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Ocean Surface Topography from Space
The Climate Data Record of Sea Level Change: Influence of Decadal Variability


Robert Nerem - (University of Colorado)

  John Fasullo
Dallas Masters
Mark Merrifield
Gary Mitchum
(Univ. Corp. for Atmospheric Research, National Center for Atmospheric Research)
(University of Colorado, Aerospace Engineering Sciences)
(School of Ocean and Earth Science and Technology (SOEST))
(University of South Florida, College of Marine Science)


Global mean sea level, as observed by TOPEX/Poseidon, Jason-1, and Jason-2, has risen at a rate of 3.1 mm/year over the last 20 years, which is nearly double the rate observed over the previous 50 years by tide gauge sea level reconstructions. It has also been observed that the rate of sea level rise was greater during the first decade of the altimeter era (3.5 mm/year) versus the second decade (2.4 mm/year). Superimposed on the overall increase, are substantial interannual variations correlated with ENSO events have also been observed. These are understood to be related to changes in land/ocean precipitation patterns during El Niño and La Niña events. One feature of this interannual variability is that sea level tends to be higher during El Niño events (as more precipitation falls over the oceans) and lower during La Niña events (more precipitation over the land). Recent findings of ours suggest strongly that a similar phenomena may occur with the warm/cold phases of the PDO and this proposal is intended to explore this hypothesis.
In a regional context, sea level trends have increased in the western tropical Pacific Ocean during the altimeter era at rates that are more than three times the global average. The best information from the tide gauge network indicates that prior to this time, sea level trends in the western tropical Pacific were below the global average. The recent changes have been shown to be related to an intensification of the trade winds in the tropical Pacific, which we suggest are also related to the Pacific Decadal Oscillation (PDO).
Together, these findings lead us to question how to interpret the altimeter sea level record since 1993. Clearly, the record spans an unusual period, as it starts just after the speedup of the trade winds and the resulting multidecadal change in the state of the tropical Pacific ocean. The unusual pattern of sea level change has a global average that increases at 3.1 mm/year, but with substantial ENSO-related interannual variability, and we argue PDO decadal variability as well. If the multidecadal changes in the trade winds are accompanied by shifting land-ocean precipitation patterns, what is the impact on global mean sea level variability during the altimeter era, and even the 20th century as a whole?
The important questions for the proposed investigation are:
  • Do changing precipitation patterns associated with the PDO lead to a mass transfer from the ocean to land, analogous to interannual ENSO events?
  • Are decadal precipitation changes large enough to impact global sea level?
  • Are shifts in the PDO masking global sea level trends, and specifically, is the recent altimeter trend biased low by PDO activity?
  • How do regional sea level patterns respond to these PDO shifts?
The overall aim of this investigation is to continue to produce a publicly-available calibrated climate data record of sea level change using the available altimeter data (TOPEX/Poseidon, Jason-1, -2, and -3, etc.), while unraveling the contributions to this record in terms of interannual, decadal, and climate signals and providing an improved context for their interpretation.

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