRM-CM5.1 includes the atmospheric model ARPEGE-Climat (v5.2), the ocean model NEMO (v3.2), the land surface scheme ISBA and the sea ice model GELATO (v5) coupled through the OASIS (v3) system. The main improvements since CMIP3 are the following. Horizontal resolution has been increased both in the atmosphere (from 2.8° to 1.4°) and in the ocean (from 2° to 1°). The dynamical core of the atmospheric component has been revised. A new radiation scheme has been introduced and the treatments of tropospheric and stratospheric aerosols have been improved. Particular care has been devoted to ensure mass/water conservation in the atmospheric component. The land surface scheme ISBA has been externalised from the atmospheric model through the SURFEX platform and includes new developments such as a parameterization of sub-grid hydrology, a new freezing scheme and a new bulk parameterisation for ocean surface fluxes. The ocean model is based on the state-of-the-art version of NEMO, which has greatly progressed since the OPA8.0 version used in the CMIP3 version of CNRM-CM. Finally, the coupling between the different components through OASIS has also received a particular attention to avoid energy loss and spurious drifts. These developments generally lead to a more realistic representation of the mean recent climate and to a reduction of drifts in a preindustrial integration. The large-scale dynamics is generally improved both in the atmosphere and in the ocean, and the bias in mean surface temperature is clearly reduced. However, some flaws remain such as significant precipitation and radiative biases in many regions, or a pronounced drift in three dimensional salinity.

/pdf/the-cnrm-cm5-1-global-climate-model-description-and-basic-vx5kg93mbd.pdf

The CNRM-CM5.1 global climate model: description and basic evaluation

A new operational ocean reanalysis system (ORAS4) has been implemented at ECMWF. It spans the period 1958 to the present. This article describes its main components and evaluates its quality. The adequacy of ORAS4 for the initialization of seasonal forecasts is discussed, along with the robustness of some prominent climate signals.

ORAS4 has been evaluated using different metrics, including comparison with observed ocean currents, RAPID-derived transports, sea-level gauges, and GRACE-derived bottom pressure. Compared to a control ocean model simulation, ORAS4 improves the fit to observations, the interannual variability, and seasonal forecast skill. Some problems have been identified, such as the underestimation of meridional overturning at 26°N, the magnitude of which is shown to be sensitive to the treatment of the coastal observations.

ORAS4 shows a clear and robust shallowing trend of the Pacific Equatorial thermocline. It also shows a clear and robust nonlinear trend in the 0–700 m ocean heat content, consistent with other observational estimates. Some aspects of these climate signals are sensitive to the choice of sea-surface temperature product and the specification of the observation-error variances. The global sea-level trend is consistent with the altimeter estimate, but the partition into volume and mass variations is more debatable, as inferred by discrepancies in the trend between ORAS4- and GRACE-derived bottom pressure.

/pdf/evaluation-of-the-ecmwf-ocean-reanalysis-system-oras4-1g4fx79wf2.pdf

Evaluation of the ECMWF ocean reanalysis system ORAS4

Possible changes in Atlantic meridional overturning circulation (AMOC) provide a key source of uncertainty regarding future climate change. Maps of temperature trends over the twentieth century show a conspicuous region of cooling in the northern Atlantic. Here we present multiple lines of evidence suggesting that this cooling may be due to a reduction in the AMOC over the twentieth century and particularly after 1970. Since 1990 the AMOC seems to have partly recovered. This time evolution is consistently suggested by an AMOC index based on sea surface temperatures, by the hemispheric temperature difference, by coral-based proxies and by oceanic measurements. We discuss a possible contribution of the melting of the Greenland Ice Sheet to the slowdown. Using a multi-proxy temperature reconstruction for the AMOC index suggests that the AMOC weakness after 1975 is an unprecedented event in the past millennium (p > 0.99). Further melting of Greenland in the coming decades could contribute to further weakening of the AMOC.

/pdf/exceptional-twentieth-century-slowdown-in-atlantic-ocean-4mff1kex9z.pdf

Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation

Supporting Online Material for Temporal Variability of the Atlantic Meridional Overturning Circulation at 26.5°N

Five years of data from a line of dynamic height moorings (DHM), bottom-pressure recorders (BPR), and pressure-equipped inverted echo sounders (PIES) near the Atlantic Ocean western boundary at 26.51N are used to evaluate the structure and variability of the Deep Western Boundary Current (DWBC) during 2004–2009. Comparisons made between transports estimated from the DHM þ BPR and those made by the PIES demonstrate that the two systems are collecting equivalent volume transport information (correlation coefficient r¼ 0.96, root-mean-square difference ¼6 Sv; 1 Sv ¼10 6 m 3 s � 1 ). Integrated to � 450 km off from the continental shelf and between 800 and 4800 dbar, the DWBC has a mean transport of approximately 32 Sv and a standard deviation during these five years of 16 Sv. Both the barotropic (full-depth vertical mean) and baroclinic flows have significant variability (changes exceeding 10 Sv) on time scales ranging from a few days to months, with the barotropic variations being larger and more energetic at all time scales. The annual cycle of the deep transport is highly dependent on the horizontal integration distance; integrating � 100 km offshore yields an annual cycle of roughly similar magnitude but shifted in phase relative to that found from current meter arrays in the 1980–1990s, while the annual cycle becomes quite weak when integrating � 450 km offshore. Variations in the DWBC transport far exceed those of the total basin-wide Meridional Overturning Circulation (standard deviations of 16 Sv vs. 5 Sv). Transport integrated in the deep layer out to the west side of the Mid-Atlantic Ridge still demonstrates a surprisingly high variance, indicating that some compensation of the western basin deep variability must occur in the eastern basin. Published by Elsevier Ltd.

/pdf/variability-of-the-deep-western-boundary-current-at-26-5-n-of4ju3gdz2.pdf

Variability of the Deep Western Boundary Current at 26.5°N during 2004–2009

Efficient monitoringof large-scale current systems for climate research requires the development of new techniques to estimate ocean transports. Here, a methodology for continuous estimation of dynamic height profiles and geostrophic currents from moored temperature sensors is presented. The technique is applied to moorings deployed in the Atlantic Deep Western Boundary Current at 26.51N, off Abaco, the Bahamas (WOCE ACM-1 array). Relative geostrophic currents are referenced usingbottom pressure sensors and available shipboard direct velocity (lowered-ADCP) sections over the period of the deployment, to obtain a time series of absolute volume transport. Comparison with direct velocity measurements from a complete array of current meters shows good agreement for the mean transport and its variablity on time scales longer than 10 days, but larger variability in the current meter derived transport at time scales shorter than 10 days. A rigorous error analysis assesses the contributions of various error sources in the geostrophic as well as direct transport estimates. Low-frequency drift of the bottom pressure sensors is found to be the largest error source in the geostrophic transport estimates and recommendations for improvement of the technique and related measurement technologies are made. r 2005 Elsevier Ltd. All rights reserved.

William E. Johns

Papers

Supporting Online Material for Temporal Variability of the Atlantic Meridional Overturning Circulation at 26.5°N

Variability of the Deep Western Boundary Current at 26.5°N during 2004–2009

Instruments and Methods Estimating ocean transports with dynamic height moorings: An application in the Atlantic Deep Western Boundary Current at 261N