First Intercomparison Experiments

The international Atmospheric Model Intercomparison Project (AMIP), in the framework of the World Climate Research Programme (WCRP) has provided a guidance for the oceanic modelling community. The aim of AMIP was to offer a comprehensive evaluation of the performance of atmospheric GCMs7 on climate and higher-frequency time-scales, and the documentation of their systematic errors. In a common modelling framework, that is, simulating the monthly variability of the atmospheric parameters for the 1990s decade, all climate modelling groups (more than 20 institutions around the world) provided their simulations in a standard way. Due to the participation of all groups in building up the assessment methodology, and the sustained reporting on evaluation of each experiment, AMIP has become the reference for atmospheric and climate performance assessment. An overview of AMIP is given in (Gates 1992).

7 GCM: Global Circulation Model.

Free coupled ocean/atmosphere numerical simulations are compared (usually monthly averaged parameters) against data (averaged in the same way), climatologies, or existing reference simulations. In particular ECMWF8, NCEP9 or COADS10 reanalysis, considered to be more realistic due to assimilation benefit. An ensemble approach was adopted. First each numerical simulation was individually evaluated (RMS11, correlation against the compared references). Then the ensemble mean, and its standard deviation were also evaluated. Ensemble approaches expect that individual simulations will present errors that are not correlated. In practice, this is not obviously true, if simulations are based on similar ocean models, similar forcings, etc However, by multiplying the number of simulations in the intercomparison, AMIP objective was clearly to get "not correlated" simulations. Such approach is presented in Fig. 23.1, taken from (Stammer et al. 2009)12 in the framework of the CLIVAR's GSOP13. Models biased similarly bring to ensemble estimates also biased.

With the era of model's eddy-permitting capacity, different ocean modelling groups started to organize intercomparison experiment. The US-German Community Modelling Effort (CME), in support of the World Ocean Circulation Experiment (WOCE) started to infer model parametrization and sensitivity studies in modelling the North Atlantic basin (for a review, see Boning and Bryan 1996). The circulation and the eddy field were described in a limited way. Several causes were identified, among them boundary conditions, the representation of water exchanges and topographic controlled flows, overturning circulation and vertical mixing..

This experiment have been followed by the DYNAMO project, dedicated to offer intercomparison among three classes of ocean models of the North Atlantic Ocean in a similar numerical experiment framework (Meincke et al. 2001). Forced identically, and configured over the same domain, a z-level, sigma-level and isopycnal vertical discretisation primitive equation models have been run in similar ways. The objective was to identify patterns of the North Atlantic Ocean circulation that were robust, and others that were sensitive to model parametrisation. Thus, one objective aimed to increase our knowledge of the Atlantic Ocean dynamics, the second was to improve ocean models, and share expertise among different modelling groups.

Simulations were eddy-permitting (1/3° horizontal resolution). As far as possible, model parametrisations (i.e., lateral and vertical mixing, bottom friction, mixed layer turbulence, bathymetry, boundary conditions) were tuned to be similar, and initial conditions were provided by the Levitus climatology (for details, see Willebrand et al. 2001). After a spin-up of 15 years, the last 5 years of the monthly-mean

8 European Centre for Medium-Range Weather Forecasts.

9 United States National Centers for Environmental Prediction.

10 Comprehensive Ocean-Atmosphere Data Set.

11 RMS: root mean square.

12 This OceanObs'09 community white paper is available at http://www.oceanobs09.net/cwp/in-dex.php.

13 Global Synthesis and Observations Panel, (http://www.clivar.org/organization/gsop/synthesis/ synthesis.php).

Unbiased Independent Models

Biased Clustered Models

Syntheses * 9

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The model closest to all other models will also be closest to the truth

No relation between truth and the model closest to all

Fig. 23.1 From Fig. 8 of (Stammer et al. 2009): evaluating model quantity from multi-ensemble of results. The arrows illustrate the general expectance that assimilation of observations moves the results closer to the truth. The left panel show the ideal situation in which the ensemble spread and the distance to the ensemble mean provide useful measures while the right panel illustrates a biased case that is more realistic for the ensemble of present day synthesis

Fig. 23.1 From Fig. 8 of (Stammer et al. 2009): evaluating model quantity from multi-ensemble of results. The arrows illustrate the general expectance that assimilation of observations moves the results closer to the truth. The left panel show the ideal situation in which the ensemble spread and the distance to the ensemble mean provide useful measures while the right panel illustrates a biased case that is more realistic for the ensemble of present day synthesis climatological forcing have been analysed following a protocol still considered nowadays for "consistency assessment" (explained later in this chapter):

• Analysis of the meridional overturning circulation, that reflects the thermohaline circulation (mean annual values). Differences were analysed in term of deep flow and outflow/overflow representations, as well as diapycnal mixing effects.

• Analysis of the overturning transport at 25°N. that also reflects the thermohaline circulation (mean annual values). Seasonal variation were also assessed in a specific study (Boning et al. 2001). At this latitude, the western boundary current as well as the return circulations of the subtropical gyre are captured. Note that a particular effort has been put by the international community in order to have a sustained observation network of the flow across the Atlantic at that latitude. The RAPID array is a sustained program that provides data since 200414 (Cunningham et al. 2007).

14 The RAPID array uses standard observational techniques—moored instruments that measure conductivity, temperature and pressure, as well as bottom pressure recorders—to measure density and pressure gradients across the North Atlantic, from which one can readily calculate the basin overturning circulation and heat transport.

Fig. 23.2 Meridional heat transport in the North Atlantic Ocean from DYNAMO intercomparison (full line = LEVEL / dashed = ISOPYCNIC / dotted = SIGMA / dash-dotted = SIGMA-2). Values and errors bars given by Macdonald and Wunsch (1996). (Taken from Fig. 9 of Willebrand et al. 2001)

• Analysis of the mean meridional heat transport, that reflects heat flux exchanges in a climatological aspect. Figure 23.2, taken from (Willebrand et al. 2001) shows that models are underestimating the transport south of 20°N, compared to hydrographic data analysis, and that level and sigma models look less efficient in representing the transport in the subtropical gyre., due to their weaker Meridional Overturning Circulation (MOC) representation.

• Analysis of mean surface circulation, associated with the mean geostrophic flow. Current at the surface and different depth are studied as well as vertically integrated transport. The Gulf Stream (transport across Florida Strait, separation at Cape Hatteras, the North West Corner flow), the North Atlantic Current, and the Azores Current representation are particularly discussed for the subtropical gyre (New et al. 2001b). A dedicated study was performed for tropical currents in the western basin (South Atlantic Current, North Brazil Current, retroflection and North Atlantic Counter Current, eddies propagating into the Carribean current system) for the mean, and seasonal variations (Barnier et al. 2001).

• Analysis of the eddy field and its variability, associated with baroclinic and barotropic instabilities. The sea surface variability, as well as the eddy kinetic energy can be compared to satellite altimetry equivalent values (Stammer et al. 2001).

• Analysis of circulation at depth: pathways of the Mediterranean Waters, that impact the thermohaline circulation in the North Atlantic Ocean (New et al. 2001a).

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