Periodic Reporting for period 1 - stratoIMPACT (The downward impact of the stratosphere in current and future climates)
Periodo di rendicontazione: 2021-08-01 al 2023-07-31
One of the most striking long-lived influences on the weather in Europe comes from the stratosphere, a highly stratified, stable layer of the atmosphere at 10 to 50 km above the surface. Extreme stratospheric events, such as sudden stratospheric warmings (SSW), can have a prolonged downward impact on the large-scale atmospheric circulation, including an equatorward shift of the tropospheric jet stream over the North Atlantic, cold spells over Scandinavia and increased rainfall over the Mediterranean. Future climate projections demonstrate great uncertainty when it comes to the stratospheric response to climate change. the model spread is considerable, particularly in winter, when the stratospheric influence is strongest. Furthermore, midlatitude storms are expected to become more intense as the storm tracks shift poleward or extend to Europe. It is therefore unclear how these changes in both the stratosphere and the troposphere may alter the tropospheric response to stratospheric forcing under climate change.
The main goal of the stratoIMPACT project is to understand the physical processes that are responsible for the impact of the stratosphere (a layer of the atmosphere, above 10 km) on surface weather and climate, and how this coupling will be modified under climate change.
The specific scientific objectives for the project are to:
1. Identify and characterize the role of the storm tracks in the downward response to stratospheric forcing in observations.
2. Investigate the mechanisms through which stratospheric anomalies reach the surface using a series of modeling experiments and state-of-the-art climate model projections.
3. Combine the results of (1) and (2) to resolve how the downward response to stratospheric forcing will be affected by climate change.
This project has addressed societal needs at regional level by two aspects: First, understanding the downward impact of the stratosphere and its influence on surface weather (e.g. the jet stream and the storm track) helps to shed light on ongoing processes in Earth’s atmosphere that affect our day-to-day weather, and reduces the uncertainty of future climate projections. Second, this work helps to quantify the risk and the uncertainty of extreme storms under climate change, thus providing an essential step forward and a solid scientific basis for advanced early warning systems (more than 1 week in advance) for extratropical cyclones and windstorms in a changing climate.
The main goal of the stratoIMPACT project is to understand the physical processes that are responsible for the impact of the stratosphere (a layer of the atmosphere, above 10 km) on surface weather and climate, and how this coupling will be modified under climate change.
The specific scientific objectives for the project are to:
1. Identify and characterize the role of the storm tracks in the downward response to stratospheric forcing in observations.
2. Investigate the mechanisms through which stratospheric anomalies reach the surface using a series of modeling experiments and state-of-the-art climate model projections.
3. Combine the results of (1) and (2) to resolve how the downward response to stratospheric forcing will be affected by climate change.
This project has addressed societal needs at regional level by two aspects: First, understanding the downward impact of the stratosphere and its influence on surface weather (e.g. the jet stream and the storm track) helps to shed light on ongoing processes in Earth’s atmosphere that affect our day-to-day weather, and reduces the uncertainty of future climate projections. Second, this work helps to quantify the risk and the uncertainty of extreme storms under climate change, thus providing an essential step forward and a solid scientific basis for advanced early warning systems (more than 1 week in advance) for extratropical cyclones and windstorms in a changing climate.
During this project, the researcher has worked on understanding the physical processes that affect the downward impact of the stratosphere, in present and future climate.
Present climate (WP1+WP2): the work performed in this part of the project has included analysis of the changes in weather and climate following extreme events in Earth’s stratosphere. For this purpose, the researcher has analyzed observational (reanalysis) data as well as model output from an intermediate-complexity model. We show that in observations, a “canonical” downward impact after Sudden Stratospheric Warming events (for example, a southward shift of the jet stream and the storm track over Europe and the North Atlantic) is linked to a strong signal that descends to the lower stratosphere. In the model, there is a stronger connection between the eastern North Pacific and the North Atlantic, leading to a more consistent response in these two basins, compared to the observations.
To better understand what physical drivers influence the downward impact of the stratosphere, we used a new dataset: sub-seasonal to seasonal forecasts from the ECMWF. These forecasts provide an extended-range (up to 46 days) forecast of the atmospheric conditions. By analyzing the stratospheric influence on midlatitude storm track in the Euro-Atlantic sector, we show that the midlatitude storm track tends to shift equatorward after Sudden Stratospheric Warming (SSW) events, with reduced cyclone frequency in northern Europe. After strong vortex events, cyclone frequency is increased in northern Europe, and reduced further south. While in observations only two thirds of SSW events have a downward impact, the forecasts tend to be over-confident and predict a storm track response more often than it occurs in reality.
Future climate (WP3): using state-of-the-art climate models from the CMIP6 project, we explored, together with colleagues, what are the changes that stratospheric polar vortex is expected to undergo in climate change conditions. In another project, we show that climate change is associated with increased storm damage over central Europe, whereis a decrease in storm damage is expected in northern and southern Europe. These results indicate that predicting the storm track response to extreme events in the stratosphere would require reducing the uncertainty in more than one layer of the atmosphere: both the stratosphere and the troposphere.
