An Observational Streamfunction in the Gulf Stream

Authors

Hyun-Sook Kim, University of Rhode Island
D. Randolph Watts, University of Rhode IslandFollow

Document Type

Article

Date of Original Version

1992

Department

Oceanography

Abstract

Contours of the main thermocline (12°C isotherm depth: Z₁₂) topography objectively generated from Inverted Echo Sounder observations in the Gulf Stream may be treated as a baroclinic, geostrophic streamfunction ψ. As preliminary steps, the authors developed techniques to estimate geostrophic velocity V_ψ = k×(g^*/f)∇Z₁₂ and geostrophic vorticity ζ_ψ =(g^*/f)∇²Z₁₂. In doing this the authors also determined a reduced gravity g^* = 1.53 cm s⁻² by least-squares fitting the estimated V_ψ to observed V_bc, velocities (i.e., fine-tuned for the ψ at 400 m relative to 3000 m).

Accuracy of the objective maps of ψ is investigated by comparing V_ψ, against V_bc directly measured from tall current meter moorings and ζ_ψ against vorticity ζ_p separately estimated using a “rigid-stream” velocity section and path curvature. The V_ψ, values, with speeds up to 100 cm s⁻¹, are correlated with V_bc at the 99% confidence level, and the two measurements differ by only 10 cm s⁻¹ rms error. Vorticity estimates, ranging between −0.44f and +0.64f, also show excellent agreement between ζ_ψ and ζ_p within 0.77 × 10⁻⁵ s⁻¹ (or 0.09f) rms difference. This study carefully documents the objective error-estimation techniques for these fields that, bemuse they are derivatives of the measured ψ field, are sensitive to noise. The objective estimates of error in V_ψ and ζ_ψ agree in each case with the rms differences from observations.

The authors also illustrate the consequent utility of the objective ψ maps by applying quasigeostrophic calculations to diagnose ageostrophic motions V_a, in strong events in the Gulf Stream. The authors found V_a/V_ψ to have peak values of 0.2–0.6. Roughly equal contributions to V_a came in events of large curvature or with high temporal rate of exchange. Vertical stretching ∂w/∂z at the 400-m level was estimated using the quasigeostrophic vorticity equation, finding downwelling as large as −4 mm s⁻¹ downstream of meander crests, and upwelling as large as 3 mm s⁻¹ downstream of meander troughs.

Citation/Publisher Attribution

Kim, H-S., & Watts, D. R. (1994). An Observational Streamfunction in the Gulf Stream. . Phys. Oceanogr., 24(12), 2639-2657. doi: 10.1175/1520-0485(1994)0242.0.CO;2

Available at: http://dx.doi.org/10.1175/1520-0485(1994)0242.0.CO;2