Measuring very low frequency sea level variations using satellite altimeter data

• Published in: Global and planetary change 1999-04, Vol.20 (2), p.157-171
• Main Author: Nerem, R.S
• Format: Article
• Language: English
• Subjects: Satellite altimeter data; Sea level variations; Very low frequency
• ISSN: 0921-8181; 1872-6364
• DOI: 10.1016/S0921-8181(98)00068-X

Very low frequency (VLF) sea level variations are an important indicator of global climate change, and their measurement can provide important information for determining the socioeconomic impact of sea level change on coastal land use. The prospect of measuring VLF sea level variations has been assessed using approximately 5 years of satellite altimeter data from the TOPEX/POSEIDON (T/P) mission, where synoptic mapping of the geocentric height of the ocean surface is routinely achieved with a point-to-point accuracy of better than 4 cm. The global mean sea level variations measured by T/P every 10 days have an RMS of 4 mm and a rate of change +3.1±1.3 mm/year, after accounting for an average instrument drift computed using the global tide gauge network by Mitchum (1998). A likely cause of the observed instrument drift is the microwave radiometer, which provides the water vapor delay correction, which results in a revised estimate of +2.5 mm/year if the error is assumed to be linearly related to the mean water vapor delay. Approximately half of this rise appears related to an increase in sea level that began in mid-1996, thus it is unlikely to be sustained over the long term. Estimates of sea level change over the major ocean basins reveal that the North Atlantic has risen over the T/P mission, and this is believed to be related to decadal changes in heat storage. Maps of the geographic variability of the observed sea level trends are currently dominated by ENSO variations, and thus the climate change signals cannot currently be isolated. These results suggest that T/P, when combined with tide gauge monitoring of the satellite instruments, is achieving the necessary accuracy to distinguish sea level rise caused by climate change from the natural `background' rate of sea level rise, although a longer time series is necessary to average out possible interannual and decadal variations. A longer time series will also reduce the errors in estimates of the altimeter calibration, providing an important constraint on any long-term instrument drift. Future research will focus on establishing a more realistic error budget for these measurements of global mean sea level, so that they can be put in the proper context with other observations of global climate change. In addition, the study of the spatial variability of the sea level rise signal will become increasingly important as a longer time series is collected.