Contribution of the pacific decadal oscillation to global mean sea level trends
B.D. Hamlington et al
Understanding and explaining the trend in GMSL has important implications for future projections of sea level rise. While measurements from satellite altimetry have provided accurate estimates of GMSL, the modern altimetry record has only now reached twenty years in length, making it difficult to assess the contribution of decadal to multi-decadal climate signals to the global trend. Here, we use a sea level reconstruction to study the twenty-year trends in sea level since 1950. In particular, we show that the Pacific Decadal Oscillation (PDO) contributes significantly to the twenty-year trends in GMSL. We estimate the PDO contribution to the GMSL trend over the past twenty years to be approximately 0.49 ± 0.25 mm/year, and find that removing the PDO contribution reduces the acceleration in GMSL estimated over the past sixty years.
"Over the last decade," in the words of Chambers et al. (2012), "numerous papers have commented on the appearance of decadal and longer period fluctuations in select tide gauge records (e.g., Feng et al., 2004; Miller and Douglas, 2007; Woodworth et al., 2009; Sturges and Douglas, 2011)," and they say that "multi-decadal fluctuations also appear in reconstructions of global mean sea level (GMSL) that are computed from tide gauge records, using quite different techniques (Holgate, 2007; Jevrejeva et al., 2008; Merrifield et al., 2009; Wenzel and Schroter, 2010; Church and White, 2011; Ray and Douglas, 2011)." And in their own study of long tide gauge records in every ocean basin, Chambers et al. find that there is, indeed, "a significant oscillation with a period around 60-years in the majority of the tide gauges examined during the 20th century."
Why is this finding so important?
First of all, the three researchers note that "an upturn in GMSL rise due to a 60-year oscillation with a minimum between 1980 and 1990 is consistent with the increased GMSL trend obtained from satellite altimetry (e.g., Nerem et al., 2010) and reconstructions since 1993." This fact, as they continue, "does not change the overall conclusion that sea level has been rising on average by 1.7 mm/year over the last 110 years." However, they rightly state that the 60-year oscillation does change "our interpretation of the trends when estimated over periods less than one-cycle of the oscillation." And, therefore, they conclude that "although several studies have suggested the recent change in trends of global (e.g., Merrifield et al., 2009) or regional (e.g., Sallenger et al., 2012) sea level rise reflects an acceleration, this must be re-examined in light of a possible 60-year oscillation [italics and bold added]," in further support of which contention they note that "there have been previous periods where the rate was decelerating, and rates along the Northeast U.S. coast have what appears to be a 60-year-period (Sallenger et al., 2012)," which they also indicate "is consistent with our observations of sea level variability at New York City and Baltimore."
As a final bit of advice in light of the results of their analysis, Chambers et al. prudently state that "one should be cautious about computations of acceleration in sea level records unless they are longer than two cycles of the oscillation," noting that this advice "applies to interpretation of acceleration in GMSL using only the 20-year record of satellite altimetry and to evaluations of short records of mean sea level from individual gauges."
Consequently, for those who buy into the storyline of these latter approaches, we can only say Buyer, Beware!
Sherwood, Keith and Craig Idso
References
Chambers, D.P, Merrifield, M.A. and Nerem, R.S. 2012. Is there a 60-year oscillation in global mean sea level?Geophysical Research Letters 39: 10.1029/2012GL052885.
Church, J.A. and White, N.J. 2011. Sea-level rise from the late 19th to the early 21st century. Surveys in Geophysics 32: 585-602.
Feng, M., Li, Y. and Meyers, G. 2004. Multidecadal variations of Fremantle sea level: Footprint of climate variability in the tropical Pacific. Geophysical Research Letters 31: 10.1029/2004GL019947.
Holgate, S. 2007. On the decadal rates of sea level change during the twentieth century. Geophysical Research Letters34: 10.1029/2006GL028492.
Jevrejeva, S., Moore, J.C., Grinsted, A. and Woodworth, P.L. 2008. Recent global sea level acceleration started over 200 years ago? Geophysical Research Letters 35: 10.1029/2008GL033611.
Merrifield, M.A., Merrifield, S.T. and Mitchum, G.T. 2009. An anomalous recent acceleration of global sea level rise.Journal of Climate 22: 5772-5781.
Miller, L. and Douglas, B.C. 2007. Gyre-scale atmospheric pressure variations and their relation to 19th and 20th century sea level rise. Geophysical Research Letters 34: 10.1029/2007GL030862.
Nerem, R.S., Chambers, D.P., Choe, C. and Mitchum, G.T. 2010. Estimating mean sea level change from the TOPEX and Jason altimeter missions. Marine Geodesy 33, Supplement 1: 435-446.
Ray, R.D. and Douglas, B.C. 2011. Experiments n reconstructing twentieth-century sea levels. Progress in Oceanography91: 496-515.
Sallenger, A.H., Doran, K.S. and Howd, P.A. 2012. Hotspot of accelerated sea level rise on the Atlantic coast of North America. Nature Climate Change: 10.1038/NCLIMATE1597, in press.
Sturges, W. and Douglas, B.C. 2011. Wind effects on estimates of sea level rise. Journal of Geophysical Research 116: 10.1029/2010JC006492.
Wenzel, M. and Schroter, J. 2010. Reconstruction of regional mean sea level anomalies from tide gauges using neural networks. Journal of Geophysical Research 115: 10.1029/2009JC005630.
Woodworth, P.L., White, N.J., Jevrejeva, S., Holgate, S.J., Church, J.A. and Gehrels, W.R. 2009. Evidence for the accelerations of sea level on multi-decade and century timescales. International Journal of Climatology 29: 777-789.
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