Uncovering the drivers of Pacific Ocean deoxygenation threatening marine ecosystems

As global temperatures rise, the world’s oceans are projected to lose up to 4% of their dissolved oxygen by 2100, according to the most recent IPCC report. During this timeframe, marine deoxygenation is expected to have devastating consequences for marine ecosystems and fisheries, especially in areas where oxygen levels are already low, such as the tropical Pacific Ocean.

Within the vast Pacific lies the North Pacific Oxygen minimum zone (OMZ), an area located offshore of Mexico and the United States of America that is the largest area of oxygen depletion in the world. These oxygen-depleted oceanic regions are expected to quadruple in size by 2100, highlighting a growing desire to understand the complex dynamics that underpin their alarming expansion.

In a recent study published in Nature Communications, Dr Laetitia Pichevin, Senior Researcher in Marine Biogeochemistry in the School, and her colleagues uncovered a critical link between oceanic patterns. They determined that the Atlantic Multidecadal Oscillation (AMO), a recurring climate pattern in the North Atlantic, is the main driver of oxygen changes in the tropical Pacific.

This discovery highlights that the AMO's influence on global climate and ocean circulation is larger than previously thought, challenging conventional understandings of the main drivers behind marine oxygen fluctuations.

Research revelations

Despite the recognised importance of the AMO on global climate and ocean characteristics, the specific impact of the AMO on the oxygen changes in the tropical Pacific had so far not been as thoroughly investigated. 

Traditional modelling efforts of marine oxygen changes infer that the Pacific Decadal Oscillation (PDO), a fluctuation in the North Pacific temperature pattern that occurs over a 50-year cycle, is the main driver in the Pacific OMZ.

However, historical records of deoxygenation in the Pacific OMZ only cover the last 70 years, making it difficult to confirm a causal link between PDO and marine oxygen fluctuation.

The difference here for Dr Pichevin and her team? Looking for clues in a unique sediment core retrieved from the Gulf of California. This core was remarkably preserved under low oxygen conditions, offering an unprecedented chance to reconstruct the history of deoxygenation in the Pacific OMZ with unparalleled temporal details throughout key climatic pre-industrial intervals.

“Our study shows that models need to account for the AMO, as the temperature and circulation patterns linked to the PDO are not enough to explain the fluctuations in oxygen in the tropical Pacific,” said Dr Pichevin.

“Therefore, basing simulations on mechanisms linked to the PDO alone would lead to inaccurate estimations of future deoxygenation intensity.”

Power of the Atlantic Multidecadal Oscillation

Due to global warming, warm sea surface temperature anomalies in the North Atlantic are getting more extreme by the year. 

The research undertaken by Dr Pichevin and her colleagues is significant because the world’s oceans are already losing oxygen in response to increased global sea surface temperatures. Marine deoxygenation has devastating consequences on marine ecosystems and fisheries, especially in areas where oxygen levels are already low, such as the tropical Pacific. 

“The ocean surface is not just becoming warmer in the North Atlantic; the North Pacific is also getting warmer and more stratified. Waters that are poorer in oxygen will be incorporated into the ocean circulation that fuels the tropical oceans at the subsurface and intermediate depths. This is where the oxygen minimum zones are located and where a loss of oxygen will be more intensely felt by living organisms.”

The power of the AMO also extends far beyond marine ecosystems.

According to Dr Pichevin, there is a growing body of evidence that the AMO has a major impact on rainfall and temperature averages throughout North America, Eurasia and Africa, and on atmospheric and marine circulations in other oceanic basins. 

This is projected to impact the intensity and frequency of the AMO, which in turn, will have a feedback effect on global weather patterns and climate change itself. 

“The projected loss in marine oxygen due to increasing ocean temperatures could be accelerated – or reduced – depending on how the atmospheric and ocean circulations in the Pacific respond to major climate oscillations such as the AMO.”

Therefore, probing these intricate meteorological relationships and feedback loops will be vital for directing further research endeavours. 

Informing future research efforts

The study showed that the intensity of deoxygenation in the region depends on whether the PDO and AMO phenomena are in or out of phase. Therefore, the findings highlighted in the paper can make it easier to prepare for the variations in oxygen loss in the tropical Pacific based on the predictable fluctuations in these two multidecadal climate oscillations.

“In this study, we investigated the mechanisms that presided over oxygen changes in the OMZ of the Pacific Ocean at multidecadal to interannual timescales. 

If we can understand how these climate oscillations modulate oxygen level prior to anthropogenic climate change, we can hope to inform models designed to forecast future changes in marine oxygen and their ecological consequences under climate change scenarios.” 

Considering the many environmental threats of expanding deoxygenation under global warming, this is an important finding for modelling future oxygen loss and their impact on ecosystems and communities.