Spatiotemporal scales of mode water transformation in the Sea of Oman

Abstract

In the Sea of Oman, mode water forms at the surface and is trapped under a warm stratified layer in summer. This capped and well-mixed oxygenated layer decouples the oxygen minimum zone from ocean surface processes. It also provides a space for remineralisation, reducing oxygen demand in the oxygen minimum zone. Several physical processes, from isopycnal and diapycnal mixing to advection, transform mode water and change its properties. We perform a volume budget analysis to investigate the mechanisms driving mode water volume change in the Sea of Oman from monthly to 3 d temporal scales using monthly climatologies derived from profiling floats and high-resolution underwater glider observations. Isopycnal and diapycnal water-mass transformations are estimated in a density-spice framework. Our study shows that mode water predominantly transforms along isopycnals, yet strong but transient diapycnal transformation occurs at shorter timescales. Moreover, fluxes between the mode water layer and its surroundings are highly sensitive to the presence of mesoscale eddies. Across eddies, diapycnal and isopycnal transformations intensify by 61 % and 45 % respectively, compared to non-eddy conditions, indicating that eddies are drivers of both lateral and vertical water mass exchanges. This study provides a novel application of the water mass transformation framework to high-resolution underwater gliders, and shows that this methodology can be used at higher resolution than traditional climatological products or models. By comparing monthly climatological products to the high-resolution glider data, we suggest that the climatological estimates are outside of the high-resolution glider mean ± standard error 40 % of the time for diapycnal and 60 % of the time for isopycnal transformation. These results highlight the intense variability occurring at small scales and can serve to inform future estimates of water mass transformation uncertainty from coarser products.