Wavenumber Spectrum in the Gulf Stream from Shipboard ADCP Observations and Comparison with Altimetry Measurements

H. Thomas Rossby,Dong-Ping Wang,Kathleen Donohue,Charles N. Flagg

doi:10.1175/2009jpo4330.1

Abstract

Abstract The wavenumber spectra for velocity and temperature in the Gulf Stream region are calculated from a decade (1994–2004) of shipboard acoustic Doppler current profiler (ADCP) measurements taken as part of the Oleander Project. The velocity and temperature spectra have comparable magnitude, in terms of the kinetic and potential energy, and both indicate a k−3 slope in the mesoscales. In contrast, the corresponding velocity spectrum determined from satellite altimetry sea surface heights yields a significantly higher energy level and a k−2 slope. The discrepancy between altimeter-derived and directly measured velocity spectra suggests that altimetric velocity probably is contaminated by noise in sea surface height measurement. Also, the k−3 slope, which appears to be in agreement with two-dimensional quasigeostrophic turbulence theory, does not support the contemporary surface quasigeostrophic theory. These results highlight large gaps in the current understanding of the nature of surface geostrophic turbulence.

Highlights

Mesoscale (10-100 km) eddies are dominant motions in the upper ocean
In this study we focus on the open ocean to be compatible with altimetry measurements
In QG theory, temperature variations are supposed to be located at the thermocline

Summary

Introduction

Mesoscale (10-100 km) eddies are dominant motions in the upper ocean They arise primarily from baroclinic instability and scale to the baroclinic deformation radius (~ 30 km in mid-latitudes). In three-dimensional homogeneous isotropic turbulence, energy cascades to smaller scales following the Kolomogroff k-5/3 law. The QG turbulence theory has been verified for the atmospheric synoptic scales using wavenumber spectra determined from the global reanalyses and direct aircraft measurements (Nastrom and Gage, 1985). The oceanographic observations, on the other hand, generally lack adequate spatial and temporal coverage, and velocity wavenumber spectra have been derived only indirectly from altimetric sea surface height (SSH) measurements. The corresponding surface slope (geostrophic velocity) spectrum of k-2.6 appears to be consistent with the k-3 QG turbulence. This is because the altimetric velocity estimate can be significantly impacted by noise in surface height measurements.

Analysis

Findings

Discussion