On the existence of subtropical
anticyclone withequivalent-barotropic structure
over Japan in August.
ENOMOTO Takeshi, ∗MATSUDA Yoshihisa† , Brian J. Hoskins‡
5 December 2000
Abstract
The equivalent-barotropic structure of Bonin highis examined with a simple GCM and re-analysis data.
The stationary Rossby wave induced in the desert region
is found to be responsible for the equivalent-barotropicstructure near Japan. The present talk begins with aclassical problem on the response to heating. Realisticsettings such as eﬀects of zonal ﬂow and orography are
gradually added.
∗Corresponding author, The University of Tokyo†
The University of Tokyo‡
The University of Reading
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Bonin highPlan of talk

1. Introduction
2. Response to localised heating• Response to heating on the equator• Response to oﬀ-equatorial heating• Role of zonal ﬂow• Eﬀect of orography
3. Response to climatological heating• Total climatological heating of August• Total but the Western Paciﬁc heating• Total but the Silk Road cooling
4. Conclusions and Discussion

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Bonin highEquivalent-barotropic structure of
Bonin high• Develops and covers Japan after the Baiu season.
• Equivalent-barotropic:– Vertically in phase– Maximum amplitude near the tropopause• Quasi-stationary: ∼ 10-15 days

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Bonin highCharts: 31 July 2000

Figure 1: 200, 500 hPa height and sea level pressure
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Bonin highObserved diabatic heating• Major heatings:– Bay of Bengal – Indian monsoon, oﬀ-equatorial– Western Paciﬁc – Asian monsoon• Cooling: Silk Road

Figure 2: Column integrated diabatic heating inAugust (ERA15 climatology) in W/m2

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Bonin highResponse to heating on the equator

Matsuno (1966)’s model:• Shallow water• Equatorial β plane• Linearised about the stationary atmosphere
Assuming Lx  Ly , the set equations become:∂φ∂u− βyv = −∂t∂x∂φβyu = −
∂y

∂u ∂v
∂φ+ gh+= 0.
∂t
∂x ∂y

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(1)
(2)
(3)

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Bonin highKelvin wave

If v = 0, Eqs. (1)-(3) become∂u∂φ=−∂t∂x∂φβyu = −∂y ∂u∂φ+ gh= 0.
∂t
∂x(4)
(5)
(6)

Equations (4) and (6) give 2 ∂ u∂ u= 0.
− gh∂t2∂x22(7)

Thus the dispersion relation for the Kelvin wave isω2c = 2 = gh,k2

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(8)
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Bonin highif the monochromatic wave solution   ûu
 v  =  v̂  exp{i(kx − ωt)}
φφ̂

(9)

is assumed, the solution is given by Eq. (4) and (5)û = u0 exp −2y2L2β,(10)√where Lβ ≡gh/β is the equatorial Rossby
radius of deformation.

Figure 3: (u, v) and φ for the Kelvin wave solution

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Bonin highRossby wave

For v = 0, we obtain2 2βk β yd v̂
−v̂ = 0+−2dyωgh2(11)from eq. (1)-(3), and (9). Substituting Y = y/Lβand v̂ = f (Y ) exp(−Y 2/2) into (11), we have√k ghdfd f+ −−1 f =0− 2YdY 2dYω2(12)Equation (12) has a solution
f (Y ) = Hn(Y )(13)provided that√k gh= 2n + 1, (n = 0).
−ωFDEPS 2000

(14)

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Bonin highThe phase speed of the Rossby wave is√

c=−gh,(2n + 1)(15)

which is slower than the phase speed of Kelvin wave
at least by a factor of 3 (n = 1).

Figure 4: (u, v) and φ for the Rossby wave solution

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Bonin highGill pattern

Solutions for heating on the equator:• Decay away from the equator as exp(−y 2 /4).
• Eastward propagating Kelvin wave
• Westward proagating Rossby wave

Figure 5: Distribution of (a)(u, v), w, (b)(u, v), p.

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Bonin high

Sverdrup balance

In the heating region,• ascent, w > 0, has the same pattern as Q.
• the ﬂow is away from the heat source, v > 0.
The vorticity equation,
βv = −f∂u ∂v+,∂x ∂y(the Sverdrup balance) requires the poleward ﬂow.

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Bonin highResponse to antisymmetric heating• Long mixed wave in the heating region.
• No motion to the east of heating.
• Westward propagating Rossby wave

Figure 6: Distribution of (a)(u, v), w, (b)(u, v), p.

