Validation of decadal simulations of mesoscale structures in the North Sea and Skagerrak Jon Albretsen and Lars Petter Røed

Norwegian Meteorological Institute met.no #2 Outline Background and motivation Models and configuration Validation results Application to ecosystem Conclusions Feistein Lighthouse

Norwegian Meteorological Institute met.no #3 Background and Motivation Switching to ROMS –To become our new NOWP model (decision based on earlier model- model comparison and validation results, LaCasce et. al, 2007) Old model:MIPOM - old code, yesterdays numerics New model:ROMS – modern code, sophisticated numerics, e.g., better conservation properties, able to run with higher vertical resolution Applications to cod fish eggs/larvae drift from the North Sea to Skagerrak –What is the chance of the spawned North Sea cod fish eggs to enter the Skagerrak?

Norwegian Meteorological Institute met.no #4 Background and Motivation Goal: –investigate the skill of the various models with respect to its ability to reproduce the statistical properties To be presented –Results from 27 year long hindcast simulations of the North Sea/Skagerrak area on eddy- permitting (4km) and eddy-resolving (1.5km) grids (period is ) using MI-POM and ROMS Validation tools –Mainly probability distributions (PDF’s), but also time series and vertical sections

Norwegian Meteorological Institute met.no #5 Computational Domains Atmospheric forcing: –ERA40 and ECMWF OA OBC: –4 km: SODA reanalysis + climatology –1.5 km nested to 4 km Tides included Rivers: –Climatology –Baltic S=12 psu No data-assimilation 1.5 km 4 km

Norwegian Meteorological Institute met.no #6 ItemMIPOMROMS Resolution4 km1.5 km4 km1.5 km # of vertical levels Long (internal) time step 150 s60 s120 s90 s Ratio of internal to external time step Horizontal dissipationSmagorinskyNo explicit diffusion Vertical mixing Mellor-Yamada 2.5 level GLS mixing scheme Horizontal advection scheme 2nd order centered3rd order upwind Surface fluxesMI-IM Standard ROMS bulk fluxes (analytic) Model facts

Norwegian Meteorological Institute met.no #7 Circulation pattern in the area of interest average ROMS 4km surface currents daily mean ( )

Norwegian Meteorological Institute met.no #8 Observations for validation Institute of Marine Research: –Current measurements (one location, valid from ) –Monthly data from the Hirtshals – Torungen section (12 stations, all years)

Norwegian Meteorological Institute met.no #9 Average current speed Observation period: Nov 1992 – Mar 1993 Model values from the exact same period (daily means) 58.37N,8.51E: Measured total depth: 120m Equil. depth: 233m (4km) and 163m (1.5km) Standard deviation Validation of current speed

Norwegian Meteorological Institute met.no #10 Average current speedStandard deviation Validation of current speed Observation period: Nov 1992 – Mar 1993 Model values from Nov-Mar all winters from (daily means)

Norwegian Meteorological Institute met.no #11 Validation of current speed Obs. period: Nov'92-Mar'93, Model: same period 26 winters: Obs. period: Nov'92-Mar'93, Model: same period: 13m 75m 13m 75m Statistical skill: the models' abilities to reproduce the statistical properties of the observed currents

Norwegian Meteorological Institute met.no #12 Validation of current direction Obs. period: Nov'92-Mar'93, Model: same period: Obs. period: Nov'92-Mar'93, Model: same period 26 winters: Currents from the NE parallel to the: - coast: 238 deg - local isobaths: 225 deg 13m 75m

Norwegian Meteorological Institute met.no #13 Observation period: Nov Mar 1993 Model values from the exact same period (daily means) 13m depth 75m depth Validation of current speed Useful to denote forecast skill

Norwegian Meteorological Institute met.no #14 M4.0 M1.5 R4.0 R1.5 Obs. Validation of hydrography Average density:

Norwegian Meteorological Institute met.no #15 M4.0 M1.5 R4.0 R1.5 Obs. Validation of geostrophic velocities Average velocity:

Norwegian Meteorological Institute met.no #16 Applications Simulate drift of cod eggs/larvae from the North Sea to Skagerrak Example from one location based on: Currents from ROMS 4km, 10m depth, 22.2.– Probability for a particle to enter Skagerrak: 92%

Norwegian Meteorological Institute met.no #17 Results – particle drift Probabilities for particles entering Skagerrak from locations in the North Sea between 1981 and 2007 at 10m depth averageAnnual-variability between 1981 and 2007

Norwegian Meteorological Institute met.no #18 Eddy resolution is crucial to get the mesoscale statistics of the circulation correct, and in particular the strength of the current jets This is brought about by the much better resolved topography when employing the 1.5 km mesh in combination with eddy resolution (particularly important regarding circulation in areas exhibiting prominent topographic features as f. ex. the Norwegian Trench cutting into the heart of the North Sea/Skagerrak area) Conclusions

Norwegian Meteorological Institute met.no #19 Conclusions MI-POM reproduces temperature and salinity well on average, but with the largest, positive salinity bias along the Norwegian coast in Skagerrak (the Baltic outflow challenge) The analytical expressions in ROMS for surface heat and salinity fluxes creates positive biases in both temperature and salinity (~1 o C warm-bias in the Skagerrak and slightly saltier than MI-POM) Applying similar surface heat and salinity flux algorithms in ROMS as in MI-POM will hopefully improve the modelled hydrography without downgrading the quality of the currents The model simulations form a valuable basis for analysis of statistical properties of the pathways important for the migration, growth and recruitment of fish stocks