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)

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Eddy Moisture Fluxes

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

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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

Conclusions

Energy transport in CMIP simulations is quasi-diffusive

Means this effect should cause a spreading of warming to lower latitudes

Several potential zonally symmetric mechanisms for an equatorward shift of the storm track due to Arctic sea ice loss

How robust though?