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Oxygen change in the Pacific depends on where you look, how deep you measure, and which timescale you mean. The tropical Pacific has strong year-to-year swings tied to El Niño and La Niña; oxygen-deficient waters also occur at different depths in the tropics and North Pacific. Along the US Pacific Northwest shelf, a long record shows more widespread summer near-bottom hypoxia, with an especially extensive event during strong upwelling in 2021. These findings describe different parts of the ocean, not one uniform basin-wide trend.
Where is oxygen changing, and at what depth?
There is no single Pacific-wide oxygen trend that captures all regions and depths. A 2020 analysis of Pacific oxygen and nutrient records from 50 to 300 meters found a strong oxygen-deficient layer in the upper tropical Pacific and a deeper one in the North Pacific. It also found non-linear trends and influences from climate variability, rather than one simple trajectory across the basin (Stramma et al., Biogeosciences, 2020).
That distinction matters because observations from different settings do not measure the same thing. Open-ocean measurements at depth describe oxygen in a broad water layer; coastal shelf studies may report the area of seafloor-adjacent water below a chosen hypoxia threshold. A coastal hypoxic-area percentage is not a measure of the Pacific’s total oxygen inventory.
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| Region | Depth and setting | Timescale and evidence | What the findings indicate |
|---|---|---|---|
| Tropical Pacific | Upper ocean, including the oxygen-deficient layer | Interannual variability in a 2026 analysis of observations and global ocean–sea-ice simulations | Oxygen patterns vary with ENSO phase and differ between eastern/central and western parts of the tropics (Eddebbar et al., Journal of Climate, 2026). |
| North Pacific | Deeper oxygen-deficient layer; the 2020 analysis covered 50–300 m | Decadal and longer records, with climate-mode influences | Oxygen trends are non-linear; the North Pacific record is influenced in part by the North Pacific Gyre Oscillation (NPGO) (Stramma et al., Biogeosciences, 2020). |
| Eastern tropical North Pacific | Tropical mid-depth waters | Mechanism proposed in a 2024 study | A study links multidecadal variability to possible changes in stratification, winds, circulation, oxygen supply, and biological demand; it is a proposed explanation, not a settled account of all Pacific deoxygenation. |
| US Pacific Northwest shelf | Near-bottom coastal water inshore of the 200 m isobath | Long observational record and a notably extensive 2021 event | The study reports increasing summer hypoxia in this coastal region, with the affected area sensitive to upwelling conditions (Barth et al., Scientific Reports, 2024). |
Why does tropical Pacific oxygen swing with El Niño and La Niña?
In the 2026 analysis by Eddebbar and colleagues, oxygen in the eastern and central tropical Pacific tends to be higher during El Niño and lower during La Niña. The western tropical Pacific tends toward the opposite pattern. NOAA’s Pacific Marine Environmental Laboratory summary, published July 14, 2026, describes one key physical connection: El Niño deepens the eastern Pacific thermocline and reduces the upwelling of oxygen-poor deeper water, while the thermocline tends to become shallower in the west. La Niña generally favors the opposite arrangement.
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The pattern is not explained by one physical lever. The 2026 study describes interacting, sometimes compensating effects from vertical advection, biological oxygen consumption, vertical mixing, and lateral transport. Their relative contributions vary across the tropical Pacific, which helps explain why a change in one part of the region need not match a change elsewhere.
An ENSO-related increase during an El Niño is an event-scale shift, not by itself evidence that a longer-term decline has reversed. Stramma and colleagues’ analysis of records since 1950 found that El Niño and La Niña often affect eastern tropical Pacific oxygen distribution during an event, but do not have a multi-year influence on trends. Event variability and longer-term change therefore need to be assessed separately.
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Why do North Pacific and eastern tropical Pacific trends differ?
Climate modes operating over longer periods can overlay changes in oxygen supply and demand. In the 50–300 m records analyzed by Stramma and colleagues, the Pacific Decadal Oscillation (PDO) influenced tropical and eastern Pacific records, while the NPGO was particularly influential in the North Pacific. In most of the eastern Pacific regions they studied, oxygen increased and nutrients decreased during negative PDO phases; the tendencies were opposite during positive phases. In the Oyashio region, multiple modes and an 18.6-year tidal cycle also overlay the longer-term pattern.
