What does the natural variability of the atmosphere tell us about the Earth’s climate and how it will respond to anthropogenic forcing?
How can simplified models help us understand the dynamics of the atmosphere?
These two questions have shaped my research, the former sharpening my focus, the latter my modus operandi.
Comprehensive Earth system models, as assessed by the Intergovernmental Panel on Climate Change (IPCC), have risen to the forefront of climate research. They are vital for making quantitative predictions of the impact of greenhouse gases and other “man made” perturbations to the climate system. These complex computer codes are a team effort – requiring the resources of a national lab – and built from a mixture of dynamics (i.e. physical laws and equations) and parameterizations (a scientific way to say “fudge factors”) constructed to account for processes that are not well understood, or which are too complex to be simulated. Unfortunately the increasing complexity of climate models has come at the expense of their transparency. My research seeks to ground these comprehensive systems in our theoretical understanding of atmospheric dynamics, with the goal of both better understanding the climate system and improving our ability to simulate and predict it.
My work on the connection between climate change and natural variability (its temporal structure on intraseasonal time scales in particular) has helped open a new front for validating and testing comprehensive climate models. As suggested by fluctuation-dissipation theory, we have found that the ability of models to capture the temporal structure of natural variability is linked to the sensitivity of their climate to external forcing (e.g. Gerber et al. 2008, Kidston and Gerber 2010, Son et. al. 2010, and Garfinkel et al. 2013. The natural variability in comprehensive model simulations can be compared against that of the real atmosphere, allowing us to assess model predictions of future climate change with observations available today.
I have also encouraged the use of idealized atmospheric models to understand the dynamics of the atmosphere, helping to form a bridge between theory and comprehensive models (e.g., Gerber and Vallis 2007, Gerber and Polvani 2009, Gerber 2012, and Cohen et al. 2013. These numerical primitive equation models live in the space between analytic pencil and paper work, which cannot always capture all the relevant physical processes, and comprehensive atmospheric models, which are often too opaque to understand thoroughly. As an example of their usefulness, the connections between natural variability and climate change discussed above were first discovered and explored in idealized models, before we knew to look for them in comprehensive models!
To be more concrete, my research largely falls into these topics, and explores how the topics themselves are interconnected.
Gerber et al. (2012)) provides an introduction to this topic written for a broader audience, outlining how the stratosphere impacts the surface climate. Gerber and Polvani (2009) focuses on the role of stratospheric natural variability in coupling and establishes an idealized model for the stratosphere-troposphere system. I’m also leading a chapter on stratosphere-troposphere coupling for the S-RIP project.
Climate variability on intraseasonal to decadal timescales
Gerber et al. (2010) and Gerber and Vallis (2007), explore intraseasonal variability of the extratropical jet streams in comprehensive and idealized models, respectively. On the other extreme, Li et al. (2014) and Li et al. (2015) show how the atmosphere forms a bridge from the tropical Atlantic to the Amundsen Sea, potentially linking decadal trends in Atlantic sea surface temperatures to changes in sea ice and surface temperature around Antarctica.
The general circulation of the atmosphere
Gerber (2012) illustrates the joint roles of tropospheric wave driving and stratospheric diabatic forcing in controlling the meridional overturning circulation of the stratosphere, or Brewer-Dobson circulation, while Cohen et al. (2013)explores interactions between resolve Rossby waves and “unresolved” (i.e. parameterized) gravity waves.
The impact of anthropogenic forcing on the atmospheric circulation
Gerber and Son (2014) shows how changes in stratospheric ozone have dominated summertime Southern Hemisphere tropospheric circulation trends in austral summer (the ozone hole moved the jet stream!), while Tandon et al. (2013) explores how the structure of tropical warming impacts the jet streams, with implications for how we understand the extratropical response to global warming and ENSO.
Some quick links to projects
The DataWave Project: An international effort to improve the representation of gravity waves in climate models
See the role of chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs) in climate explained in literally 90 seconds.