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Observed variability in the upper layers at the Equator, 90°E in the Indian Ocean during 2001–2008, 1: zonal currents

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Abstract

The observed variability of zonal currents (ZC) at the Equator, 90°E shows a strong seasonal cycle in the near-surface 40–350 m water column with periodic east–west reversals most pronounced at semiannual frequency. Superposed on this, a strong intraseasonal variability of 30–90 day periodicity is also prominently seen in the near-surface layer (40–80 m) almost throughout the year with the only exception of February–March. An eastward flowing equatorial undercurrent (EUC) is present in the depth range of 80–160 m during March–April and October–November. The observed intraseasonal variability in the near-surface layer is primarily determined by the equatorial zonal westerly wind bursts (WWBs) through local frictional coupling between the zonal flow in the surface layer and surface zonal winds and shows large interannual variability. The eastward flowing EUC maintained by the ZPG set up by the east–west slope of the thermocline remotely controlled by the zonal wind (ZW) and zonally propagating wave fields also shows significant interannual variability. This observed variability on interannual time scales appears to be controlled by the corresponding variability in the alongshore winds off the Somalia coast during the preceding boreal winter, the ZW field along the equator, and the associated zonally propagating Kelvin and Rossby waves. The salinity induced vertical stratification observed in the near-surface layer through barrier layer thickness (BLT) effects also shows a significant influence on the ZC field on intraseasonal time scale. Interestingly, among all the 8 years (2001–2008), relatively weaker annual cycle is seen in both ZC in the 40–350 m water column and boreal spring sea surface temperature (SST) only during 2001 and 2008 along the equator caused through propagating wave dynamics.

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Acknowledgments

Highest appreciation is placed on record for the excellent compilation by several persons and organizations of all the data sets utilized in this study. The JAMSTEC is responsible for the deployment of ADCP and TRITON CTD moorings in the eastern EIO. The PMEL is responsible for the deployment of ADCP moorings in the central EIO. The authors are grateful to Dr M. S. Girishkumar for his help with the alongshore winds off the Somalia coast. The authors thank Prof D. Sengupta for bringing the OAFlux data to their attention. The Graphics are generated with GrADS. The encouragement and the facilities provided by the Director General, JAMSTEC are gratefully appreciated. This research was carried out while the first author held a Visiting Senior Scientist position at JAMSTEC with the support of JEPP (Japan Earth Observation Promotion Program) of MEXT (Ministry of Education, Culture, Sports, Science & Technology), Japan.

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  1. ICMPO, IITM, Pune, India

    R. R. Rao

  2. JAMSTEC, Yokosuka, Japan

    T. Horii, Y. Masumoto & K. Mizuno

  3. University of Tokyo, Tokyo, Japan

    Y. Masumoto

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  1. R. R. Rao
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  2. T. Horii
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  3. Y. Masumoto
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  4. K. Mizuno
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Correspondence to R. R. Rao.

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This paper is a contribution to the special issue on Ocean estimation from an ensemble of global ocean reanalyses, consisting of papers from the Ocean Reanalyses Intercomparsion Project (ORAIP), coordinated by CLIVAR-GSOP and GODAE OceanView. The special issue also contains specific studies using single reanalysis systems.

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Rao, R.R., Horii, T., Masumoto, Y. et al. Observed variability in the upper layers at the Equator, 90°E in the Indian Ocean during 2001–2008, 1: zonal currents. Clim Dyn 49, 1077–1105 (2017). https://doi.org/10.1007/s00382-016-3234-0

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