Fast and Slow: The
Dynamics of Superrotation
Phenomena in Planetary
Atmospheres:
V. Slow rotators
Peter Read
University of Oxford, UKPlan• Local and global super-rotation in atmospheres around slowly
rotating planetary bodies• Titan• Observations• Models and mechanisms• Venus• Observations• Models and mechanisms• Tidally-locked exoplanets• Equatorial jets?
• Mechanisms• Conclusions, synthesis and outlook

(Sharkey et al., 2021)

Key features of Titan
Parameter

Titan

Earth

Radius (km)

2575

6371

Rotation period (Earth days)

16

1

Surface gravity (m s-2)

1.35

9.81

Surface pressure (bar)1.5

194% N2
5% CH478% N2
21% O2~.1% H2O

Insolation (W m-2)

14.9

1368

Obliquity (deg)

27

23.45

Eccentricity

0.029

0.0167

Length of year (Earth years)

29

1

Atmospheric composition

Late NH
summerSpringequinox

Early NH
summerNH summer
solstice(Hörst, 2017)(Fulchignoni et al., 2005)

Key features of Titan• Complicated radiative budget
and T profile• Greenhouse-like troposphere

Stratosphere• Heated via surface• Anti-greenhouse stratosphere• Heated directly via hazes• Complicated chemistry andthermodynamics• Optically thick photochemicalhazes in stratosphere• Condensible constituents (CH4)with clouds, rain, lakes….

• Strong seasonal variability

TroposphereHow do we know Titan’s atmospheresuper-rotates?

[Sharkey et al. 2021]• Observations• BUT very few cloud/haze features totrack• IR remote sensing [Voyager, Cassini]• T via gradient thermal wind equation• Not valid close to the equator!

• Stellar occultations• Central flash senses 𝜌(𝜑, 𝑧 ≈ 0.25 ℎ𝑃𝑎)

• Doppler spectroscopy in IR, visible
and microwaves• Senses p ~ 0.1 – 1 hPa• Huygens descent probe

How do we know Titan’s atmospheresuper-rotates?
Late NH
summer[Sharkey et al. 2021] Springequinox• Observations• BUT very few cloud/haze features totrack• IR remote sensing [Voyager, Cassini]

Early NH
summer• T via gradient thermal wind equation• Not valid close to the equator!

• Stellar occultations• Central flash senses 𝜌(𝜑, 𝑧 ≈ 0.25 ℎ𝑃𝑎)

• Doppler spectroscopy in IR, visible andmicrowaves• Senses p ~ 0.1 – 1 hPa• Huygens descent probe• 𝜑 ≈ 10o S• >100 m s-1 prograde winds above 100 km
altitude(Bird et al. 2005)

NH summer
solstice

How do we know Titan’s atmospheresuper-rotates?
• Observational coverage is sparseand incomplete• Need to rely significantly onmodels and simulations(Hourdin et al. 1995)• Simple GCMs

• Cases where ℛ ≈ 10 are relevant totime-mean climatology• Comprehensive GCMs

• Realistic parameterizations of radiativetransfer, sub-grid dynamics, clouds,
hazes and even chemistry• Variable success in capturing superrotation….?
• Though see Hourdin et al. [1995] forearly sucess[Here 𝜇 = 𝑚/

!
"

Ω𝑎! ]

How do we know Titan’s atmospheresuper-rotates?
𝑇'𝑢'(Hourdin et al. 1995)• Comprehensive
GCMs• Variable success in
capturing superrotation….?
• Seasonal variations
in T, 𝑢! and Y

N
SummerN
SpringΨHow do we know Titan’s atmospheresuper-rotates?
(Hourdin et al. 1995)• Comprehensive
GCMs• Variable success in
capturing superrotation….?
• Seasonal variations
in T, 𝑢! and Y• T, 𝑢! compare
reasonably wellwith (sparse!)
observations….

T𝑢'How do we know Titan’s atmospheresuper-rotates?
TitanWRF
IPSL• Comprehensive GCMs
• Most successful forsuper-rotation from 3
main groups?
• IPSL [France]• TitanWRF [US/UK]• TAM [US]• [Though see also
TitanCAM [US] and Kölngroup [Germany]]• Reasonable consistency
between models…?

