Regional Ocean Modelling in Southern Africa

Sea Surface Temperature [oC]

Acknowledgements:

N. Chang, UCT; L. Debreu, IMAG; P. Florenchie, BCLME; J. Lefevre, IRD; J.R.E. Lutjeharms, UCT; E. Machu, IRD;

P. Marchesiello, IRD; F. Shillington, UCT, J. Veitch, UCT.

1 - Some modelling tools used at IRD (Institut de Recherche pour le Developpement)

1.1 - ROMS_AGRIF: ROMS with embedding

1.2 - ROMSTOOLS: A Series of Tools for the Pre- and Post-

processing of Oceanic Regional ROMS Simulations.

2 - SAfE (Southern Africa Experiments)

2.1 - Climatology experiment (Process studies).

2.2 - Inter-annual experiment.

2.3 - Nested models.

Regional Ocean Modelling in Southern Africa

ROMS_AGRIF: Embedding capabilities

"from the large scale to the coastal applications"ROMS_AGRIF:

- Arbitrary number of embedded grids.

- Parent/child exchanges during run time.

- OpenMP/MPI compliant.

- Enforcement of mass conservation in

agreement with ROMS time stepping.

- Biology.

- 2way nesting in progress.

After Penven, P., L. Debreu, P. Marchesiello and J.C. McWilliams, Application of the ROMS embedding procedure for the Central California Upwelling System, 2006, Ocean Modelling, 12, 157-187.

ROMS_TOOLS: A toolbox for ROMS pre- and post-processinghttp://www.brest.ird.fr/Roms_tools/index.html

Goal: to simplify the design and analysis of ROMS model configurations

An observation: Similarities for the choices in different ROMS applications.

Possibility to use a systematic approach

Strategy:

Global datasets:

etopo2, WOA2001, TPX06,

DASILVA Climatology

ROMS input netcdf files:

roms_grd.nc, roms_frc.nc, roms_blk.nc,

roms_ini.nc, roms_clm.nc, roms_bry.ncROMS_AGRIF

ROMS output netcdf files: roms_his.nc, roms_avg.nc

ROMS_GUI

VisualizationDiagnostic tools

Analysis

OPENDAP servers:

NCEP, GFS, ECCO, SODA

ROMSTOOLS

roms_gui: visualization.

Inter-annual simulations

~/Roms_tools/Run/run_roms_inter.csh

make_OGCM.m , make_NCEP.m

Use of OPENDAP to get automatically the necessary data over the web

(OGCMs: ECCO, SODA, surface forcing: NCEP).

Real-time coastal system

~/Roms_tools/Run/run_roms_forecast.csh

make_forecast.m

Hypothesis:

Large scale: slow.

On the shelf: winds

http://sea.uct.ac.za/penven/RealTime/index.html

Real Time ROMS Ocean Simulations in the Southern Benguela

Model grid: 1/9 degree resolution (i.e. 10 km); 72 X 94 X 32 grid points.

Lateral boundary conditions: ECCO.

Surface forcing: GFS.

One model run every day:

1 day hindcast + 8 days forecast.

2 - SAfE (Southern Africa Experiments)

image source: J. Lutjeharms, 2006.

Upwelling

Agulhas Current

Benguela upwelling system: influence of the Agulhas Current ?

Agulhas Current:

- Velocities up to 2 m.s-1

- 70 Sverdrups.

- Sources: South-Western Indian Ocean.image source: J. Lutjeharms, 2006.

SAfE strategy:

- Parent model which resolves both the Agulhas (from its sources to the spawning of Agulhas Rings) and the Benguela.

- Coastal zooms depending on the scientific interest.

Large region: intermediate resolution (20-25 km) for the parent model.

easy to test new ideas or parameters

¼o resolution (i.e. 20-25 km)

244 X 192 X 32 grid points

Bottom topography: GEBCO

Boundary conditions (climatology run): WOA 2001

Surface forcing (climatology run): COADS

SAfE parent configuration

SAfEAVISO

Annual mean SSH [cm]:

RMS SSH [cm]:

Upstream of the Agulhas Retroflection: - spurious recirculation. - spurious high level of variability.

AVISO SAfE

Problem: High variability upstream of the Agulhas Retroflection.

Surface layer eddy kinetic energy [log (m2/s2)] (adapted from Wallcraft et al., 2002).

NLOM: Global layered ocean model at 1/32°resolution (Wallcraft et al., 2003).

Wallcraft, A. J., Hurlburt, H. E., Rhodes, R. C. and J. F. Shriver, 1/32o global ocean modeling and prediction, Intern. report, 2002.

