A Comparison of Finestructure Spectra from the Main Thermocline

Abstract

Large numbers of STD casts taken during five cruises to the same location in the central North Pacific permitted the separation of the temperature profile into mean T¯ (z) and fluctuating T′(z) components. Spectra of the T′ (z) records were ensemble-averaged by cruise and compared with similar spectra from one cruise to the South Pacific and with those from the MODE site. Two wavenumber ranges with power law behavior were found: for 0.002 cpm <k<(0.06−0.1) cpm the spectra fall with slopes slightly less than k⁻², for 0.1 cpm<kk⁻³. Spectra in the low k range were consistent with formation of T′(z) by internal waves. The break in slope at 0.1 cpm suggests that either the internal wave dynamics change at that scale or the smaller fluctuations are true finestructure. The spectral levels remained the same at the North Pacific location suggesting that the internal wave energy levels are stationary in time at that location. However, there was a factor of 10 variation of the levels at the three different sites. The vertical displacement of an isotherm about its mean depth was found to have a normal distribution, consistent with the assumption that internal waves are a Gaussian process. The average number of zero crossings of the displacement between 200 and 712 m varied between 3.5 to 5.