[Newman et al. 2011]𝑢'[Lebonnois et al. 2012]

N
SpringN
Winter

How do we know Titan’s atmospheresuper-rotates?
TitanWRF
IPSL• Comprehensive GCMs
• Most successful forsuper-rotation from 3
main groups?
• IPSL [France]• TitanWRF [US/UK]• TAM [US]• [Though see also
TitanCAM [US] and Kölngroup [Germany]]• Reasonable consistency
between models…?

[Newman et al. 2011]+Ψ[Lebonnois et al. 2012]

N
SpringN
WinterMechanisms for Titan’s atmospheric
Lebonnois et al. [2012]super-rotation• GRW-type scenario• Poleward AM transport by mean
meridional circulation
Balanced by• Equatorward AM transport by(transient) eddies• Common to all main models intime-mean• Seasonal variations?
• Mechanisms to generate
eddies?

• Vertical AM transport?

• Seems weak in comparison tohorizontal eddies• Tides?

Mechanisms for Titan’s atmosphericsuper-rotation• Seasonal variations in 𝑢!

• AND AM transport

Lebonnois et al. [2012]

Mechanisms for Titan’s atmospheric
Newman et al. [2011] 𝜕𝑞' /𝜕𝑦 & 𝜕𝑀/𝜕𝑡super-rotation• GRW-type scenario:
Mechanisms to generate
eddies?
• Barotropic instabilities at
midlatitudes?
• 𝜕𝑞2 /𝜕𝑦 must change sign
• Highly variable in time?
• Intermittent bursts of
instabities• Other (baroclinic?)
instabilities at high latitudes
in troposphere only
• [Lebonnois et al. 2012]?
• Transports AM poleward….?

Mechanisms for Titan’s atmospheric
Bézard et al. [2018]super-rotation

B. Bézard et al. / Icarus 302 (2018) 437–450• Vertical AM transport?

• Seems weak in comparison to horizontal
eddies• Tides?

• Thermal• Radiative time constant too long forsignificant diurnal tide?
$! %"Radiative timescale 𝜏!"# ≈;%&'(#$$(*+,)[transmissivity 𝜖]Or𝜏!"# ≈ 𝑇/radiative cooling rate
• Weak westward diurnal waves found insome models [Newman et al. 2011]• Models don’t seem to need it to reproduce Fig. 6. Vertical profiles of radiative relaxation time in Titan’s atmosphere. The solid
super-rotation….
line corresponds to damping out a Gaussian temperature perturbation having a full

• Gravitational?

width at half maximum of one pressure scale height. The dotted line shows theradiative time constant calculated by Achterberg et al. (2011) at 5°S using the direct cooling-to-space approximation for the radiative cooling. The dashed line represents the temperature divided by the cooling rate, following the approach ofStrobel et al. (2009).

Fig. 7. Comparison of a temments around 6°N and nort
model profiles calculated foris included in the energy equMechanisms for Titan’s atmospheric
Bézard et al. [2018]super-rotation• Vertical AM transport?
• Seems weak in comparison tohorizontal eddies• Tides?
• Gravitational?
• Excites m=2 wave

Mechanisms for Titan’s atmospheric
Bézard et al. [2018]super-rotation• Vertical AM transport?
• Seems weak in comparison tohorizontal eddies• Tides?
• Gravitational?
• Excites m=2 wave
• Travels around planet• Effect on circulation (AMtransport etc.….)?
• Seems to be weak or negligible
in most (but not all) models

Titan: Mechanisms for super-rotation?
• GRW scenario seems to work effectively in most models• Driven mainly by barotropic instabilities at mid-latitudes• Intermittent rather than continuous?
• Other instabilities?

• Thermal tides may be present [Newman et al. 2011] though don’t
contribute much
• Gravitational tides….?

Key features of Venus
Parameter

Venus

Earth

Radius (km)

6052

6371

Rotation period (Earth days)

243.7

1

Surface gravity (m s-2)

8.87

9.81

Surface pressure (bar)921

96.5% CO2
3.5% N2

78% N2
21% O2~.1% H2O

Insolation (W m-2)

2617

1368

Obliquity (deg)

177.4

23.45

Eccentricity

0.00680.0167

Length of year (Earth years)

0.615

1

Atmospheric composition[Picciali et al. 2012][Taylor & Grinspoon 2009]

Key features of Venus• Complicated radiative budget and T
profile• Deep, greenhouse-like troposphere• Heated via surface• Nearly isothermal stratosphere [z >80 km]• Complicated chemistry andthermodynamics• Condensible constituents (H2SO4)
with ubiquitous clouds• Featureless in the visible but withclear markings in UV• Unknown UV absorber….?
• Deep atmosphere may be supercritical for CO2….

