Download Atmosphere and Ocean as Dynamic Drivers of Polar Climate

Atmosphere and Ocean as Dynamic Drivers of Polar
Climate Sensitivity
Hansi A. Singh
Applicant, NOAA CGC Postdoctoral Fellowship
Research Statement
Background & Motivation
The polar regions only occupy a small fraction of the the globe, but their role in moderating
climate globally is incontestable (Lindzen & Farrell, 1980; Zachos et al. , 2001). Today, the
Arctic is undergoing large-scale climatic changes that are unparalleled elsewhere, including
sharply declining sea ice (Comiso, 2002) and rapidly rising surface temperatures (Jones
et al. , 1999; Kaufman et al. , 2009), at least partly due to anthropogenic forcings. In
the Antarctic, on the other hand, changes have been more mixed: while Southern Ocean
temperatures are rising (Gille, 2008) and regions of the West Antarctic Ice Sheet are melting
(Pritchard et al. , 2012), sea ice area around the Antarctic continent has expanded modestly
(Parkinson & Cavalieri, 2012) and the East Antarctic Ice Sheet is gaining mass (Boening
et al. , 2012). The aim of the proposed research is to understand why the Arctic is especially
sensitive to anthropogenic climate perturbations, particularly on short time scales, and why
the Antarctic is much less so. Pursuing these lines of inquiry will help improve prediction
of polar climate change on a range of time scales, and help facilitate understanding of how
polar climates fit into the framework of planetary climate as a whole.
Past studies have suggested that enhanced climate sensitivity in the Arctic is due to
stronger local feedbacks, though there is some disagreement regarding which local feedbacks are most crucial. Winton (2006) implicated stronger local longwave feedbacks due to
temperature, water vapor, and clouds. The crucial role played by longwave feedbacks has
been substantiated by other studies, which find that greater atmospheric precipitable water
(Chen et al. , 2011; Ghatak & Miller, 2013) and positive cloud feedbacks (Intrieri et al. ,
2002; Vavrus, 2004) may play a role in strong regional climate sensitivity in the Arctic. On
the other hand, Pithan & Mauritsen (2014) found that both the local lapse rate feedback and
the ice-albedo feedback were key. While some studies have implicated the ice-albedo feedback in augmenting polar climate sensitivity (Screen & Simmonds, 2010a,b), experiments
with locked albedo (Graverson & Wang, 2009) and with warm climate states (Alexeev et al.
, 2005; Rose & Ferreira, 2013) indicate that enhanced polar climate sensitivity is also characteristic of climates where there is no ice-albedo feedback. Other studies have also recognized
the importance of the positive lapse rate feedback in the high latitudes (Boe et al. , 2009;
Bintanja et al. , 2011).
Nevertheless, there remain many puzzling aspects of polar climate sensitivity that suggest
a more global etiology than this local one. As already noted, the Arctic is very sensitive to
anthropogenic climate forcing while the Antarctic is much less so (Turner et al. , 2007), hinting that the very different mean states of these two polar regions, due to different land-ocean
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Research Statement
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configurations, high-latitude orography, and ocean basin layout, may play a role in mediating these very different perturbation responses. In the northern hemisphere, the midlatitude
Rocky and Himalayan mountain ranges make the jet and storm tracks very azonal (Manabe
& Terpstra, 1974), with these circulations extending particularly far northwards over the
Atlantic basin. The only region where surface waters sink to great depths in the northern
hemisphere is in the Greenland-Iceland-Norwegian Seas (Schmitz & McCartney, 1993), and
the pole itself is ocean-covered but flanked by sizable continents. In the ocean-dominated
southern hemisphere, on the other hand, orographic driving is minimal, and the jet and
storm track are strongly zonal and relatively seasonally-invariant (Peixoto & Oort, 1992).
The south pole itself is covered by the thick Antarctic ice sheet (over 3 km high in some
regions), which creates a distinct atmospheric circulation (Egger, 1992, 1994). Deep sinking
of surface waters occurs around the entire Antarctic continent, and the Southern Ocean is
the most important region for ocean heat uptake globally (Purkey & Johnson, 2010).
Given the differences in the large-scale atmosphere and ocean dynamics of the northern
and southern hemisphere polar regions, which are, in turn, due to differences in orography
and continental configuration, it is, perhaps, not surprising that the northern and southern
polar regions respond so differently to anthropogenic forcings. Indeed, many of the strong
local feedbacks that characterize the Arctic may be linked to how the large-scale dynamics
of the northern hemisphere responds uniquely to anthropogenic forcings. Both enhanced
longwave and lapse rate feedbacks in the Arctic, for example, may be linked to changes in
how dry static energy and moisture are transported to the high latitudes; receding sea ice,
which causes a strong ice-albedo feedback, may be linked to increased ocean heat transport.
