Thomas Reichler and Bowen Zhao University of Utah, Salt Lake City, USA

ROLE OF THE MIDDLE ATMOSPHERE FOR LOW-FREQUENCY CLIMATE VARIABILITY

Changing View of the Stratosphere

ozone solar cycle volcanoes QBO

Changing View of the Stratosphere

ozone solar cycle volcanoes QBO

Changing View of the Stratosphere

ozone solar cycle volcanoes QBO

• In the past, polar vortex underwent pronounced decadal variations

Low-frequency Variability in the Stratosphere

U @ 10 hPa 60N

U1

0 (

m/s

)

• In the past, polar vortex underwent pronounced decadal variations

• Origin of this variability is unclear

Low-frequency Variability in the Stratosphere

SSWs U @ 10 hPa 60N

U1

0 (

m/s

)

• NAM power spectra in control simulations

• Enhanced variability from ocean coupling, both in troposphere and stratosphere

Stratosphere-Troposphere-Ocean Coupling

coupled SSTs

pre

ssu

re

period (yrs)

climatological SSTs

pre

ssu

re

period (yrs)

power

What Causes Variations in Vortex Strength?

Predictions of SH zonal mean u in models with ozone recovery

Son et al. (2008)

1. Thermal forcings in the stratosphere

– natural (volcanoes, sun) or human (GHGs, ozone)

2. Wave forcing from below

– natural or human

Hypothesis

tropospheric perturbation

T

NAO-like response

N

sea-ice, ocean, land

O

Internal Coupled Variability

QBO BDC

O3

stratospheric polar vortex

S

Hypothesis

– sun

– volcanoes

– humans (O3, GHGs)

tropospheric perturbation

T

NAO-like response

N

sea-ice, ocean, land

O

Internal Coupled Variability

QBO BDC

O3 External Forcings

stratospheric polar vortex

S

Hypothesis

– sun

– volcanoes

– humans (O3, GHGs)

tropospheric perturbation

T

NAO-like response

N

sea-ice, ocean, land

O

Internal Coupled Variability

QBO BDC

O3

Closed feedback? • sign • preferred time scale • mechanisms • consequences

External Forcings

stratospheric polar vortex

S

+/-

slow

fast

11 S N

• Stratospheric impact is focused on NAO

– over the North Atlantic, the latitudinal position of the jet favors strong tropospheric eddy feedback (Garfinkel et al. 2013)

– this sustains and amplifies the stratospheric forcing

Stratosphere → NAO

• Much of past research was on the downward influence of intraseasonal vortex variability (SSWs)

• But any change in polar vortex strength impacts the NAO:

Stratosphere → NAO

Gerber & Polvani (2009):

latitudinal change of tropospheric jet (red) associated with vortex strength (blue)

11 N O

• Strong vortex

• +NAO

• Anomalous fluxes of heat, momentum, and fresh water at the ocean surface

• This impacts the ocean, at the surface and below

• Requirement: low-frequency NAO component

Observations: NAO → Ocean

contours: SLP anomalies vectors: wind stress anomalies shading: sensible and latent heat fluxes

Strong Vortex Composites

Observations: NAO → Ocean

• Observed impact of strong vortex anomalies on zonal wind stress and SSTs on the North Atlantic ocean

NAM

Composite of 20 Strong Vortex Events

TAU SST

(over North Atlantic)

Climate Model: NAO → Ocean

• Multi-year-long polar vortex variations:

– drive NAO

Climate Model: NAO → Ocean

• Multi-year-long polar vortex variations:

– drive NAO

– cool North Atlantic SSTs

– create anomalous sinking into deep ocean

Climate Model: NAO → Ocean

• Multi-year-long polar vortex variations:

– drive NAO

– cool North Atlantic SSTs

– create anomalous sinking into deep ocean

– drive oscillation in the MOC and change basin-wide SSTs (AMO)

• Similar in other models

• Still unclear what drives vortex variations

• Reichler et al. (2012) – SSW-induced AMOC modulation – temperature driven: -NAO warms SSTs – key: collocation of NAO nodal point with

downwelling region

• Menary and Scaife (2014) – solar-induced AMOC modulation – salinity driven: complicated chain of events, no

clear NAO involvement

• Scaife et al. (2013); Gray et al. (2013) – solar-induced SST signal – extended memory of ocean heat content

anomalies through re-emergence mechanism – SST signal feeds back on NAO – similar ideas: Schimanke et al. (2011)

Proposed mechanisms: S → N → O

S: solar signal

11 O T

• Certain regions are particularly important for stratospheric impact – Eastern Europe (H) – North Pacific (L)

• Related to structure of stationary waves (#1 & 2)

Ocean → Troposphere

Precursor Regions

Garfinkel & Hartmann

2011

• Anomalous SSTs and sea ice perturb geopotential height Z

11 T S

Change in planetary waves impact the polar vortex:

• Propagation of waves

– zonal wind and static stability in the tropopause region is critical

• climate change related change in temperatures

• intensifying, poleward, & upward moving westerlies

• Generation of waves

– surface forcings

• ocean: ENSO, AMO, PDO, warming SSTs

• sea ice changes

• Eurasian snow cover

Troposphere → Stratosphere

Summary

• Coupling of the stratosphere with other climate components may also involve the ocean

• This may create closed feedback loops

• Ocean may be initiator and/or recipient of low-frequency variability

tropospheric perturbation

TNAO-like response

N

sea-ice, ocean, land

O

closed feedback

loop?

QBO BDC

O3stratospheric polar vortex

S

Thank You