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LT Keir D. Stahlhut, 13 SEP 2005

Account of the paper, “Stability of the (Western) Sargasso Sea Subtropical Frontal Zone (SFZ),” by Halliwell, Peng, and Olson (1994). LT Keir D. Stahlhut, 13 SEP 2005. Cross Section with depth of the Subtropical Frontal Zone (SFZ). Sargasso Sea area.

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LT Keir D. Stahlhut, 13 SEP 2005

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  1. Account of the paper, “Stability of the (Western) Sargasso Sea Subtropical Frontal Zone (SFZ),” by Halliwell, Peng, and Olson (1994). LT Keir D. Stahlhut, 13 SEP 2005

  2. Cross Section with depth of the Subtropical Frontal Zone (SFZ) Sargasso Sea area Basically three characteristically different water types….

  3. Depth of 26C Isotherm Current SST

  4. NOV 2004 AVHRR SST 7-Day composite (from JHAPL) shows front nicely…..

  5. Analysis Strategy • Demonstrate the mean SFZ is (baroclinically) unstable • Characterize properties of the unstable eddies using 3-layer model • Estimate growth rates and wavelengths • Consider absolute stability properties • Analyze evolution of turbulent (non-linear) eddy field after some finite amplitude; compare these amplitudes to satellite data

  6. Stability analysis…. Equation to be solved (non-dimensional PV equations for small perturbations) Obtain the eigenvalue equation….. Where these are the coefficients….. Solution “Form”

  7. Stability analysis continued…. • Turns out that…. • This analysis predicts instability regions, dominated by wavelengths 150-200 km (agrees with 3.9Rd rule) • Growth rate depends primarily on Shear between top two layers (processes that effect seasonal thermocline are very important) • Further “absolute stability” analysis by “Ripa’s version” of “Arnol’d’s Theorems” also show instability

  8. “Case 1”--- Idealized representation of the large scale separating thermocline structure of SFZ “Case 2”---upper interface intersects the surface near the center of the frontal zone Numerical Model set up…. • Three active layers • Infinitely long, zonally oriented Beta plane channel, bounded by solid walls to N/S • Hydrostatic, quasi-geostrophic balance

  9. “Case 1”--- Idealized representation of the large scale separating thermocline structure of SFZ • This figure shows increased eddy variability over time • “Most unstable” wavelengths are ~ 150 -200 km (agrees with 3.9Rd rule) • After day 125, non-linear energy transfer takes place, eventually becoming a true “cascade regime” (think Time-Series) Larger amplitudes of sea surface elevation at later time

  10. Characteristic Westward propagation after day 200 is ~ 4km/day, but varies with latitude

  11. “Case 2”--- transition to higher turbulence, higher sea surface elevation amplitude occurs more rapidly than for Case 1 • “Most unstable” wavelengths are ~ 150 -200 km (agrees with 3.9Rd rule) • Results are “comparable” to satellite and XBT data for the area • Thus, model results suggest that baroclinic energy conversion and atmospheric forcing contribute roughly equal (order of magnitude) to the eddy variability within the SFZ

  12. Results/Conclusions: • Linear theory suggests the SFZ should be unstable to larger disturbances, with “most unstable” wavelengths being ~150 to 200 km. This was also confirmed by “absolute stability” theory. • Numerical modeling confirmed predictions of linear theory in early stages. After this, non-linear effects caused energy to transfer in wavenumber space. • “Case 2” of this modeling, where upper interface intersected the surface developed more rapidly, and the developed eddies were confined to south of the surface front. • Processes that act to steepen the seasonal thermocline of the SFZ are very important. • Mesoscale Oceanography is fun.

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