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Bonin highResponse to oﬀ-equatorial heating• Symmetric + antisymmetric (Gill, 1980)• Dominatly Rossby wave to the west
• Warm region to the west
• Descent to the west (Rodwell and Hoskins, 1996)

Figure 7: Distribution of (a)(u, v), w, (b)(u, v), p.
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Bonin high

Why Sahara is dry in the summer?

• No Hadley cell in NH summer.
• But strong descent over the desert region.
• Zonal(West-East) temperature gradient due tooﬀ-equatorial heating.

Figure 8: Observed JJA (a)ω, (b)-(d) each term of
thermodynamic equation, and (u, v), p on θ = 325K
(Rodwell and Hoskins, 1996).
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Bonin highMonsoon-Desert mechanism• Monsoon heating creates a warm anomaly to thewest.
• The isentropic surface lowers.
• The westerly descends along the isentropics
surface.
pressureIsentropic surface

Westerly

Isentropic surface

Warm
HWest

EastFigure 9:
Schematic illustration
Monsoon-Desert mechanism

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Bonin highModel and Data

Reading Univ. spectral model:• Horizontal truncation wavenumber: 42• Num of vertical layers: 15• No physical processes
Climatology of August from the ECMWF reanalysis (1979-93):• Zonally-averaged winds and temperature, or• Globally-averaged temperature in each layer• Diabatic heating

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Bonin highExperiments on responseto localised heating• Indian heating (80E–110E, 10S–35N)• Eﬀect of oﬀ-equatorial heating:
Heating at the equator and at 20N• Role of the jet– How does oﬀ-equatorial heating interact withthe jet?
– Re-examination of the Monsoon-Desert
mechanism• Eﬀect of orography

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Bonin highResponse to the Indian heating
relocated to the equator

Figure 10: ψ and ω at Day 5 (top) and 10 (bottom)

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Bonin highResponse to the Indian heating

Figure 11: ψ and ω at Day 5 (top) and 10 (bottom)

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Bonin highResponse to the Indian heatingin the presence of the jet• Descent and ascent along the jet• Northeastward propagation of stationary Rossby
Wave

Figure 12: ψ at σ = 0.23 and ω at σ = 0.5
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Bonin highDescent along the isentropes∂ ūR ∂ T̄=−∂z
H ∂yThe jet is associated with the isentropes descending
toward the equator.

Figure 13: φ–p section of (v, ω), θ at λ = 81E
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Bonin highVorticity balance∂ζ ū+ βv  = −(f + ζ)D∂x• In the upper troposphere, the zonal advection isbalanced by the planetary vorticity advection andthe stretching of vortex tube.
• In the lower troposphere, the shrinking of vortex
tube is balanced by the planetary vorticity
advection.
Vorticity advection by zonal wind
HH
Planetary vorticity advection
Stretching of vortex tube
Planetary vorticity advection

Shrinking of vortex tube
LFigure 14: Schematic illustration of vorticity balance

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Bonin highThermodynamic balance∂ T̄ 
∂T+v +ū∂x∂y

pp0κQ∂θ=ω∂p cp

At 40N on the 500 hPa surface• ū ∼ 8 m/s, v  ∼ 4 m/s• |Tx | ∼ 0.05 K/1000km < |T̄y | ∼ 0.4 K/1000km• Thus the primary balance exists between
meridional and vertical advection at the centreof decent.

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Bonin highRole of the jet

The zonal (eastward) basic ﬂow• creates a meridional tilt of isentropes.
• advects negative voriticity eastward.
As a result, the meridional ﬂow of disturbance hasa vertical component, which induces the stationary
Rossby wave.

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Bonin highResponse to the Indian heatingin the presence of the jet and orography

Figure 15: ψ at σ = 0.23 and ω at σ = 0.5

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Bonin highResponse to the Indian heatingin the presence of the jetand orography shifted −45◦

Figure 16: ψ at σ = 0.23 and ω at σ = 0.5

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Bonin highStationary Rossby wave propagation

Figure 17: Wave activity ﬂux on ψ (k ≥ 3) forwithout, with, and shifted orography runs
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Bonin highEﬀect of orography• The orography modiﬁes distribution of verticalmotion thus Rossby wave induction.
• Cancellation of the mechanical ascent and thethermally-induced explains why descent does notexist near the forcing and implies involvement ofother processes.
• Weakening of vertical motion suppresses theRossby wave induction.
• The descent near the Aral sea has a realistic
location and distribution.

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Bonin highWhich diabatic heating is important?

