Articles | Volume 14, issue 15
Biogeosciences, 14, 3615–3632, 2017
Biogeosciences, 14, 3615–3632, 2017

Research article 01 Aug 2017

Research article | 01 Aug 2017

Physical control of interannual variations of the winter chlorophyll bloom in the northern Arabian Sea

Madhavan Girijakumari Keerthi1, Matthieu Lengaigne2,3, Marina Levy2, Jerome Vialard2, Vallivattathillam Parvathi1, Clément de Boyer Montégut4, Christian Ethé2, Olivier Aumont2, Iyyappan Suresh1, Valiya Parambil Akhil1, and Pillathu Moolayil Muraleedharan1 Madhavan Girijakumari Keerthi et al.
  • 1Physical Oceanography Division, CSIR-National Institute of Oceanography (CSIR-NIO), Goa, India
  • 2Sorbonne Universités (UPMC, Univ Paris 06)-CNRS-IRD-MNHN, LOCEAN Laboratory, IPSL, Paris, France
  • 3Indo-French Cell for Water Sciences, IISc-NIO-IITM-IRD Joint International Laboratory, NIO, Goa, India
  • 4IFREMER, Univ. Brest, CNRS, IRD, Laboratoire d'Océanographie Physique et Spatiale, IUEM, 29280, Brest, France

Abstract. The northern Arabian Sea hosts a winter chlorophyll bloom, triggered by convective overturning in response to cold and dry northeasterly monsoon winds. Previous studies of interannual variations of this bloom only relied on a couple of years of data and reached no consensus on the associated processes. The current study aims at identifying these processes using both  ∼  10 years of observations (including remotely sensed chlorophyll data and physical parameters derived from Argo data) and a 20-year-long coupled biophysical ocean model simulation. Despite discrepancies in the estimated bloom amplitude, the six different remotely sensed chlorophyll products analysed in this study display a good phase agreement at seasonal and interannual timescales. The model and observations both indicate that the interannual winter bloom fluctuations are strongly tied to interannual mixed layer depth anomalies ( ∼  0.6 to 0.7 correlation), which are themselves controlled by the net heat flux at the air–sea interface. Our modelling results suggest that the mixed layer depth control of the bloom amplitude ensues from the modulation of nutrient entrainment into the euphotic layer. In contrast, the model and observations both display insignificant correlations between the bloom amplitude and thermocline depth, which precludes a control of the bloom amplitude by daily dilution down to the thermocline depth, as suggested in a previous study.

Short summary
The northern Arabian Sea hosts a winter chlorophyll bloom, which exhibits strong interannual variability. The processes responsible for this interannual variation of the bloom are investigated using observations and a model. The interannual fluctuations of the winter bloom are largely related to the interannual mixed-layer depth (MLD) anomalies, which are driven by net heat flux anomalies. MLD controls the bloom amplitude through a modulation of nutrient turbulent fluxes into the mixed layer.
Final-revised paper