In just over 20 years, the northward shift of the subtropical jet stream — a high-altitude airflow — caused by climate change has reduced primary production in the northwestern Mediterranean by about 40%. This marked reduction — the highest ever described — affects the base of the marine food web and could significantly impact living resources, ecosystem health and marine dynamics in this region of the Mediterranean.
This is one of the main conclusions of an article published in the journal Progress in Oceanography, whose main authors are Julia Crespín and Miquel Canals, from the Faculty of Earth Sciences at the University of Barcelona, and Jordi Solé, from the Institute of Marine Sciences (ICM-CSIC) in Barcelona and the Centre for Ecological Research. Experts Miquel Canals and Jordi Solé are director and member, respectively, of the UB Chair on Sustainable Blue Economy.
Primary production in the oceans consists of the synthesis of organic matter by photosynthetic organisms (phytoplankton), which capture carbon dioxide (CO2) from the atmosphere and release oxygen (O2). The effect of the persistent displacement of the jet stream on primary production could affect fisheries and local and regional economies in the medium to long term. Reduced primary production can also alter the ocean’s carbon cycle — less phytoplankton means less CO2 uptake by the ocean — and this could accentuate the effects of climate change.
This study is pioneering in establishing a systematic link between the northward shift of the subtropical jet stream and the decline in marine primary production. Apart from the biological impact, the authors warn that the modification of wind patterns could also alter the surface and subsurface temperature of marine waters in the medium and long term, among other parameters, and significantly change the oceanographic dynamics of the northwestern Mediterranean.
Fast air currents circulating at high altitude
The subtropical jet stream is a high-altitude airflow that transports cold and heat at high speed, regulates the regional climate and modulates the position and intensity of surface winds, especially those of the northern quadrant, such as the tramontane and the mistral. Due to climate change, this current has progressively shifted northward, causing atmospheric patterns to change in the northwestern Mediterranean.
“The surface atmospheric circulation and associated marine dynamics are the processes most affected by the northward displacement of the jet stream. Northerly winds drive the upwelling of nutrient-rich deep waters, so the alteration of their behaviour has considerably weakened the intensity of this upwelling and, therefore, has reduced the supply of nutrients to the sea surface”, explains Miquel Canals, professor at the Department of Earth Dynamics.
“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”, the expert notes.
How does climate change affect primary production?
Previous studies published in 2022 had reported decreases of 22% in marine primary production in deep waters of the Spanish Mediterranean, “but this was not related to the phenomenon of jet currents”, says UB researcher Julia Crespín, first author of the article. “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”.
“It should also be taken into account that the Mediterranean is a closed basin with very specific atmospheric and oceanographic characteristics, which can make the impact detected more accentuated”, Crespín points out. “Therefore, it will depend on future studies to find out if there are comparable situations in other parts of the world where the jet stream is also moving persistently”.
As part of the study, the authors have analysed several key variables to understand the effects on atmospheric and marine processes of continuous northward migration of the jet stream. “Specifically, the position of the jet stream in the study area (to know the location and intensity of the atmospheric flows), the Ekman transport (a wind-induced movement of water mass towards the surface, which is critical for the upwelling of deep water to the surface levels of the water column), the northerly wind stress on the sea surface (a determining factor for the formation and maintenance of upwelling in the study area) and the depth-integrated chlorophyll concentration derived from satellite measurements, which are indicators of the amount of phytoplankton, and thus, of marine primary production”, notes expert Jordi Solé (ICM-CSIC).
“With these variables — adds Solé — 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”.
New perspectives on winds and ocean dynamics
Several studies had described the effects of global change on ocean and nutrient exchange, and how increased temperature or water stratification affect marine primary production. One of the dilemmas for the scientific community revolves around the effects of upwelling-favourable winds. According to studies, winds will increase with climate change and will drive upwelling of nutrient-rich deep waters. However, others postulate the opposite effect, i.e. warming of the surface layers of the ocean will intensify the stratification of the water column, decreasing the frequency, intensity and efficiency of upwelling processes and the availability of nutrients at the surface.
“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”, the authors explain.
“We consider that the change in the wind pattern is the factor that conditions the behaviour 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 behaviour of both the atmosphere and the ocean (and their interaction) as the key to understanding its future evolution”.
In this context, the northwestern Mediterranean presents a key scenario to study these effects and to better understand possible future trajectories of marine ecosystems in other regions of the world.
From continental forests to marine ecosystems
The study of the role of jet streams on the climate and ecosystem evolution of the Earth is becoming more popular. This line of research opens a relevant dimension to understand how the planet works and what are the interactions that determine global change, but also those determined by global change itself. This phenomenon is not just subject to the marine environment: it affects from forests to marine ecosystems, as stated in studies on the effects of the changes in the position of the jet stream on the production and growth of trees in European forests (Nature Communications, 2022, and Nature, 2024).
“Never before has there been as much data as there is today, or as powerful tools to manage, analyse 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”.
“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. We understand this is a line of work that promises major advances in the coming years”, the team concludes.