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Seasonal Surface Chlorophyll a Variability in the Seychelles–Chagos Thermocline Ridge


Affiliations
1 National Institute of Ocean Technology, Chennai 600 100, India
2 National Centre for Antarctic and Ocean Research, Goa 403 804, India
 

Seychelles–Chagos Thermocline Ridge (SCTR, 5°–10°S, 50°–75°E) in the southwestern tropical Indian Ocean is a unique area that experiences year-round upwelling. This is a response to the upward Ekman pumping prevalent in the region. Satellite data, model data and objectively analysed Argo temperature/salinity data have been used to study the seasonal surface chlorophyll a (chl a) variability in SCTR. Variability of surface chl a concentration in SCTR showed a weak semiannual signature. The western part of SCTR (WSCTR, 50°–62°E) is characterized by higher chl a concentration than the eastern part (ESCTR, 63°–75°E). Average chl a concentration in WSCTR/ESCTR showed a primary peak in July– August (~0.26/~0.16 mg/m3) and a secondary peak in January (~0.14/~0.12 mg/m3). Minimum chl a concentration (~0.12/~0.1 mg/m3) was observed during March– April and December–January. The high amplitude of chl a variability observed during July–August is associated with weak stratification and deep mixed layer depth (MLD). Deep MLD reaching to nutrient-rich thermocline entrains nutrients to the surface and thereby increases the surface chl a concentration. However, the low surface chl a concentration is a result of shallow MLD in the region. The deep MLD (30–40 m) observed during June–October is dominated by wind mixing and supported by buoyancy mixing. Shallow MLD (<30 m) observed during rest of the year is due to weak wind mixing and high surface buoyancy. The high surface buoyancy is a manifestation of ocean surface warming and presence of low saline surface waters in the SCTR region.

Keywords

Buoyancy Flux, Chlorophyll a, Climatology, Wind Mixing.
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  • Hermes, J. C. and Reason, C. J. C., Annual cycle of the South Indian Ocean (Seychelles–Chagos) thermocline ridge in a regional ocean model. J. Geophys. Res., 2008, 113, C04035; doi:10.1029/2007JC004363.

  • Vialard, J. et al., Air–sea interactions in the Seychelles–Chagos thermocline ridge region. Bull. Am. Meteorol. Soc., 2009, 90, 45–61.

  • Mccreary, J. P., Kundu, P. K. and Molinari, R. L., A numerical investigation of dynamics, thermodynamics and mixed-layer processes in the Indian Ocean. Prog. Oceanogr., 1993, 31, 181–244.

  • Yokoi, T., Tozuka, T. and Yamagata, T., Seasonal variation of the Seychelles dome. J. Climate, 2008, 21, 3740–3754.

  • Kawamiya, M. and Oschlies, A., Formation of a basin-scale surface chl pattern by Rossby waves. Geophys. Res. Lett., 2001, 28, 4139–4142.

  • Schott, F. A., Xie, S. P. and Mccreary, J. P., Indian Ocean circulation and climate variability. Rev. Geophys., 2009, 47, RG1002.

  • Mccreary, J. P. et al., Biophysical processes in the Indian Ocean. In Indian Ocean Biogeochemical Processes and Ecological Variability, Geophysical Monograph Series 185, American Geophysical Union, Washington, DC, 2009, pp. 9–32.

  • Resplandy, L., Vialard, J., Lévy, M., Aumont, O. and Dandonneau, Y., Seasonal and intraseasonal biogeochemical variability in the thermocline ridge of the southern tropical Indian Ocean. J. Geophys. Res., 2009. 114, C07024.

  • Beckmann, A. and Hense, I., Beneath the surface: characteristics of oceanic ecosystems under weak mixing conditions – a theoretical investigation. Prog. Oceanogr., 2007, 75, 771–796.

  • Wilson, C. and Qiu, X., Global distribution of summer chl blooms in the oligotrophic gyres. Prog. Oceanogr., 2008, 78, 107–134.

  • Vinayachandran, P. N. and Saji, N. H., Mechanisms of South Indian Ocean intraseasonal cooling. Geophys. Res. Lett., 2008, 35, L23607.

  • Waliser, D. E., Murtugudde, R., Strutton, P. and Li, J. L., Subseasonal organization of ocean chl: prospects for prediction based on Madden–Julian Oscillation. Geophys. Res. Lett., 2005, 32, L23602.

