The Max-Planck-Institute global ocean/sea ice model with orthogonal curvilinear coordinates

Résumé The ocean engine of NEMO (Nucleus for European Modelling of the Ocean) is a primitive equation model adapted to regional and global ocean circulation problems. It is intended to be a flexible tool for studying the ocean and its interactions with the others components of the earth climate system over a wide range of space and time scales. Prognostic variables are the three-dimensional velocity field, a linear or non-linear sea surface height, the temperature and the salinity. In the horizontal direction, the model uses a curvilinear orthogonal grid and in the vertical direction, a full or partial step z-coordinate, or s-coordinate, or a mixture of the two. The distribution of variables is a three-dimensional Arakawa C-type grid. Various physical choices are available to describe ocean physics, including TKE, GLS and KPP vertical physics. Within NEMO, the ocean is interfaced with a sea-ice model (LIM v2 and v3), passive tracer and biogeochemical models (TOP) and, via the OASIS coupler, with several atmospheric general circulation models. It also support two-way grid embedding via the AGRIF software. Le moteur océanique de NEMO (Nucleus for European Modelling of the Ocean) est un modèle aux équations primitives de la circulation océanique régionale et globale. Il se veut un outil flexible pour étudier sur un vaste spectre spatiotemporel l’océan et ses interactions avec les autres composantes du système climatique terrestre. Les variables pronostiques sont le champ tridimensionnel de vitesse, une hauteur de la mer linéaire ou non, la temperature et la salinité. La distribution des variables se fait sur une grille C d’Arakawa tridimensionnelle utilisant une coordonnée verticale z à niveaux entiers ou partiels, ou une coordonnée s, ou encore une combinaison des deux. Différents choix sont proposés pour décrire la physique océanique, incluant notamment des physiques verticales TKE, GLS et KPP. A travers l’infrastructure NEMO, l’océan est interfacé avec des modèles de glace de mer, de biogéochimie et de traceurs passifs, et, via le coupleur OASIS, à plusieurs modèles de circulation générale atmosphérique. Il supporte également l’emboı̂tement interactif de maillages via le logiciel AGRIF.

NEMO ocean engine

Numerical experiments are conducted with a coarse-resolution global ice-ocean model in order to determine to what degree the sea ice-ocean exchanges of heat, salt/freshwater, and momentum control the general circulation of the world ocean on long timescales. These experiments reveal that the formation of North Atlantic Deep Water (NADW) in the model results from the strong heat losses that occur at the oceanic surface in the high-latitude North Atlantic. The large-scale ice-ocean interactions have nearly no influence on this process. In particular, neglecting the freshwater flux associated with the southward ice transport at Fram Strait does not impact seriously on the salinity of the Greenland and Norwegian Seas. At equilibrium the absence of this freshwater flux is balanced by an enhanced oceanic freshwater transport from the Arctic. Furthermore, it appears that the model NADW formation does not critically depend on the media (ice or ocean) transporting the freshwater. Besides, both the salt/freshwater and heat exchanges between sea ice and ocean are crucial in the Southern Ocean for the deep water production, properties, and export. The large amount of brine released during ice formation on the model Antarctic continental shelf leads to very high salinities there. The resulting dense shelf waters are then transported toward great depths after some mixing with ambient waters, finally forming the Antarctic Bottom Water body. On the other hand, the net ice melting associated with ice convergence in some areas, such as the southwestern Pacific, stabilizes the water column and forbids deep mixing in these regions. Furthermore, the contact with the ice imposes that the polar surface waters must be maintained very close to their freezing point temperature. Our results suggest that this process takes an important part in increasing the density of the Antarctic Bottom Water. We also show that the modifications of the stress at the ocean surface induced by the internal ice stress have only a regional effect.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999JC900215

Importance of ice-ocean interactions for the global ocean circulation: A model study

