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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