Periodic Reporting for period 2 - SD4SP (Stratospheric Dynamics for Seasonal Prediction)
Reporting period: 2025-04-03 to 2026-04-02
Summary of the context and overall objectives of the project
The Stratospheric Dynamics for Seasonal Prediction (SD4SP) project aims to bridge the gap between stratospheric theory and operational forecasting to provide more reliable seasonal climate services for the North Atlantic – European (NAE) region. While the WMO Global Framework for Climate Services recognizes seasonal forecasting as a high-impact tool for sectors such as agriculture, water management, and energy, current General Circulation Models (GCMs) struggle to outperform climatological persistence in the extratropics. SD4SP addresses the critical need for improved predictability by isolating the stratosphere as a source of climate memory at seasonal timescales. By identifying and correcting model biases in stratosphere-troposphere coupling, this project establishes a pathway to impact where refined dynamical representations translate into trustworthy, actionable information for decision-makers and forecast providers.
The project investigates the distinct contributions of the tropical and polar stratosphere to the NAE predictive skill. Using two independent state-of-the-art GCMs—CanESM5.1 and SPS4/CMCC-CAM6—SD4SP employs a rigorous experimental suite of nudging protocols and novel diagnostic tools. Specifically, the project utilizes a unique 3D relative vorticity framework to reconcile ENSO-driven stratospheric pathways and explain the phase-dependent morphology of Extreme Stratospheric Events (ESEs), such as the prevalence of "split" Sudden Stratospheric Warmings (SSWs) during La Niña versus "displacements" during El Niño. Furthermore, idealized nudging experiments are used to calibrate the extratropical sensitivity to the Quasi-Biennial Oscillation (QBO) amplitude and latitudinal extension, and other extratropical model biases.
Preliminary results indicate that while GCMs currently suffer from "false alarms" and a misrepresentation of wave-number 2 patterns, a "perfect ensemble" forecast confirms that accurate stratospheric representation significantly enhances NAO predictability. SD4SP demonstrates that the timely occurrence of ESEs is the linchpin for utilizing stratospheric information effectively. By advancing the simulation of the Holton-Tan effect and ENSO/QBO-NAE teleconnections, SD4SP provides a roadmap for the next generation of GCMs. The ultimate goal is to transform the models’ stratosphere from a source of stochastic noise into a robust driver of regional predictability, thereby enhancing the resilience of socio-economic sectors vulnerable to North Atlantic climate variability.
The project investigates the distinct contributions of the tropical and polar stratosphere to the NAE predictive skill. Using two independent state-of-the-art GCMs—CanESM5.1 and SPS4/CMCC-CAM6—SD4SP employs a rigorous experimental suite of nudging protocols and novel diagnostic tools. Specifically, the project utilizes a unique 3D relative vorticity framework to reconcile ENSO-driven stratospheric pathways and explain the phase-dependent morphology of Extreme Stratospheric Events (ESEs), such as the prevalence of "split" Sudden Stratospheric Warmings (SSWs) during La Niña versus "displacements" during El Niño. Furthermore, idealized nudging experiments are used to calibrate the extratropical sensitivity to the Quasi-Biennial Oscillation (QBO) amplitude and latitudinal extension, and other extratropical model biases.
Preliminary results indicate that while GCMs currently suffer from "false alarms" and a misrepresentation of wave-number 2 patterns, a "perfect ensemble" forecast confirms that accurate stratospheric representation significantly enhances NAO predictability. SD4SP demonstrates that the timely occurrence of ESEs is the linchpin for utilizing stratospheric information effectively. By advancing the simulation of the Holton-Tan effect and ENSO/QBO-NAE teleconnections, SD4SP provides a roadmap for the next generation of GCMs. The ultimate goal is to transform the models’ stratosphere from a source of stochastic noise into a robust driver of regional predictability, thereby enhancing the resilience of socio-economic sectors vulnerable to North Atlantic climate variability.
Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far
ENSO Teleconnection Analysis
To isolate the stratospheric pathway of the El Niño-Southern Oscillation (ENSO) teleconnection, a novel diagnostic approach utilizing three-dimensional relative vorticity was implemented. This framework characterizes the stratospheric polar vortex and elucidates its interaction with tropospheric Rossby wave trains forced by ENSO. This analysis reconciles disparate findings in current literature and provides a mechanistic explanation for the phase-dependent morphology of Sudden Stratospheric Warmings (SSWs): specifically, the higher frequency of wave-number 2 split SSWs during La Niña, in contrast to the displacement SSWs predominantly associated with El Niño.
