High resolution ocean nesting within a flexible model infrastructure

Figure 1: Schematic of the FOCI modelling system.

Unique capability to refine ocean model resolution in specific area of interest in a state-of-the-art global coupled climate model.

In occasion of the ESM Annual Meeting 2018 in Leipzig, we present a new coupled climate model termed the Flexible Ocean and Climate Infrastructure (FOCI, Matthes et al., in prep.). FOCI is based on the MPI-ESM and consists of ECHAM6.3 (Stevens et al., 2013) as atmosphere model linked through the coupling software OASIS3-MCT (Valcke et al., 2013) to the NEMO3.6 ocean model (Madec et al., 2016). FOCI has in addition several optional modules for more specific applications. The atmosphere of FOCI can be run optionally with interactive chemistry (Schulz et al., 2018) and for the ocean a biogeochemistry module is available [see ESM contribution from D. Ehlert]. A unique feature of FOCI is the possibility to refine the ocean grid in order to resolve the meso-scale in an area of interest, based on the Adaptive Grid Refinement in Fortran (AGRIF, Debreu et al., 2008), while the global ocean outside the area of interest remains non-eddying. With the so-called nesting approach we are able to perform multi-decadal to centennial climate simulations with an eddying ocean. Figure 1 summarizes the modeling system FOCI schematically.

Two high resolution ocean nest configurations are presented, one covering the South Indian and Atlantic Ocean (INALT10X) as well as the northern North Atlantic (VIKING10). Both configurations run separately to address their specific scientific questions. VIKING10 for example investigates paths and fates of large meltwater inputs from the Greenland ice sheet and its climate response, whereas INALT10X focuses on the non-linear dynamics of the Agulhas region around South Africa which is a key region for the global meridional overturning circulation (MOC) and therefore large-scale climate pattern. Meso-scale eddies shed off in the retroflection zone of the western boundary Agulhas current and transport warm and salty waters from the Indian into the South Atlantic Ocean, thereby affecting the upper limb of the MOC. With FOCI and INALT10X we aim to deploy a comprehensive understanding of the main mechanisms at play in a fully coupled system.

Here, we show both the technical details of FOCI and a first validation of the nested model components using several configurations of FOCI. In FOCI the atmosphere model runs at a T63 resolution, ~1.8˚, with 95 vertical levels resolving the stratosphere and the mesosphere up to 80km. The nominal global ocean resolution is 1/2˚ on an ORCA05 tri-polar grid and 46 vertical levels. Horizontal resolution can be refined by a factor of 5 to a 1/10˚ in the nest area. The selected two-way-nesting technique ensures an information transfer from the finer nest grid to the coarser global grid.

First results show that the model in its basic unnested configuration runs stable under pre-industrial control as well as historical forcing, and produces a climate mean state and variability that compares well with observations, reanalysis products and other climate models. The control run (without nesting) has been integrated over 1500 years, while historical conditions extend from 1850-2012. The Altantic MOC at 26.5˚N is stable throughout the entire unnested FOCI as well as over a 150 year nested INALT10X integration, both under piControl and historical conditions. Volume transport through the Drake Passage in the southern Ocean settles at a rather low level of ~80 Sv in the standard FOCI configuration, but strengthens when resolution increases in INALT10X. Here, values of ~100 Sv compare well to the broad observational range.

Some long-standing biases in climate models are reduced in the nested FOCI configurations. One of the common biased quantities is the simulated sea surface temperature (SST) in current climate models at standard resolution, Fig. 2 a,d. While in the northern North Atlantic SSTs are too low, SSTs in the equatorial tropical Atlantic are too high. Climate models participating in the current phase of the climate model intercomparison project (CMIP5) share these flaws with similar magnitudes as in FOCI, up to -6˚C and +6˚C, respectively. Increasing the ocean resolution from 1/2˚ to 1/10˚ in the two nested FOCI simulations substantially reduces both biases, most pronounced the North Atlantic cold bias, Fig. 2 b,e.

References

Debreu, L., et al., 2008 “AGRIF: Adaptive grid refinement in Fortran”, Computers & Geosciences

Madec, G., 2016, “NEMO ocean engine”, Note du Pôle modélisation, Inst. Pierre-Simon Laplace

Matthes, K. et al., FOCI reference paper, in prep.

Schultz, M. G., et al., 2018, “The chemistry–climate model ECHAM6.3-HAM2.3-MOZ1.0”, Geosci. Model Dev.

Stevens, B., et al., 2013, “Atmospheric component of the MPI-M Earth System Model: ECHAM6”, Journal of Advances in Modeling Earth Systems

Valcke, S., 2013, “The OASIS3 coupler: a European climate modelling community software”, Geoscientific Model Development

Figure 2: Some long-standing biases in climate models are reduced in the nested FOCI configurations. One of the common biased quantities is the simulated sea surface temperature (SST) in current climate models at standard resolution (a,d). While in the northern North Atlantic SSTs are too low, SSTs in the equatorial tropical Atlantic are too high. Climate models participating in the current phase of the climate model intercomparison project (CMIP5) share these flaws with similar magnitudes as in FOCI, up to -6˚C and +6˚C, respectively. Increasing the ocean resolution from 1/2˚ to 1/10˚ in the two nested FOCI simulations substantially reduces both biases, most pronounced the North Atlantic cold bias (b,e).