Wallcraft, A. J., Kara, A. B., Hurlburt, H. E. and P. A. Rochford, The NRL Layered Global Ocean Model(NLOM) with an embedded

mixed layer submodel: formulation and tuning, J. Atmos. Ocean. Tech., 2003, 20, 1601-1615.

1/16° NLOM (1980-1989) 1/32° NLOM (1980-1989)

Topex/Poseidon (1993-2001)

POP: Global z-coordinate ocean model at 1/10°resolution (Maltrud and McClean, 2005).

Maltrud, M. E. and J. L. McClean, An eddy resolving 1/10o ocean simulation, Ocean Modelling, 2005, 8, 31-34.

Surface height variability [cm] (adapted from Maltrud and McClean, 2005).

1/10° POP (1991- 1993) 0.28° POP (1995- 1996) TOPEX/ERS (1993- 1997)

OFES: Global z-coordinate ocean model at 1/10°resolution (Sasaki et al., 2005).

Figures adapted from Sasaki et al., 2005.

Sasaki, H., Komori, N., Takahashi, K., Masumoto, Y. and H. Sakuma, Fifty years Time-integration of Global Eddy-

resolving Simulation, Intern. Report, 2005.

RMS SSH [cm] (years 46 - 50) mean SSH [cm]

Snapshot of sea surface height

[cm] and velocity vectors at 100m.

RMS sea surface elevation [cm]:

SAfE – Smagorinsky

(a=0.1)

SAfE– Smagorinsky

(a=0.05)

SAfE – Smagorinsky

(a=0.025)

SAfE – Smagorinsky

(a=0.0125)

SAfEAVISO

Smoother topography +

Parametrisation of shear induced horizontal turbulent mixing (Smagorinski, 1963):

New SAfE simulations

Final SAfE climatology experiment:

Penven, P., J. Lutjeharms and P. Florenchie, Madagascar: a pacemaker for the Agulhas Current system ? Geophys. Res. Lett., 2006, 33, L17609, doi:10.1029/2006GL026854.

image source: J. Lutjeharms, 2006.

SAfE annual mean transport (1500m - surface)

Madagascar: a pacemaker for the Agulhas Current System ?

Penven, P., J. Lutjeharms and P. Florenchie, Madagascar: a pacemaker for the Agulhas Current system ? Geophys. Res. Lett., 2006, 33, L17609, doi:10.1029/2006GL026854.

SAfE Inter-annual simulation Boundary conditions: SODA (Carton et al., 2005)

Surface forcing: NCEP/NCAR (bulk formula).

Time period: 1958-2001 (+ 2 years spin-up).

SST [oC] in St Helena Bay [33oS-31oS; 17oE-19oE]:

1 - Annual mean:

SAfE ~ SODA: 17.4oC (0.5 higher than OI-SST).

2 - Seasonal cycle:

SAfE: winter warm bias (no summer bias).

3 - Non seasonal anomalies:

RMS(OI-SST - SODA) = 0.56oC

RMS(OI-SST - SAfE) = 0.45oC

Correlation Coefficients:

OI-SST/SODA: 0.62; OI-SST/SAfE: 0.58

4 - Annual anomalies:

RMS(OI-SST - SODA) = 0.29oC

RMS(OI-SST - SAfE) = 0.2oC

Correlation Coefficients:

OI-SST/SODA: 0.61; OI-SST/SAfE: 0.65

SODA

SAfE

OI-SST

Without data assimilation, in the Benguela, SAfE perform at least as well as SODA.

Nested model for the Agulhas Bank (N. Chang, UCT PhD student)

• Zoom in SAfE.

• Resolution: 1/12o (i.e. ~ 8 km).

• Analysis of the dynamics of the

cool ridge, the seasonal cycle, the

effects of the wind.

Nested model for the West Coast (J. Veitch, UCT PhD student)

• Zoom in SAfE.

• Resolution: 1/12o (i.e. ~ 8 km).

• Comparisons between the South

Benguela (seasonal upwelling) and

the North Benguela (perennial

upwelling).

Biological application (collaboration with V. Kone & E. Machu, IRD)

Conclusion

• ROMS: • Generic and robust regional oceanic model.

• ROMSTOOLS:

• Systematic approach.

• Climatological, long term inter-annual and quasi real time

applications.

• SAfE:

• Spurious recirculation upstream of the Agulhas Retroflection.

• Role of Madagascar.

• Dynamics of the cool ridge.

• Dynamics of the Benguela upwelling system.

• Applications to fisheries.