Responses of Horizontally Expanding Oceanic Oxygen Minimum Zones to Climate Change Based on Observations

Abstract

Due to climate change, global oceanic dissolved oxygen (DO) has been decreasing, and oxygen minimum zones (OMZs) have been expanding. Here, we estimate the annual global and regional OMZ areas using geostatistical regression combined with Monte Carlo. From 1960 to 2019, annual global OMZ20 (DO < 20 μmol/kg) and OMZ60 (DO < 60 μmol/kg) areas cover 5%–14% and 15%–32% of the global ocean, respectively. The global and most regional OMZ areas after the late 2000s were all significantly larger than those in previous years. Most oceanic regions likely experienced significant expansions in OMZ areas, especially the North Pacific. However, the equatorial Pacific with the largest OMZ area showed insignificant horizontal expansion. The expanding OMZ areas of most oceans responded more quickly and effectively to the surrounding sea temperatures than to the sea surface temperatures. Our research highlights the need for marine resource management to account for the challenges from OMZs.

Satellite-Based Global Sea Surface Oxygen Mapping and Interpretation with Spatiotemporal Machine Learning

“…The results of the AOU map (Figure 5a) show a spatial distribution similar to that of the long-term recorded data from WOD (Figure S7). The undersaturated areas of DO in the low-latitude to midlatitude of the ocean surface coincide with the known distribution of oxygen minimum zones (OMZs), 3,48 including Midwest Pacific, North Indian Ocean, etc. Furthermore, we found extensive areas of surface deoxygenation in the Northeastern Pacific, with the deoxygenation spatial range highly consistent with the OMZs of 80 μmol/kg threshold.…”

On the Origins of Open Ocean Oxygen Minimum Zones

“…While it is clear that OMZs are expanding (Stramma et al., 2010; Zhou et al., 2022), the driving mechanisms are still an area of active research. The warming of the surface ocean impacts the oxygen concentrations directly by decreasing the solubility of oxygen, and indirectly by increasing ocean stratification.…”

“…OMZs are usually found at depths between 100 and 900 m with O 2 concentrations as low as 4.5 μmol kg −1 (Karstensen et al., 2008). Direct observations show that OMZs are expanding (Stramma et al., 2010; Zhou et al., 2022), yet the underlying mechanisms are unclear. While global warming is expected to decrease ocean ventilation in a more stratified ocean (Keeling & Garcia, 2002), the myriad feedbacks involved result in highly uncertain responses of OMZs (Frölicher et al., 2020; Fu, Primeau, et al., 2018).…”

Recent Developments in Oxygen Minimum Zones Biogeochemistry

“…Even if the emissions were to stop, deoxygenation will continue over centuries in the deep ocean (Long et al, 2019) as the decline of O 2 committed by historical emissions is estimated to be 3 times larger than current O 2 loss (Oschlies, 2021). In the tropical thermocline, ocean deoxygenation has been shown to cause the expansion and shoaling of OMZs Zhou et al, 2022). This can lead to a substantial reduction in habitat of sensitive marine organisms, including commercial fish species such as tunas and billfishes (Stramma et al, 2012b).…”

“…However, these changes could also be linked to interannual variability characterizing denitrification in the region , rather than reflecting long-term trends (Naqvi, 2019). A recent analysis of historical observations by Zhou et al (2022) has revealed an expansion of the area occupied by all major OMZs, including the Arabian Sea OMZ, over the last 3 to 4 decades. Goes et al (2020) reported evidence of increased winter stratification (and reduced winter convection) in the northern Arabian Sea, together with a decrease in the N:P ratios, indicative of an increase in denitrification over the recent decades.…”