• No seasonal variability• But other variability often seen

Venus in UV [Credit: JAXA][Taylor & Grinspoon 2009]

How do we know Venus’s atmospheresuper-rotates?
• Observations• Tracking cloud features in the UV andIR• Prograde winds up to 100 m s-1 or more
at cloud tops, even on the equator• Entry probes• Doppler tracking as probes descend• Venera (USSR) and Pioneer Venus (USA)sampled different latitudes• Balloons• Doppler spectroscopy• O2 (airglow) and NO emissions• IR remote sounding

How do we know Venus’s atmospheresuper-rotates?
• Observations• Tracking cloud features in the UV andIR• Prograde winds up to 100 m s-1 or more
at cloud tops, even on the equator

UV cloud tops(~65 km)980 nm(~60 km)• Entry probes• Doppler tracking as probes descend• Venera (USSR) and Pioneer Venus (USA)sampled different latitudes• Balloons• Doppler spectroscopy• O2 (airglow) and NO emissions• IR remote sounding

1.74 µm(~45 km)

How do we know Venus’s atmospheresuper-rotates?
• Observations• Tracking cloud features in the UV and IR• Prograde winds up to 100 m s-1 or more atcloud tops, even on the equator• Very large local super-rotation s• Entry probes• Doppler tracking as probes descend• Venera (USSR) and Pioneer Venus (USA)sampled different latitudes• Balloons• Doppler spectroscopy• O2 (airglow) and NO emissions• IR remote sounding

UV cloud tops(~65 km)

UV cloud tops(~65 km)980 nm
980 nm(~60 km)(~60 km)1.74 µm(~45 km)1.74 µm(~45 km)

How do we know Venus’s atmospheresuper-rotates?
• Observations• Tracking cloud features in the UV and IR• Prograde winds up to 100 m s-1 or more atcloud tops, even on the equator• Very large local super-rotation s• Zonal AND meridional velocities• Entry probes• Doppler tracking as probes descend• Venera (USSR) and Pioneer Venus (USA)sampled different latitudes• Balloons• Doppler spectroscopy• O2 (airglow) and NO emissions• IR remote sounding

How do we know Venus’s atmospheresuper-rotates?
• Observations• Tracking cloud features in the UV andIR• Prograde winds up to 100 m s-1 or more
at cloud tops, even on the equator• Very large local super-rotation s• Entry probes• Doppler tracking as probes descend• Venera (USSR) and Pioneer Venus (USA)sampled different latitudes• Balloons• Doppler spectroscopy• O2 (airglow) and NO emissions• IR remote sounding

How do we know Venus’s atmospheresuper-rotates?
[Schofield & Taylor 1980]• Observations• Tracking cloud features in the UV andIR• Prograde winds up to 100 m s-1 or more
at cloud tops, even on the equator• Very large local super-rotation s• Entry probes• Doppler tracking as probes descend• Venera (USSR) and Pioneer Venus (USA)sampled different latitudes• Balloons• Doppler spectroscopy• O2 (airglow) and NO emissions• IR remote sounding

How do we know Venus’s atmospheresuper-rotates?
• Observations• Tracking cloud features in the UV andIR• Prograde winds up to 100 m s-1 or more
at cloud tops, even on the equator• Very large local super-rotation s• Entry probes• Doppler tracking as probes descend• Venera (USSR) and Pioneer Venus (USA)sampled different latitudes• Balloons• Doppler spectroscopy• O2 (airglow) and NO emissions• IR remote sounding

How do we know Venus’s atmospheresuper-rotates?
• Observations• Spatial coverage for (u,v) is reasonably good within/above main cloud decks• Poor at deeper levels• No vertical velocities….
• Local time coverage?

• Also need to rely on models and simulations• Explore hypotheses for dynamical mechanisms• Predict circulation in places inaccessible to observations• Simple GCMS
• Comprehensive GCMs

Simple GCMs for Venus’s atmosphere• Full PEs but with simplified
parameterisations• Linear relaxation to Teq• Surface drag• Dry convection• No diurnal cycle?

• Success reproducing realistic
super-rotation has been highlyvariable• Based on GRW scenario• Depends on model accuracy andconservation properties

Comprehensive GCMs for Venus’s
atmosphere• Full PEs with more realistic
parameterisations• Full radiative transfer• Boundary layer turbulence• Surface topography• Clouds and chemistry?

