Natural climate variability modulates the expansion of oxygen minimum zone in the tropical Pacific

Decrease in oceanic oxygen is predicted from ocean heat uptake and increased stratification in the warming climate. The extent of oxygen minimum zone (OMZ) in the eastern tropical Pacific is particularly sensitive to the ocean deoxygenation due to its nonlinear response to the regional oxygen inventory. Ocean time series observations show strong signature of decadal fluctuation superimposed over the long-term decreasing trend. Here, we use a combination of idealized and complex models to illustrate the physical and biogeochemical mechanisms behind the recent expansion of the tropical Pacific OMZ. Oxygen variability for the late 20th century is simulated using a three-dimensional ocean biogeochemistry model, reproducing the expansion of tropical Pacific OMZ since 1980s. Decreasing solubility and increasing apparent oxygen utilization together drive the long-term declining trend. However, these two components tend to compensate one another on interannual timescales due to their co-variation with El Nino Southern Oscillation (ENSO). Upper ocean heat content, biological oxygen utilization and upwelling of thermocline water are significantly influenced by the ENSO cycle and associated circulation changes. Furthermore, regional budget analysis reveals the important role of physical oxygen supply via equatorial zonal jets. A simple autoregressive model is constructed to illustrate the importance of decadal thermocline ventilation, integrating anomalies in physical supply and biological consumption, generating the multi-decadal variability of OMZ. Our results indicate that natural climate variability can significantly modulate the long-term ocean deoxygenation on decadal timescales through many, often competing, processes which are then reddened due to the finite memory of thermocline waters.