How a wandering jet stream is draining marine life in the Mediterranean Sea

The subtropical jet stream – a fast-moving air current high above the Earth – is behind a dramatic drop in primary production in the northwestern Mediterranean.

According to a recent study, the drop in productivity is linked to a gradual northward shift in this high-altitude wind belt, which regulates weather and drives the upwelling of nutrient-rich water.

The research was conducted by experts at the University of Barcelona and the Institute of Marine Sciences (ICM-CSIC).

The team paired satellite data on ocean color with atmospheric observations. The analysis revealed that a subtle change in wind patterns can ripple through the water column and reshape an ecosystem.

A jet stream on the move

The surface atmospheric circulation and associated marine dynamics are the processes most affected by the northward displacement of the jet stream, noted study co-author Professor Miquel Canals.

“Northerly winds drive the upwelling of nutrient-rich deep waters, so the alteration of their behavior has considerably weakened the intensity of this upwelling and, therefore, has reduced the supply of nutrients to the sea surface,” explained Professor Canals.

“Deep water reaching the ocean surface is essential to nourish phytoplankton, which is the basis of the marine food web. Consequently, the weakening of upwelling at the regional level has directly impacted marine primary production, causing a decline of up to 40% in just over two decades.”

When those winds weaken, the supply of nutrients from the deep slows down – a change that hits phytoplankton hardest. The entire food web, from tiny zooplankton to commercially valuable fish, feels the squeeze.

Jet stream changes and marine life

Earlier studies hinted at falling productivity in parts of the Spanish Mediterranean. Julia Crespín, first author of the current study, noted that the previous research confirmed similar losses – but not related to the phenomenon of jet currents. She added that the broader literature is mixed.

“Apart from that, several studies deal with the relationship between climate change and marine primary production with contradictory results: while some point to a decrease in primary production, others indicate an increase,” said Crespin.

The new work links the dots by focusing on the jet stream’s drift – a trend also seen in other climate records.

Tracking wind, water, and life

To tease out the link between a drifting jet stream and marine life, the researchers tracked four key variables. According to study co-author Jordi Solé, these variables included the position of the jet stream in the study area and the Ekman transport – a wind-induced movement of water mass towards the surface.

The team also tracked northerly wind stress on the sea surface, which is a determining factor for the formation of upwelling in the study area, and the depth-integrated chlorophyll concentration.

Chlorophyll levels in the water serve as an indicator of phytoplankton abundance, and therefore reflect marine primary production.

“With these variables, we have established relationships between changes in the atmosphere and the biological response of the surface ocean, which has helped us to better understand how the jet stream ultimately affects marine primary production,” said Solé.

The cost of a moving jet stream

Less phytoplankton means the ocean absorbs less carbon dioxide, so the atmospheric budget worsens. Fisheries may also struggle as lower rations at the base of the web work their way up the chain.

“Our study provides a new perspective by showing that, at least in the northwestern Mediterranean, the northward shift of the jet stream is weakening the mechanisms and interactions that drive upwelling, which has contributed to a significant decline in marine primary production,” explained the researchers.

“We consider that the change in the wind pattern is the factor that conditions the behavior of the ecosystem, beyond dichotomies about its intensification or the effect of temperature.”

“Thus, we point to a totally new factor that focuses especially on the dynamic behavior of both the atmosphere and the ocean (and their interaction) as the key to understanding its future evolution.”

Looking beyond the Mediterranean

The Mediterranean is a semi-enclosed basin with unique winds and currents, so the team cautions against a one-size-fits-all conclusion. Still, the jet stream is drifting in many regions, and similar studies may reveal comparable effects on marine life elsewhere.

The researchers emphasized that today’s tools – from satellites to high-resolution climate models – are finally catching up with the complexity of ocean-atmosphere systems.

“Never before has there been as much data as there is today, or as powerful tools to manage, analyze and interpret it as there are today.”

“In this context, it is crucial to continue research in this line of work, combining data from in situ observation, from remote sensors such as those installed on satellites, and from high-resolution climate models to better understand the physical and biological mechanisms involved, the interactions created, the interannual variability and the potential effects they generate.”

The experts also noted that these interactions may not be constant year-to-year, and their global relevance is still unfolding.

“It will also be necessary to further study interannual variability and the mechanisms that explain why this connection is stronger in some years than in others, as well as to explore how such changes may affect other marine regions around the world,” the researchers concluded.

“We understand this is a line of work that promises major advances in the coming years.”

The full study was published in the journal Progress In Oceanography.

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