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The Stratosphere: Role in Climate and Seasonal Weather Prediction

Prof Piers Forster (SEE), Dr Ryan Neely III (SEE), Dr Dan Marsh (NCAR, Boulder, U.S.A.)

Contact email: p.m.forster@leeds.ac.uk

Overview and scientific need

The stratosphere is the atmospheric layer located just above the troposphere. It is crucial for life in Earth because it contains 90% of the atmospheric ozone that shields life against harmful solar radiation. The stratosphere also affects surface weather and it may be the key to being able to make seasonal forecasts of European weather. In this project the student will undertake cutting edge simulations to understand the role of the stratosphere in seasonal forecasts. The simulations will be done with the state-of-the-art WACCM model that accounts for the upper air effects of solar radiation. The simulations will be designed in collaboration with the National Center for Atmospheric Research in Boulder, Colorado, and the candidate is expected to spend time working with scientists there.

NASA image of the Space Shuttle flying through the upper atmosphere (2010)

One hundred years ago the stratosphere was considered dynamically inactive, but pioneering studies found that this layer experiences dramatic changes in winter, and connects the equator and the poles through a meridional overturn known as the Brewer-Dobson circulation (Dobson et al., 1930; Brewer, 1949). Later work attributed the origin of the stratospheric dynamics and variability to tropospheric phenomena. However, more recent studies have found that the stratosphere impacts the lower atmosphere. These studies demonstrate the existence of two-way vertical interactions between the stratosphere and the troposphere (Baldwin et al. 2003). The coupling occurs through two pathways:

  1. From the troposphere to the stratosphere (bottom-up coupling). During the winter of each hemisphere planetary-scale waves propagate towards the stratosphere only when its background winds are westerlies and lower than a certain threshold (e.g., Shepherd 2000). The breaking and subsequent dissipation of the waves modulate the stratospheric flow, decelerating the polar vortex, an intense westerly wind jet in the winter polar stratosphere that is generated by radiative cooling during the polar night. Thus, stratospheric winter variability is dominated by episodes of weakening and strengthening of the polar vortex. The time scales of these events are longer than those of the troposphere, due to the longer radiative relaxation times in the stratosphere (e.g., Limpasuvan et al. 2005).
  2. From the stratosphere to the troposphere (top-down coupling). Changes in the stratosphere can in turn propagate downward, affecting the troposphere and surface. Unlike the bottom-up coupling, this top-down coupling is not well understood (e.g., Gerber et al. 2012). The downward signal is often well characterized by anomalies of the Annular Mode (AM), which captures the dominant pattern of intraseasonal to interannual variability in the extratropical atmosphere (e.g., Thompson et al. 2002). In the stratosphere, the AM (NAM in the northern hemisphere, SAM in the southern hemisphere) measures the intensity of the polar vortex, while its tropospheric counterpart, called the Arctic Oscillation (AO) / North Atlantic Oscillation (NAO) in the northern hemisphere, and the Antarctic Oscillation (AAO) in the southern hemisphere), modulates the meridional position of the extratropical jet stream and the trajectories of the storms. Thus, the downward propagation of stratospheric anomalies affects the temperature and precipitation over continental regions (e.g., Baldwin and Dunkerton 2001).

As the stratospheric variability is associated with wave-mean flow interactions, it depends on the wave activity coming from the troposphere but also on the background flow conditions of the stratosphere, which determine the pathways the waves can propagate through. Thus, the sources of stratospheric variability can reside in the troposphere (i.e., the bottom-up coupling) but also in other agents capable of modulating the basic state of the upper atmosphere. Such disturbances to be examined in this project may include large volcanic eruptions, sudden stratospheric warmings (which are extreme phenomena associated with a dramatic warming of the winter polar stratosphere and an abrupt reversal of the zonal wind) solar events (such as flares and coronal mass ejections) and hurricanes.

Further to investigating stratospheric processes and their role in climate, this project will also examine the role in climate these coupling processes have across the whole atmosphere when the atmosphere experiences a significant disturbance.