QBO Idealized Experimental Suite (CanESM5.1)
The investigation into Quasi-Biennial Oscillation (QBO) teleconnections was conducted through a series of idealized experiments using CanESM5.1. Given that the model does not exhibit a spontaneous QBO, a 200-year Control (CTL) integration was performed in ocean-coupled mode under constant 2005 radiative conditions. Nudging experiments were subsequently branched from the CTL run, initiated on the first of September, November, or December to evaluate seasonal sensitivity.
A) Nudging Protocols and Parameter Sensitivity
The experimental strategy involved a systematic calibration of nudging parameters to optimize the tropical-extratropical response:
i) Vertical and Latitudinal Extension: Experiments targeted the distinct roles of the upper and lower QBO cells. The vertical sensitivity was tested by comparing nudging up to the model top versus a 10 hPa limit (per the QBOi protocol). Latitudinally, three configurations were evaluated (qboi-fit, qboi, and qboi-20) to determine the impact of tropical width on extratropical signals.
ii) Nudging Strength and Domain: A 24-hour nudging timescale was selected after a sensitivity analysis of 6 to 120 hours. By applying nudging in spectral space to only the zonal-mean vorticity, the model maintained observed tropical zonal winds while allowing meridional wind components and wave dynamics to evolve freely.
B) Target State Characterization Several approaches were utilized to define the target stratospheric state:
i) Amplified Snapshots: Initial composites of EQBO and WQBO phases from reanalysis required a two-fold amplification of their profiles to successfully trigger a statistically significant extratropical response.
ii) Evolving Anomalies: To improve model realism, QBO anomalies were superimposed onto the model’s own climatology. This approach incorporated seasonality and found that while WQBO anomalies were sufficient at observed magnitudes, EQBO anomalies required amplification to produce a robust teleconnection.
iii) Temporal Constraints: Alternative strategies included "Initialization by Nudging" (nudging only in October to observe the subsequent free-running decay) and "Late-Winter Nudging" (commencing in December) to evaluate the QBO impact on a mature polar vortex.
Idealized Hindcast Experiments
The study evaluates hindcast experiments from CanESM (1980–2020) and SPS4/CMCC-CAM6 (1993–2016), both initialized in November using ERA5 atmospheric conditions. A key methodological feature is the use of nudging experiments (free vs. climatological) to isolate the stratospheric contribution to tropospheric predictive skill. An additional set using “perfect stratosphere” conditions in CanESM was also facilitated by its lower computational cost relative to SPS4. This enabled an assessment of the potential skill derived from the stratosphere under idealized conditions, highlighting the significant gains in regional forecast skill that remain attainable through improved stratospheric representation.
A) Comparative Model Performance
Paradoxically, despite its lower spatial and vertical resolution, CanESM exhibits superior predictive skill (ACC) in the North Atlantic-European (NAE) region compared to SPS4, particularly during lead month 0 and late winter. While both models underestimate teleconnection strength relative to ERA5, the initialization of tropical winds in CanESM is sufficient to sustain a weak Holton-Tan effect for several weeks.
B) ENSO-Driven Predictability and Stratospheric Noise
ENSO teleconnections enhance predictability in the subtropical Atlantic across all experiments. Notably, suppressing stratospheric variability (climatological nudging) improves late-winter performance by reducing the "noise" associated with the final warming. However, results from the "perfect stratosphere" hindcasts suggest that capturing the full range of stratospheric variability could significantly extend surface predictability up to lead month 4.
C) QBO Influence and Bias Mitigation
A comparative analysis of QBO phases reveals that free-running models often introduce positive biases over the Euro-Mediterranean region due to mismanaged stratospheric variability. In contrast, suppressing this variability reduces continental biases, highlighting a fundamental divergence between model-simulated stratospheric dynamics and observed reality.
D) Impact of Extreme Stratospheric Events (ESEs) The predictive skill of the North Atlantic Oscillation (NAO) is intrinsically linked to the accurate simulation of ESEs, such as Sudden Stratospheric Warmings (SSWs) and, the more overlooked, Strong Polar Vortex (SPV) events. The models struggle with the timing and frequency of these events, leading to frequent false alarms. The research demonstrates that when the ensemble is subsetted to include only members that accurately predict the timing of ESEs, NAO predictability increases significantly. This confirms that while the physical coupling between the stratosphere and troposphere is present in the models, the utility of stratospheric information is currently limited by the models' inability to synchronize the occurrence of extreme events with observations.