  • Jayakumar, A. and Gnanaseelan, C., Study the mechanism of surface chl a variability in the southern tropical Indian Ocean using an OGCM. Mar. Geodesy, 2012, 35, 246–256; doi:10.1080/01490419.2011.637874.

  • Daria, H. and Tong, L., Mechanisms controlling seasonal mixed layer temperature and salinity in the southwestern tropical Indian Ocean. Dyn. Atmos. Oceans, 2011, 51, 77–93.

  • Conkright, M. E., Locarnini, R. A., Garcia, H. E., O’Brien, T. D., Boyer, T. P., Stephens, C. and Antonov, J. I., World Ocean Atlas 2001: Objective Analyses, Data Statistics, and Figures, CD-ROM Documentation, National Oceanographic Data Center, Silver Spring, MD, 2002, p. 17.

  • Shigeki, H., Ohira, T. and Nakamura, T., A monthly mean data set of global oceanic temperature and salinity derived from Argo float observations. JAMSTEC Report of Research Development, 2008, vol. 8, pp. 47–59.

  • Simons, R. A., ERDDAP – The Environmental Research Division’s Data Access Program, NOAA/NMFS/SWFSC/ERD, Pacific Grove, CA, USA; http://coastwatch.pfeg.noaa.gov/erddap

  • Bonjean, F., and Lagerloef, G. S. E., Diagnostic model and analysis of the surface currents in the tropical Pacific Ocean. J. Phys. Oceanogr., 2002, 32, 2938–2954.

  • Yu, L., Jin, X. and Weller, R. A., Global Flux Datasets from the Objectively Analyzed Air-sea Fluxes (OAFlux) Project: latent and sensible heat fluxes, ocean evaporation, and related surface meteorological variables. OAFlux Project Technical Report OA-200801, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA, 2008, p. 64.

  • Strickland, J. D. H. and Parsons, T. R., A practical handbook of seawater analysis. Can. Bull. Fish. Aquat. Sci., 1972, 167, 310.

  • Weller, R. A., Baumgartner, M. F., Josey, S. A., Fischer, A. S. and KIndle, J. C., A one-year record of atmospheric forcing from the Arabian Sea. Deep Sea Res., 1998, 45, 1961–1999.

  • Pond, S. and Pickard, G. L., Introductory Dynamic Oceanography. Pergamon Press, New York, 1978.

  • Gill, A. E., Atmosphere–Ocean Dynamics, Acadamic Press, New York, USA, 1982, p. 662.

  • George, J. V. et al., Role of physical processes in chlorophyll distribution in the western tropical Indian Ocean. J. Mar. Syst., 2013, 113, 1–12.

  • Xie, S. P., Annamali, H., Schot, F. A. and McCreary Jr, J. P., Structure and mechanisms of South Indian Ocean climate variability. J. Climate, 2002, 15(8), 864–878.

  • McCreary, J. P., Equatorial beams. J. Mar. Res., 1984, 42, 395–430.

  • Matano, R. P., Beier, E. J., Strub, P. T. and Tokmakian, R., Large-scale forcing of the Agulhas variability: the seasonal cycle. J. Phys. Oceanogr., 2002, 32, 1228–1241.

  • Saji, N. H., Xie, S.-P. and Tam, C.-Y., Satellite observations of intense intraseasonal cooling events in the tropical south Indian Ocean. Geophys. Res. Lett., 2006, 33, L14704; doi:10.1029/2006GL026525.

  • Murtugudde, R., Mccreary, J. P. and Busalacchi, A. J., Oceanic processes associated with anomalous events in the Indian Ocean with relevance to 1997–1998. J. Geophys. Res., 2000, 105, 3295–3306.

  • Wiggert, J. D., Murtugudde, R. G. and Mcclain, C. R., Processes controlling interannual variations in wintertime (northeast monsoon) primary productivity in the central Arabian Sea. Deep Sea Res., Part II, 2002, 49, 2319–2343.

  • Keerthi, M. G., Lengaigne, M., Vialard, J., De Boyer Montegut, C. and Muraleedharan. P. M., Interannual variability of the tropical Indian Ocean mixed layer depth. Climate Dyn., 2013, 40, 743–759.

  • New, A., Alderson, S., Smeed, D. and Stansfield, K., On the circulation of water masses across the Mascarene Plateau in the South Indian Ocean. Deep Sea Res., 2007, 54, 42–74.

