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 1981-2007) 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 2005-2007

– 1.5 km nested to 4 km

• Tides included• Rivers:

– Climatology– Baltic S=12 psu

• No data-assimilation

1.5 km4 km

Norwegian Meteorological Institute met.no

#6

Item MIPOM ROMSResolution 4 km 1.5 km 4 km 1.5 km

# of vertical levels 26 21 32 32

Long (internal) time step

150 s 60 s 120 s 90 s

Ratio of internal to external time step

30 40 30 30

Horizontal dissipation Smagorinsky No explicit diffusion

Vertical mixingMellor-Yamada

2.5 levelGLS mixing scheme

Horizontal advection scheme

2nd order centered 3rd order upwind

Surface fluxes MI-IMStandard ROMS bulk

fluxes (analytic)

Model facts

Norwegian Meteorological Institute met.no

#7

Circulation pattern in the area of interest

1981-2007 average

ROMS 4kmsurface currents

daily mean (2007-3-9)

Norwegian Meteorological Institute met.no

#8

Observations for validation

• Institute of Marine Research: – Current

measurements (one location, valid from 27.10.1992-4.4.1993)

– 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 1993Model 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 speed Standard deviation

Validation of current speed

Observation period: Nov 1992 – Mar 1993Model values from Nov-Mar all winters from 1981-2007

(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

13m

75m

75m

Norwegian Meteorological Institute met.no

#13

Observation period: Nov 1992 - Mar 1993 Model values from the exact same period (daily

means)

13m depth 75m depth

Validation of current speed

Useful to denote forecast skill

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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 fromROMS 4km,10m depth, 22.2.–1.5. 2006

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

1981-2007-average

Annual-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 (~1oC 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