Meteorology: weak polar vortex makes weather more predictable
LUDWIG-MAXIMILIANS-UNIVERSITÄT MÜNCHEN
Events in the stratosphere are making long-range weather in Northern Europe easier to forecast, researchers at LMU have discovered.
Weather is a chaotic system and predicting weather conditions several weeks in advance poses considerable challenges. The accuracy of such long-range forecasts remains generally quite low. Accordingly, even moderate improvements can prove valuable for various sectors. For instance, farmers rely on these forecasts to determine optimal sowing and harvesting times, energy providers use them to anticipate fluctuations in renewable energy production, and public health officials use them to prepare for outbreaks of diseases such as malaria or dengue fever, which are correlated with specific weather conditions.
Researchers at LMU are now investigating a phenomenon that has its origin in the stratosphere, the layer of our atmosphere situated 15 to 50 kilometers above our heads. “Previous work has shown that during Northern winter the state of the circulation in the polar stratosphere may provide useful information for improved long-range forecasts, especially for weather over the North Atlantic and Eurasia,” explains Thomas Birner, Professor of Theoretical Meteorology at LMU. In particular, when the polar vortex (a band of strong eastward circumpolar flow at stratospheric levels) strongly weakens or breaks down, the North Atlantic jetstream tends to shift southward and the likelihood of cold spells over Eurasia increases. Such vortex breakdowns are relatively rare events that only happen approximately every other winter. But its time has come round again: “One such event is currently unfolding with corresponding expected impacts on Eurasian weather in the coming weeks.”
And now for the weather: cold, but less chaotic
In a study published recently in the journal Communications Earth & Environment, LMU meteorologists highlight an additional aspect of stratospheric influence on long-range weather forecasts: Weak polar vortex states, such as the one currently prevailing, are typically followed by reduced uncertainty of 3-5 week forecasts over Northern Europe. The authors found that ensembles of forecasts show a reduced range of possible weather conditions by about 25%. Such ensembles are made up of a large number of individual forecasts, which typically diverge at longer forecasting periods. After weak polar vortex events there is less spread among these forecasts over Northern Europe, making the weather more predictable.
“We attribute this reduced forecast uncertainty to the southward shift of the North Atlantic jetstream,” says Jonas Spaeth, doctoral student at LMU’s Meteorological Institute and lead author of the new study. The associated southward shift of the tracks of winter storms, which are the main source of forecast uncertainty during this season, causes less storm activity and thereby reduced forecast uncertainty over Northern Europe. Conversely, forecast uncertainty increases over Southern Europe.
“Our study sheds light on meteorological phenomena where uncertainty of weather forecasts several weeks in advance systematically reduces or increases,” says Jonas Spaeth. “Furthermore, it underscores how the practical use of long-range forecasts can benefit from a deeper understanding of the remote coupling across different atmospheric regions.”
JOURNAL
Nature
ARTICLE TITLE
Stratospheric impact on subseasonal forecast uncertainty in the Northern extratropics
ARTICLE PUBLICATION DATE
14-Mar-2024
Advances and challenges in understanding compound weather and climate extremes
This study is led by Prof. Zengchao Hao (College of Water Sciences, Beijing Normal University) and Prof. Yang Chen (State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences). In the context of global warming, many extremes, such as heatwaves, heavy precipitation, and droughts, have become increasingly frequent and intense, as expected theoretically. Somewhat unexpectedly, these extremes have also exhibited tightened linkage in both time and space, constituting compound weather and climate extremes with larger impacts. During the past decade, compound events received considerable attention, with much progress in event typology, impacts, changes and risks already made.
By synthesizing ~350 peer-reviewed papers, the authors thoroughly documented definition and impacts, physical mechanisms, historical/future changes as well as attribution evidences, with respect to 13 reported and relatively well-studied compound events. Some of these events are specific to East Asian monsoonal regions. They also pointed out deficiencies and gaps of existing studies on each of these events. At the end of the review, they attempted to identify data and methodological challenges common to the field and came up with outlooks on the future directions of the emerging topic.
More specifically, they laid out their review by the order of definition, mechanisms, changes and attribution. For each of reviewed events, the authors adopted an impact-centric approach to introduce the definition by illustrating how the fashion of compounding aggregated and amplified impacts. Distinct from previous reviews on some types of compound events focusing largely on long-term changes, the new review assigned a large volume of space to the underlying physical processes, especially from the dynamic (including monsoon dynamics) and multi-sphere interactive perspectives. Despite rapid development of event attribution methods, attribution dedicated to compound events remains in its infancy. The author team also tried to assess confidence of attribution conclusions for compound events, where are available, and to figure out the source of attribution uncertainties.
In the prospect section, the authors introduced unique requirements on data and methodological design to study compound events, as opposed to those for univariate extremes. In particular, much longer records, both in observations and simulations, are warranted to sufficiently sample compound events. For statistical fitting, model evaluation and metrics constructed for attribution, inter-event dependence or coincidence should be factored into. Regarding mechanistic diagnosis, potential interactions amongst drivers of different scales and spheres, and exposure and vulnerability altered by the concurrence and/or sequence of extremes need to be addressed in a proper manner. As with attribution, given the difficulty in simulating the interacting and cascading processes in most of free-running attribution models, a storyline scheme that is able to faithfully reproduce the dynamically unfolding of events represents a promising way forward. For the projection and risk assessment, internal variability of various scales should be no longer treated as noise; but rather seen as an indispensable driver that is equivalently important to external forcings in shaping the compounding patten of events.
The review might be used as a general guide to identify and analyze compound events, and also adds some incentives for multi-disciplinary collaborations to better prepare against and adapt to compound hazards in a changing climate.
See the article:
Hao Z, Chen Y. 2024. Research progresses and prospects of multi-sphere compound extremes from the Earth System perspective. Science China Earth Sciences, https://doi.org/10.1007/s11430-023-1201-y
JOURNAL
Science China Earth Sciences
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