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

Abstract

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.

Reconstructing Global Chlorophyll-a Variations Using a Non-linear Statistical Approach

“…Although successfully used in a variety of biogeochemical studies (e.g., Bopp et al, 2005;Gehlen et al, 2006;Lengaigne et al, 2007;Schneider et al, 2008;Steinacher et al, 2010;Tagliabue et al, 2010;Séférian et al, 2013;Aumont et al, 2015;Keerthi et al, 2017;Parvathi et al, 2017 and references therein), the ability of the PISCES model to reproduce satellite surface Chl is briefly illustrated in section "Evaluation of Chl PISCES at global scale. "…”

Robust Projected Weakening of Winter Monsoon Winds Over the Arabian Sea Under Climate Change

“…The evaporative cooling associated with the cold and dry IWM winds induces convective overturning in the northern AS, contributing to the ventilation of the nearly anoxic subsurface waters (Morrison et al, 1999). The convective overturning also entrains nutrients into the surface euphotic layer (Keerthi et al, 2017;Prasanna Kumar et al, 2001), leading to a prominent chlorophyll bloom (e.g., Banse & McClain, 1986). This bloom contributes substantially to the total annual AS primary productivity (Madhupratap et al, 1996;Marra & Barber, 2005) and occurs when the winter oceanic conditions are more favorable for fishing activities than the summer leading to the highest fish catch .…”

Evidence of Eddy‐Enhanced Winter Chlorophyll‐a Blooms in Northern Arabian Sea: 2017 Cruise Expedition

“…It has recently been determined that multiple mechanisms control the existence of winter blooms in the AS (e.g., Banerjee & Kumar, 2015; Keerthi et al, 2017; Madhupratap et al, 1996). The convective mixing in response to ocean surface cooling is identified as the primary trigger of the blooms.…”

“…In situ measurements have captured changes in specific thermohaline layers during winter blooms periods (e.g., Bauer et al, 1991; Gundersen et al, 1998; Keerthi et al, 2017; Madhupratap et al, 1996; Prasanna Kumar et al, 2001). For example, during the winter blooms of 2007–2008, the mixed layer depth (MLD) deepened to ~70 m (from the annual mean state of ~30 m) and the nitracline deepened to ~90 m (from the annual mean state of ~50 m) (Keerthi et al, 2017). Both the eddies and turbulent convective mixing are capable of inducing vertical transport and further affecting the local thermohaline stratification (McGillicuddy et al, 2008).…”