Kessler, Spillane, McPhaden, Harrison 1995 Abstract

Scales of variability in the equatorial Pacific inferred from the Tropical Atmosphere- Ocean (TAO) buoy array

Kessler, W.S., M.C. Spillane, M.J. McPhaden and D.E. Harrison, 1996

Journal of Climate, 9, 2999-3024.

The highly temporally-resolved time series from the TAO moored buoy array are used to evaluate the scales of thermal variability in the upper equatorial Pacific. The TAO array consists of nearly 70 deep-ocean moorings arranged nominally 15 degrees longitude and 2 degrees latitude apart across the equatorial Pacific. The bulk of the data from the array consists of daily averages telemetered in real time, with some records up to 15 years long. However, at several sites more finely resolved data exist, in some cases with resolution of 1 minute. These data form the basis for spectral decomposition spanning virtually all scales of variability from the Brunt-Vaisala frequency to the El Nino-Southern Oscillation timescale. The spectra are used to define the signal to noise ratio as a function of sample rate and frequency, and to investigate the effects of aliasing that results from sparser sampling such as ship-based observational techniques. The results show that the signal to noise ratio is larger in the east, mostly because the low-frequency signals are larger there. The noise level for SST varies by as much as a factor of ten among the locations studied, while noise in thermocline depth is relatively more homogeneous over the region. In general, noise due to aliased high-frequency variability increases by roughly a factor of ten as the sample rate decreases from daily to 100-day sampling. Time scales were estimated in frequency bands by integration of the autocorrelation functions. For variability with periods between daily and about 150 days, the time scale is about 5 days for both surface and subsurface temperatures everywhere in the region. Horizontal space scales were estimated from crossªcorrelations among the buoys. Zonal scales of low-frequency SST variations along the equator were half the width of the Pacific, larger than those of thermocline depth (about 30-40 degrees longitude). In the east, meridional scales of low-frequency SST were large (greater than about 15 degrees latitude), associated with the coherent waxing and waning of the equatorial cold tongue, whereas in the west these scales were shorter. Thermocline depth variations had meridional scales associated with the equatorial waves, particularly in the east. Spatial scale estimates reported here are generally consistent with those found from the ship of opportunity XBT data sets when the ENSO signals in the records of each data set are taken into account. However, if signals with periods of one to two months are to be properly sampled, then sampling scales of 1-2 degrees latitude by 8-10 degrees longitude, with a 5-day time scale, are needed.

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