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. In each case I've provided example
articles.
- Stratosphere-troposphere interactions
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
decadel trends in Atlantic sea surface tempretures 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.
All of my published work can
be found here, and provides a more complete view of my research.