"Saltness Battle": Atlantic Ocean VS Pacific Ocean, the salinity gap is quietly widening!

Produced by: Science Popularization China

Author: Keke (popular science creator)

Producer: China Science Expo

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The ocean covers 71% of the Earth's surface and is the largest body of water on Earth. The salt dissolved in seawater gives the ocean unique physical and chemical properties, affecting global climate, marine ecosystems and human activities.

Recently, a study by Wang Fan's team at the Institute of Oceanology, Chinese Academy of Sciences, revealed a surprising discovery: the difference in salinity between the Atlantic and Pacific Oceans has been increasing over the past half century. This discovery not only challenges our traditional understanding of ocean changes, but also provides a new perspective for understanding global climate change.

Ocean salinity: a key factor affecting global climate

Ocean salinity refers to the content of dissolved salts in seawater, usually expressed in parts per thousand (‰) or practical salinity units (PSU). As one of the basic characteristics of seawater, salinity has an important impact on ocean circulation, climate system and the distribution of marine life.

As the two largest oceans on Earth, the Atlantic and Pacific Oceans have a particularly striking difference in salinity. The Atlantic Ocean is generally salinier than the Pacific Ocean, and this difference affects the water mass properties, circulation structure, and biogeochemical cycles of the two oceans, and thus affects the global climate system.

However, against the backdrop of global climate change, ocean salinity is undergoing profound changes.

Wang Fan's team's research quantitatively revealed for the first time the strengthening trend and spatial structure of the Atlantic-Pacific salinity contrast (APSC) since the mid-20th century, providing a new perspective for our understanding of ocean change. This study not only demonstrates the complexity of ocean salinity changes, but also emphasizes the key role of the ocean in the global climate system.

Salinity of various waters

(Image source: Wikipedia)

Surprising discovery: Salinity differences are increasing

By analyzing observational data at depths of 0-2000 meters in the ocean, the research team found that the Atlantic-Pacific salinity difference has shown a clear strengthening trend since 1965. Specifically, the salinity of the Atlantic Ocean has generally increased, while the salinity of the Pacific Ocean has generally decreased . This trend shows a clear meridional structure, which is particularly prominent in the subtropical regions of the northern and southern hemispheres. In particular, between 20° and 40° north latitude, the salinity difference has increased by 5.9±0.6%.

This discovery means that the "salinity gap" between the Atlantic and Pacific Oceans is deepening. This change not only affects the physical properties of the ocean, but may also have a profound impact on the global climate system. The increase in salinity differences may lead to significant changes in water mass exchange, heat transfer and biogeochemical cycles between the two oceans, thus affecting the global climate pattern.

Global average ocean salinity

(Image source: Wikipedia)

Drivers of salinity change: more than just surface freshwater fluxes

Traditionally, it is believed that freshwater flux from the ocean surface (such as precipitation and evaporation) is the main factor affecting salinity changes. However, the research of Wang Fan's team shows that in addition to these known factors, wind fields and ocean warming also play an important role in salinity changes . The research team revealed a series of key findings through innovative analytical methods (thermodynamic mode decomposition) and model simulations.

First, long-term changes in wind fields lead to variations in wind-driven ocean circulation, which changes the distribution of salt in the ocean. This change may lead to increased salinity in some areas and decreased salinity in other areas. Second, the warming of the ocean surface causes the source of water mass formation to migrate to higher latitudes, further affecting the distribution of salinity. This migration may change the traditional process of water mass formation and mixing, thereby affecting the salinity structure of the entire ocean.

These processes work together to produce a "salinity redistribution" effect, which is a key factor in the strengthening trend of salinity differences and the formation of their meridional structures. It is worth noting that although these processes occur in both the Atlantic and Pacific Oceans, due to the differences in the climatic salinity distribution of the two oceans, they ultimately produce different effects, leading to a further increase in salinity differences. This discovery emphasizes the importance of ocean dynamics in salinity changes and provides a new perspective for our understanding of ocean changes.

Change in average ocean salinity from 0 to 2000 m since 1965 based on observational data (unit: psu)

(Image source: Reference 1)

Widespread impacts of salinity change: from sea level to marine ecology

The strengthening of salinity differences is not only an interesting scientific phenomenon, but may also have broad and far-reaching impacts on the global ocean and climate system. Studies have found that this increase in salinity differences may exacerbate the trend of sea level rise in the Pacific Ocean. Because fresh water is less dense than salt water, the Pacific Ocean with reduced salinity may experience more significant sea level rise, which poses a potential threat to coastal areas and low-lying islands.

In addition, salinity changes may also enhance ocean stratification (the phenomenon that the physical, chemical and biological properties of seawater are layered with depth), affecting the vertical mixing of seawater. The strengthening of ocean stratification may reduce the exchange between surface and deep water bodies, affecting the transport of nutrients and the balance of marine ecosystems. Such changes may have a profound impact on the marine food chain and fishery resources.

What is more worthy of attention is that the strengthening of salinity differences may aggravate the hypoxia and acidification of the Pacific subsurface water. The enhancement of seawater stratification may reduce the oxygen supply in deep water bodies, leading to the formation of hypoxic zones in certain areas, threatening the survival of marine life. At the same time, the intensification of ocean acidification may have a negative impact on calcified organisms (such as corals and shellfish), thereby affecting the stability of the entire marine ecosystem.

These potential impacts highlight the importance of studying ocean salinity changes. By gaining a deeper understanding of these changes and their driving factors, scientists can better predict future ocean changes and provide a scientific basis for responding to climate change and protecting marine ecosystems.

Conclusion

This work not only reveals the trend of strengthening salinity differences between the Atlantic and Pacific Oceans, but also clarifies the important role of wind-driven ocean circulation variations and water mass source migration in salinity changes. These findings are of great reference value for improving climate models and enhancing climate prediction capabilities.

However, there is still a long way to go in the study of ocean salinity changes. Future research scientists may do includes: in-depth exploration of the long-term impact of salinity changes on marine ecosystems and global climate; improving climate models to increase the accuracy of simulations of ocean salinity changes; and developing new observation technologies and methods to obtain more comprehensive and accurate ocean salinity data.

In addition, interdisciplinary collaboration will play a key role in this. The joint efforts of oceanographers, climate scientists, and ecologists will help us more fully understand changes in ocean salinity and their impacts on the Earth system.

In general, the progress in the study of ocean salinity changes has provided us with new perspectives and tools to deal with global climate change. By deeply understanding the changes in the ocean, we can better predict and respond to future environmental challenges and contribute to the sustainable development of the earth.

References:

1.Ying Lu et. al., North Atlantic–Pacific salinity contrast enhanced by wind and ocean warming

2.Nature Climate Change, Ocean dynamics contribute to the amplifying Atlantic–Pacific salinity contrast