Evaluation of CMIP6 DECK Experiments With CNRM‐CM6‐1

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Vertical mixing is often regarded as the Achilles' heel of ocean models. In particular, few models include a comprehensive and energy-constrained parameterization of mixing by internal ocean tides. Here, we present an energy-conserving mixing scheme which accounts for the local breaking of high-mode internal tides and the distant dissipation of low-mode internal tides. The scheme relies on four static two-dimensional maps of internal tide dissipation, constructed using mode-by-mode Lagrangian tracking of energy beams from sources to sinks. Each map is associated with a distinct dissipative process and a corresponding vertical structure. Applied to an observational climatology of stratification, the scheme produces a global three-dimensional map of dissipation which compares well with available microstructure observations and with upper-ocean finestructure mixing estimates. This relative agreement, both in magnitude and spatial structure across ocean basins, suggests that internal tides underpin most of observed dissipation in the ocean interior at the global scale. The proposed parameterization is therefore expected to improve understanding, mapping, and modeling of ocean mixing.Plain Language Summary When tidal ocean currents flow over bumpy seafloor, they generate internal tidal waves. Internal waves are the subsurface analog of surface waves that break on beaches. Like surface waves, internal tidal waves often become unstable and break into turbulence. This turbulence is a primary cause of mixing between stacked ocean layers-a key process regulating ocean currents and biology and a key ingredient of computer models of the global ocean. In this article, a three-dimensional global map of mixing induced by internal tidal waves is presented. This map incorporates a large variety of energy pathways from the generation of tidal waves to turbulence, accounting for the conservation of energy. The map is compared to available observations of turbulence across the globe and found to reproduce with good fidelity the main patterns identified in observations. This relatively good agreement suggests that internal tidal waves are the main source of turbulence in the subsurface ocean and implies that the map may serve a range of applications. In particular, the three-dimensional map provides an efficient and realistic means to represent mixing by internal tidal waves in global ocean models.

Section: Discussionmentioning

confidence: 99%

A Parameterization of Local and Remote Tidal Mixing

Lavergne

Vic

Madec

et al. 2020

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“…In order to support research on these overarching topics, the CNRM‐CERFACS climate group aims at contributing to a large number of MIPs using three different models (Table ): the standard resolution (i.e., ~1° or ~140 km) AOGCM of sixth generation (CNRM‐CM6‐1, Voldoire et al ()), the high‐resolution (i.e., ~0.5° or ~50 km) AOGCM (CNRM‐CM6‐HR), and the standard (i.e., ~1° or ~140 km) resolution Earth system model of second generation (CNRM‐ESM2‐1, this study). These three models have been designed to tackle the overarching scientific questions as proposed by CMIP6 using an alternative mapping (with respect to CMIP5) pending on model resolution and complexity.…”

Section: Introductionmentioning

confidence: 99%

“…This paper provides key information relative to CNRM‐ESM2‐1 contribution to CMIP6. In particular, we document the development of CNRM‐ESM2‐1, which uses the physical‐dynamical core of the ocean‐atmosphere coupled climate model CNRM‐CM6‐1 as detailed in Voldoire et al () and accounts for several Earth system feedbacks enabled by interactive atmospheric chemistry and aerosols, as well as interactive land and ocean carbon cycles. We also detail the modeling setup used for CMIP6 including the use of recommended forcing and the spin‐up strategy.…”

Section: Introductionmentioning

confidence: 99%

Evaluation of CNRM Earth System Model, CNRM‐ESM2‐1: Role of Earth System Processes in Present‐Day and Future Climate

