New paper finds sea level trends are biased by natural ocean oscillations

A new paper published in Geophysical Research Letters finds that much of the observed variation in Pacific Ocean sea levels is explained by natural ocean oscillations such as the Pacific Decadal Oscillation [PDO] and El Nino Southern Oscillation [ENSO]. The authors caution that sea level trends over the short period observed by satellites [less than 20 years] can be erroneously biased by this natural variability.

Top graph shows rapid sea level changes in the Western Pacific and little to no sea level change in the Eastern Pacific since 1993 are largely due to natural variability from ocean oscillations [ENSO and PDO]

GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L21701, 8 PP., 2012

doi:10.1029/2012GL053240

Sea level trends, interannual and decadal variability in the Pacific Ocean

Key Points

• Sea level linear trend over short period is complicated by climate variability
• We separate interannual and decadal sea level variability from trend in Pacific
• Decadal sea level [natural] variability can be erroneously aliased into sea level trend

Xuebin Zhang

Centre for Australian Weather and Climate Research, a Partnership between CSIRO and the Bureau of Australia Meteorology, Melbourne, Victoria, Australia

CSIRO Wealth from Oceans Flagship, Hobart, Tasmania, Australia

CSIRO Marine and Atmospheric Research, Hobart, Tasmania, Australia

John A. Church

Centre for Australian Weather and Climate Research, a Partnership between CSIRO and the Bureau of Australia Meteorology, Melbourne, Victoria, Australia

CSIRO Wealth from Oceans Flagship, Hobart, Tasmania, Australia

CSIRO Marine and Atmospheric Research, Hobart, Tasmania, Australia

Linear trend analysis is commonly applied to quantify sea level change, often over short periods because of limited data availability. However, the linear trend computed over short periods is complicated by large-scale climate variability which can affect regional sea level on interannual to inter-decadal time scales. As a result, the meaning of a local linear sea level trend over the short altimeter era (since 1993; less than 20 years) is unclear, and it is not straightforward to distinguish the regional sea level changes associated with climate change from those associated with natural climate variability. In this study, we use continuous near-global altimeter measurements since 1993 to attempt to separate interannual and decadal sea level variability in the Pacific from the sea level trend. We conclude that the rapid rates of sea level rise in the western tropical Pacific found from a single variable linear regression analysis are partially due to basin-scale decadal [natural] climate variability. The negligible sea level rise, or even falling sea level, in the eastern tropical Pacific and US west coast is a result of the combination of decreasing of sea level associated with decadal [natural] climate variability and a positive sea level trend. The single variable linear regression analysis only accounts for slightly more than 20% of the observed variance, whereas a multiple variable linear regression including filtered indices of the El Nino-Southern Oscillation and the Pacific Decadal Oscillation accounts for almost 60% of the observed variance.