 Possible solar effects on climate from Grey et al., 2010

Your exploration of the atmosphere will based on simulations created with the latest version of NCAR’s Whole Atmosphere Community Climate Model (WACCM). WACCM is a state-of-the-art model that is a core component of NCAR Community Earth System Model which has been successfully used by numerous studies examining climate and the upper atmosphere. The simulations will be created by designing a set of numerical experiments that target a set of specific hypothesis developed at the onset of your project to target the objectives set out below.

Climate models with a well-resolved stratosphere (the so-called high-top models; such as WACCM) are able to simulate the observed 20th century climate change better than most models without detailed stratospheric dynamics (low-top models). The top-down coupling can also influence future climate projections. In the Northern Hemisphere, for example, future changes simulated by high-top models have significantly different projections of precipitation and storminess for Europe (e.g., Scaife et al. 2011; Marsh et al. 2013). Thus, improving our understanding of the processes involved in the coupling of the upper atmosphere to the surface are key to improving our ability to make accurate regional climate predictions.

Scientific objectives

This project aims to systematically investigate the role large upper atmospheric disturbances have on surface climate. In particular, you will improve our understanding of upper atmospheric variability and the associated vertical coupling as a way to improve the characterization of the climate responses to external forcings and internal variability. More specifically, the initial goals of this project are envisioned to be:

  1. Characterize the effects of external (solar and volcanic) changes on the stratosphere and their associated effects on the surface, quantifying the differences arising from uncertainties in the external forcings.
  2. Test how the properties of the stratosphere may affect European climate, particularly the predictability of European winter weather.
  3. Better characterize the impact of the upper atmosphere on the surface climate.

Financial support

In addition to the stipend and research money that comes with a NERC studentship, this work will be supported by the Priestley International Centre for Climate and the U.S.’s National Center for Atmospheric Research. This includes the opportunity to time in at NCAR in Boulder, Colorado to design the model simulations and incorporate new physics or chemistry into the model where appropriate.

Potential for high impact outcome

This project will provide a novel and synergistic view of climate and upper atmospheric processes in order to: 1) better understand the role of the upper atmosphere in the climate’s response to large internal and external forcings; 2) improve the current understanding climate by incorporating the added value of the whole atmosphere, current knowledge of the external forcings and recent advances in modelling their interactive roles; 3) improve seasonal forecasts.

Training

You will work directly under the supervision of Prof Piers Forster, Dr Ryan Neely III and Dr Dan Marsh. You will also become an active member of each PIs’ research group and, thus, benefit from working within an active and multidisciplinary group of scientists within Physics, Chemistry, the School of Earth and Environment and the National Centre for Atmospheric Science.

This project will equip you with the necessary expertise to become a leader in the next generation of atmospheric scientists, ready to carry out your own programme of innovative scientific research. These skills will be developed by a mixture of hands on experience, attending external training courses, and taking part in the Leeds – York NERC Doctoral Training Partnership programme. This includes access to a broad spectrum of training workshops put on by the Faculty that consist of a range of extensive training workshops that will help you manage your degree and prepare for your viva (http://www.emeskillstraining.leeds.ac.uk/).

Specifically, this project will provide highly transferrable scientific training in a range of growth areas/skills including: i) numerical modelling; ii) communication of these results to the broader scientific community at international meetings and iii) working in a team with a broad range of expertise both within and outside of Leeds.

Requirements

A good first degree (1st or good 2i), or a good Masters degree in a physical or mathematical discipline, such as mathematics, physics, geophysics, engineering or meteorology is required. Experience in computer programming (e.g. UNIX, Python, Matlab, IDL, R…) and fieldwork experience is an advantage.

Contact Information

Contact is strongly encourage before application so that we may discuss your interests and project specifics. Help with the application process may also be provided. Enquires should be made by contacting Piers Forster (p.m.forster@leeds.ac.uk) or Ryan Neely III (R.Neely@leeds.ac.uk) in the first instance.

Related undergraduate subjects:

  • Applied mathematics
  • Astronomy
  • Atmospheric science
  • Chemistry
  • Computer science
  • Computing
  • Earth science
  • Earth system science
  • Engineering
  • Geochemistry
  • Geophysical science
  • Geophysics
  • Geoscience
  • Mathematics
  • Mechanical engineering
  • Meteorology
  • Natural sciences
  • Oceanography
  • Physical science
  • Physics
  • Remote sensing