Three-dimensional stirring of thermohaline fronts

This study investigates the stirring of the thermohaline anomalies in a fully turbulent quasigeostrophic stratified flow. Temperature and salinity fields are permanently forced at large scales and are related to density by a linear equation of state. We show, using some inherent properties of quasi-geostrophic turbulence, that the 3-D ageostrophic circulation is the key dynamical characteristic that governs the strength and the spatial distribution of small-scale thermohaline fronts that are strongly density compensated. The numerical simulations well illustrate the formation by the mesoscale eddy field of sharp thermohaline fronts that are mainly located in the saddle regions and around the eddy cores and have a weak signature on the density field. One important aspect revealed by the numerical results is that the thermohaline anomalies experience not only a direct horizontal cascade but also a significant vertical cascade. One consequence of this 3-D cascade is that the ultimate mixing of the thermohaline anomalies will not be necessarily maximum at the depth where the large-scale temperature and salinity anomalies are maximum. Some analytical arguments allow us to identify some of the mechanisms that drive this 3-D cascade

SEA SURFACE DYNAMIC HEIGHT TOPOGRAPHY AND THE NORTH EQUATORIAL COUNTERCURRENT AS INFERRED FROM A LINEAR MODEL

Abstract. Special attention is paid to the computed North Equatorial Countercurrent. We conclude that most of the observed seasonal variations can be explained using simple linear theory

Resolving and parameterising the ocean mesoscale in earth system models

Purpose of Review. Assessment of the impact of ocean resolution in Earth System models on the mean state, variability, and future projections and discussion of prospects for improved parameterisations to represent the ocean mesoscale. Recent Findings. The majority of centres participating in CMIP6 employ ocean components with resolutions of about 1 degree in their full Earth Systemmodels (eddy-parameterising models). In contrast, there are alsomodels submitted toCMIP6 (both DECK and HighResMIP) that employ ocean components of approximately 1/4 degree and 1/10 degree (eddy-present and eddy-rich models). Evidence to date suggests that whether the ocean mesoscale is explicitly represented or parameterised affects not only the mean state of the ocean but also the climate variability and the future climate response, particularly in terms of the Atlantic meridional overturning circulation (AMOC) and the Southern Ocean. Recent developments in scale-aware parameterisations of the mesoscale are being developed and will be included in future Earth System models. Summary. Although the choice of ocean resolution in Earth System models will always be limited by computational considerations, for the foreseeable future, this choice is likely to affect projections of climate variability and change as well as other aspects of the Earth System. Future Earth System models will be able to choose increased ocean resolution and/or improved parameterisation of processes to capture physical processes with greater fidelity

Challenges and Prospects in Ocean Circulation Models

We revisit the challenges and prospects for ocean circulation models following Griffies et al. (2010). Over the past decade, ocean circulation models evolved through improved understanding, numerics, spatial discretization, grid configurations, parameterizations, data assimilation, environmental monitoring, and process-level observations and modeling. Important large scale applications over the last decade are simulations of the Southern Ocean, the Meridional Overturning Circulation and its variability, and regional sea level change. Submesoscale variability is now routinely resolved in process models and permitted in a few global models, and submesoscale effects are parameterized in most global models. The scales where nonhydrostatic effects become important are beginning to be resolved in regional and process models. Coupling to sea ice, ice shelves, and high-resolution atmospheric models has stimulated new ideas and driven improvements in numerics. Observations have provided insight into turbulence and mixing around the globe and its consequences are assessed through perturbed physics models. Relatedly, parameterizations of the mixing and overturning processes in boundary layers and the ocean interior have improved. New diagnostics being used for evaluating models alongside present and novel observations are briefly referenced. The overall goal is summarizing new developments in ocean modeling, including: how new and existing observations can be used, what modeling challenges remain, and how simulations can be used to support observations.Peer reviewe

