EMPIRICAL ORTHOGONAL FUNCTION-ANALYSIS OF GLOBAL TOPEX POSEIDON ALTIMETER DATA AND IMPLICATIONS FOR DETECTION OF GLOBAL SEA-LEVEL RISE/

Two years of TOPEX/POSEIDON altimeter data are examined to determine t he dominant spatial features and timescales of sea surface height vari ability in the global oceans and to estimate the rate of global sea le vel rise. Empirical orthogonal function (EOF) decomposition of 69 cycl es of TOPEX altimeter data into the significant modes of variability r eveals dominant annual and interannual timescales. The annual modes in clude the hemispheric-scale changes in steric height due to seasonal h eating variations, changes in the strength of the major current system s in the equatorial Pacific, and the reversing monsoonal circulation i n the Indian Ocean. The interannual modes capture oscillations in the tropical Pacific characteristic of recent El Nino events. A a-year his tory of the change in mean sea level derived from TOPEX altimeter data reveals a rise of 5.2 mm/yr. By analyzing the contribution of each EO F mode to global mean sea level variations, we find that 82% of the ri se in mean sea level is caused by a single interannual mode of variabi lity. Altimeter data spanning only 2 years, however, are insufficient to resolve a complete El Nino-Southern Oscillation (ENSO) cycle which dominates the interannual EOF modes. Thus most of the rise in mean sea level derived from TOPEX altimetry is an artifact of incomplete tempo ral sampling of interannual variability. When a longer time series of TOPEX altimeter data is obtained and a complete ENSO cycle is observed , a significant reduction in the rate of global mean sea level rise es timated from TOPEX altimetry is expected. Most of the remaining rise i n global mean sea level is explained by the annual EOF modes, suggesti ng a possible connection between sea level rise and changes in the ste ric component of sea surface height.