Observations of Reduced Ventilation in Meridional Overturning Circulation: Evidence From Physical and Biogeochemical Changes in Repeat Observations Along 110°E

Two hydrographic voyages conducted 56 years apart (in 1963 and 2019) along 110°E in the southeast Indian Ocean reveal significant long-term changes in water mass properties. We focus on physical and biogeochemical changes in Subtropical Water (STW), Subantarctic Mode Water (SAMW), and Antarctic Intermediate Water (AAIW)—key components of the Indian Ocean shallow and global overturning circulations. Changes in temperature and salinity are decomposed into spice (along-isopycnal) and heave (vertical movement of isopycnals) components. STW has become warmer and saltier on isopycnals. In contrast, SAMW and upper AAIW have cooled and freshened primarily due to spice changes, suggesting salinity-driven processes originating from their respective source regions. Meanwhile, the lower AAIW shows warming and salinification, indicating temperature-driven changes and distinct source variability. The observed thickening of the SAMW and AAIW layers, combined with increased apparent oxygen utilizations, nitrate, and phosphate concentrations, points to reduced ventilation and enhanced stratification. These changes imply a slowdown in circulation and altered nutrient cycling consistent with anthropogenic climate change impacts.