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Pacific-Origin Rossby Waves Modulated by Atlantic SSTs in CMIP6 Large Ensembles

This study utilizes CMIP6 large ensembles to demonstrate that Atlantic sea surface temperature anomalies modulate the propagation of Pacific-origin Rossby waves into Europe during boreal summer by altering upper-level jet structures, thereby explaining inter-model variability in trans-basin teleconnections and European summer climate predictability.

Original authors: Ramon Fuentes-Franco, Julia F. Lockwood, Nick Dunstone, Adam A. Scaife, Torben Koenigk

Published 2026-08-13

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Ramon Fuentes-Franco, Julia F. Lockwood, Nick Dunstone, Adam A. Scaife, Torben Koenigk

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Great Weather Web: How Oceans Talk to Each Other

Imagine the Earth's atmosphere as a giant, invisible trampoline made of air. When you push down on one side of this trampoline, a ripple travels across the surface, eventually bumping into things on the other side. In the world of climate science, these ripples are called Rossby waves. They are massive, slow-moving waves in the air that carry weather patterns from one part of the globe to another, linking distant regions like the Pacific Ocean and Europe. Think of them as the planet's way of sending a text message: "Hey, it's hot here, so it might get rainy over there."

These waves are triggered by things like warm or cold patches in the ocean, which act like the finger pushing the trampoline. Scientists have long known that the Pacific Ocean is a master sender of these messages, often dictating weather patterns across the Northern Hemisphere. However, there's a catch: sometimes the message gets through loud and clear, and other times it gets garbled or lost entirely. This is a big deal because if we can't predict how these waves travel, we can't accurately forecast summer heatwaves, droughts, or heavy rains in places like Europe. The big question has always been: what decides whether the message arrives, and why does it sometimes get mixed up?

The Paper's Story: A Cosmic Game of "Whisper Down the Lane"

In this study, researchers Ramon Fuentes-Franco and his team at the Met Office and SMHI decided to play detective with the world's most advanced climate models. They used a special tool called "CMIP6 Large Ensembles," which is like running the same weather simulation 30 to 65 times with slightly different starting conditions to see what usually happens. Their goal? To figure out why the weather messages sent from the northeastern Pacific sometimes arrive in Europe as a clear signal and other times as a confusing mess.

The Main Discovery: The Atlantic is the Gatekeeper
The team found that while all the models agreed on how the wave starts in the Pacific, they completely disagreed on what happened when the wave tried to cross the Atlantic Ocean to reach Europe. It turns out the Atlantic Ocean isn't just a passive body of water; it's the bouncer at the club.

The researchers discovered that the temperature of the Atlantic Sea Surface (SST) acts like a switch for the "waveguide"—the invisible tunnel in the upper atmosphere that the waves travel through.

When the Atlantic is Cold: The waveguide gets strong and straight, like a well-paved highway. The Rossby waves from the Pacific zoom across the ocean and hit Europe with full force, creating distinct weather patterns (like wetter, stormier summers in Northern Europe).
When the Atlantic is Warm: The waveguide gets bumpy and weak, like a road full of potholes. The waves get scattered, lose their energy, or get blocked entirely before they can reach Europe.

The Jet Stream Glitch
The paper also points out that the models themselves have a few "bugs" in their code. Specifically, many of the models get the speed and position of the upper-level jet stream (the fast-moving river of air high in the sky) wrong. If a model's jet stream is too strong or in the wrong place, it bends the wave path, causing the weather signal to arrive in the wrong country or with the wrong intensity. The study suggests that fixing these jet stream biases is key to making our weather forecasts better.

What They Ruled Out
The authors explicitly argue against the idea that the models are fundamentally "broken" or that they can't simulate the Pacific waves at all. In fact, the models are great at generating the waves in the Pacific. The problem isn't the start of the journey; it's the middle of the journey. The paper also rules out the idea that the connection between the Pacific and Europe is a fixed, unchangeable rule. Instead, they show it is "state-dependent," meaning it changes based on the current temperature of the Atlantic.

How Sure Are They?
The team is very confident in their simulations. They didn't just look at one model; they looked at seven different models with hundreds of total simulations. They found that the "cold Atlantic = strong connection" and "warm Atlantic = weak connection" pattern is a robust feature across all of them. However, they note that the exact strength of this effect varies from model to model, suggesting that while the mechanism is real, the exact amount of change might still need more research. They also highlight that the "cold Atlantic" effect is particularly strong in models that use specific ocean components, hinting that the size of the temperature swings in the ocean matters just as much as the direction of the temperature.

The Takeaway
So, if you want to know if a Pacific weather event will ruin your summer picnic in London, you can't just look at the Pacific. You have to check the Atlantic first. If the Atlantic is cold, the Pacific message will likely get through loud and clear. If it's warm, the message might get lost in the noise. This study suggests that to predict European summers accurately, we need to understand this dynamic dance between the two oceans and the jet stream that connects them, rather than treating them as separate players.