Similarly, weaker local feedbacks over the Antarctic may be linked to unique circulation
responses in the southern hemisphere, which alter alter atmospheric and oceanic energy
transport differently than in the Arctic.
The goal of the proposed work will be to describe exactly how changes in the large-scale
dynamics, particularly how energy converges in the polar regions, mediates greater climate
sensitivity. While previous studies have identified various local factors that enhance polar
climate sensitivity, they do not elucidate the roles of ocean and atmospheric dynamics in
mediating the polar climate response to a forcing. How do changes in meridional energy
transport from the lower latitudes, either by the ocean or the atmosphere, enhance polar
climate sensitivity? Similarly, how does the relative partitioning between changes in ocean
energy transport and atmospheric energy transport affect the polar climate response? Finally, what role do changes in energy transport play in enhancing local feedbacks, such as
that due to ice or water vapor? I propose to investigate the role of large-scale dynamics in mediating polar climate sensitivity by focusing, in turn, on oceanic and atmospheric
mechanisms. I describe two projects that I will complete below.
I: Understanding the Role of Ocean Dynamics
I will begin my work by considering the role of ocean dynamics, particularly ocean heat
transport and heat uptake, in moderating polar climate sensitivity. The polar regions are a
strongly coupled system, and interactions between the atmosphere, ocean, and intervening
sea ice orchestrate the climate response. Sea ice is particularly sensitive to changes in oceanic
energy transport (Holland & Bitz, 2003), suggesting that at least some warming attributable
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to the ice-albedo feedback may involve an oceanic mechanism; on the other hand, sea ice loss
itself may also play a role in moderating ocean heat transport into the high latitudes (Deser
et al. , 2015). The rate of ocean heat uptake strongly affects transient climate sensitivity, with
slower warming corresponding to more rapid high latitude heat uptake (Winton et al. , 2010);
Rose et al. (2014) found ocean heat uptake in the high latitudes to be particularly effective
in damping transient polar climate sensitivity. Intriguingly, a GCM with a slab ocean model
(with prescribed ocean heat transport and uptake, or q-fluxes) responded to CO2 -doubling
with equal warming over the Arctic and Antarctic, unlike the fully-coupled case in which
the Arctic warmed much more strongly than the Antarctic (Bitz et al. , 2006), indicating
that the differing ocean circulation responses may help explain the different sensitivities of
the northern and southern polar regions. While one feedback study has suggested very little
ocean impact on polar warming (Pithan & Mauritsen, 2014), others show a strong positive
correlation between changes in ocean heat transport and the polar climate response (Holland
& Bitz, 2003; Hwang et al. , 2011).
Although polar warming is clearly coupled to ocean dynamics, there have been few mechanistic studies that explore how transient and steady-state changes in ocean dynamics with
CO2 forcing impact the polar climate response. Once heat is absorbed by a column of water
in the high latitude oceans, how much is transported into the deep ocean and how much contributes to in situ warming? What is the role of changes in circulation, either changes in the
mean meridional circulation or synoptic-scale ocean eddies, in this movement of heat? How
do circulation changes compare to the role of changes in temperature gradients? And how do
changes in high latitude ocean heat uptake translate into greater heat convergence into the
polar ocean surface waters, where such changes can alter surface heat fluxes and melt sea ice?
Finally, what is the process by which the polar oceans attain a new steady state following a
period of large ocean heat uptake accompanied by circulation adjustments? These questions
highlight two important considerations: (1) how the polar oceans themselves adjust over
time to a forcing; and (2) how the polar atmosphere responds to the ocean’s adjustment
process.
In order to understand the role of ocean dynamics in mediating polar climate sensitivity,
I propose to conduct a series of experiments with the Community Earth System Model
(CESM). The first experiment will be a fully-coupled CO2 -doubling experiment, which will
be integrated to quasi-equilibrium (i.e. the net top-of-atmosphere anomaly is of the same
magnitude as that of the preindustrial control simulation). This experiment will be used
to study transient adjustments in ocean dynamics, as increased ocean heat uptake (and
correspondingly lower transient climate sensitivity) gradually gives way to a new steadystate ocean heat transport. A time-dependent heat budget analysis of the high latitude
oceans will be performed in order to highlight links between heat uptake, heat transport,
and the polar climate response, including sea ice retreat and local evaporation increases.
In a second set of experiments, CESM will be run in a slab ocean configuration (Hurrell
et al. , 2013), some with CO2 doubled; in these slab ocean experiments, both the ocean
heat transport and deep ocean heat uptake will be adjusted by varying the prescribed qfluxes. The q-fluxes will be linearly interpolated between the preindustrial control ocean
heat transport and the quasi-equilibrium CO2 -doubled ocean heat transport, and will be
superposed with different spatial patterns and rates of ocean heat uptake. These slab ocean
experiments will be compared to the fully-coupled runs, and will be used to understand how
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the atmosphere responds to changes in high latitude ocean heat uptake and transport.
