Seasonal and spatial variations in spice generation in the South Indian Ocean salinity maxima

Abstract

Spiciness anomalies generated in the salinity maxima region are important for several atmospheric and oceanic factors as they move along the geostrophic pathways towards the equator and resurface. Subduction and injection mechanisms are responsible for the spiciness generation in the South Indian Ocean (SIO) salinity maxima region. Using ECCO data from 1992 to 2017, here we study monthly variations of spiciness associated with both of these mechanisms in the SIO salinity maxima region. Using a Lagrangian approach, we estimated the monthly evolution of the subduction rate. A maximum subduction rate of 35–38 m/mon occurs during September and consequently decreases towards the end of the year. The effective subduction rate in the salinity maxima region shows the dominance of temporal induction (mixed layer tendency) term, with a sharp gradient in total subduction rate along the 30∘S associated with large mixed layer depth variation. Further, a high Turner angle (> 66∘) to the south of 30∘S confirms the generation of spiciness by injection mechanism. We found that the decrease in mixed layer salt (MLS) coincides with the increase in salinity below the mixed layer. To explore the significance of MLS changes in spiciness generation, we further addressed the monthly evolution of spiciness through MLS budget. Our results suggest that the entrainment and meridional advection terms are key to monthly variations in MLS changes and thus the spiciness.