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Patterns of Indian Ocean sea-level change in a warming climate


Global sea level has risen during the past decades as a result of thermal expansion of the warming ocean and freshwater addition from melting continental ice1. However, sea-level rise is not globally uniform1,2,3,4,5. Regional sea levels can be affected by changes in atmospheric or oceanic circulation. As long-term observational records are scarce, regional changes in sea level in the Indian Ocean are poorly constrained. Yet estimates of future sea-level changes are essential for effective risk assessment2. Here we combine in situ and satellite observations of Indian Ocean sea level with climate-model simulations, to identify a distinct spatial pattern of sea-level rise since the 1960s. We find that sea level has decreased substantially in the south tropical Indian Ocean whereas it has increased elsewhere. This pattern is driven by changing surface winds associated with a combined invigoration of the Indian Ocean Hadley and Walker cells, patterns of atmospheric overturning circulation in the north–south and east–west direction, respectively, which is partly attributable to rising levels of atmospheric greenhouse gases. We conclude that—if ongoing anthropogenic warming dominates natural variability—the pattern we detected is likely to persist and to increase the environmental stress on some coasts and islands in the Indian Ocean.

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Figure 1: Tide-gauge-observed and HYCOM-simulated annual mean sea level anomalies (SLAs) and their Kendall Theil trends26 during 1961–2008.
Figure 2: Time series of annual mean SLA from HYCOM during 1961–2008 and from satellite-observed sea level for 1993–2008.
Figure 3: Kendall Theil trends of surface wind stress, Ekman pumping velocity and time series of SST.
Figure 4: A schematic diagram showing the mechanisms for the Indo-Pacific warm-pool warming to cause the Indian Ocean sea-level change.


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This work was completed when W.H. was on sabbatical leave at NCAR. She thanks CCR/NCAR for providing partial summer salary support. Thanks also go to K. Trenberth, A. Timmermann, C. Deser and A. Dai for their insightful comments and stimulating discussions. W.H., J-W.W. and L.L.T. are supported by NSF CAREER award OCE 0847605 and NASA OSTST award NNX08AR62G. Portions of this study were supported by the Office of Science (BER), US Department of Energy, Cooperative Agreement No. DE-FC02-97ER62402, and the National Science Foundation. NCAR is sponsored by the National Science Foundation. J.T.F’s participation is sponsored by NASA Award No. NNX07AKG82G. We thank NCAR CISL for computational support.

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W.H. led the project and did the main analyses and primary HYCOM experiment, J.T.F. analysed atmospheric reanalysis products, A.H. processed the climate-model results, J-W.W. processed the tide-gauge data, X-W.Q. carried out the AGCM experiments, L.L.T. did the extended HYCOM run, A.W. helped to design and run all HYCOM experiments, S.Y. carried out the POP model experiment and G.A.M., B.R., W.G.L., J.L. and T.S. contributed to the scientific results through stimulating discussions and analyses. All authors contributed extensively to writing the paper and analysing the results.

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Correspondence to Weiqing Han.

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Han, W., Meehl, G., Rajagopalan, B. et al. Patterns of Indian Ocean sea-level change in a warming climate. Nature Geosci 3, 546–550 (2010).

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