Ck. Tai, FREQUENCY PERIODOGRAMS OF ALTIMETRIC SEA-LEVEL AND THEIR GEOGRAPHICALVARIATION FROM 2 YEARS OF TOPEX POSEIDON DATA/, J GEO RES-O, 101(C4), 1996, pp. 8933-8941
Frequency periodograms are estimated from 2 years of TOPEX/POSEIDON al
timetric sea levels at crossovers (XOs), where the height measurements
are twice as abundant than at non-XO paints. It is shown that the ext
ra set of height measurements is best utilized to reduce aliasing rath
er than to double the temporal resolution. Least squares fitting with
Fourier series handles the problem of missing data and unequal spacing
. A global average. power spectrum (area-weighted from spectra at XOs)
and average spectra from low-variability as well as high-variability
areas are obtained. In addition, regional average spectra are produced
for various western boundary currents as well as the Antarctic Circum
polar Current and northeastern equatorial Pacific. All spectra are red
and (with few exceptions) have a strong annual peak. Strong annual pe
aks are present for the northern hemisphere western boundary currents,
where enhanced air-sea interaction results in large annual heating/co
oling effects. A mechanism of winter cooling proposed by Huang [1990]
appears to be consistent with the seasonal movements and intensity of
the surface jet, augmenting the annual peak. In contrast, the annual p
eak is less pronounced in the southern hemisphere western boundary cur
rents, presumably because of smaller annual heating/cooling effects. D
etrending of the height time series before the Fourier analysis shows
the 2-year sea level trends over the global ocean. The global average
is about 4.5 mm/yr after correction for instrumental drift. The data h
ave been treated with orbit and tide error reduction schemes. The freq
uency spectrum of the orbit correction is white.(as expected) and prov
es that the orbit error reduction scheme utilized here does not attenu
ate the ocean signal.