To isolate the stratospheric pathway of the El Niño-Southern Oscillation (ENSO) teleconnection, a novel diagnostic approach utilizing three-dimensional relative vorticity was implemented. This framework characterizes the stratospheric polar vortex and elucidates its interaction with tropospheric Rossby wave trains forced by ENSO. This analysis reconciles disparate findings in current literature and provides a mechanistic explanation for the phase-dependent morphology of Sudden Stratospheric Warmings (SSWs): specifically, the higher frequency of wave-number 2 split SSWs during La Niña, in contrast to the displacement SSWs predominantly associated with El Niño.
QBO Idealized Experimental Suite (CanESM5.1)
The investigation into Quasi-Biennial Oscillation (QBO) teleconnections was conducted through a series of idealized experiments using CanESM5.1. Given that the model does not exhibit a spontaneous QBO, a 200-year Control (CTL) integration was performed in ocean-coupled mode under constant 2005 radiative conditions. Nudging experiments were subsequently branched from the CTL run, initiated on the first of September, November, or December to evaluate seasonal sensitivity.
A) Nudging Protocols and Parameter Sensitivity
The experimental strategy involved a systematic calibration of nudging parameters to optimize the tropical-extratropical response:
i) Vertical and Latitudinal Extension: Experiments targeted the distinct roles of the upper and lower QBO cells. The vertical sensitivity was tested by comparing nudging up to the model top versus a 10 hPa limit (per the QBOi protocol). Latitudinally, three configurations were evaluated (qboi-fit, qboi, and qboi-20) to determine the impact of tropical width on extratropical signals.
ii) Nudging Strength and Domain: A 24-hour nudging timescale was selected after a sensitivity analysis of 6 to 120 hours. By applying nudging in spectral space to only the zonal-mean vorticity, the model maintained observed tropical zonal winds while allowing meridional wind components and wave dynamics to evolve freely.
B) Target State Characterization Several approaches were utilized to define the target stratospheric state:
i) Amplified Snapshots: Initial composites of EQBO and WQBO phases from reanalysis required a two-fold amplification of their profiles to successfully trigger a statistically significant extratropical response.
ii) Evolving Anomalies: To improve model realism, QBO anomalies were superimposed onto the model’s own climatology. This approach incorporated seasonality and found that while WQBO anomalies were sufficient at observed magnitudes, EQBO anomalies required amplification to produce a robust teleconnection.
iii) Temporal Constraints: Alternative strategies included "Initialization by Nudging" (nudging only in October to observe the subsequent free-running decay) and "Late-Winter Nudging" (commencing in December) to evaluate the QBO impact on a mature polar vortex.
Idealized Hindcast Experiments
The study evaluates hindcast experiments from CanESM (1980–2020) and SPS4/CMCC-CAM6 (1993–2016), both initialized in November using ERA5 atmospheric conditions. A key methodological feature is the use of nudging experiments (free vs. climatological) to isolate the stratospheric contribution to tropospheric predictive skill. An additional set using “perfect stratosphere” conditions in CanESM was also facilitated by its lower computational cost relative to SPS4. This enabled an assessment of the potential skill derived from the stratosphere under idealized conditions, highlighting the significant gains in regional forecast skill that remain attainable through improved stratospheric representation.
A) Comparative Model Performance
Paradoxically, despite its lower spatial and vertical resolution, CanESM exhibits superior predictive skill (ACC) in the North Atlantic-European (NAE) region compared to SPS4, particularly during lead month 0 and late winter. While both models underestimate teleconnection strength relative to ERA5, the initialization of tropical winds in CanESM is sufficient to sustain a weak Holton-Tan effect for several weeks.
B) ENSO-Driven Predictability and Stratospheric Noise
ENSO teleconnections enhance predictability in the subtropical Atlantic across all experiments. Notably, suppressing stratospheric variability (climatological nudging) improves late-winter performance by reducing the "noise" associated with the final warming. However, results from the "perfect stratosphere" hindcasts suggest that capturing the full range of stratospheric variability could significantly extend surface predictability up to lead month 4.
