ORCA family: global ocean with tripolar grid

The ORCA family is a series of global ocean configurations that are run together with the LIM sea-ice model (ORCA-LIM) and possibly with PISCES biogeochemical model (ORCA-LIM-PISCES), using various resolutions. The appropriate &namcfg namelist is available in CONFIG/ORCA2_LIM/EXP00/namelist_cfg for ORCA2 and in CONFIG/SHARED/README_other_configurations_namelist_namcfg for other resolutions

Figure: ORCA mesh conception. The departure from an isotropic Mercator grid start poleward of 20. The two "north pole" are the foci of a series of embedded ellipses (blue curves) which are determined analytically and form the i-lines of the ORCA mesh (pseudo latitudes). Then, following Madec and Imbard [1996], the normal to the series of ellipses (red curves) is computed which provide the j-lines of the mesh (pseudo longitudes).

ORCA tripolar grid

The ORCA grid is a tripolar is based on the semi-analytical method of Madec and Imbard [1996]. It allows to construct a global orthogonal curvilinear ocean mesh which has no singularity point inside the computational domain since two north mesh poles are introduced and placed on lands. The method involves defining an analytical set of mesh parallels in the stereographic polar plan, computing the associated set of mesh meridians, and projecting the resulting mesh onto the sphere. The set of mesh parallels used is a series of embedded ellipses which foci are the two mesh north poles (Fig. 16.1). The resulting mesh presents no loss of continuity in either the mesh lines or the scale factors, or even the scale factor derivatives over the whole ocean domain, as the mesh is not a composite mesh.

Figure: Top: Horizontal scale factors ($e_1$, $e_2$) and Bottom: ratio of anisotropy ($e_1 / e_2$) for ORCA 0.5 mesh. South of 20a Mercator grid is used ($e_1 = e_2$) so that the anisotropy ratio is 1. Poleward of 20, the two "north pole" introduce a weak anisotropy over the ocean areas ($< 1.2$) except in vicinity of Victoria Island (Canadian Arctic Archipelago).

The method is applied to Mercator grid ($i.e.$ same zonal and meridional grid spacing) poleward of 20, so that the Equator is a mesh line, which provides a better numerical solution for equatorial dynamics. The choice of the series of embedded ellipses (position of the foci and variation of the ellipses) is a compromise between maintaining the ratio of mesh anisotropy ($e_1 / e_2$) close to one in the ocean (especially in area of strong eddy activities such as the Gulf Stream) and keeping the smallest scale factor in the northern hemisphere larger than the smallest one in the southern hemisphere. The resulting mesh is shown in Fig. 16.1 and 16.2 for a half a degree grid (ORCA_R05). The smallest ocean scale factor is found in along Antarctica, while the ratio of anisotropy remains close to one except near the Victoria Island in the Canadian Archipelago.

ORCA pre-defined resolution

The NEMO system is provided with five built-in ORCA configurations which differ in the horizontal resolution. The value of the resolution is given by the resolution at the Equator expressed in degrees. Each of configuration is set through the namcfg namelist, which sets the grid size and configuration name parameters (Tab. 16.1). .

Table 16.1: Set of predefined parameters for ORCA family configurations. In all cases, the name of the configuration is set to "orca" ($i.e.$ cp_cfg = orca).

Horizontal Grid	jp_cfg	jpiglo	jpjglo
4	4	92	76
2	2	182	149
1	1	362	292
00.5	05	722	511
00.25	025	1442	1021

The ORCA_R2 configuration has the following specificity : starting from a 2 ORCA mesh, local mesh refinements were applied to the Mediterranean, Red, Black and Caspian Seas, so that the resolution is 11there. A local transformation were also applied with in the Tropics in order to refine the meridional resolution up to 0.5at the Equator.

The ORCA_R1 configuration has only a local tropical transformation to refine the meridional resolution up to 1/3 at the Equator. Note that the tropical mesh refinements in ORCA_R2 and R1 strongly increases the mesh anisotropy there.

The ORCA_R05 and higher global configurations do not incorporate any regional refinements.

For ORCA_R1 and R025, setting the configuration key to 75 allows to use 75 vertical levels, otherwise 46 are used. In the other ORCA configurations, 31 levels are used (see Tab. 4.2 and Fig. 4.6).

Only the ORCA_R2 is provided with all its input files in the NEMO distribution. It is very similar to that used as part of the climate model developed at IPSL for the 4th IPCC assessment of climate change (Marti et al., 2009). It is also the basis for the NEMO contribution to the Coordinate Ocean-ice Reference Experiments (COREs) documented in Griffies et al. [2009].

This version of ORCA_R2 has 31 levels in the vertical, with the highest resolution (10m) in the upper 150m (see Tab. 4.2 and Fig. 4.6). The bottom topography and the coastlines are derived from the global atlas of Smith and Sandwell (1997). The default forcing uses the boundary forcing from Large and Yeager [2004] (see §7.5.1), which was developed for the purpose of running global coupled ocean-ice simulations without an interactive atmosphere. This Large and Yeager [2004] dataset is available through the GFDL web site. The "normal year" of Large and Yeager [2004] has been chosen of the NEMO distribution since release v3.3.

ORCA_R2 pre-defined configuration can also be run with an AGRIF zoom over the Agulhas current area ( key_ agrif defined) and, by setting the appropriate variables in &namcfg, see CONFIG/SHARED/namelist_ref a regional Arctic or peri-Antarctic configuration is extracted from an ORCA_R2 or R05 configurations using sponge layers at open boundaries.