Frequency periodograms of altimetric sea level and their geographical variation from 2 years of TOPEX/POSEIDON data

Abstract

Frequency periodograms are estimated from 2 years of TOPEX/POSEIDON altimetric sea levels at crossovers (XOs), where the height measurements are twice as abundant as at non‐XO points. It is shown that the extra set of height measurements is best utilized to reduce aliasing rather 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 Circumpolar Current and northeastern equatorial Pacific. All spectra are red and (with few exceptions) have a strong annual peak. Strong annual peaks are present for the northern hemisphere western boundary currents, where enhanced air‐sea interaction results in large annual heating/cooling 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 peak is less pronounced in the southern hemisphere western boundary currents, presumably because of smaller annual heating/cooling effects. Detrending 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 have been treated with orbit and tide error reduction schemes. The frequency spectrum of the orbit correction is white (as expected) and proves that the orbit error reduction scheme utilized here does not attenuate the ocean signal.