  • Sengupta, D. G. N., Bharath Raj and Shenoi, S. S. C., Surface freshwater from Bay of Bengal runoff and Indonesian throughflow in the tropical Indian Ocean. Geophys. Res. Lett., 2006, 33, L22609.

  • Foltz, G. R., Vialard, J., Praveen Kumar, B. and McPhaden, M. J., Seasonal mixed layer heat balance of the southwestern tropical Indian Ocean. J. Climate, 2010, 23, 947–965; doi:10.1175/2009JCLI3268.1.

  • Yokoi, T., Tozuka, T. and Yamagata, T., Seasonal and interannual variations of the SST above the Seychelles Dome. J. Climate, 2012, 25, 800–814.

  • Latelier, R. M., Karl, D. M., Abott, M. R. and Bidigare, R. R., Light driven seasonal patterns of chlorophyll and nitrate in the lower euphotic zone of the North Pacific Subtropical Gyre. Limnol. Oceanogr., 2004, 49(2), 508–519.

  • Anitha, G., Ravichandran, M. and Sayanna, R., Surface buoyancy flux in Bay of Bengal and Arabian Sea. Ann. Geophys., 2008, 26, 395–400; doi:10.5194/angeo-26-395-2008.

  • New, A. L., Stansfield, K., Smythe-wright, D., Smeed, D. A., Evans, A. J. and Alderson, S. G., Physical and biochemical aspects of the flow across the Mascarene Plateau in the Indian Ocean. Philos. Trans. R. Soc. London, Ser. A, 2005, 363, 151–168.

  • Konyaev, K. V., Sabinin, K. D. and Serebryany. A. N., Large-amplitude internal waves at the Mascarene Ridge in the Indian Ocean. Deep Sea Res. Part I, 1995, 42, 2075–2091.

  • Morozov, E. G. and Vlasenko, V. I., Extreme tidal internal waves near the Mascarene Ridge. J. Mar. Syst., 1996, 9, 203–210.

  • Wunsch, C. and Ferrari. R., Vertical mixing, energy, and the general circulation of the oceans. Annu. Rev. Fluid Mech., 2004, 36, 281–314.


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  • Seasonal Surface Chlorophyll a Variability in the Seychelles–Chagos Thermocline Ridge

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Authors

Jenson V. George
National Institute of Ocean Technology, Chennai 600 100, India
M. Nuncio
National Centre for Antarctic and Ocean Research, Goa 403 804, India
N. Anilkumar
National Centre for Antarctic and Ocean Research, Goa 403 804, India
Racheal Chacko
National Centre for Antarctic and Ocean Research, Goa 403 804, India
D. Rajashekhar
National Institute of Ocean Technology, Chennai 600 100, India

Abstract


Seychelles–Chagos Thermocline Ridge (SCTR, 5°–10°S, 50°–75°E) in the southwestern tropical Indian Ocean is a unique area that experiences year-round upwelling. This is a response to the upward Ekman pumping prevalent in the region. Satellite data, model data and objectively analysed Argo temperature/salinity data have been used to study the seasonal surface chlorophyll a (chl a) variability in SCTR. Variability of surface chl a concentration in SCTR showed a weak semiannual signature. The western part of SCTR (WSCTR, 50°–62°E) is characterized by higher chl a concentration than the eastern part (ESCTR, 63°–75°E). Average chl a concentration in WSCTR/ESCTR showed a primary peak in July– August (~0.26/~0.16 mg/m3) and a secondary peak in January (~0.14/~0.12 mg/m3). Minimum chl a concentration (~0.12/~0.1 mg/m3) was observed during March– April and December–January. The high amplitude of chl a variability observed during July–August is associated with weak stratification and deep mixed layer depth (MLD). Deep MLD reaching to nutrient-rich thermocline entrains nutrients to the surface and thereby increases the surface chl a concentration. However, the low surface chl a concentration is a result of shallow MLD in the region. The deep MLD (30–40 m) observed during June–October is dominated by wind mixing and supported by buoyancy mixing. Shallow MLD (<30 m) observed during rest of the year is due to weak wind mixing and high surface buoyancy. The high surface buoyancy is a manifestation of ocean surface warming and presence of low saline surface waters in the SCTR region.

Keywords


Buoyancy Flux, Chlorophyll a, Climatology, Wind Mixing.

References





DOI: https://doi.org/10.18520/cs%2Fv114%2Fi04%2F868-878