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Generation and Initial Evolution of a Mode Water  -S Anomaly

Introduction & Motivation Variations in the S Pacific Salinity Maximum The South Pacific Eastern Subtropical Mode Water Modeling Study Seasonal Mode Water Evolution Potential Vorticity  -S Anomaly Turner Angle Double-Diffusion Microstructure Mixing Parameterization

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Generation and Initial Evolution of a Mode Water  -S Anomaly

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  1. Introduction & Motivation • Variations in the S Pacific Salinity Maximum • The South Pacific Eastern Subtropical Mode Water • Modeling Study • Seasonal Mode Water Evolution • Potential Vorticity • -S Anomaly • Turner Angle • Double-Diffusion • Microstructure Mixing Parameterization • Salinity Flux Estimates • 1-D “Model” • Conclusions • Simple 1-D “Model” Goes Some Way in Matching -S Anomaly Evolution • However, Advection Must Play a Part Generation and Initial Evolution of a Mode Water -S Anomaly Gregory Johnson NOAA/Pacific Marine Environmental Laboratory

  2. SE Pacific Salinity Max Variations Figures after Kessler (1999) S max Subducted in SE Pacific S max Advected Towards Equator • Significant -S Variation along 165E • Salinity Changes of Order 0.4 • Few Locations so Well Measured • Variations Related to Advection • Difficult to Find Cause at Source • Next cut along 103W . . .

  3. South Pacific Eastern Subtropical Mode Water After Wong & Johnson (2003) WOCE P18 Section Data Along 103W in 1994 Region of Small d/dz 25.6 < < 24.8 kg m-3 • Region Sits Below S Maximum • Formed in High E-P Region • Winter Evaporation & Cooling • dS/dz Also Reduced Here • Note dS/dz Destabilizing • Warm Salty Over Cold Fresh

  4. SPESTMW (Continued) After Wong & Johnson (2003) Potential Vorticity Minimum Capped Over in Austral Fall Spreads Equatorward of Formation Region Wide -S Property Range • High Turner Angle • Winter Evaporation & Cooling • Warm Salty Over Cold Fresh • (Tu > 77 = Density Ratio < 1.6) • Potential for Double Diffusion • Just Austral Fall Data • Well After Subduction

  5. Figures of Yeager & Large (2004) • Look on = 25.5 kg m-3 • RMS S (10-2 PSS-78) • Strong Signal In SPESTMW • Propagates Equatorward • Linked to Spiciness Modeled -S Variability • Follow Anomaly Equatorward • Subducted Around 1967-1968 • On Equator 6-7 Years Later • Reduced in Magnitude • Appropriate Diffusivity? • Numerical & Parameterized • Double Diffusion not Enabled . . .

  6. After Johnson (in press) • Just Downstream of High Turner Angle (Spicy) SPESTMW Formation Region • Winter Surface Waters Contoured as 1.0 < R < 2.0 at 0.2 Intervals • WMO IDs 4900451 (cyan) & 4900454 (magenta) • -Deployed January 2004 & Analyzed into July 2005 • -Profiles Every 10 days • -71 Data Points • 100-dbar Spacing at 2000 dbar • Reduces to 8-dbar Spacing by 160-dbar Floats as a Time-Series

  7. Mar 2004 (Black o’s) • Typical of Central Waters • Salinity Destabilizing • Anomaly Near 24.8 kg m-3? • Oct 2004 (Magenta +’s) • Maximum Ventilation • Mixed Layer to 25.0 kg m-3 • Temp Cold But . . . • Upper -S Pulled Salty •  < 25.2 kg m-3 • Cooling with Evaporation • Mar 2005 (Cyan ◊’s) • Austral Fall Stratification • Strong Anomaly • Near 25.0 kg m-3 • Anomaly Also Denser •  > 25.2 kg m-3 • Double Diffusion? • Downward S Flux • Rotated -S Curve A Condensed Preview of the Time-Series

  8. Potential Vorticity Time-Series Seasonal Mixed Layer Evolution Deeper & Denser Mar-Oct Abrupt Spring Restratification Gradually Lighter Until Fall Maximum Spring Ventilation Pr > 150 dbar ≈ 25.0 kg m-3 • Late Spring PV Reset • Low PV Replenished • 2004 Ventilation vs. 2003 • PV Min Lower • PV Min Thicker • Stronger Ventilation?