Other proposed mechanisms concern how circulation, stratification, and biology interact. A 2024 Nature Communications study proposes a multidecadal link between North Atlantic temperature variability and deoxygenation in the northern tropical Pacific. Its analysis discusses possible effects involving stratification, trade winds, equatorial circulation, and the Equatorial Undercurrent, which can alter oxygen supply as well as biological oxygen demand. This is a mechanism advanced by that study, not a single established explanation for oxygen change throughout the Pacific.
A 2016 Nature Geoscience study adds a different possible pathway: in its model and sensitivity experiments, aerosol pollution increased iron deposition, which boosted regional productivity and respiration and accelerated tropical Pacific mid-depth oxygen decline. The result supports the possibility that biogeochemical processes can contribute alongside circulation and climate variability; it does not establish aerosol deposition as the cause of change in every Pacific region.
What does the Pacific Northwest shelf record show?
The coastal record provides a concrete example of long-term change combined with sensitivity to a particular season’s conditions. Barth and colleagues’ 2024 study defined near-bottom hypoxia as dissolved oxygen below 61 µmol kg−1. For the area inshore of the 200 m isobath, it reported the following fractions of near-bottom water hypoxic during summer upwelling conditions:
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| Period or event | Reported hypoxic fraction | Qualification |
|---|---|---|
| 1950–1980 | 2% | Average during the summer upwelling season; the study describes this as nearly absent. |
| 2009–2018 | 24% | Average during the summer upwelling season. |
| 2021 | 56% | Observed during unusually strong upwelling; not a typical-year average. |
In summer 2021, the study also mapped near-bottom hypoxia across nearly half of the continental shelf inshore of the 200 m isobath, an area of 15,500 km2. A mid-shelf ribbon of water below 50 µmol kg−1 extended 450 km off north-central Oregon and Washington. Those measurements describe a specific coastal event and region, not the open Pacific as a whole.
The authors’ maps since 1950 show a consistent trend toward lower oxygen, with spatial patterns related to shelf width and other regional features. They say the longer-term increase in hypoxia is consistent with increased upwelling-favorable wind forcing under climate change. The contrast between the period averages and 2021 also shows why a long-term record and individual event conditions should not be conflated.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What can—and can’t—be concluded about Pacific oxygen loss?
The evidence supports regional and depth-specific change, not a claim that oxygen is falling everywhere in the Pacific at the same rate. ENSO can shift tropical oxygen substantially from year to year; decadal modes influence other records; and coastal near-bottom hypoxia responds to local ocean conditions and upwelling. Warming and ventilation changes, circulation, and biological oxygen consumption are among the processes discussed in the studies, but their importance is not uniform across the basin.
The studies summarized here do not provide a harmonized, current basin-wide rate of Pacific oxygen loss. A rate would require comparable measurements over defined regions, depths, and periods; the cited findings instead describe distinct layers, records, models, and coastal thresholds. The most accurate map-like picture is therefore a patchwork: different oxygen-deficient layers offshore, event-driven swings in the tropics, and a documented long-term coastal change in the Pacific Northwest.
Quick Recap
Sources
- Eddebbar et al., “ENSO-Driven Variability of Oxygen Content and Distribution in the Tropical Pacific,” Journal of Climate 39(5), 1333–1353 (2026).
- NOAA Pacific Marine Environmental Laboratory, “Oxygen in the Tropical Pacific responds to El Niño Forcing,” July 14, 2026.
- Barth et al., “Widespread and increasing near-bottom hypoxia in the coastal ocean off the United States Pacific Northwest,” Scientific Reports 14, 3798 (2024); NOAA Ocean Acidification Program page dated February 15, 2024.
- Stramma et al., “Trends and decadal oscillations of oxygen and nutrients at 50 to 300 m depth in the equatorial and North Pacific,” Biogeosciences 17(3), 813–831 (2020).
- “North Atlantic temperature control on deoxygenation in the northern tropical Pacific,” Nature Communications (2024).
- “Acceleration of oxygen decline in the tropical Pacific over the past decades by aerosol pollutants,” Nature Geoscience (2016).
