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

[摘要] In this paper we examine time-series measurements of near-surface chlorophyllconcentration from a mooring that was deployed at 80.5°E on theequator in the Indian Ocean in 2010. These data reveal at least six strikingspikes in chlorophyll from October through December, at approximately 2-week intervals, that coincide with the development of the fall Wyrtki jetsduring the transition between the summer and winter monsoons. Concurrentmeteorological and in situ physical measurements from the mooring reveal thatthe chlorophyll pulses are associated with the intensification of eastward windsat the surface and eastward currents in the mixed layer. These observationsare inconsistent with upwelling dynamics as they occur in the Atlantic andPacific oceans, since eastward winds that force Wyrtki jet intensificationshould drive downwelling. The chlorophyll spikes could be explained by twoalternative mechanisms: (1) turbulent entrainment of nutrients and/orchlorophyll from across the base of the mixed layer by wind stirring orWyrtki jet-induced shear instability or (2) enhanced southward advection ofhigh chlorophyll concentrations into the equatorial zone. The first mechanismis supported by the phasing and amplitude of the relationship between windstress and chlorophyll, which suggests that the chlorophyll spikes are theresult of turbulent entrainment driven by synoptic zonal wind events. Thesecond mechanism is supported by the observation of eastward flows over theChagos–Laccadive Ridge, generating high chlorophyll to the north of theequator. Occasional southward advection can then produce the chlorophyllspikes that are observed in the mooring record. Wind-forced biweekly mixedRossby gravity waves are a ubiquitous feature of the ocean circulation inthis region, and we examine the possibility that they may play a role inchlorophyll variability. Statistical analyses and results from the OFAM3 (Ocean Forecasting Australia Model, version 3) eddy-resolving model provide support for both mechanisms. However, the modeldoes not reproduce the observed spikes in chlorophyll. Climatologicalsatellite chlorophyll data show that the elevated chlorophyll concentrationsin this region are consistently observed year after year and so arereflective of recurring large-scale wind- and circulation-induced productivityenhancement in the central equatorial Indian Ocean.

[学科分类] 地球化学与岩石