1. Ocean Modelling and High Performance Computing Andrew Coward Cluster Computing Summer School 2009

2. Ocean Modelling and High Performance Computing Introduction and rationale Historical perspective The NEMO model Example results

3. main warming is at mid-depth - unobservable from space the best observed basin-wide, full depth hydrographic section …… 3 times in 35 yr! Evidence of change: In-situ observations Temperature change at 1.5-2.5km off Bermuda Atlantic temperature change ( o C) at 24N (1957-1992) Introduction and rationale

4. Recipe for an ocean model: 1.Derive mathematical equations describing the ocean’s evolution from an initial state subject to surface forcing. 2.Discretize equations on a spatial grid (3-dimensional). 3.Obtain initial state from observations. 4.Obtain time varying surface forcing from observations or Numerical Weather Prediction program. 5.Integrate equations forward in time from initial state. 6.Test and develop model in hindcast mode. Ocean models are needed to “fill-in the gaps” and provide a predictive capability Introduction and rationale

5. Greenland-Scotland Topography Greenland Iceland Scotland 1. Complex domains Introduction and rationale: the need for resolution in ocean models

6. Ocean eddy (IR) note difference in horizontal scales Observations from space Atmospheric depression (IR) 1000 km 2. Small scales of motion Introduction and rationale: the need for resolution in ocean models

7. 1/4 o 1o1o 1/12 o 1o1o Satellite observed sea surface temperatureSimulated sea surface temperature

8. Cray X/YMP 1/2 o x 1/4 o Southern Ocean Model (circa 1990) The other changing environment: 5M gridcells Cray T3D/E Origin 3800 1/4 o x 1/4 o Global Ocean Model (mid nineties) } 37M gridcells IBM Regatta } 1/12 o x 1/12 o Global Ocean model 608M gridcells ? EvolutionEvolution Cray XMP/YMP autotasking parallelism 8 processors memory slab window with SSD asynchronous "putwa's and getwa's" Typical performance: 20 model days in 12 hours using 512 HPCx processors Storage requirement ~ 1TB per model year Historical perspective

9. Recipe for a High Performance ocean model:. 1.Discretize equations on a spatial grid (3-dimensional). 2.Decompose grid into multiple overlapping tiles 3.Introduce a message-passing harness to exchange information between tiles. 4.Obtain time varying surface forcing from observations or Numerical Weather Prediction program. 5.Integrate equations forward in time from initial state. 6.Test and develop model in hindcast mode. A separate processor computes values in each differently coloured patch

10. NEMO example 16x16 domain decomposition requiring 221 processors

11. Agulhas Sea Surface Temperature Sea Surface Temperature Range: 11 o C to 25 o C

12. The role of eddies in global heat transport