The Atlantic meridional overturning circulation in high resolution models

The Atlantic meridional overturning circulation (AMOC) represents the zonally integrated stream function of meridional volume transport in the Atlantic Basin. The AMOC plays an important role in transporting heat meridionally in the climate system. Observations suggest a heat transport by the AMOC of 1.3 PW at 26°N ‐ a latitude which is close to where the Atlantic northward heat transport is thought to reach its maximum. This shapes the climate of the North Atlantic region as we know it today. In recent years there has been significant progress both in our ability to observe the AMOC in nature and to simulate it in numerical models. Most previous modeling investigations of the AMOC and its impact on climate have relied on models with horizontal resolution that does not resolve ocean mesoscale eddies and the dynamics of the Gulf Stream/North Atlantic Current system. As a result of recent increases in computing power, models are now being run that are able to represent mesoscale ocean dynamics and the circulation features that rely on them. The aim of this review is to describe new insights into the AMOC provided by high‐resolution models. Furthermore, we will describe how high‐resolution model simulations can help resolve outstanding challenges in our understanding of the AMOC

OMIP contribution to CMIP6: experimental and diagnostic protocol for the physical component of the Ocean Model Intercomparison Project

The Ocean Model Intercomparison Project (OMIP) is an endorsed project in the Coupled Model Intercomparison Project Phase 6 (CMIP6). OMIP addresses CMIP6 science questions, investigating the origins and consequences of systematic model biases. It does so by providing a framework for evaluating (including assessment of systematic biases), understanding, and improving ocean, sea-ice, tracer, and biogeochemical components of climate and earth system models contributing to CMIP6. Among the WCRP Grand Challenges in climate science (GCs), OMIP primarily contributes to the regional sea level change and near-term (climate/decadal) prediction GCs. OMIP provides (a) an experimental protocol for global ocean/sea-ice models run with a prescribed atmospheric forcing; and (b) a protocol for ocean diagnostics to be saved as part of CMIP6. We focus here on the physical component of OMIP, with a companion paper (Orr et al., 2016) detailing methods for the inert chemistry and interactive biogeochemistry. The physical portion of the OMIP experimental protocol follows the interannual Coordinated Ocean-ice Reference Experiments (CORE-II). Since 2009, CORE-I (Normal Year Forcing) and CORE-II (Interannual Forcing) have become the standard methods to evaluate global ocean/sea-ice simulations and to examine mechanisms for forced ocean climate variability. The OMIP diagnostic protocol is relevant for any ocean model component of CMIP6, including the DECK (Diagnostic, Evaluation and Characterization of Klima experiments), historical simulations, FAFMIP (Flux Anomaly Forced MIP), C4MIP (Coupled Carbon Cycle Climate MIP), DAMIP (Detection and Attribution MIP), DCPP (Decadal Climate Prediction Project), ScenarioMIP, HighResMIP (High Resolution MIP), as well as the ocean/sea-ice OMIP simulations

North Atlantic simulations in Coordinated Ocean-ice Reference Experiments phase II (CORE-II). Part I: Mean states

Simulation characteristics from eighteen global ocean–sea-ice coupled models are presented with a focus on the mean Atlantic meridional overturning circulation (AMOC) and other related fields in the North Atlantic. These experiments use inter-annually varying atmospheric forcing data sets for the 60-year period from 1948 to 2007 and are performed as contributions to the second phase of the Coordinated Ocean-ice Reference Experiments (CORE-II). The protocol for conducting such CORE-II experiments is summarized. Despite using the same atmospheric forcing, the solutions show significant differences. As most models also differ from available observations, biases in the Labrador Sea region in upper-ocean potential temperature and salinity distributions, mixed layer depths, and sea-ice cover are identified as contributors to differences in AMOC. These differences in the solutions do not suggest an obvious grouping of the models based on their ocean model lineage, their vertical coordinate representations, or surface salinity restoring strengths. Thus, the solution differences among the models are attributed primarily to use of different subgrid scale parameterizations and parameter choices as well as to differences in vertical and horizontal grid resolutions in the ocean models. Use of a wide variety of sea-ice models with diverse snow and sea-ice albedo treatments also contributes to these differences. Based on the diagnostics considered, the majority of the models appear suitable for use in studies involving the North Atlantic, but some models require dedicated development effort

