Bio‐physical coupling and ocean dynamics in the central equatorial Indian Ocean during 2006 Indian Ocean Dipole

Kumar, S. Prasanna; David, T. Divya; Byju, P.; Narvekar, Jayu; Yoneyama, Kunio; Nakatani, Naoki; Ishida, Akio; Horii, Takanori; Masumoto, Yukio; Mizuno, Keisuke

doi:10.1029/2012gl052609

Cited by 12 publications

(6 citation statements)

References 14 publications

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“…The annual cycle of sea surface temperature is dominated by solar heating in the summer of each hemisphere, and by cooling through evaporative heat loss driven by monsoon winds, and by upwelling along the western side of the basin during the southwest (summer) monsoon (Rao and Sivakumar, 2000). The mean distribution of sea surface salinity is characterized by salty waters in regions dominated by evaporation: the southern hemisphere subtropics, Arabian Sea and Persian Gulf, and fresh waters in the Bay of Bengal and around Indonesia, where precipitation exceeds evaporation and river runoff is significant (Schott and McCreary, 2001).…”

Section: Spatial Contextmentioning

confidence: 99%

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

et al. 2015

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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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Section: Spatial Contextmentioning

confidence: 99%

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

et al. 2015

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“…Yanai waves (also known as mixed Rossby-gravity waves) contribute to intraseasonal variability and play a major role in driving convergent heat flux at the equator (Nagura et al, 2014;Smyth et al, 2015), upwelling (Horii et al, 2011;Kumar et al, 2012;Masumoto et al, 2008;Sengupta et al, 2004;Tsai et al, 1992) and, possibly, mixing in the abyssal ocean (Holmes et al, 2016). Although Yanai waves have been studied in the Atlantic and Pacific Oceans (Ascani et al, 2010;Bunge et al, 2007;Farrar & Durland, 2012;Lyman et al, 2007;Shinoda, 2012), we focus on the equatorial Indian Ocean, where prevalent oceanic, atmospheric, and biological patterns differ from other basins (Schott & McCreary, 2001;Schott et al, 2009).…”

Section: Introductionmentioning

confidence: 99%

Generation of Quasi‐Biweekly Yanai Waves in the Equatorial Indian Ocean

Arzeno

Giddings

Pawlak

et al. 2020

Geophysical Research Letters

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The spatial and temporal structure of quasi‐biweekly Yanai waves in the Indian Ocean and their relationship to wind stress are uniquely described using satellite observations of sea level anomalies and wind velocities, previously approximated only using theory, numerical models, and discrete mooring observations. Yanai waves represent a significant contribution to equatorial antisymmetric sea level anomaly variability in the 10–20 day band. Robust climatologies produced using 17 years of data (spanning 45–95°E and 10°S to 10°N) reveal a clear spatiotemporal pattern and are consistent with Yanai wave generation in the western equatorial Indian Ocean during monsoon seasons. Spectral and correlation analyses imply that Yanai waves are linearly forced by wind stress patterns with similar wavelengths. A new method of assembling data composites shows the first full structure of a vertical mode‐2 Yanai wave moving across the equatorial Indian Ocean under westward propagating wind vortices.

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“…3c). This biweekly periodicity in the meridional velocities with upward phase propagation indicates the presence of mixed Rossby-gravity waves (Kumar et al, 2012). These waves are present throughout the year and also at other equatorial moorings.…”

Section: Mooring Time Seriesmentioning

confidence: 79%

“…• E in late 2006 that revealed deepening of the surface layer, nitracline and subsurface chlorophyll maximum during the fall Wyrtki jet period (Kumar et al, 2012). However, these data also revealed a biweekly shoaling of the lower thermocline and the depth of the chlorophyll maximum, associated with the passage of mixed Rossby-gravity waves.…”

Section: Introductionmentioning

confidence: 94%

Biogeochemical variability in the equatorial Indian Ocean during the monsoon transition

Strutton¹,

Coles²,

Hood³

et al. 2014

Preprint

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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 in October through December, with approximately 2 week periodicity, 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 intensification of eastward winds at the surface and eastward currents in the mixed layer. These observations are inconsistent with upwelling dynamics as occurs 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 horizontal advection of high chlorophyll concentrations into the convergent 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 satellite chlorophyll observations that reveal a clear connection between the increased chlorophyll concentrations at the mooring location and larger-scale topographic wake effects from the Chagos–Lacadive Ridge upstream. The biweekly periodicity of the chlorophyll spikes appears to be related to the presence of mixed Rossby-gravity waves, also known as Yanai waves, which can be seen throughout the time-series as a biweekly periodicity in the meridional velocities with upward phase propagation. Consistent with hypothesis 2, eastward flows over the Chagos–Lacadive Ridge generate high chlorophyll concentrations to the north of the equator and periodic southward advection in the meridional flows associated with Yanai waves produces the chlorophyll spikes that are observed in the mooring record. Yanai waves may also contribute to vertical shear across the base of the mixed layer that could help support entrainment. The OFAM3 eddy-resolving model suggests that both of our proposed mechanisms may be important. 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.

show abstract

Bio‐physical coupling and ocean dynamics in the central equatorial Indian Ocean during 2006 Indian Ocean Dipole

Cited by 12 publications

References 14 publications

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

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

Generation of Quasi‐Biweekly Yanai Waves in the Equatorial Indian Ocean

Biogeochemical variability in the equatorial Indian Ocean during the monsoon transition

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