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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 12, issue 8 | Copyright

Special issue: Current biogeochemical and ecosystem research in the Northern...

Biogeosciences, 12, 2367-2382, 2015
https://doi.org/10.5194/bg-12-2367-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 23 Apr 2015

Research article | 23 Apr 2015

Biogeochemical variability in the central equatorial Indian Ocean during the monsoon transition

P. G. Strutton1, V. J. Coles2, R. R. Hood2, R. J. Matear3, M. J. McPhaden4, and H. E. Phillips1 P. G. Strutton et al.
  • 1Institute for Marine and Antarctic Studies, University of Tasmania, and Australian Research Council Centre of Excellence for Climate System Science, Hobart, Australia
  • 2Horn Point Laboratory, University of Maryland Center for Environmental Sciences, Cambridge, USA
  • 3Commonwealth Scientific and Industrial Research Organisation, Marine and Atmospheric Research, Hobart, Australia
  • 4National Oceanic and Atmospheric Administration, Pacific Marine Environmental Laboratory, Seattle, WA, USA

Abstract. In this paper we examine time-series measurements of near-surface chlorophyll concentration from a mooring that was deployed at 80.5°E on the equator in the Indian Ocean in 2010. These data reveal at least six striking spikes in chlorophyll from October through December, at approximately 2-week intervals, that coincide with the development of the fall Wyrtki jets during the transition between the summer and winter monsoons. Concurrent meteorological and in situ physical measurements from the mooring reveal that the chlorophyll pulses are associated with the intensification of eastward winds at the surface and eastward currents in the mixed layer. These observations are inconsistent with upwelling dynamics as they occur in the Atlantic and Pacific oceans, since eastward winds that force Wyrtki jet intensification should drive downwelling. The chlorophyll spikes could be explained by two alternative mechanisms: (1) turbulent entrainment of nutrients and/or chlorophyll from across the base of the mixed layer by wind stirring or Wyrtki jet-induced shear instability or (2) enhanced southward advection of high chlorophyll concentrations into the equatorial zone. The first mechanism is supported by the phasing and amplitude of the relationship between wind stress and chlorophyll, which suggests that the chlorophyll spikes are the result of turbulent entrainment driven by synoptic zonal wind events. The second mechanism is supported by the observation of eastward flows over the Chagos–Laccadive Ridge, generating high chlorophyll to the north of the equator. Occasional southward advection can then produce the chlorophyll spikes that are observed in the mooring record. Wind-forced biweekly mixed Rossby gravity waves are a ubiquitous feature of the ocean circulation in this region, and we examine the possibility that they may play a role in chlorophyll variability. Statistical analyses and results from the OFAM3 (Ocean Forecasting Australia Model, version 3) eddy-resolving model provide support for both mechanisms. However, the model does not reproduce the observed spikes in chlorophyll. Climatological satellite chlorophyll data show that the elevated chlorophyll concentrations in this region are consistently observed year after year and so are reflective of recurring large-scale wind- and circulation-induced productivity enhancement in the central equatorial Indian Ocean.

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In 2010, a first-of-its-kind deployment of biological sensors on a mooring in the central Indian Ocean revealed interesting variability in chlorophyll (a proxy for ocean productivity) at timescales of about 2 weeks. Using the mooring data with satellite observations and a biogeochemical model, it was determined that local wind mixing and entrainment, rather than mixed Rossby gravity waves, were likely responsible for much of the observed variability.
In 2010, a first-of-its-kind deployment of biological sensors on a mooring in the central Indian...
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