atmospheric energy transports, polar amplification and...
Post on 27-Mar-2018
217 Views
Preview:
TRANSCRIPT
D A R G A N M . W . F R I E R S O N
U N I V E R S I T Y O F W A S H I N G T O N , D E P A R T M E N T O F A T M O S P H E R I C S C I E N C E S
C O L L A B O R A T O R S : Y E N - T I N G H W A N G ( U W , S O O N T O B E S C R I P P S & N A T I O N A L T A I W A N U N I V E R S I T Y ) ,
J E N K A Y ( N C A R )
Atmospheric Energy Transports, Polar Amplification and Midlatitude Climate
Can Polar Regions Affect Climate Elsewhere?
Absolutely!!
There’s much recent work on how climate changes in the extratropics can affect far away locations
As extreme examples, Arctic changes can affect tropical rainfall and even the storm tracks over the Southern Ocean
Can Sea Ice Affect Tropical Precipitation?
Yes! Work by Chiang & Bitz demonstrated this
Strong sensitivity of tropical rain bands to Arctic sea ice increases alone
Rain band shifts away from cooling
Many subsequent studies have confirmed this idea
Moistening
Drying
Change in precipitation,
Last Glacial Maximum sea ice minus
current sea ice (Chiang and Bitz 2005)
NH cooling affects SH jet stream?
Yes! Recent study shows an “interhemispheric teleconnection”:
Poleward shift of SH jet stream in response to NH extratropical cooling alone
From Ceppi, Hwang, Liu, Frierson, and Hartmann 2013, JGR
Surface zonal wind change (contours) and control (shading) from cooling at 50 N
Arctic Influence on Other Latitudes
These are just a few examples in a growing body of literature that takes high latitude influences on other latitudes as a given…
Mechanisms for these studies are based on atmospheric energy transports
So let’s discuss energy transports…
Poleward Energy Transports
Outside of the tropics, the atmosphere transports much more heat than the ocean
Trenberth and Fasullo (2008)
Northward energy transport
Atmospheric Energy Transports
Dry and latent energy transport both contribute to the atmospheric poleward transport
Latent energy transport Dry static energy transport
Source: Trenberth and Stepaniak (2003)
Total transport
When water vapor condenses, it releases latent heat Movement of moisture is important for the energy budget
Dry and Moist Energy Divergence
Components of divergence of energy transport:
Heating from dry static energy transport dominates closer to the North Pole
Latent is smaller poleward of 60o N
Source: Trenberth and Stepaniak (2003)
H
ea
tin
g
the
atm
os
C
oo
lin
g t
he
a
tmo
s
Water Vapor and Global Warming
With global warming, the atmospheric moisture content is increasing
7% increase per degree warming at constant relative humidity
Increased atmospheric heat transport as a mechanism for polar amplification?
More latent energy transported into the high latitudes, where it condenses and releases heat
Shown to be partly significant for polar amplification in GCMs with surface albedo feedback suppressed (e.g., Alexeev et al 2005, Graversen and Wang 2009)
Energy Transports and Arctic Amplification
Does more energy transport lead to more Arctic amplification in GCMs?
From Hwang, Frierson, and Kay 2011
Arc
tic
am
pli
fica
tio
n
No! More polar amplification is associated with less heat transport into the Arctic
Atmospheric transport change across 70 N
Results from CMIP3 simulations: 10 models using A1B scenario 10 models using A2 scenario
Latent and Dry Static Energy Transports
Decomposition into latent and dry static energy:
Latent always increases
Dry always decreases
Latent is similar among models Dry static energy transport causes most of the variation
From Hwang, Frierson, and Kay 2011
Total Atmospheric Energy Transport
Sum of latent transport and dry static energy transport:
While some models increase, many decrease (i.e., transport less energy into the Arctic)
From Hwang, Frierson, and Kay 2011
Why the Anticorrelation?
