Comparing Global Sea Level Rise Estimates from Satellite Altimetry and a Global Ocean Reanalysis: 1993-2001
Laury Miller(1) and Bruce C. Douglas(2)
Lab for Satellite Altimetry,
Silver Spring, MD,
(2) Florida International University, University Park, Miami, FL, United States
Satellite altimeter observations show that global mean sea level has been rising over the past decade at a rate of about 3 mm/yr, well above the centennial rate of 1.8 mm/yr. This has been occurring despite the presence of large geographical variations, including broad areas of falling sea level. Here we investigate the global and regional nature of this signal by comparing satellite altimeter measurements of sea level change between 1993 and 2001 with estimates of the steric component of sea level change for the same period based on the SODA 1.2 reanalysis of global temperature and salinity (Carton et al., 2005).
A map comparison of the two trend data sets shows broad geographical similarities, including high positive rates (>10 mm/yr) throughout much of the western Pacific and eastern Indian Oceans, negatives in the eastern tropical Pacific, and positives in the North Atlantic. Surprisingly, the reanalysis rates tend to have higher absolute values than the altimeter rates, particularly in the tropical Pacific. Analyzing the data sets in three zonal bands (66N to 30N, 30N to 30S, 30S to 66S) reveals distinct latitudinal differences. The northern and equatorial bands exhibit roughly similar average altimeter rates of sea level rise, at 2.5 and 2.3 mm/yr, respectively, and similar levels of correlation (~0.7) between altimeter trends and reanalysis trends on a local (grid point) basis. The southern band shows the highest average altimeter rate, at 3.9 mm/yr, suggesting that much of the increase between the centennial global rate determined from tide gauges and the 1993-2001 global altimeter-derived rate is due to rapid changes in the Southern Ocean. However, a local comparison shows that the reanalysis trends are poorly correlated with the altimeter trends in this band, making it difficult to distinguish between steric and eustatic contributions in the band of greatest sea level rise. The poor correlation between the two data sets is probably due the lack of in-situ hydrographic observations in the Southern Ocean, a situation which no longer exists because of the advent of the Argo program, coincidently in 2001.