Biological production in the Indian Ocean upwelling zones –Part 1: refined estimation via the use of a variable compensation depth in ocean carbon models

Mohanan Geethalekshmi Sreeush,Koneru Venkata Siva Rama Prasad,Vinu Valsala,Raghu Murtugudde,Sreenivas Pentakota

doi:10.5194/bg-15-1895-2018

Abstract

Abstract. Biological modelling approach adopted by the Ocean Carbon-Cycle Model Intercomparison Project (OCMIP-II) provided amazingly simple but surprisingly accurate rendition of the annual mean carbon cycle for the global ocean. Nonetheless, OCMIP models are known to have seasonal biases which are typically attributed to their bulk parameterisation of compensation depth. Utilising the criteria of surface Chl a-based attenuation of solar radiation and the minimum solar radiation required for production, we have proposed a new parameterisation for a spatially and temporally varying compensation depth which captures the seasonality in the production zone reasonably well. This new parameterisation is shown to improve the seasonality of CO2 fluxes, surface ocean pCO2, biological export and new production in the major upwelling zones of the Indian Ocean. The seasonally varying compensation depth enriches the nutrient concentration in the upper ocean yielding more faithful biological exports which in turn leads to accurate seasonality in the carbon cycle. The export production strengthens by ∼ 70 % over the western Arabian Sea during the monsoon period and achieves a good balance between export and new production in the model. This underscores the importance of having a seasonal balance in the model export and new productions for a better representation of the seasonality of the carbon cycle over upwelling regions. The study also implies that both the biological and solubility pumps play an important role in the Indian Ocean upwelling zones.

Highlights

The Indian Ocean is characterised by the unique seasonally reversing monsoon wind systems which act as the major physical drivers for the coastal and open-ocean upwelling processes
The regions we considered are (1) western Arabian Sea (WAS; 40–65◦ E, 5◦ S–25◦ N), (2) Sri Lanka Dome (SLD; 81–90◦ E, 0–10◦ N), (3) Sumatra coast (SC; 90–110◦ E, 0–10◦ S) and (4) Seychelles–Chagos thermocline ridge (SCTR; 50–80◦ E, 5–10◦ S; Fig. 1)
A spatially and temporally varying Zc parameterisation as a function of solar radiation and Chl a is implemented in the biological pump model of OCMIP-II for a detailed analysis of biological fluxes in the upwelling zones of the Indian Ocean

Summary

Introduction

The Indian Ocean is characterised by the unique seasonally reversing monsoon wind systems which act as the major physical drivers for the coastal and open-ocean upwelling processes. The major upwelling systems in the Indian Ocean are (1) the western Arabian Sea (WAS; Ryther and Menzel, 1965; Smith, 2001; Sarma, 2004; Wiggert et al, 2005, 2006; Murtugudde et al, 2007; McCreary et al, 2009; Naqvi et al, 2010; Prasanna Kumar et al, 2010; Roxy et al, 2015), (2) the Sri Lanka Dome (SLD; Vinayachandran and Yamagata, 2008; Vinayachandran et al, 2004), (3) the Java and Sumatra coasts (SC; Murtugudde et al, 1999; Susanto et al, 2001; Osawa and Julimantoro, 2010; Xing et al, 2012) and (4) the Seychelles–Chagos thermocline ridge (SCTR; Murtugudde and Busalacchi, 1999; Dilmahamod et al, 2016, Fig. 1). Sreeush et al.: Biological production in the Indian Ocean upwelling zones – Part 1

Methods

Results

Conclusion