  9. Salinity Anomaly Time-Series Pick Reference -S Curves (Blue Vertical Lines) S Anomalies Relative to Curves S Anomaly Around 0.3 PSS-78 Salty & Warm Water Subducted • Subsequent Evolution • Max Anomaly Reduces • Anomaly Also Moves Denser • Result of Salt-Fingering? • Patchiness • Mesoscale? • Advection? • Winter 2004 Stronger Than 2003?

  10. Turner Angle Time-Series Contours: R ≤ 2.0 at 0.1 intervals Wintertime Latent Cooling with . . . Strong Evaporation Salinity Anomaly Favors Large Turner Angle Double Diffusion • Seasonal Anomaly Evolution • Again Tu Maximum Eroded • Migrates Downward • Similar to the S Anomaly • Interannual Variations • 2004 Exceeds 2003

  11. Parameterize Salt Fingering Mixing Use an Ad-Hoc Parameterization Decreased Stability ->Increased Mixing After Yeager & Large (2004, Eq. B1) St. Laurent & Schmitt (1999) Data Assume That For 1 < R < 2.05 (90 < Tu < 71): Ks (R) = 2.410-4  F + 0. 110-4 m2 s-1 With F = [1 - (R - 1)/(2.05 - 1)]3 And Elsewhere: Ks = 0.110-4 m2 s-1

  12. Use Previous Parameterization • Admittedly Ad-Hoc & Uncertain • Salinity Flux Below S Anomaly • Large Diapyncal Flux Downward • Significant Fraction of Anomaly Size Over a Year Diapycnal Salinity Flux Time-Series • Seasonal Anomaly Evolution • Flux Decays with Time • Zero-Crossing Denser with Time • Similar to Other Fields • Interannual Variations . . . • 2004 Stronger than 2003 • Next: Follow  = 25.35 kg m-3

  13. Model S Anomaly on = 25.35 kg m-3 Find Diapycnal Salt Flux on Isopycnal Integrate with Time (1-D) Compare Mapped Salinities Pick Best Agreement Integration Constant • Seasonal Anomaly Evolution • Anomaly Ramps up in Spring • Decays slowly thereafter • Delayed for 4900454 • Weaker Northern Winter Ventilation

  14. 1-D Model is Surprisingly Good • However, Advection is Present • Short-term Variations (Eddies) • Mean Circulation • Mixed Layer Slumping? • In January 2004 Data Were Few • Difficult Even to Map Anomaly • More Difficult to Trace Anomaly • In January 2006 Data Are Many • Anomaly Mapping & Tracing Almost Possible? What about Advection?

  15. Observational and Modeling Studies Reveal SE Pacific -S Variations • Warm Salty Water Subducted in SE Subtropical Pacific • Spiciness Enables Large -S Variation in Eastern STMW • Anomalies May Even Reach the Equator, Upwell, and Influence SST • Argo Floats Allow Local Studies of Seasonal Mode Water Evolution • Potential Vorticity • -S Anomaly • Turner Angle • Double-Diffusion May Be Important • Microstructure Mixing Parameterization • Salinity Flux Estimates from 1-D “Model” Match Observations Pretty Well • Advection Must Play a Role Over Longer Time-Scales • Next Steps: Mapping Anomalies & Tracing Their Evolution • Growth of Array Begins to Make this Realistic • Requires Data From a Continuous Argo Float Array • Must Maintain Array over Several Years • For Mapping Anomalies & Tracing Them Equatorward • For Analyzing Interannual Variations Conclusions

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