Transport is responding to temperature gradients
Polar amplification causes weaker temperature gradients, this causes less dry static energy transport into the Arctic
More gradient = more transport (i.e., transport is diffusive)
Let’s look at a couple of examples for illustrative purposes…
Comparison of Extreme Cases
CCCMA (T63) has less increase in flux into high latitudes, MPI has more increase
Factor of two difference in total atmospheric flux in SH
These are slab ocean 2xCO2 experiments, for illustrative purposes
Sea Ice and Cloud Induced Heating
More ice melts in CCCMA
Cloud-induced cooling in MPI
Feedback terms calculated with approximate piecewise radiative perturbation (APRP) method (Taylor et al 2007)
Heating from Sea Ice + Clouds
CCCMA has more net heating in SH high latitudes: Energy transports increase less MPI has cooling in SH b/w 45-65 degrees: Energy transports increase more
Our Argument
We claim:
Latent energy transport always increases (due to warming)
Differences in energy fluxes are due to differences in heating
Forcing by ice-albedo, clouds, aerosols, or ocean heat uptake
Take sea ice as an example:
More sea ice melting => more absorbed SW at high latitudes => less flux into that region
Can be modeled with a (moist) energy balance model
Moist Energy Balance Model
Goal: predict the change in atmospheric energy transport across 65o N
We also predict clear-sky radiation
Assume diffusive transport of moist static energy
Flux proportional to the gradient
Diffusivity is assumed to be:
Constant with latitude
Not changing with climate change
The same for every model
Polar Energy Transports with Global Warming
Energy balance model is accurate at predicting transports given cloud, ice, ocean uptake/ transport changes
We don’t predict surface temperature – need a characterization of lapse rate feedback. (work in progress with Sarah Kang)
See Hwang, Frierson & Kay 2011 for details
Works in Lower Latitudes Too
Can also predict transports at 40o N/S (below)
Ice-albedo, aerosols, clouds & ocean uptake as heatings
We’ve also used to study cross-equatorial energy transports (e.g., Frierson and Hwang 2012)
Midlatitude transport predictions: Captures differences among models, & between slab and coupled simulations
Hwang and Frierson (2010)
Implications for Polar Amplification
Implies that polar amplification is determined primarily by local processes
Moisture transports can cause some amplification, but doesn’t explain model-to-model spread
Other studies have shown that local feedbacks are most important (e.g., Kay et al 2012, Pithan et al, in prep)
Role of the Ocean?
Ocean heat transport (calculated approximately here) is fairly well-correlated with polar amplification – could this drive stronger feedbacks?
Or is the ocean change driven by the amplification?
Change in ocean heat transport
Implications of the Diffusive Framework
Implies that polar warming should spread to lower latitudes
Warmer Arctic warmer midlatitudes
Models with more Arctic warming have anomalous dry static energy transport southward – back towards the midlatitudes
Happens relatively independently to the storm track amplitude/location change within model
Impact of Annular Mode Changes
Our energy transport results were based on century-long global warming though, was only explaining the spread of models, etc Energy transports are by no means the whole story…
Annular modes are important: If the storm tracks shift equatorward (negative phase AO), can definitely have pronounced local cooling patterns Can even happen coincident with quasi-diffusive energy
transport…
Mechanisms for an equatorward shift?
What Determines Storm Track Location?
Much science since CMIP3 on why storm tracks shift poleward with global warming
Some have studied shifts with polar amplification though (e.g., Butler et al 2010)
Two categories of mechanisms:
Where eddies grow
Where waves propagate
Eddies grow where temperature gradients are large & static stability is small
Changes in Eddy Growth/Baroclinicity
Change in potential temperature in CMIP3 multi-model mean global warming simulations:
Large decrease in lower tropospheric temperature gradient in winter. Also decrease in static stability though.
Frierson (2006)
Very seasonal pattern though!
Changes in Wave Propagation
I’ve focused in the past on changes in critical line dynamics (where waves break) See Chen & Held (2007) for application to ozone depletion
With deceleration of high latitude thermal winds, I’d expect more wave breaking at higher latitudes This would likely shift the circulation equatorwards (negative
phase AO)
Baroclinicity and wave propagation are two zonally symmetric mechanisms for an equatorward shift response to sea ice loss...
Need for Intense Study/Simulations
We should perform more studies with a range of models to better understand connections between rapid sea ice loss and midlatitude dynamics
Will we see a similar burst of studies as with the large poleward shift literature after AR4?
I hope so! I’d encourage an inclusion of energy budget diagnostics in such studies
top related