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

Abstract

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.

Tintinnid community structure in the eastern equatorial Indian Ocean during the spring inter‑monsoon period

“…There is no equatorial upwelling in the equatorial Indian Ocean, which is considered a typical tropical ultraoligotrophic ocean and is relatively far from coastal influences (Schott et al 2002). The eastward-propagating spring Wyrtki jets are strongest from April to May (Murtugudde et al 2000); the main biogeochemical impact of the jets is a depression of the thermocline and nitracline on the eastern side of the basin and a decrease in primary production in May (Wiggert et al 2006, Strutton et al 2015.…”

Biogeochemical and ecological impacts of boundary currents in the Indian Ocean

“…Satellite SST and chlorophyll images reveal dramatic eastward advection of cool (< 28° C) chlorophyll rich upwelled water by the Southwest Monsoon Current (Vinayachandran, 2004;de Vos et al, 2014; Figure 12). Chlorophyll-rich waters from the southwestern coast of India are also advected by the Southwest Monsoon Current towards Sri Lanka during the SWM (Vinayachandran, 2004;Strutton et al, 2015). Satellite-derived surface chlorophyll concentrations along the southern coast of Sri Lanka can exceed 10 mgChla m -3 in July/August during the SWM, compared to much lower concentrations in January during the NEM when the Northeast Monsoon Current flows westward (de Vos et al, 2004; Figure 5).…”

The role of wind gusts in upper ocean diurnal variability

“…GOTM also estimates downward irradiance at different ocean depths, I ( z ), following Paulson and Simpson [], i.e., , with I 0 the solar radiation. In our simulations, A = 0.58, , and , consistent with water type I as classified by Jerlov [] and with clear water conditions in the region [ Li et al ., ; Matthews et al ., ; Strutton et al ., ]. Clear water conditions were observed during the period of interest by a glider at 80°E, between 3°S and 4°S [ Webber et al ., , Figure c], with the mean 0–10 m chlorophyll concentration less than 1 mg m −3 .…”