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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