• Reproduce realistic T
structure• And velocity fields near cloudtops• Some deficiencies in 𝑢! (𝑧) indeep atmosphere (z < 50 km)?

[Lebonnois et al. 2018]

Comprehensive GCMs for Venus’s [Lebonnois et al. 2018]atmosphere• Full PEs with more realistic
parameterisations• Full radiative transfer• Boundary layer turbulence• Surface topography• Clouds and chemistry?

• Reproduce realistic T
structure• And velocity fields near cloudtops• Some deficiencies in 𝑢! (𝑧) indeep atmosphere (z < 50 km)?

Venus super-rotation: key questions• Mechanisms to accelerate and maintain super-rotation?
• Structure and role of meridional overturning circulations?
• Which wave modes dominate eddy AM transport?
• Origin of principal wave modes?
• Instabilities? [Which?]• Tides?
• Topography?

• Impact of waves on observable features?
• Clouds, temperatures etc….

• How to verify models?

• Which observations?
• Feasibility and role of data assimilation….?

Venus super-rotation: key questions• Which kinds of waves? [Summary from Lebonnois et al. 2016]• Thermal tides• Semi-diurnal• Diurnal

Contribute significantly to vertical AM transport• Equatorial planetary waves• Rossby• Periods ~6-20 d, mainly at high-mid latitudes• Kelvin• Period ~7 d, centred on equator• MRG….

• Period ~ 16 d

• Inertia-gravity waves• High frequencies and short wavelengths• Mixture depends to some extent on the model and large-scale atmosphericstructure, static stability etc.!

Venus super-rotation: key questions

Observed: [Del Genio & Rossow 1990]• Which kinds of waves?
• Thermal tides• Semi-diurnal• Diurnal• Equatorial planetary waves• Rossby• Periods ~5 d, mainly at high-mid latitudes• Kelvin• Period ~4 d, centred on equator• MRG….

• Period ~ 16 d

• Inertia-gravity waves• High frequencies and short wavelengths• Mixture depends to some extent on themodel and large-scale atmosphericstructure, static stability etc.!

Venus super-rotation: key questions• Which kinds of waves?
• Thermal tides• Semi-diurnal• Diurnal• Equatorial planetary waves• Rossby• Periods ~5.1 d, mainly at high-mid latitudes• Kelvin• Period ~3.8 d, centred on equator• MRG….

• Period ~ 16 d

• Inertia-gravity waves• High frequencies and short wavelengths• Mixture depends to some extent on themodel and large-scale atmosphericstructure, static stability etc.!

K

R

Observed: [Imai et al. 2019]

Impact of planetary waves on clouds: origin of‘Y’-feature?
𝜑
NP

E

SP
Credit: Lee et al. [2010]lImpact of planetary waves on clouds: origin of‘Y’-feature?
• Produced by vertical and horizontal
advection of cloud tracers by Kelvinand Rossby(?) waves within thecloud layers• Generated in SGCM by Lee et al.
[2010]• Confirmed in observations andexplicit transport model by Nara et al.
[2019]

Credit: Imai et al. [2010]

Venus super-rotation: key questions• Origin of waves• Thermal tides• Semi-diurnal• Diurnal

Directly forced in and above main cloud decks [p < 1bar]• Equatorial planetary waves• Rossby• Barotropic and baroclinic instabilities• Kelvin• ??Barotropic Rossby-Kelvin instability??

• MRG….

• ??Ditto??

• Inertia-gravity waves• Local convection and KH instabilities?

• Mixture depends to some extent on the model and large-scale atmosphericstructure, static stability etc.!
• Mechanisms also vary significantly with location:• Tropics vs midlatitudes or polar regions?
• Cloud decks vs upper levels or deep atmosphere?

Venus super-rotation: key questions Credit: [Sugimoto et al. 2014]• Origin of waves• Thermal tides• Semi-diurnal• Diurnal• Equatorial planetary waves• Rossby•

Barotropic and baroclinic instabilities•??Barotropic Rossby-Kelvin instability??

•??Ditto??

• Kelvin• MRG….

• Inertia-gravity waves• Local convection and KH instabilities?

• Mixture depends to some extent on themodel and large-scale atmospheric structure,static stability etc.!

• Mechanisms also vary significantly with location:• Tropics vs midlatitudes or polar regions?
• Cloud decks vs upper levels or deep atmosphere?

