Climatological Seasonal Cycle of Global Ocean Oxygen, Heat and Apparent Oxygen Utilization Content Anomalies in the Surface Mixed Layer

Mean monthly climatological mixed layer depth (MLD) combined with temperature, dissolved oxygen, and apparent oxygen utilization (AOU) are used to produce global estimates of the seasonal variability of ocean heat content anomaly (OHCA), O ₂ content anomaly (O2CA), and AOU content anomaly (ACA) in the surface mixed layer. Linear regression analyses show that the highest correlation occurs when O2CA lags OHCA by one month, whereas the highest correlation occurs when ACA lags OHCA by 2-3 months. The O2CA is negatively correlated, while the ACA is positively correlated with the OHCA in the mixed layer. The O 2 -heat ratio in the surface mixed layer is about -1.85 nmol/J in the subtropical and subpolar regions, which is on the same order of magnitude due to the O ₂ solubility effect alone. The solubility effect is the primary driver for the seasonal cycle of the O ₂ inventory in the mixed layer, and thus subject to changes in ocean warming. The 1-month lag between O2CA and OHCA suggests the O ₂ inventory quickly responds to heat content changes on seasonal time scales due to strong mixing in the mixed layer. The 2-3 month lag between ACA and OHCA suggests oxygen changes through biological activities take a longer time following OHC changes in relation to physical changes through O ₂ solubility. Our analysis indicates that the deoxygenation rate in the mixed layer, estimated from the regression analysis, is approximately -2.2 Tmol/year based on the O ₂ -heat ratio in the mid-latitudes, accounting for 6±2% of the global deoxygenation for the time period 1955-2019.