Figure 1 – A weather model representation of the North Pacific jet stream, with North America on the right hand side of the image. Credit: https://earth.nullschool.net
Wavy bands of fast flowing air, called jet streams, are some of the most recognisable features of the Earth’s atmospheric circulation (figure 1). They have a critical impact on weather in temperate regions by directing the flow of air and interacting with storm systems.
Jet stream variability can also drive extreme weather. For example, a northward-shifted jet stream contributed to the extremely hot and dry European summer of 2022 (Patterson et al, 2024), while a weakened or shifted North Pacific jet stream reduces rainfall in California, increasing wildfire risk (Wahl et al 2019).
Given the significant role of jet streams in our weather, it is important to understand how they might be changing with climate change. Even subtle shifts to the latitude of the mean jet streams, or changes to their speed, could have large impacts on extreme rainfall, with substantial implications for society.
Under future scenarios of increasing greenhouse gases, climate models generally project a poleward shift of the jet streams, with some variation between regions and seasons (Ossó et al 2024). Broadly speaking, this is due to an increase in the temperature gradient between the equator and poles, which regulates the speed and latitude of the jets.
However, understanding whether or not climate change has already affected jet streams is made challenging by their natural variability. Low frequency sea surface temperature variability resulting from changes to ocean circulation patterns can shift the jets on multi-decadal timescales. In fact, the way in which the Pacific jet changes will likely be affected by how climate change projects onto natural patterns of variability like El Niño Southern Oscillation.
A recent paper by Keel et al (2024) showed that the wintertime North Pacific jet stream has shifted northwards since the beginning of the satellite-era in 1979. This northward shift likely contributed to the drying trend which made the January 2025 wildfires in Los Angeles more likely. Keel et al (2024) suggested that this shift may bear the signature of anthropogenic climate change.
I addressed this hypothesis and related questions in a new paper with my colleague, Chris O’Reilly (Patterson and O’Reilly, 2025). We wanted to know whether climate model simulations, forced with past variations of greenhouse gases, could capture this trend. These computer models are a climate scientist’s laboratory, and if the models could capture this jet trend we could use them to investigate the cause.
Figure 2 – Trends in a winter (December-January-February) North Pacific jet stream index over the period 1979-2023, for an ensemble of climate models (histogram and frequency distribution) and two observation-based datasets (cross and triangle). Numbers show the percentile at which the observations lie relative to the models, i.e. they exceed all of the model trends.
Surprisingly, none of the 180 simulations we looked at could reproduce the magnitude of the North Pacific jet trend (figure 2). One reason for this is the differing behaviour of the tropical Pacific Ocean in the models compared with the real climate system.
Figure 3 – Trends in sea surface temperatures (DJF, 1979-2023). The trend is shown for a) Observations (HadISST) b) the mean over trends in all CMIP6 model simulations. Hatching in a) indicates grid-points at which HadISST trends lie outside of the middle 95% of the CMIP6 ensemble.
Whilst most of the sea surface has warmed over the recent decades, the tropical Pacific has warmed little or even cooled in some seasons (figure 3a). In contrast climate models tend to show an even greater warming in this region, relative to the global average (figure 3b). It is unclear whether this discrepancy arises because the models aren’t responding properly to greenhouse gases in this region, or that this is just an expression of natural variability.
In any case, we found that accounting for the differing trends could explain some but not all of the more northward jet shift in the real climate, compared to the models. It is possible that the models also don’t show enough jet stream variability on long timescales or do not respond correctly to greenhouse gas variations.
So can we say that climate change is responsible? We extended the time series of jet stream variability back before 1979 using long datasets. The longer datasets indicate that while the jet has shifted northwards since 1979, it had shifted southward over the prior thirty years at a similar rate.
This doesn’t mean that climate change wasn’t involved. However, it does suggest that the recent jet trend has not emerged from natural variability. It is clear that more work is required to understand the drivers of this recent northward trend in the North Pacific jet and the implications for future climate change in North America.
References
Keel, T., Brierley, C., Edwards, T. and Frame, T.H., 2024. Exploring uncertainty of trends in the North Pacific Jet position. Geophysical Research Letters, 51(16), p.e2024GL109500. Doi: https://doi.org/10.1029/2024GL109500
Patterson, M. and O’Reilly, C.H., 2025. Climate models struggle to simulate observed North Pacific jet trends, even accounting for tropical Pacific sea surface temperature trends. Geophysical Research Letters, 52(4), p.e2024GL113561. Doi: https://doi.org/10.1029/2024GL113561
Patterson, M., Befort, D.J., O’Reilly, C.H. and Weisheimer, A., 2024. Drivers of the ECMWF SEAS5 seasonal forecast for the hot and dry European summer of 2022. Quarterly Journal of the Royal Meteorological Society, 150(765), pp.4969-4986. Doi: https://doi.org/10.1002/qj.4851
Ossó, A., Bladé, I., Karpechko, A., Li, C., Maraun, D., Romppainen-Martius, O., Shaffrey, L., Voigt, A., Woollings, T. and Zappa, G., 2024. Advancing Our Understanding of Eddy-driven Jet Stream Responses to Climate Change–A Roadmap. Current Climate Change Reports, 11(1), p.2. Doi: https://doi.org/10.1007/s40641-024-00199-3
Wahl, E.R., Zorita, E., Trouet, V. and Taylor, A.H., 2019. Jet stream dynamics, hydroclimate, and fire in California from 1600 CE to present. Proceedings of the National Academy of Sciences, 116(12), pp.5393-5398. Doi: https://doi.org/10.1073/pnas.1815292116