Venus super-rotation: key questions• Mechanisms to accelerate andmaintain super-rotation?
• Structure and role of meridional
overturning circulations?
• Complex, multi-cellular structure• Transports AM upwards andpolewards [green]• Transient eddies transport AM
downwards and equatorwards

Vertical transport

Horizontal transport

𝜔𝑚
No diurnal
cycle[Lebonnois et al. 2010]

Venus super-rotation: key questions• Mechanisms to accelerate andmaintain super-rotation?
• Structure and role of meridional
overturning circulations?
• Complex, multi-cellular structure• Transports AM upwards andpolewards• Enhanced/modified by diurnal
cycle

Vertical transport

Horizontal transport𝜔𝑚With diurnal
cycle[Lebonnois et al. 2010]

Venus super-rotation: key questions• Mechanisms to accelerate andmaintain super-rotation?
• Structure and role of meridional
overturning circulations?
• Complex, multi-cellular structure• Transports AM upwards andpolewards• Enhanced/modified by diurnal
cycle

𝜔𝑚

• Role of waves (with tides)?
• Equatorward transport of m?
• Downward transport of m?

With diurnal
cycle[Lebonnois et al. 2010]

Venus super-rotation: keyquestions• Mechanisms to accelerate andmaintain super-rotation?

• Structure and role of meridional
overturning circulations?

• Complex, multi-cellular structure• Transports AM upwards andpolewards• Enhanced/modified by diurnal cycle

With diurnal
cycle• Role of waves?

• Equatorward transport of m
• Downward transport of m?

• Zonal momentum budget𝜕𝑢"= −𝒖∗ . ∇𝑚 − ∇. 𝑭 + 𝑋"
𝜕𝑡Mean mer. EP flux
circulation. convergence[Mendonça & Read 2016]

Venus super-rotation: keyquestions[Mendonça & Read 2016]−∇. 𝑭• Mechanisms to accelerate andmaintain super-rotation?

• Structure and role of meridional
overturning circulations?

• Complex, multi-cellular structure• Transports AM upwards andpolewards• Enhanced/modified by diurnal cycle• Role of waves?

With diurnal
cycle<0.4VD

0.4-0.6 VD• Equatorward transport of m
• Downward transport of m?

• Zonal momentum budget𝜕𝑢""= −𝒖∗ . ∇𝑚 − ∇. 𝑭[+𝑋]𝜕𝑡Mean mer. EP flux
circulation. convergence• Which wave modes?

0.6-0.9 VD> 0.9 VD

Venus super-rotation: key
Tides
questions• Zonal momentum budget𝜕𝑢""= −𝒖∗ . ∇𝑚 − ∇. 𝑭[+𝑋]𝜕𝑡Mean mer. EP flux
circulation. convergence• Observational verification?
• Cloud-level velocity and Tb from
Akatsuki measurements• Horinouchi et al. [2020]• Thermal tides converge westward
AM in tropics, accelerating superrotation• Transient waves diverge westward
AM, decelerating super-rotation?

Credit: Horinouchi et al. [2020]
Transients𝜕𝑢' /𝜕𝑡Venus super-rotation: keyquestions• Zonal momentum budget𝜕𝑢""= −𝒖∗ . ∇𝑚 − ∇. 𝑭[+𝑋]𝜕𝑡• Observational verification?

• Cloud-level velocity and Tb from
Akatsuki measurements

Credit: Horinouchi et al. [2020]• Horinouchi et al. [2020]• Thermal tides converge westward
AM in tropics, accelerating superrotation• Transient waves diverge westward
AM, decelerating super-rotation?

• Suggests a non-classical GRW
scenario, dominated by thermaltides?

?? Possible
negative feedback??

Venus super-rotation: Summary• Venus super-rotation is the most dramatic exemplar of atmosphericwave-zonal flow interactions• Complex circulation involving multi-cellular meridional overturningand a rich spectrum of waves• Numerical models/GCMs are able to simulate many aspects of thiscirculation, but not all!
• Deep atmosphere and surface interactions?

• Seems to follow a GRW-style mechanism near cloud decks,
dominated by thermal tides with smaller contributions from otherwaves• Consensus between models and observations?
• Variability in time….?

Super-rotation on tidally-locked exoplanets• Orbital period ~ few days
• Rotation likely to be locked
in 1:1 resonance with orbit,
due to gravitational tides• Phase curves fromsecondary transits indicate
eastward displacement of
sub-stellar hot spot
• Advected by strong (>1000
m s-1) eastward (superrotating) flow…?
• Mechanisms...?