To summarize, episodes of stratosphere-troposphere coupling have a long-lived influence on surface weather, especially in winter. Using a combination of reanalysis data, idealized models and operational forecast models, we are able to study what processes determine if there is going to be an impact of the stratosphere on surface weather in Europe and the North Atlantic, or not, and how well it is represented in forecast models. By addressing these fundamental questions, the goal of this research project was to improve the predictability of climate extremes on sub-seasonal to seasonal timescales, which is critical for a better management of their associated impacts on infrastructure and human lives.
Present climate (WP1+WP2): the work performed in this part of the project has included analysis of the changes in weather and climate following extreme events in Earth’s stratosphere. For this purpose, the researcher has analyzed observational (reanalysis) data as well as model output from an intermediate-complexity model. We show that in observations, a “canonical” downward impact after Sudden Stratospheric Warming events (for example, a southward shift of the jet stream and the storm track over Europe and the North Atlantic) is linked to a strong signal that descends to the lower stratosphere. In the model, there is a stronger connection between the eastern North Pacific and the North Atlantic, leading to a more consistent response in these two basins, compared to the observations.
To better understand what physical drivers influence the downward impact of the stratosphere, we used a new dataset: sub-seasonal to seasonal forecasts from the ECMWF. These forecasts provide an extended-range (up to 46 days) forecast of the atmospheric conditions. By analyzing the stratospheric influence on midlatitude storm track in the Euro-Atlantic sector, we show that the midlatitude storm track tends to shift equatorward after Sudden Stratospheric Warming (SSW) events, with reduced cyclone frequency in northern Europe. After strong vortex events, cyclone frequency is increased in northern Europe, and reduced further south. While in observations only two thirds of SSW events have a downward impact, the forecasts tend to be over-confident and predict a storm track response more often than it occurs in reality.
Future climate (WP3): using state-of-the-art climate models from the CMIP6 project, we explored, together with colleagues, what are the changes that stratospheric polar vortex is expected to undergo in climate change conditions. In another project, we show that climate change is associated with increased storm damage over central Europe, whereis a decrease in storm damage is expected in northern and southern Europe. These results indicate that predicting the storm track response to extreme events in the stratosphere would require reducing the uncertainty in more than one layer of the atmosphere: both the stratosphere and the troposphere.
To summarize, episodes of stratosphere-troposphere coupling have a long-lived influence on surface weather, especially in winter. Using a combination of reanalysis data, idealized models and operational forecast models, we are able to study what processes determine if there is going to be an impact of the stratosphere on surface weather in Europe and the North Atlantic, or not, and how well it is represented in forecast models. By addressing these fundamental questions, the goal of this research project was to improve the predictability of climate extremes on sub-seasonal to seasonal timescales, which is critical for a better management of their associated impacts on infrastructure and human lives.
In stratoIMPACT, we investigated the long-lived impact of the stratosphere on surface weather, and what dynamical factors are important for the stratosphere to affect our day-to-day weather. We also explored what makes successful forecasts? Focusing on storm track and extratropical cyclones after stratospheric extreme events, and collaborated with colleagues and students towards a risk assessment of the damage associated with changes in the storm track in future climate.
The stratoIMPACT project has improved our understanding of the links between persistent atmospheric circulation patterns, as forced by the downward impact of the stratosphere, and mid-latitude weather and climate. These findings have the potential to provide more accurate forecasts of intense storm impacts and help to reduce the risk against damage incurred by changes in the storm track and associated extratropical cyclones.
IMPACT: In a changing climate, even small shifts in the storm track position can have a significant influence on regional weather and associated extreme events. However, while midlatitude weather is dominated by cyclones, it is particularly challenging to predict weather in the midlatitudes beyond ten days. Skillful predictions of extratropical storms, on timescales of several weeks to months, rely on the connection between storm tracks and large-scale atmospheric variability modes, such as the stratosphere, thus providing a potential for extending the predictability limits of storms to timescales longer than a few weeks. These predictions can provide early warnings for society and reliable information for decision makers in sectors such as public health, energy, ecosystems and water security.
The stratoIMPACT project has improved our understanding of the links between persistent atmospheric circulation patterns, as forced by the downward impact of the stratosphere, and mid-latitude weather and climate. These findings have the potential to provide more accurate forecasts of intense storm impacts and help to reduce the risk against damage incurred by changes in the storm track and associated extratropical cyclones.
IMPACT: In a changing climate, even small shifts in the storm track position can have a significant influence on regional weather and associated extreme events. However, while midlatitude weather is dominated by cyclones, it is particularly challenging to predict weather in the midlatitudes beyond ten days. Skillful predictions of extratropical storms, on timescales of several weeks to months, rely on the connection between storm tracks and large-scale atmospheric variability modes, such as the stratosphere, thus providing a potential for extending the predictability limits of storms to timescales longer than a few weeks. These predictions can provide early warnings for society and reliable information for decision makers in sectors such as public health, energy, ecosystems and water security.