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Subtropical Mode Water: the result of air-sea interaction?

Sources and Predictability of Anomalies in North Atlantic Subtropical Mode Water Formation Using the Walin Framework ( A CLIMODE contribution) Kathryn A. Kelly University of Washington Western Boundary Current Air-sea Interaction Working Group Workshop January 2009.

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Subtropical Mode Water: the result of air-sea interaction?

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  1. Sources and Predictability of Anomalies in North Atlantic Subtropical Mode Water Formation Using the Walin Framework(A CLIMODE contribution)Kathryn A. KellyUniversity of WashingtonWestern Boundary Current Air-sea Interaction Working Group Workshop January 2009

  2. Subtropical Mode Water:the result of air-sea interaction? STMW: thick isothermal layer (18C “EDW”), south of Gulf Stream Conventional wisdom (Worthington, 1976): STMW is formed by heat losses from cold air outbreaks over the warm Gulf Stream

  3. Subtropical Mode Water:What does it contribute to air-sea interaction? WBC advection brings warm water into region Heat is stored south of WBC (recirculation gyre) Accumulated heat is fluxed to atmosphere Large heat storage <=> Low STMW volume Shenfu Dong, Ph D thesis, 2004

  4. Heat Content and Mode Water in the Gulf Stream Region Low heat content <=> thick 18C layer Subtropical Mode Water <=> heat deficit 50-year correlation in GS (Kwon thesis) Low heat content <=> low SSH

  5. Heat Content and Air-Sea Heat Fluxin Gulf Stream Sea Surface Height (proxy for heat content) and turbulent flux: SSH leads by 3 months in STMW region => Heat storage forces heat loss (seasonal-interannual)

  6. The Walin Framework for Mode Water CLIMODE: understanding mode water formation & dispersion Volume flux is a function of air-sea fluxes B and diffusion D (focus here only on formation)

  7. Transformation and Formation(using temperature, not density) Transformation F - add water to a temperature class Formation M - difference between water entering & waterleaving Outcrop area A - area of a given temperature interval at sea surface (NOT Walin volume flux) Net surface heat flux Q

  8. Causes and Predictability of Formation Examineanomalies over many years to answer: • Does formation depend more on “cold air outbreaks” (Q) or on pre-existing ocean conditions (stratification, A)? • How predictable is formation rate M?

  9. 2004 2007 Stratification, SST and Outcrop Area • Below mixed layer water is 18C • Higher stratification gives SST>18C (warmer) • Low stratification => larger 18C outcrop

  10. Formation Estimates • Integrate net surface heat flux over SST outcrop • Problem: flux resolution is low (prior to 2002) • Solution: EOF decomposition (and CCA) to match resolution

  11. Turbulent Heat Flux Example High-resolution: QuikSCAT/AMSR /ECMWF Low-resolution: NCEP2 variables run through COARE Reconstructed flux using ~24 CCA modes

  12. SST Example “Match” resolution of SST to flux fields Degrade SST: only ~24 CCA modes

  13. Wintertime (JFM) Fluxes

  14. Annual Formation Rates: 1990-2007(only in western Atlantic) Most formation occurs in JFM (red vs black) Sensitive to outcrop (blue vs black) Use 17.5-18.5C And JFM mean

  15. Interannual Variations Q Interannual variations in heat loss much smaller than outcrop size (Possibly from poor Q estimates) A M

  16. How Does Formation M Depend on Q and A? (17.5-18.5C outcrop heat loss & outcrop area) Outcrop area more important than Qnet!

  17. Regression of 18C formation M on Q and A(18C JFM anomalies) • M depends only on outcrop area: 58% variance reduction from A alone Q contribution not significant (F test) • Q and A anti-correlated:  = -0.38, large A (colder) gives less heat loss (A contains information about Q, no additional information from uncorrelated part of Q)

  18. Preconditioning: December SST

  19. Preconditioning: can December observations predict JFM formation M? 19.5C outcrop heat loss & outcrop area December heat loss has more skill than outcrop area Both make significant contributions to prediction

  20. Causes and Prediction of Interannual Variations in Mode Water Formation Conclusions: • STMW is a heat (deficit) reservoir for air-sea interaction • STMW formation depends more on outcrop area (ocean stratification) than on heat loss (e.g., cold air outbreaks) • Heat loss is correlated with outcrop area (and with heat content): STMW <=> memory in climate system • December heat loss and outcrop area can predict JFM formation anomalies <=> some predictability

  21. Unresolved Issues in Mode Water Limitations of this study: • Dispersion of STMW was not considered (volume depends on formation minus dispersion) • Short record: only 17 years (extend!) Future work: • Are formation estimates consistent with volume changes? • Does this analysis apply to the North Pacific STMW?

  22. Wintertime (JFM) SST

  23. Which outcrop best describes SST? Outcrops anticorrelated with SST: December 20C and March 18C

  24. Can December outcrop area A predict JFM heat loss Q? • Dec. 19.5C outcrop area A has predictive skill for JFM heat loss, Q

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