Experience with ROMS for Downscaling IPCC Climate Models 2008 ROMS/TOMS European Workshop, Grenoble, 6-8 October Bjørn Ådlandsvik, Paul Budgell, Vidar Lien Institute of Marine Research, Bjerknes Centre for Climate Research Bergen, Norway

Contents Background Climate models and downscaling Critique North Sea downscaling Intermediate scale downscaling and the Barents Sea Conclusions

Background Norway's two largest export industries, petroleum activity and fisheries, are based on the continental shelves and are heavily influenced by climate. There is therefore considerable interest in the development of future climate scenarios for ocean climate in the North Sea and Barents Sea.

Global climate models AOGCM  Atmosphere/Ocean General Circulation Model  Atmosphere/Ocean Global Climate Model IPCC AR4, approx. 20 AOGCMs

Regional use of AOGCMs AOGCMs lack necessary resolution for shelf and coastal seas  Topographic features  Coastal upwelling  Mesoscale eddies  Exchange shelf sea – deep ocean AOGCMs lack important physics for coastal seas  Tidal mixing

Dynamical downscaling Force a regional model with atmospheric forcing and lateral boundary description from an AOGCM. Presently: One-way coupling, no feedback to AOGCM Intent:  Provide consistent high resolution regional scenarios of present and future climate, for use in marine ecological effect studies

Forcing scenarios 20C3M  20 th Century Climate in Coupled Models  Historical greenhouse gas concentrations  Used for validation and control A1B  “Moderate” greenhouse forcing for 21 th century  The most widely used IPCC scenario

Critique The results from the AOGCMs are not good enough for regional studies Downscaling can't improve errors in atmospheric circulation affecting the regional scale Downscaled results may therefore be badly biased, and may give misleading input to further assessment

North Sea Downscaling Validation: 20C3M,  Ådlandsvik and Bentsen, Ocean Dynamics 2007 Future scenario: A1B,  Ådlandsvik, TellusA, 2008 AOGCM:  Bergen Climate Model, BCCR-BCM  Ocean component: MICOM  Atmosphere: ARPEGE Regional Ocean Model  ROMS

Model domain

Model setup Atmospheric forcing  Daily averaged BCM surface fluxes Ocean lateral boundary forcing  Monthly averaged BCM fields  8 tidal constituents  Boundary scheme: FRS + Flather/Chapman Fresh water  Climatological run-off modulated by BCM precipitation  Baltic = large river, salinity = 18  Relaxation of Sea Surface Salinity towards BCM

Sea surface temperature average for March 1978 BCMROMSClimatology

ROMS SST Change

North Sea Conclusions I BCM does a good job with integrated values for the North Sea Some problems due to low resolution but also isopycnal coordinates on shallow shelf sea. Downscaling works technically, with a factor ten in resolution.

North Sea Conclusions II Downscaling provides added value by improving the BCM results where most needed  Improved regional details, incl. Coastal Current  Improved Atlantic Inflow  Improved winter temperature  Improved vertical structure, incl. surface salinity:

Conclusions III Future scenario:  Warming of the North Sea, maximum in winter Yearly mean: BCM +1.0°C, ROMS +1.4°C  Slightly increased Atlantic Inflow, max increase in August Yearly mean: +0.2 Sv = +15 %

Problem in the Barents Sea Most AOGCMs have too much ice in the 20 th century climate When the ice dissapeares in future climate, it gives an unrealistic warming The atmosphere behaves very differently if it has sea ice or open water at surface boundary With no feedback to the atmosphere, the fluxes lead to strong cooling and too much ice in the regional model

Intermediate downscaling Atlantic-Arctic domain Get the open boundaries far away from ice-infected area Stretched coordinates  Resolution 10 km in Nordic Seas

Model set-up Daily atmospheric forcing from GISS AOM, one of the three best models for ice in Arctic and Barents Sea. (Overland and Wang, 2007)‏ Control: 20C3M, (incl. 5 y spinup)  Initial and boundary states from SODA, climatology for Future: A1B, (incl. 5 y spinup)‏  Initial and boundary = SODA + GISS future - GISS present

IMR GISS AOM Temperature at 100 m - March ROMSSODA

GISS AOM Sea Ice Concentration - March Observed GISS AOM - Obs ROMS ROMS - Obs

Barents Sea Winter Temperature 20C3MA1BA1B - 20C3M

Barents Sea Summer Temperature 20C3MA1BA1B - 20C3M

Barents Sea Ice Concentration

Barents Sea Conclusions Downscaling provide added value by  More realistic temperature and ice cover  But: Still problems with ice in east Future climate  Slight warming in western Barents Sea  Unrealistic strong warming in eastern Barents  Decreased inflow from 2.2 to 2.1 Sv,  Small change in heat transport

Overall experience Downscaling provides added value regionally to results from AOGCMs. Regional results must be validated for 20 th century. Results must be used with care, in particular where the validation fails. For robust conclusions we need to downscale from several AOGCMs.