Technical Summary: Pacific-Origin Rossby Waves Modulated by Atlantic SSTs in CMIP6 Large Ensembles

Problem Statement
Atmospheric teleconnections originating in the Pacific Ocean significantly influence Northern Hemisphere summer climate through Rossby wave trains. However, the downstream impact of these waves over Europe exhibits pronounced variability across time and climate models. While the generation of Rossby waves by Rossby Wave Sources (RWS) in the northeastern Pacific is a known mechanism, the consistency of their propagation across the North Atlantic and into Europe varies widely among models. Furthermore, the role of Atlantic Sea Surface Temperature (SST) anomalies in modulating this trans-basin propagation remains a critical area of investigation, particularly regarding how the background state of the Atlantic affects the efficiency of wave transmission to Europe.

Methodology
The study utilizes multiple CMIP6 Large Ensembles (LEs) to assess the robustness of atmospheric responses to RWS in the northeastern Pacific. The analysis covers the boreal summer (June–July–August, JJA) period from 1951 to 2014. The models analyzed include CanESM5, EC-Earth3, GISS-E2-1-G, IPSL-CM6A-LR, MIROC6, NorCPM1, and UKESM1-0-LL, with ensemble member counts ranging from 17 to 65 per model.

Key diagnostic tools include:

Rossby Wave Source (RWS) Calculation: Computed diagnostically from the 200 hPa wind field using the formulation $RWS = -\nabla \cdot (v_\chi \zeta)$ , where $v_\chi$ is the irrotational wind component and $\zeta$ is the absolute vorticity. This isolates vorticity anomalies generated by divergent outflows interacting with background vorticity.
Teleconnection Analysis: Correlation patterns between northeastern Pacific RWS anomalies and 500 hPa geopotential height (Z500) are computed for each ensemble member and compared against ERA5 reanalysis.
Jet Stream Bias Assessment: Zonal ( $u_a$ ) and meridional ( $v_a$ ) wind biases at 200 hPa are evaluated relative to ERA5 to determine their influence on waveguide structure.
SST Modulation Composites: The study stratifies ensemble members into "cold" and "warm" North Atlantic SST states (based on anomalies in the region 57°W–30°W, 37°N–67°N) to analyze how the Atlantic background state alters the Pacific-to-Europe teleconnection.

Key Contributions and Results

Robust Generation, Variable Propagation:
CMIP6 large ensembles consistently reproduce a circumglobal Rossby wave train emanating from the northeastern Pacific, confirming that the generation mechanism is robust across models. However, substantial inter-model spread emerges downstream over the North Atlantic and Europe. The uncertainty is not primarily associated with the generation of the wave train in the Pacific but rather with the maintenance and phasing of the wave as it crosses the Atlantic.
The Role of Jet Biases:
The inter-model spread in the European response is closely linked to biases in the upper-tropospheric jet structure.
- Models with realistic jet structures (e.g., EC-Earth3, CanESM5) maintain the correct phase and amplitude of the wave train into Europe.
- Models with biases in jet strength or meridional position (e.g., UKESM1-0-LL, IPSL-CM6A-LR) exhibit systematic shifts in the teleconnection node. For instance, a northward extension or strengthening of the jet can displace the downstream trough eastward, while a weaker jet may cause the signal to decay before reaching Europe.
- Local wind biases over Europe were found to be more strongly correlated with the positioning of the teleconnection node than basin-scale jet biases alone.
Atlantic SSTs as Modulators:
A central finding is that Atlantic SST anomalies regulate the efficiency of trans-basin wave propagation through a state-dependent waveguide mechanism:
- Cold Atlantic Conditions: Strengthen the upper-level jet and enhance meridional shear. This creates a more coherent and efficient waveguide, facilitating the eastward extension of the Pacific-origin wave train into Europe and amplifying the European response.
- Warm Atlantic Conditions: Weaken and disrupt the waveguide structure. This limits the eastward propagation of wave energy, effectively suppressing the European response to Pacific forcing.
- The study notes that the strongest European summer responses occur when Pacific and Atlantic anomalies align (e.g., negative IPO concurrent with negative AMV), suggesting constructive interference, whereas opposing phases may lead to destructive interference.
Internal Variability vs. Structural Bias:
Analysis of individual ensemble members reveals that the ERA5-observed teleconnection strength falls within the range of internal variability produced by the models. This indicates that discrepancies between a single model realization and observations are often due to the magnitude of internal variability rather than a systematic inability of the models to reproduce the teleconnection physics.

Significance and Claims
The paper claims that the European summer climate response to Pacific variability cannot be understood in isolation. Instead, it emerges from the interaction between Pacific forcing, the state of Atlantic SSTs, and the background atmospheric circulation (specifically the upper-level jet).

The study underscores that:

The non-stationarity of the Pacific–Europe teleconnection is dynamically explained by the modulation of the waveguide by Atlantic SSTs.
Improving the representation of mean jet biases and Atlantic SST conditions in climate models is critical for reducing inter-model spread and enhancing the predictability of European summer climate.
Large ensembles are essential for separating the forced signal from internal variability, revealing that the "mismatch" between models and observations is often a feature of the system's sensitivity to background flow rather than a fundamental model failure.

These findings suggest that accurate seasonal to decadal predictability of European summer climate depends not only on Pacific variability but also on the accurate representation of Atlantic SST conditions and the resulting upper-level jet structure.

The Great Weather Web: How Oceans Talk to Each Other

The Paper's Story: A Cosmic Game of "Whisper Down the Lane"

Technical Summary: Pacific-Origin Rossby Waves Modulated by Atlantic SSTs in CMIP6 Large Ensembles

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