C) QBO Influence and Bias Mitigation
A comparative analysis of QBO phases reveals that free-running models often introduce positive biases over the Euro-Mediterranean region due to mismanaged stratospheric variability. In contrast, suppressing this variability reduces continental biases, highlighting a fundamental divergence between model-simulated stratospheric dynamics and observed reality.
D) Impact of Extreme Stratospheric Events (ESEs) The predictive skill of the North Atlantic Oscillation (NAO) is intrinsically linked to the accurate simulation of ESEs, such as Sudden Stratospheric Warmings (SSWs) and, the more overlooked, Strong Polar Vortex (SPV) events. The models struggle with the timing and frequency of these events, leading to frequent false alarms. The research demonstrates that when the ensemble is subsetted to include only members that accurately predict the timing of ESEs, NAO predictability increases significantly. This confirms that while the physical coupling between the stratosphere and troposphere is present in the models, the utility of stratospheric information is currently limited by the models' inability to synchronize the occurrence of extreme events with observations.
Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)
ENSO extratropical teleconnection
For the first time, the ENSO-NAE teleconnection has been explored in a 3-Dimensional framework. Figure 3 shows the January-February 3-D structure of the stratospheric polar vortex during El Niño and La Niña using relative vorticity in the ERA5 reanalysis. The ENSO-forced wavetrain crossing North America interacts with the Canadian center of rotation of the polar vortex, that intensifies during La Niña, and weakens during El Niño. The result is the apparent displacement of the polar vortex to Eurasia during El Niño, while a more elongated vortex with two strong centers of rotation is present for La Niña. These opposite configurations over Canada from the troposphere to the middle stratosphere is key to explain the preference of split type sudden stratospheric warmings (SSWs) during La Niña versus the displacement type SSWs more prone during El Niño. More details will be found in Palmeiro et al. (2025a, to be submitted).
QBO extratropical teleconnection
The extratropical response to the Quasi-Biennial Oscillation (QBO) exhibits distinct temporal and structural characteristics depending on the phase, with the response to the easterly phase (EQBO) manifesting earlier than that of the westerly phase (WQBO). Key parameters for tuning these teleconnections within numerical models include the amplitude and latitudinal extension of the QBO; notably, the extratropical response demonstrates greater sensitivity to QBO amplitude than to the specific width of the nudging zone. In the EQBO phase, the weakening of the stratospheric polar vortex is particularly sensitive to amplitude. Furthermore, shifts in the zero-wind line appear critical: a poleward shift is required for vortex strengthening, though this remains a challenge for nudging schemes. While the WQBO teleconnection develops more slowly and is associated with enhanced equatorward wave propagation at lower latitudes, the EQBO response in the CanESM model appears earlier and is seemingly independent of upward wave propagation, suggesting that the conservation of potential vorticity may be the governing mechanism. Using the QBOi models (QBO-resolving), results show the QBO extratropical teleconnection is ussually improved if the observed QBO is nudged in models, mostly due to the deficiencies in the models to capture the adequate amplitude and width of the tropical winds. Other model biases as the stratospheric polar vortex structure and strength are also a source for the missrepresentation of the teleconnections.
ENSO and QBO Impacts on Predictive Skill
The representation of the Quasi-Biennial Oscillation (QBO) and El Niño-Southern Oscillation (ENSO) teleconnections remains a significant challenge for predictive skill in the North Atlantic-European (NAE) region. In the SPS4 model, the impact of the QBO is notably absent, and the combined influence of ENSO and the QBO is either significantly underestimated or missing entirely. Consequently, rather than providing a source of predictability, stratospheric variability in the model often introduces stochastic noise that obscures meaningful signals. This deficiency suggests that the tropical-extratropical pathways are not sufficiently resolved, as a more robust representation of stratospheric dynamics is theorized to enhance forecast skill and extend the window of predictability. Current results indicate that while the potential for improved NAE forecasting exists, there remains substantial room for structural model improvements to capture these large-scale atmospheric drivers.
Extreme Stratospheric Events Influences on Predictive Skill
The fidelity of modeled Extreme Stratospheric Events (ESEs), such as Sudden Stratospheric Warmings (SSWs), is critical for seasonal forecasting, yet biases in wave-driving mechanisms persist. While ENSO phases typically modulate the morphology of SSWs—with El Niño favoring displacements and La Niña favoring splits—there is a suspected underrepresentation of wave-number 2 patterns (splits) within the model that requires further investigation. These structural inaccuracies, alongside a high frequency of "false alarms" regarding SSWs and Strong Polar Vortex (SPV) events, likely degrade the signal-to-noise ratio in the extratropics. However, a "perfect member" analysis reveals that when ensemble members with the correct stratospheric evolution are isolated, there is a marked improvement in North Atlantic Oscillation (NAO) predictability. This confirms that stratosphere-troposphere coupling is physically active within the model, suggesting that improving the initialization and representation of the stratosphere could significantly unlock higher forecast skill.
