Generation and Initial Evolution of a Mode Water -S Anomaly

Gregory JohnsonNOAA/Pacific Marine Environmental Laboratory

•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

SE Pacific Salinity Max Variations

•Figures after Kessler (1999)•S max Subducted in SE Pacific•S max Advected Towards Equator

•Significant -S Variation along 165E•Salinity Changes of Order 0.4•Few Locations so Well Measured•Variations Related to Advection•Difficult to Find Cause at Source•Next cut along 103W . . .

South Pacific Eastern Subtropical Mode Water

•After Wong & Johnson (2003)•WOCE P18 Section Data

–Along 103W 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

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

Modeled -S Variability•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

•Follow Anomaly Equatorward–Subducted Around 1967-1968–On Equator 6-7 Years Later–Reduced in Magnitude

•Appropriate Diffusivity?–Numerical & Parameterized–Double Diffusion not Enabled . . .

Floats as a Time-Series•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

A Condensed Preview of the Time-Series

•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

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?

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?

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

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) = 2.410-4 F + 0. 110-4 m2 s-1

With F = [1 - (R - 1)/(2.05 - 1)]3

And Elsewhere:Ks = 0.110-4 m2 s-1

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

•Use Previous Parameterization•Admittedly Ad-Hoc & Uncertain•Salinity Flux Below S Anomaly•Large Diapyncal Flux Downward•Significant Fraction of Anomaly Size Over a Year

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

What about Advection?•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?

Conclusions

•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