Causes for the reversal of North Indian Ocean decadal sea level trend in recent two decades
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Using satellite and in-situ observations, ocean reanalysis products and model simulations, we show a distinct reversal of the North Indian Ocean (NIO, north of 5°S) sea level decadal trend between 1993–2003 and 2004–3013, after the global mean sea level rise is removed. Sea level falls from 1993 to 2003 (Period I) but rises sharply from 2004 to 2013 (Period II). Steric height, which is dominated by thermosteric sea level of the upper 700 m, explains most of the observed reversal, including the spatial patterns of sea level change. The decadal change of surface turbulent heat flux acts in concert with the change of meridional heat transport at 5°S, with both being driven by decadal change of surface winds over the Indian Ocean, to cause sea level fall during Period I and rise during Period II. While the effect of surface net heat flux is consistent among various data sets, the uncertainty is larger for meridional heat transport, which shows both qualitative and quantitative differences amongst different reanalyses. The effect of the Indonesian Throughflow on heat content and thus thermosteric sea level is limited to the South Indian Ocean, and has little influence on the NIO. Our new results point to the importance of surface winds in causing decadal sea level change of the NIO.
KeywordsNorth Indian Ocean Sea level Decadal change Surface winds
The authors are grateful for all the organizations and persons who made the datasets used in this research freely available. We thank two anonymous reviewers to critically go through the manuscript and provide us valuable suggestions to improve the content of the manuscript. Special thanks to Drl Jerome Vialard for his valuable suggestions to improve the manuscript. AVISO monthly sea level anomaly maps are downloaded from ftp://ftp.aviso.altimetry.fr/global/delayed-time/grids/climatology/monthly_mean. NOCS v2.0 heat flux data, Ishii and Japan reanalysis data (JRA-55) are available at http://rda.ucar.edu/datasets/. EN4_v2a objective analysis data, ORAS4 reanalysis, OAFlux flux data, wind data (ERA-Interim, NCEP and CCMP), monthly HadISST are downloaded from http://apdrc.soest.hawaii.edu/. Tropflux data is available at http://www.incois.gov.in/tropflux/. ERA-Interim evaporation and precipitation data is found at http://apps.ecmwf.int/datasets/data/interim-full-mnth/. The encouragement and facilities provided by the Director, ESSO-INCOIS are gratefully acknowledged. The authors wish to acknowledge the use of the Ferret program (NOAA) for analysis and graphics in this paper. The authors gratefully acknowledge the financial support provided by the Earth System Science Organization, Ministry of Earth Sciences, and the government of India, to conduct this research. The National Monsoon Mission Directorate award number SSC-03-002 was awarded to Weiqing Han at the University of Colorado, in collaboration with ESSO-INCOIS. Weiqing Han is also partly supported by NSF AGS 1446480 and NASA OVWST NNX14AM68G. This is ESSO-INCOIS contribution No. 0275.
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