Challenges and Prospects in Ocean Circulation Models

We revisit the challenges and prospects for ocean circulation models following Griffies et al. (2010). Over the past decade, ocean circulation models evolved through improved understanding, numerics, spatial discretization, grid configurations, parameterizations, data assimilation, environmental monitoring, and process-level observations and modeling. Important large scale applications over the last decade are simulations of the Southern Ocean, the Meridional Overturning Circulation and its variability, and regional sea level change. Submesoscale variability is now routinely resolved in process models and permitted in a few global models, and submesoscale effects are parameterized in most global models. The scales where nonhydrostatic effects become important are beginning to be resolved in regional and process models. Coupling to sea ice, ice shelves, and high-resolution atmospheric models has stimulated new ideas and driven improvements in numerics. Observations have provided insight into turbulence and mixing around the globe and its consequences are assessed through perturbed physics models. Relatedly, parameterizations of the mixing and overturning processes in boundary layers and the ocean interior have improved. New diagnostics being used for evaluating models alongside present and novel observations are briefly referenced. The overall goal is summarizing new developments in ocean modeling, including: how new and existing observations can be used, what modeling challenges remain, and how simulations can be used to support observations

Effets des vents fluctuants et de la topographie sur la turbulence oceanique a moyenne echelle

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

OBSERVATION ET MODELISATION DE LA VARIABILITE SAISONNIERE DANS L'OCEAN ATLANTIQUE EQUATORIAL PROFOND

LES OBSERVATIONS COURANTOMETRIQUES DANS LA PARTIE CENTRALE DE L'OCEAN ATLANTIQUE EQUATORIAL PROFOND REVELENT LA PRESENCE DE FLUCTUATIONS A PERIODES ANNUELLE ET SEMI-ANNUELLE SUR LA COMPOSANTE ZONALE DE LA VITESSE ESSENTIELLEMENT. L'ANALYSE DES DONNEES MONTRE QUE LA STRUCTURE SPATIO-TEMPORELLE DU SIGNAL BASSE-FREQUENCE EST COHERENTE AVEC LA DYNAMIQUE DES ONDES EQUATORIALES. UNE ETUDE DE PROCESSUS EST MENEE POUR COMPRENDRE LA REPONSE D'UN BASSIN EQUATORIAL AU FORCAGE PERIODIQUE PAR LA TENSION ZONALE DU VENT. L'ENERGIE DU VENT ATTEINT L'OCEAN PROFOND PAR L'INTERMEDIAIRE DE LA PROPAGATION VERTICALE D'ONDES EQUATORIALES. UNE ONDE DE ROSSBY DE MODE MERIDIEN 1, EXCITEE PRES DE LA SURFACE A LA FRONTIERE EST DU BASSIN, DOMINE LA REPONSE DANS LA PARTIE OUEST DU BASSIN. ELLE TRAVERSE LE BASSIN D'EST EN OUEST ET SE REFLECHIT A LA FRONTIERE OUEST EN UNE ONDE DE KELVIN. DANS LA PARTIE EST DU BASSIN, LA REPONSE EST PLUS COMPLEXE CAR PLUSIEURS ONDES D'AMPLITUDES COMPARABLES SE SUPERPOSENT. LA REPONSE, PEU SENSIBLE AUX PARAMETRES DU FORCAGE, PEUT ETRE CONSIDEREE COMME LINEAIRE DANS LES COUCHES PROFONDES DU MODELE. SON AMPLITUDE DEPEND DE LA TAILLE DU BASSIN ET DE LA FREQUENCE DU FORCAGE ; UN PHENOMENE DE RESONANCE APPARAIT D'AILLEURS A PERIODE SEMI-ANNUELLE. LA TRANSMISSION DE L'ENERGIE, DE LA SURFACE VERS LES COUCHES PROFONDES, SE FAIT DANS LA PARTIE EST DU BASSIN LORS DE LA REFLEXION D'UNE ONDE DE KELVIN, FORCEE EN SURFACE, EN ONDES DE ROSSBY LONGUES. LES COURANTS MOYENS, PLUTOT QUE LA STRATIFICATION, APPARAISSENT COMME UNE GENE A LA PROPAGATION DES ONDES DANS LES COUCHES DE SURFACE. LA PRESENCE D'UN RELIEF SOUS-MARIN NE MODIFIE PAS LA NATURE DE LA REPONSE. IL EST SIMPLEMENT UNE BARRIERE A LA PROPAGATION DES ONDES INCIDENTES. CETTE ETUDE DE PROCESSUS PERMET DE COMPRENDRE LES PREMIERS RESULTATS D'UN MODELE HAUTE-RESOLUTION DE L'OCEAN ATLANTIQUE EQUATORIAL.BREST-BU Droit-Sciences-Sports (290192103) / SudocSudocFranceF