Biogeochemical variability in the central equatorial Indian Ocean during the monsoon transition
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 inte...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2015-04-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/12/2367/2015/bg-12-2367-2015.pdf |
| _version_ | 1819105266294063104 |
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| author | P. G. Strutton V. J. Coles R. R. Hood R. J. Matear M. J. McPhaden H. E. Phillips |
| author_facet | P. G. Strutton V. J. Coles R. R. Hood R. J. Matear M. J. McPhaden H. E. Phillips |
| author_sort | P. G. Strutton |
| collection | DOAJ |
| description | 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. |
| first_indexed | 2024-12-22T02:19:31Z |
| format | Article |
| id | doaj.art-2dc32292b1b24321a5faf1540cc1ad15 |
| institution | Directory Open Access Journal |
| issn | 1726-4170 1726-4189 |
| language | English |
| last_indexed | 2024-12-22T02:19:31Z |
| publishDate | 2015-04-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Biogeosciences |
| spelling | doaj.art-2dc32292b1b24321a5faf1540cc1ad152022-12-21T18:42:10ZengCopernicus PublicationsBiogeosciences1726-41701726-41892015-04-011282367238210.5194/bg-12-2367-2015Biogeochemical variability in the central equatorial Indian Ocean during the monsoon transitionP. G. Strutton0V. J. Coles1R. R. Hood2R. J. Matear3M. J. McPhaden4H. E. Phillips5Institute for Marine and Antarctic Studies, University of Tasmania, and Australian Research Council Centre of Excellence for Climate System Science, Hobart, AustraliaHorn Point Laboratory, University of Maryland Center for Environmental Sciences, Cambridge, USAHorn Point Laboratory, University of Maryland Center for Environmental Sciences, Cambridge, USACommonwealth Scientific and Industrial Research Organisation, Marine and Atmospheric Research, Hobart, AustraliaNational Oceanic and Atmospheric Administration, Pacific Marine Environmental Laboratory, Seattle, WA, USAInstitute for Marine and Antarctic Studies, University of Tasmania, and Australian Research Council Centre of Excellence for Climate System Science, Hobart, AustraliaIn 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.http://www.biogeosciences.net/12/2367/2015/bg-12-2367-2015.pdf |
| spellingShingle | P. G. Strutton V. J. Coles R. R. Hood R. J. Matear M. J. McPhaden H. E. Phillips Biogeochemical variability in the central equatorial Indian Ocean during the monsoon transition Biogeosciences |
| title | Biogeochemical variability in the central equatorial Indian Ocean during the monsoon transition |
| title_full | Biogeochemical variability in the central equatorial Indian Ocean during the monsoon transition |
| title_fullStr | Biogeochemical variability in the central equatorial Indian Ocean during the monsoon transition |
| title_full_unstemmed | Biogeochemical variability in the central equatorial Indian Ocean during the monsoon transition |
| title_short | Biogeochemical variability in the central equatorial Indian Ocean during the monsoon transition |
| title_sort | biogeochemical variability in the central equatorial indian ocean during the monsoon transition |
| url | http://www.biogeosciences.net/12/2367/2015/bg-12-2367-2015.pdf |
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