Argo Floats and Modelling the North Pacific Ocean

Michael W. StaceyDepartment of PhysicsRoyal Military College of CanadaKingston, Ontario, K7K 7B4Stacey-m@rmc.ca

15 June 2006

Major Collaborators:

Jennifer Shore (RMC)Dan Wright (BIO)Keith Thompson (Dalhousie)Howard Freeland (IOS)Bill Crawford (IOS)

MotivationMeteorologists have provided regular weather forecasts for years, using atmospheric observations and numerical models.

Observations of the sub-surface ocean are much harder to make, but oceanographers are now at the stage where they can begin to make comprehensive observations in almost real-time.

These observations are obviously useful in providing better information about the state of the ocean. Also, they provide useful information to scientists studying climate change, since the oceans influence the climate.

Basic Requirements

1. Observations

- Argo Floats

- Satellite observations

- etc

2. Computational Resources

- HPCVL

Outline• Argo float program• Numerical Model• Spectral Nudging• Model simulations of the North Pacific Ocean• Zoom in on the Northeast Pacific Ocean• Eddy formation and propagation in the NE Pacific• Summary

Argo Floats (images from http://www.argo.ucsd.edu/)

Argo floats recordsalinity, temperatureand pressure

Approx $15K/Float,plus another $15K forhandling and running

- Each floats lasts for about 140 cycles, or 4 years.- Observations can be downloaded from the Argo website free of charge.

- Each floats lasts for about 140 cycles, or 4 years.- Observations can be downloaded from the Argo website free of charge.

Approximately 100,000 profiles/yr once 3000 deployed

Isopycnals are deepening in the NE Pacific

200

100

0

Pre

ssur

e (d

bar)

2001 2002 2003 2004 2005 Last update 28th M ar. 2006

The ModelThe ModelParallel Ocean Program (POP)

-Finite difference, numerical model, parallelized at Los Alamos, modified at the Bedford Institute of Oceanography (Halifax) for the Atlantic Ocean, modified at RMC for the North Pacific Ocean.

-Uses ‘Spectral Nudging’ to prevent drift of the ‘mean’ model fields.

-Run at HPCVL. A single, twenty year simulation takes about 10 days of CPU time, using 20 processors.

- Horizontal Resolution: approx. 30 km -Vertical Resolution (23 layers): Layer Depths (m)Top 200 m Bottom 4000 m1. 10 9. 260 16. 22002. 20 10. 360 17. 27003. 35 11. 510 18. 3200 4. 55 12. 710 19. 37005. 75 13. 985 20. 42006. 100 14. 1335 21. 47007. 135 15. 1750 22. 52008. 185 23. 5700

-

Spectral Nudging

- Available observations of temperature and salinity are temporally averaged to obtain the observed ‘climatological mean’. - Want the model to have the same ‘mean’ as the observations. Energy for the eddies comes from the energy in the ‘mean’ flow, so need reasonable ‘mean’ in order to get the eddy field right. - Models can be constrained to have the same ‘mean’ as the observations by ‘nudging’ them towards the climatology as the simulation proceeds through time, but because the observed ‘mean’ is ‘smooth’, standard nudging suppresses the formation of eddies in models. - Spectral nudging constrains only the ‘mean’ component of the model to remain close to the climatology, so eddies can form in the model.

Standard Nudging vs. Spectral Nudging

Standard nudging prevents model T and S from drifting away from climatology but suppresses eddy variability.

Spectral nudging constrains the mean T and S fields while keeping higher frequency variations.

Elevation (cm

)

Elevation (cm

)

Sea Surface Height

Temperature at 100 m

Northeast Pacific (Gulf of Alaska)

Sea Surface Height

Temperature at 100 m

‘Snapshot’ of the Sea-Surface Temperature off the Coast of B.C.

Observation (Thomson & Gower, 1998) Model

- Eddy trains form during the winter.

- The eddies are about 200 km across and at least 500 m deep.

- They propagate towards the west at about 3 km/day.

Thomson and Gower (1998)

Yelland and Crawford, 2005

Size(Diameter)

Speed Reference

160-200 km 3.0 ± 0.5 km/d Thomson and Gower, 1998

160 km 2.5 km/d Crawford et al., 2000

150-370 km 1 km/d Yelland and Crawford, 2005

200-300 km 4.4 ± 0.6 km/d Our results

Eddy Sizes and Speeds

Standard Deviation of the sea surface height variability.

Stacey et al (2006)

Crawford et al., (2000)

Summary - The Argo program has significantly improved our ability to make observations of the sub- surface ocean. It is a multi-national program for which Canada is a significant contributor.

- HPCVL has provided the computational resources for simulations of the North Pacific Ocean to take place at RMC.

Thank YouReferencesCrawford, W. R., J. Y. Cherniawsky, and M. G. G. Foreman (2000), Multi-year meanders and eddies in the Alaskan Stream as observed by TOPEX/Poseidon altimeter, Geophys. Res. Letters, 27, 1025-1028.

Stacey, M. W., J. Shore, D. G. Wright, and K. R. Thompson (2006), Modeling events of sea-surface variability using spectral nudging in an eddy permitting model of the northeast Pacific Ocean, J Geophys. Res., 111, C06037, doi:10.1029/2005JC003278.

Thomson, R. E., and J. F. R. Gower (1998), A basin-scale oceanic instability event in the Gulf of Alaska,. J. Geophys. Res., 103, 3033-3040.

Yelland, D., and W. R. Crawford (2005), Currents in Haida Eddies, Deep-Sea Res. II, 52, 875-892.