II: Understanding the Role of Atmospheric Energy Transport
Like changing ocean heat transport, changing atmospheric energy transport may also mediate polar climate sensitivity. Indeed, at interannual time scales, Arctic surface temperatures
are well-correlated to poleward atmospheric energy convergence (Yang et al. , 2010). While
poleward transport of atmospheric dry static energy is expected to decrease with warming, moisture transport from the lower latitudes is expected to increase (Langen & Alexeev,
2007; Lu & Cai, 2010; Hwang & Frierson, 2010). This increase in moisture transport is
linked to a poleward shift in general circulation features (Yin, 2005; O’Gorman, 2010) and
increased eddy length scales (Riviere, 2011; Lorenz, 2014), which tie polar climate sensitivity
to planetary-scale dynamical changes with CO2 -induced warming. Increased moisture transport may contribute to enhanced longwave warming at the surface directly through radiative
impacts and condensational heating, and indirectly through cloud feedbacks (Winton, 2006;
Ghatak & Miller, 2013). Nevertheless, several studies have found a negative correlation between changes in atmospheric moist static energy transport and polar climate sensitivity
(Hwang & Frierson, 2010; Pithan & Mauritsen, 2014). Furthermore, other studies suggest
that moisture-induced warming in the Arctic is primarily due to locally-enhanced evaporation rather than moisture transport from lower latitudes (Screen & Simmonds, 2010b;
Bintanja & Selten, 2014), a puzzling finding given that Arctic warming is largest in winter,
when local evaporation is inhibited by sea ice cover.
In order to understand how changes in atmospheric dynamics affect polar climate sensitivity, I propose to run a series of modeling experiments with standard dry and moist dynamical
cores (Held & Suarez, 1994) to understand how changes in the eddy length scale and jet latitude impact moist static energy (MSE) transport. In these experiments, I will modify the
eddy length scale and jet latitude (for an example illustrating how the eddy length scale and
jet latitude may be adjusted in a dynamical core, see Lorenz, 2014), and compute how these
changes impact dry static energy transport and moisture transport by eddies, given varying background temperature gradients (surface, mid-tropospheric, and upper-tropospheric),
moisture gradients, and static stability profiles. These experiments will provide an overview
of how the robust poleward migration of general circulation features with warming, which
characterizes anthropogenic forcing by greenhouse gas emissions, will impact poleward MSE
transport. These experiments will also help constrain the vertical distribution of changes in
MSE transport, which may be useful for understanding the high latitude lapse rate feedback,
as described below.
In order to understand the high latitude lapse rate feedback and its link to changes in
atmospheric MSE transport, I will use NCAR’s Column Radiation Model (CRM) to compute
the surface warming and top-of-atmosphere net impact of moisture (both vapor and cloud)
and heating at different levels in the atmospheric column. The background state will be an
atmospheric temperature inversion, as found in the Arctic in winter. Given that erosion of
this inversion has been associated with rapid surface warming in models (Bintanja et al. ,
2011), such experiments will elucidate the conditions in which MSE transport from the lower
latitudes may invoke such abrupt erosion of the surface inversion. Results from the previous
experiments using dynamical cores, in which the effect of poleward migration of atmospheric
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general circulation features and increases in eddy length scales on the MSE transport have
been quantified, will help constrain at which atmospheric levels MSE transport changes are
most likely to be found. Experiments with the CRM can be used to assess which, if any,
MSE transport changes may affect the high latitude vertical temperature profile.
Summary
In this proposal, I have presented two distinct series of experiments for isolating the impact of
atmospheric and oceanic dynamical mechanisms in mediating polar climate sensitivity. This
approach relies, in part, on the use of models that lie between full GCMs and simple energy
balance models in the complexity spectrum; while polar climate sensitivity has already been
studied extensively with the use of box models (Flannery, 1984; Cai, 2005) and fully-coupled
GCMs (Winton, 2006; Pithan & Mauritsen, 2014), there is little study in the literature using
models that fall between these in the complexity spectrum. As I have argued here, such
intermediate complexity models may be ideal tools for thorough, mechanistic studies of how
changes in atmospheric and oceanic dynamic mechanisms may drive polar climate sensitivity.
In addition to the specific experiments outlined above, I will also use the archive of GCM
experiments in the Fifth Community Climate Model Intercomparison Project (CMIP5), data
from a variety of atmospheric observational and reanalysis products, and output from the
Large Ensemble Community Project (Kay et al. , 2015) to clarify the role of the dynamical
mechanisms elucidated by the experiments described above. In particular, I will consider
the extent to which these mechanisms operate in the full Earth system, as in the real world
or as simulated in a full atmosphere-ocean GCM, compared to more idealized models. Such
comparisons serve to provide a reality-check on the modeling experiments described earlier,
and will also permit study of how dynamic mechanisms differ across models, and between
models and observations.
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