Key Questions
How does the stratosphere influence surface weather and climate?
Background
The stratosphere is the region of the atmosphere between ~10-50 km above the Earth’s surface that includes the ozone layer. Its fluid flow differs from the underlying troposphere. Ozone heating cause temperatures to increase with height, so it is statically stable. There is also very little water vapour, clearly evident from an aircraft window with beautiful sunny, blue skies once we emerge above the clouds. Winds are much stronger than in the troposphere, with huge variability in winter when they can reverse from >80ms-1 westerlies to >50ms-1 easterlies in just a few days. These sporadic events are called Stratospheric Sudden Warmings (SSWs) because of associated large adiabatic temperature changes. They involve large-scale wave-mean flow interactions, possibly involving resonance, but there is still much we do not understand about their internal generation mechanisms. Their frequency is known to be influenced by external factors including greenhouse gas increases, explosive volcanic eruptions, 11-yr solar cycle variations and El Nino events. SSWs are known to influence surface weather and improved understanding of their fundamental mechanisms and representation in forecast models will improve seasonal and decadal-scale weather predictions.
Aims of the Project
To improve our fundamental understanding of dynamical variability of the stratosphere and how it influences surface weather and climate over Europe and the North Atlantic.
Project Description
A number of different aspects can be included in this project, depending on the candidate’s interests. These include
-
The influence of the 11-year solar cycle on surface weather and climate: a mechanism via the solar cycle influence on stratospheric ozone > temperature > winds > SSWs > surface is just one of several proposed mechanisms of influence; there is little understanding of which is the most influential route and models are currently unable to adequately capture the observed signals.
-
The influence of explosive volcanic eruptions on surface weather and climate: the widely accepted mechanism here is that volcanic aerosol particles lead to heating of the equatorial stratosphere, increased latitudinal temperature gradients, changed winds and their influence on the underlying troposphere, the mechanisms of which are not well understood; models do not capture the observed volcanic signal well, although there are very few recent well-observed eruptions, so validation against observations is also challenging.
-
Stratosphere-Troposphere Interactions: although there are well observed influences from the stratosphere onto the troposphere following SSWs, not all SSWs have a significant influence at the surface and the exact mechanism for the influence and the important influencing factors are not well understood; there is conflicting evidence for whether the exact nature of the large-scale stratospheric flow is important e.g. whether the SSW is dominated by wavenumber 1 or 2 or whether the local impact on synoptic-scale baroclinic waves is more important.
Methods to be used
All of the projects require a mixture of observational data analysis and climate model experimentation. The observational data consist primarily of ‘reanalyses’ i.e. best estimates of the observed winds, temperatures, heating terms, circulation tendencies, wave interactions etc by merging available observed quantities into forecast models. The model experiments will be performed using the state-of-the-art Met Office Hadley Centre Unified Climate Model.
Specialised skills required
The project will involve analysis of fluid flow, so a background in physics / maths is essential. Extensive diagnostic will require statistical skills.
Please contact Prof. Lesley Gray (Lesley.gray@physics.ox.ac.uk), Prof. Tim Woollings (tim.woollings@physics.ox.ac.uk) or Dr. Scott Osprey (scott.osprey@physics.ox.ac.uk) if you are interested in this project.