• Wave-zonal flow response toa stationary day-nightheating patternKnutson et al. (2007)

Super-rotation on tidally-locked exoplanets• Mechanisms...?

• Wave-zonal flow response to astationary day-night heating pattern• Applies for motion on scales 𝐿~𝐿"#[equatorial deformation radius]
𝑁𝐻𝑎 $/&𝐿"# ~2Ω
• Where LRE << a we can apply
equatorial b-plane• Used by Matsuno [1966] and Gill
[1980] to obtain analytical solutions
for linear response• With weak damping, takes the formof a superposition of Kelvin andRossby modes• “Fleur-de-lis” pattern!

[2023]

Super-rotation on tidally-locked exoplanets• Mechanisms...?
• Similar ”fleur-de-lis’ patterns
appear in tidally locked
exoplanet GCM simulations• But accompanied by strong,
prograde equatorial jets
• Width ~ LRE• Matsuno-Gill pattern also occurs
in slowly rotating tidally-locked
circulations on planetary scale• But no longer a simple RossbyKelvin wave pattern• How to predict the strength of 𝑢! ?

Heng & Showman [2015]

Super-rotation on tidally-locked
exoplanets• Interpret GCM simulated
circulation via Helmholtz
decomposition of velocity field• 𝒖 = 𝒖789 + 𝒖:;<Following Hammond & Lewis [2021]• Example for tidally-locked
terrestrial planet

Super-rotation on tidally-locked
exoplanets• Interpret GCM simulated
circulation via Helmholtz
decomposition of velocity field• 𝒖 = 𝒖789 + 𝒖:;<Following Hammond & Lewis [2021]• Example for tidally-locked
terrestrial planet• Divergent component dominated by
Kelvin-like wave
• Rotational component dominated byzonal jet + Rossby-like wave

Super-rotation on tidally-locked
exoplanets• Interpret GCM simulated circulation
via Helmholtz decomposition ofvelocity field• 𝒖 = 𝒖'() + 𝒖*+,Following Hammond & Lewis [2021]• Example for tidally-locked terrestrial
planet• Divergent component dominated by
Kelvin-like wave
• Rotational component dominated byzonal jet + Rossby-like wave

• Divergent component roughly
axisymmetric about sub-stellar/antistellar axis

Super-rotation on tidally-locked exoplanets• How to predict the strength of zonal jet 𝑢! ?
• Estimate following Hammond et al. [2020]• Approximate zonal mean momentum Eq on the equator by
𝜕𝑢2
𝜕𝑢2
𝜕 ! !
1
𝜕
≈ −𝜔
9
−
𝑢𝜔 −
𝑢! 𝑣′cos " 𝜑
"
𝜕𝑡
𝜕𝑝 𝜕𝑝
𝑎cos 𝜑 𝜕𝜑
MV

SV

SH

=+
>
• Solve linear equations for waves + 𝑢! on a b-plane and set =< =0 so

1
𝑢! ≈ * SV + SH 𝑑𝑝𝜔,Super-rotation on tidally-locked
exoplanets• How to predict the strength of 𝑢! ?
• Estimate following Hammond et al. [2020]? =• Approximate 𝑢! ~ − +𝑢B 𝜔 B ;@ =A
CD𝑤′~ E. ?; and
@/ F
CFD
𝑢′~
≈ . .
?
E ?

• Leading to2.53𝜋𝑎𝑔G𝜎 H/I H/I
𝑢! ~
𝐹J
I
𝑅𝑁 𝜌J𝑐A• Scaling ~consistent with GCMs….?

Summary/Conclusions• Venus & Titan both seem to super-rotate via a form of GRW
mechanism at cloud level• Barotropic instabilities generate dominant waves on Titan• Mixture of thermal tides and barotropic/baroclinic instabilities on Venus?

• Common occurrence of Rossby and Kelvin waves associated withvariability• Rossby & Kelvin waves play important roles in accelerating zonal jets
in tidally-locked planets• Matsuno-Gill phase-locked pattern• Scaling of zonal wind magnitude….?

Open issues• Super-rotating bifurcations and Earth climate change….?
• Super- or sub-critical?

• Mechanism for gas/ice giant equatorial jets?
• Deep or shallow?

• Verification of what drives Titan’s super-rotation?
• Which waves are active?
• Observations….?

• Venus super-rotation• Time variations?
• Deep atmosphere processes?

• How valid/useful are scaling theories for super-rotation?