On the use of tide gauges to determine altimeter drift

TOPEX measurements of sea level variability have been compared to tide gauge measurements from 40 sites and to dynamic topography measurements computed from temperatures recorded at 23 Tropical Ocean-Global Atmosphere (TOGA)-Tropical Atmosphere-Ocean (TAO) buoys in the eastern Pacific and mean temperature-salinity profiles. Buoy data in the western Pacific were not used because of large long-term slopes in the data that appear to be due to interannual salinity variations. The relative drift between TOPEX and the two different in situ sets of data agree within 1 mm yr -1 , with a weighted average of -2.6 mm yr -1 and an estimated uncertainty of 1.5 mm yr -1 , if values from an internal calibration of the TOPEX altimeter are applied. The consistency of the two relative drifts suggests that the slope is due at least in part to a drift in the TOPEX measurement. A substantial portion of this drift may be due to a drift in the TOPEX microwave radiometer (TMR), since comparisons with three independent external measurements indicate a drift in sea level due to the TMR measurement of about -2 mm yr -1 .