SUMMER HEMISPHERE NEAR-EQUATORIAL
SURFACE WESTERLIES IN THE TROPICS
Example of near-equatorial westerlies in the Indian Ocean
during boreal winter
Why are there equatorial westerlies?
Are they important?
•
Equatorial westerlies appear in observations
and model reconstructions in the summer
hemispheres of all three ocean basins
•
What causes them? Are they the result of
Coriolis turning or are they the indicative
physics of equatorial dynamics
•
I argue that the latter option is the case and, in
order to understand the monsoon, one must
understand these physics
•
Furthermore, in the South Indian Ocean, these
westerlies may produce McCreary’s
thermocline dome.
References:
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Winds at 925hPa
DJF
American Monsoons
JJA
West African Monsoon
Asian Monsoon
Austral Monsoon
Low-level
westerly
component
occurring with
convection
Converging
easterlies into
ITCZ
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η=0
Converging
easterlies
Surface
westerlies
Mean surface pressure (February) and η=ζ+f=0
η=0
η=0
In regions of strong cross-equatorial p-gradient,
the η=0 contour lies in the summer hemisphere
Mean surface pressure (July) and η=ζ+f=0
What is the significance of an off-equator η=0 contour?
Consequences of the location of the η=0 contour
η=0
Low p η>0
η<0
f >0
EQU
f <0
High p
• When there is a X-equ pgradient (CEPG), anticyclonic
vorticity is advected across the
equator by the divergent wind
• This may be inertially
(symmetrically) unstable!
divergent wind α CEPG
High p
• When there is no CEPG, there
is no cross-equatorial divergent
EQU wind and the system is stable.
η>0
η=0
Low p
η<0
High p
More consequences….
divergent wind
Low p
η=0
SST max
η>0
CONV
mslp min
Convection (ITCZ)
DIV
f >0
f <0
Surface westerlies
EQU
EQU
η<0
High p
MONSOON
LARGE CEPG
•
Convection is off equator on poleward side of η=o contour
•
Not colocated with mslp min or SST max
•
Surrounded by converence/divergence doublet
•
Coincides with surface westerly jet
•
Highly variable and transient and deepest convection!
More consequences….
WARM POOL CONVECTION
High p
η>0
η=0
Low p
EQU
η<0
High p
SSTmax
Convection (ITCZ) EQU
mslp min
SMALL OR ZERO CEPG:
Collocation of SST maximum, mslp minimum
Convergence located near equator with ITCZ
Locally stable
No westerlies
Two tropical worlds:
• Large CEPG:
Australian monsoon and west SIO
East PAC (summer)
East NATL and West Africa
NIO and South Asian monsoon (*)
• Small/zero CEPG:
Central Pacific warm pool
(* The summer monsoon has unique character)
Cross-sections of MSLP, SST, OLR, abs. vort
Vast difference between
west and east Pacific: esp.
in the CEPG
February 1992: Absolute vorticity 10**-5 /s
July 1992: Absolute vorticity 10**-5 /s
Annual cycle of zero absolute vorticity: regions of instability
Correlations of latitude of zero abs. vort. line and CEPG
February: Divergent wind field
July: Divergent wind field
February: Divergence
July: Divergence
Indian Ocean winter 55E-85E (Feb)
• Divergent wind bisected by η=0
• Strong upward motion
poleward side of η=0
• Divergence/convergence
doublet around η=0
Western Atlantic/Africa ATL 30W-0E (July)
Indian Ocean 55E-85E (July)
Here we note some
differences with the structure
spread out across a much
larger latitude belt
Tomas and Webster’s simple (naïve?) view of why
we have two circulations: Simultaneous dry and
moist (this caused by inertial instability). Also
thought that position of ITCZ was steady
Shallow circulation
also noted and
studied by Zhang et
al. 2004
Toma and Webster (2009a,b) argue that regions of large
CEPG are sources of easterly waves.
Transients
∂η
− V ⋅ ∇η = η∇ ⋅V + αζ
∂t
Absolute vort equation
−V ⋅ ∇η = η∇ ⋅V + αζ
advection
Steady state
stretching dissipation
β = df / dy η = f + ς
−V ⋅ ∇η → − β vd
η∇ ⋅V → f ∇ ⋅V
− β v = f ∇ ⋅V + αζ
latitude where
advection cancelled
out by stretching
tan ϕ e = (vd a ∇gv )
tan ϕ e = (vd a ∇gv )
Where vd (divergent wind is given by
d(CEPG)/dy
QuickTime™ and a
decompressor
are needed to see this picture.
Tomas and Webster’s simple (naïve?) view of why
we have two circulations: Simultaneous dry and
moist (this caused by inertial instability). Also
thought that position of ITCZ was steady
Shallow circulation
also noted and
studied by Zhang et
al. 2004
Toma and Webster (2009a,b) argue that regions of large
CEPG are sources of easterly waves.
QuickTime™ and a
decompressor
are needed to see this picture.
Absolute vorticity equation:
∂η
= 0 = V g∇η + η∇gV + αζ
∂t
Now at η = 0,∇gV → 0 so that η∇gV → 0
V g∇η + αζ = 0
Break velocity into divergent and rotational parts:
V = Vχ + Vψ
So that
Vψ g∇η + Vχ g∇η = −αζ
η=0
But Vψ is othogonal to ∇η so that Vψ g∇η = 0
Then :
Vχ g∇η = −αζ
η=0
Now, Vχ is a maximum at η = 0
So that for a given α , ζ
η=0
must also be a
maximum at η = 0
∂u
i.e.,
must be a maximum (maximum shear)!
∂y
At η = 0,Vχ g∇η > 0 so that ζ max < 0 so that
westerlies must increasenorth of the η = 0 contour.
So regions of equatorial westerlies (such as the turning
of the wind into west Africa) is the result of inertially
unstable dynamics and NOT Coriolis turning
What happens in the IO where the η=0 contour resides
(and does not move for the entire season). This is
because the summer IO is inertially stable because of the
very strong subsidence in the west IO.
Potential temperature sections across the IO:
All other regions spawn
disturbances of the
inertial period of the
mean ITCZ position
except IO. I.e., easterly
waves
One wonders if this
stability allows the
accumulation of energy
(and hence CAPE) to
fuel MJO or MISO.