For the first time, the ENSO-NAE teleconnection has been explored in a 3-Dimensional framework. Figure 3 shows the January-February 3-D structure of the stratospheric polar vortex during El Niño and La Niña using relative vorticity in the ERA5 reanalysis. The ENSO-forced wavetrain crossing North America interacts with the Canadian center of rotation of the polar vortex, that intensifies during La Niña, and weakens during El Niño. The result is the apparent displacement of the polar vortex to Eurasia during El Niño, while a more elongated vortex with two strong centers of rotation is present for La Niña. These opposite configurations over Canada from the troposphere to the middle stratosphere is key to explain the preference of split type sudden stratospheric warmings (SSWs) during La Niña versus the displacement type SSWs more prone during El Niño. More details will be found in Palmeiro et al. (2025a, to be submitted).
QBO extratropical teleconnection
The extratropical response to the Quasi-Biennial Oscillation (QBO) exhibits distinct temporal and structural characteristics depending on the phase, with the response to the easterly phase (EQBO) manifesting earlier than that of the westerly phase (WQBO). Key parameters for tuning these teleconnections within numerical models include the amplitude and latitudinal extension of the QBO; notably, the extratropical response demonstrates greater sensitivity to QBO amplitude than to the specific width of the nudging zone. In the EQBO phase, the weakening of the stratospheric polar vortex is particularly sensitive to amplitude. Furthermore, shifts in the zero-wind line appear critical: a poleward shift is required for vortex strengthening, though this remains a challenge for nudging schemes. While the WQBO teleconnection develops more slowly and is associated with enhanced equatorward wave propagation at lower latitudes, the EQBO response in the CanESM model appears earlier and is seemingly independent of upward wave propagation, suggesting that the conservation of potential vorticity may be the governing mechanism. Using the QBOi models (QBO-resolving), results show the QBO extratropical teleconnection is ussually improved if the observed QBO is nudged in models, mostly due to the deficiencies in the models to capture the adequate amplitude and width of the tropical winds. Other model biases as the stratospheric polar vortex structure and strength are also a source for the missrepresentation of the teleconnections.
ENSO and QBO Impacts on Predictive Skill
The representation of the Quasi-Biennial Oscillation (QBO) and El Niño-Southern Oscillation (ENSO) teleconnections remains a significant challenge for predictive skill in the North Atlantic-European (NAE) region. In the SPS4 model, the impact of the QBO is notably absent, and the combined influence of ENSO and the QBO is either significantly underestimated or missing entirely. Consequently, rather than providing a source of predictability, stratospheric variability in the model often introduces stochastic noise that obscures meaningful signals. This deficiency suggests that the tropical-extratropical pathways are not sufficiently resolved, as a more robust representation of stratospheric dynamics is theorized to enhance forecast skill and extend the window of predictability. Current results indicate that while the potential for improved NAE forecasting exists, there remains substantial room for structural model improvements to capture these large-scale atmospheric drivers.
Extreme Stratospheric Events Influences on Predictive Skill
The fidelity of modeled Extreme Stratospheric Events (ESEs), such as Sudden Stratospheric Warmings (SSWs), is critical for seasonal forecasting, yet biases in wave-driving mechanisms persist. While ENSO phases typically modulate the morphology of SSWs—with El Niño favoring displacements and La Niña favoring splits—there is a suspected underrepresentation of wave-number 2 patterns (splits) within the model that requires further investigation. These structural inaccuracies, alongside a high frequency of "false alarms" regarding SSWs and Strong Polar Vortex (SPV) events, likely degrade the signal-to-noise ratio in the extratropics. However, a "perfect member" analysis reveals that when ensemble members with the correct stratospheric evolution are isolated, there is a marked improvement in North Atlantic Oscillation (NAO) predictability. This confirms that stratosphere-troposphere coupling is physically active within the model, suggesting that improving the initialization and representation of the stratosphere could significantly unlock higher forecast skill.