Using altimeter measurements for quantitative assessment of high resolution ocean models

1. Using altimeter measurements for quantitative assessment of highresolution ocean models LuAnne Thompson School of Oceanography Kathryn A. Kelly Applied Physics Laboratory University of Washington Also Wei Cheng (UW) E. Joseph Metzger(NRL)

2. What questions can we answer with high resolution (non assimilating) models? • How well does the model reproduce the characteristics of WBCs? • What are the predictable components of the WBC heat budget?

3. Plan for analysis • Look at the Kuroshio Extension in a 0.08 deg model (HYCOM, run at NRL) • Use satellite SST and SSH comparisons • Compare KE strength and path • Examine possible causes for Low frequency variations (wind forcing) • Examine the upper ocean heat budget • Conclusions

4. SST in high and low resolution models One degree ocean model 1/10 degree Ocean model

5. SST:the Kuroshio Extension

6. RMS SSH over the KE RMS SSH From one degree Model (max 0.2m) RMS SSH from obs (max 0.4 m)

7. Kuroshio Extension Path and Intensity SSH profile Error function Fit error function to SSH to get axis and strength

8. SSH Variability and Mean Path Magnitude of anomalies is good Obs suggest more penetration of the jet

9. Jet Strength: Mean and Anomalies Teague et al 1990 Mean Jet Strength is Represented well Low frequency variations differ

10. Both show very low path variation (1993-1994 & 2001-2002); HYCOM is less stable Annual Path Variations & Stability HYCOM Altimeter Path analysis adapted from Qiu and Chen, 2005

11. Std dev. of path latitude Transporth Stronger jet <=> more stable path Correlations between path stability and strength Correlationweak but significant r = -0.4 (marginal) Correlation high r = -0.7 (significant)

12. Ocean Response to Wind Stress Curl Strengthens subpolar gyre curl • EOF 1 of wind stress curl from ECMWF (27%): • Negative correlation with observed h (wind leads by one year) • However, not correlated with h from HYCOM near KE

13. Mean SST comparison SST agreement with small warming in northwest, and cooling in the southeast

14. HYCOM path crosses (its own) 21C contour: Mean Path and 21C Contour

15. Net Surface Heat Flux Comparison => Flux correction does not cause low-frequency differences Mean difference: compensates for SST differences SSH response to heat flux anomalies small

16. Upper Ocean Heat Budget Heat storage rate = surface flux + lateral flux + isotherm motion Advection dominates in both -- model advection larger and not correlated with Vivier et al result

17. What does the model do correctly? • SSH anomaly strength • Jet path and mean strength • Strong current is (weakly) correlated with stable path • Dominance of advection in heat budget

18. What differences can we see? • Small path errors • Path is more unstable than observed (weaker correlation between current strength and path stability) • Low frequency (wind-forced) SSH variations obscured by unstable jet • Advection dominates heat budget (as in observations), but anomalies too large

19. Questions • What controls the jet stability? Is it predictable? • Is there intrinsic variability in the model/real ocean that masks the predictable wind-forced variations? • What are the dynamics that control the one year lag in the adjustment of the WBC to the wind? Are these dynamics missing/masked in the model? • Is the path stability related to the penetration?