Beyond the maximum gradient: a consecutive-threshold criterion for nitracline detection in the Northwestern Indian Ocean
Year: 2026
Type: article
Abstract: Depth of the nitracline is an important indicator of the vertical distribution and availability of nitrate in the upper ocean. While the conventional definition of the nitracline as the depth of maximum nitrate gradient (dN/dz) has been widely accepted, this study introduces a complementary consecutive-gradient criterion to detect the onset of a sustained nitrate gradient zone. A total of 358 profiles from five Bio-Argo floats in the northwestern Indian Ocean, were analysed. Further, the nitracline depth was derived using three different methods: maximum gradient, single threshold, and consecutive threshold method. Of the three, the consecutive-threshold method provides a consistent approach for detecting the onset of a sustained nitrate gradient zone. It maintains a continuous nitrate gradient and is less affected by random spikes or noise. Hence, a consecutive gradient criterion is proposed for nitracline detection, defining the depth where dN/dz ≥ 0.1 µmol kg− 1 m− 1 is sustained over a 14 m continuous vertical span. Further, nitracline depth showed a strong association with the 26 °C isotherm (D26), which was used as an empirical thermal proxy when D26 > MLD. Under strong mixing, when D26 < MLD, the 23.5 °C isotherm (D23.5) showed a stronger association and was used as an alternative proxy. This will provide a practical thermal proxy for estimating nitracline depth in regions where direct nitrate measurements are unavailable.
Source: Scientific Reports
Language: English
Subfield: Oceanography
Field: Earth and Planetary Sciences
Domain: Physical Sciences
SDG: Life below water
Open Access status: gold
Estimated APC paid: $2,690
OpenAlex ID: W7223137520