Séférian

Nabat

Michou

et al. 2019

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457

167

This study introduces CNRM-ESM2-1, the Earth system (ES) model of second generation developed by CNRM-CERFACS for the sixth phase of the Coupled Model Intercomparison Project (CMIP6). CNRM-ESM2-1 offers a higher model complexity than the Atmosphere-Ocean General Circulation Model CNRM-CM6-1 by adding interactive ES components such as carbon cycle, aerosols, and atmospheric chemistry. As both models share the same code, physical parameterizations, and grid resolution, they offer a fully traceable framework to investigate how far the represented ES processes impact the model performance over present-day, response to external forcing and future climate projections. Using a large variety of CMIP6 experiments, we show that represented ES processes impact more prominently the model response to external forcing than the model performance over present-day. Both models display comparable performance at replicating modern observations although the mean climate of CNRM-ESM2-1 is slightly warmer than that of CNRM-CM6-1. This difference arises from land cover-aerosol interactions where the use of different soil vegetation distributions between both models impacts the rate of dust emissions. This interaction results in a smaller aerosol burden in CNRM-ESM2-1 than in CNRM-CM6-1, leading to a different surface radiative budget and climate. Greater differences are found when comparing the model response to external forcing and future climate projections. Represented ES processes damp future warming by up to 10% in CNRM-ESM2-1 with respect to CNRM-CM6-1. The representation of land vegetation and the CO 2 -water-stomatal feedback between both models explain about 60% of this difference. The remainder is driven by other ES feedbacks such as the natural aerosol feedback.

“…In the standard GCM configuration of CNRM‐CM6‐1, this interactive aerosol scheme is not activated and is replaced by monthly AOD fields of BC, OM, SO

_{4}

, DD, and SS varying each year. As described in Voldoire et al (), these AOD fields have been calculated in preliminary CNRM‐CM6‐1 AMIP‐type (atmosphere only) historical and future simulations using the interactive aerosol scheme described above and observed SSTs and sea ice (CMIP6 data). This historical simulation is named amip‐hist‐aod in the present study (see Table 2).…”

Section: Methodsmentioning

confidence: 99%

Present‐Day and Historical Aerosol and Ozone Characteristics in CNRM CMIP6 Simulations

Michou

Nabat

Saint‐Martin

et al. 2020

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Characteristics and radiative forcing of the aerosol and ozone fields of two configurations of the Centre National de Recherches Météoroglogiques (CNRM) and Cerfacs climate model are analyzed over the historical period (1850-2014), using several Coupled Model Intercomparison Project 6 (CMIP6) simulations. CNRM-CM6-1 is the atmosphere-ocean general circulation model including prescribed aerosols and a linear stratospheric ozone scheme, while the Earth System Model CNRM-ESM2-1 has interactive tropospheric aerosols and chemistry of the midtroposphere aloft. The representations of aerosols and ozone in CNRM-CM6-1 are issued from simulations of CNRM-ESM2-1, and this ensures some comparability of both representations. In particular, present-day anthropogenic aerosol optical depths are similar (0.018), and their spatial patterns correspond to those of reference data sets such as MACv2 and MACv2-SP despite a negative bias. Effective radiative forcings (ERFs) have been estimated using 30-year fixed sea surface temperature simulations (piClim) and several calls to the radiative scheme. Present-day anthropogenic aerosol ERF, aerosol-radiation ERF, and aerosol cloud ERF are fully within CMIP5 estimates and, respectively, equal to −1.10, −0.36, and −0.81 W m −2 for CNRM-CM6-1 and −0.21, −0.61, and −0.74 W m −2 for CNRM-ESM2-1. Additional CMIP6-type piClim simulations show that these differences are mainly due to the interactivity of the aerosol scheme whose impact is confirmed when assessing the response of both climate model configurations to rising CO 2 . Present-day stratospheric ozone ERF, equal to −0.04 W m −2 , is in agreement with that of the CMIP6 ozone. No trend appears in the ozone ERF over the historical period although the evolution of the total column ozone is correctly simulated. Plain Language SummaryThe manuscript documents the Météo-France Centre National de Recherches Météorologiques aerosol-chemistry modeling contributions to the sixth Coupled Model Intercomparison Project that supports the sixth IPCC Assessment Report of climate change. It establishes that their results are suitable for use by the scientific community in the analysis of the sixth Coupled Model Intercomparison Project experiments. The authors provide an evaluation of the model performance in both present-day and historical (1850-2014) contexts, as well as a detailed analysis of the model calculated effective radiative forcing due to ozone and aerosols.