Analysis of the mean annual cycle of the dissolved oxygen anomaly in the World Ocean

A global climatology of the dissolved oxygen anomaly (the excess over saturation) is created with monthly resolution in the upper 500 m of the ocean. The climatology is based on dissolved oxygen, temperature and salinity data archived at the National Oceanographic Data Center. Examination of this climatology reveals statistically significant annual cycles throughout the upper 500 m of the World Ocean, though seasonal variations are most coherent in the North Atlantic, where data density is greatest. Vertical trends in the phase and amplitude of the annual cycle are noted. The cycle in surface waters is characterized by a summer maximum and a winter minimum, consistent with warming and high rates of photosynthesis during the summer, and cooling and entrainment of oxygen-depleted water during the winter. In low and middle latitudes, the amplitude increases with depth and the maximum occurs later in the year, a trend consistent with the seasonal accumulation of oxygen associated with the shallow oxygen maximum. At a depth that varies between about 30 and 130 m, the phase of the annual cycle undergoes an abrupt shift. We call this depth the oxygen nodal depth. Below the nodal depth, the annual cycle is characterized by an early-spring maximum and a late-fall minimum, consistent with a cycle dominated by respiration during the spring and summer and replenishment of oxygen from the atmosphere by ventilation during the fall and winter. Below the nodal depth, the amplitude of the annual cycle generally decreases with depth, indicative of decreasing respiration and ventilation rates, or less seasonality in both processes. We postulate that the nodal depth in middle and high latitudes corresponds closely to the summertime compensation depth, where photosynthesis and net community respiration are equal. With this interpretation of the nodal depth and a simple model of the penetration of light in the water column, a compensation light intensity of 1 W m -2 (4 μE m -2 s -1 ) is deduced, at the low end of independent estimates. Horizontal trends in the phase and amplitude of the annual cycle are also noted. We find that the nodal depth decreases toward the poles in both hemispheres and is generally greater in the Southern Hemisphere, patterns found to be consistent with light-based estimates of the compensation depth. The amplitude of the annual cycle in the oxygen anomaly increases monotonically with latitude, and higher latitudes lag lower latitudes. In the North Atlantic and North Pacific, the amplitude of the annual cycle tends to increase from east to west at all depths and latitudes, as expected considering that physical forcing has greater seasonal variability in the west. The tropics and the North Indian Ocean have features that distinguish them from other regions. Below about 75 m, these waters have pronounced annual cycles of the oxygen anomaly that are shown to be caused mainly by wind-driven adiabatic displacements of the thermocline. A semiannual cycle of the oxygen anomaly is found in the surface waters of the North Indian Ocean, consistent with the known semiannual cycle of surface heat flux in this region.