. The main advancements of the Beijing Climate Center (BCC) climate system model from
phase 5 of the Coupled Model Intercomparison Project (CMIP5) to phase
6 (CMIP6) are presented, in terms of physical parameterizations and model
performance. BCC-CSM1.1 and BCC-CSM1.1m are the two models involved in CMIP5,
whereas BCC-CSM2-MR, BCC-CSM2-HR, and BCC-ESM1.0 are the three models configured for
CMIP6. Historical simulations from 1851 to 2014 from BCC-CSM2-MR (CMIP6) and
from 1851 to 2005 from BCC-CSM1.1m (CMIP5) are used for models assessment.
The evaluation matrices include the following: (a) the energy budget at
top-of-atmosphere; (b) surface air temperature, precipitation, and atmospheric circulation for
the global and East Asia regions; (c) the sea surface temperature (SST) in the
tropical Pacific; (d) sea-ice extent and thickness and Atlantic Meridional
Overturning Circulation (AMOC); and (e) climate variations at different timescales, such as the global warming trend in the 20th century, the stratospheric
quasi-biennial oscillation (QBO), the Madden–Julian Oscillation (MJO), and the diurnal
cycle of precipitation. Compared with BCC-CSM1.1m, BCC-CSM2-MR shows
significant improvements in many aspects including the tropospheric air
temperature and circulation at global and regional scales in East Asia and
climate variability at different timescales, such as the QBO, the MJO, the diurnal cycle
of precipitation, interannual variations of SST in the equatorial Pacific,
and the long-term trend of surface air temperature.

/pdf/the-beijing-climate-center-climate-system-model-bcc-csm-the-1agm2ik24a.pdf

The Beijing Climate Center Climate System Model (BCC-CSM): the main progress from CMIP5 to CMIP6

A Technical Guide to MOM4

The concept of age in marine modelling I. Theory and preliminary model results

In the World Ocean, densest waters found on the continental shelves induce density driven downsloping currents that can influence the deep ocean water masses properties. This process is poorly represented in z-coordinate ocean models, especially in Ocean General Circulation Model (OGCM) with coarse resolution in both horizontal and vertical directions. Consequently, continental shelves appear to be too isolated from the open ocean, whereas the density remains too low in the deep ocean. This study presents a simple parameterization of downsloping flow designed for z-coordinate, coarse resolution ocean model. At the shelf break, when the density on the shelf is higher than that in the neighbouring deep water column, a downsloping current is set up. This current is linearly related to the horizontal density gradient between the two adjacent boxes, using a prescribed coefficient. For simplicity, a uniform value of the coefficient is used here, although it should ideally vary in space. From the shelf, the downsloping flow is assumed to go downward along the slope until it reaches a level of equal density. An upward return flow of equal magnitude maintains the conservation of mass. This parameterization has been implemented in an OGCM and two experiments, with and without this scheme, have been integrated until equilibrium using restoring boundary conditions. The impact of the downsloping parameterization on the global ocean is dominated by the improvement of the Antarctic bottom water circulation and water mass properties. The parameterization increases the density of the deep ocean and tends to reduce the intensity and depth of the North Atlantic deep water circulation, which is in better agreement with observations. As a result of a higher exchange with the open ocean, the properties of continental shelf waters are also improved, with a marked reduction of the Antarctic shelves salinities. Therefore, this simple parameterization leads to a significant improvement of the model results, at little computational cost.

Parameterization of density-driven downsloping flow for a coarse-resolution ocean model in z-coordinate

It is generally accepted that fresh-water fluxes due to ice accretion or melting profoundly influence the formation of Antarctic bottom water (AABW). This is investigated by means of a global, three-dimensional ice-ocean model. Two model runs were conducted. At the high southern latitudes, the control experiment exhibits positive (i.e. towards the ocean) fresh-water fluxes over the deep ocean, and large negative fluxes over the Antarctic continental shelf, because of the intense ice-production taking place in this region. The salinity of shelf water can increase in such a way that deep-water formation is facilitated. The simulated net fresh-water flux over the shelf has an annual mean value of -1 m a-1. This flux induces a transport of salt to bottom waters, which corresponds to an increase of their salinity estimated to be around 0.05 psu. In the second model run, the fresh-water fluxes due to ice melting or freezing are neglected, leading to a rearrangement of the water masses. In particular, the AABW-formation rate decreases, which allows the influence of North Atlantic deep water (NADW) to increase. As NADW is warmer and saltier than AABW, the bottom-water salinity and temperature become higher.

/pdf/impact-of-sea-ice-formation-on-the-properties-of-antarctic-svuyrse295.pdf

Jean Michel Campin

Papers

The concept of age in marine modelling I. Theory and preliminary model results

Parameterization of density-driven downsloping flow for a coarse-resolution ocean model in z-coordinate

Impact of sea-ice formation on the properties of Antarctic Bottom Water

An Orthogonal Curvilinear System for a World Ocean Model