Changes in AOU are not consistent across spatial domains. Notably, the Northern Hemisphere AOU spikes in the mid-1990s, while Southern Hemisphere AOU significantly decreases in the mid-2000s. This could be due to less dense data in the Southern Hemisphere. Since the 80s, regions of major O decrease (AOU increase) include the subpolar North Pacific, equatorial Atlantic, and eastern equatorial Pacific. Regions of O increase (AOU decrease) include the western subtropical North Pacific and eastern subpolar North Atlantic.