• Indian heating, jet, and orography: no Bonin high• Other regions of diabatic heating/cooling:
1. Western Paciﬁc (110-180E, 5-25N)– PJ pattern (Nitta, 1987)
2. Japan (120-150E, 30-60N)
3. Silk Road (0-120E, 30-60N)

Fig 2: Column integrated diabatic heating in August(ERA15 climatology) in W/m2

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Bonin highComparison: Upper troposphere

Figure 18: ψ at σ = 0.23 for W Paciﬁc, Japan, andSilk Road runs.
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Bonin highComparison: Surface pressure

Figure 19: Sea level p for W Paciﬁc, Japan, andSilk Road runs.
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Bonin highStationary Rossby wave from the desert

Figure 20: λ-t section of ψ 

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Bonin highComparison: Wave activity ﬂux and ψ Figure 21: Wave activity ﬂux on ψ  (k ≥ 3) at
σ = 0.23 for W Paciﬁc, Japan and Silk Road runs.
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Nitta (1987)

400 hPa height anomaly

Cloud amounts

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Bonin highEﬀect of other heating/cooling• The Western Paciﬁc heating appears to beimportant in the low levels and to induce thePJ pattern, which is not of primary importanceto the equivalent-barotropic structure near Japan.
• The local diabatic forcing appears to enhance theupper tropospheric anticyclone over Japan. Its
eﬀect near the surface is minor.
• The Silk Road cooling is of primary importancein stationary Rossby wavepacket induction. The
development of decay of anticyclone has timescaleof ∼ 10 days, which is consistent with observation.

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Bonin highExperiments with climatological heating

Having understood the role of various
heating/cooling heat source, the jet, and orography,
the climatological heating is used:
1. Total
2. Total but the Western Paciﬁc heating
3. Total but the Silk Road (incl. Japan) cooling

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Bonin highComparison: Upper troposphere

Figure 22: ψ at σ = 0.23 for the climatology, control
and no W Paciﬁc runs.
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Bonin highComparison: Upper troposphere

Figure 23: ψ at σ = 0.23 for the climatology, control
and no Silk Road runs.
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Bonin highComparison: Surface pressure

Figure 24: Sea level p for the climatology, control
and no W Paciﬁc runs.
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Bonin highComparison: Surface pressure

Figure 25: Sea level p for the climatology, control
and no Silk Road runs.
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Bonin highTime evolution of ψ  in the Control

Figure 26: λ-t section of ψ  at φ = 38N

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Bonin highTime evolution of ψ in the no W Paciﬁc run

Figure 27: λ-t section of ψ  at φ = 38N
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Bonin highTime evolution of ψ in the no Silk Road run

Figure 28: λ-t section of ψ 
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Bonin highComparison: Wave activity ﬂux and ψ at φ = 38N

Figure 29: Wave activity ﬂux on ψ (k ≥ 3) at σ = 0.23
for the control, no W Paciﬁc and no Silk Road runs
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Bonin highComparison: Isentropic PV

Figure 30: IPV on 350 K at Day 10 for the control,
no W Paciﬁc, and no Silk Road runs

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Bonin highComparison: Isentropic PV

Figure 31: IPV on 350 K at Day 15 for the control,
no W Paciﬁc, and no Silk Road runs

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Bonin highSummary of climatological heating
experiments• The equivalent-barotropic anticyclone near Japanis successfully simulated with a simple GCM.
• The existence of anticyclone without the Western
Paciﬁc heating shows the lack of importance ofthis heating.
• Removal of the Silk Road cooling weakens theanticyclone: the remote descent creates the Boninhigh.
• The develpment of anticyclone can be understoodin terms of negative PV anomaly over Japan.

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Bonin highMechanism for the Silk Road pattern
and Bonin highNorth
The Asia jet

40
Descent

H30

HQ

2030

60

90120

EastFigure 32: Schematic diagram illustrating mechanismfor the Silk Road pattern and Bonin high

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Bonin highConclusions

Base upon the foregoing dynamical consideration,
climatology, and numerical experiments, thefollowing hypothesis is proposed:• The Indian heating, oﬀ the equator during thesummer monsoon, creates the Asia jet and zonal
temperature gradient.
• The descent near the Aral sea and East
Mediterranean sea induces the stationary Rossby
wave on the jet.
• The Rossby wavepacket propagates along the jet
converges near Japan and creates the Bonin high.
• The Bonin high is enhanced by the local diabaticforcing. The PJ pattern may be in phase.
Corresponding author: ENOMOTO Takeshi eno@aos.eps.s.u-tokyo.ac.jp

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