Section 5: Kelvin waves
1. Introduction2. Shallow water theory3. Observation4. Representation in GCM5. Summary
5.1. Introduction• Equatorial waves:
• Trapped near equator• Propagate in zonal-vertical directions• Coriolis force changes sign at the equator
• Can be oceanic or atmospheric.
• Diabatic heating by organized tropical convection can excite atmospheric equatorial waves, wind stress can excite oceanic equatorial waves.
• Atmospheric equatorial wave propagation is remote response to localized heat source.
• Oceanic equatorial wave propagation can cause local wind stress anomalies to remotely influence thermocline depth and SST.
• Described by the shallow water theory.
5.1. Introduction• Kelvin waves were first identified by William Thomson
(Lord Kelvin) in the nineteenth century.
• Kelvin waves are large-scale waves whose structure "traps" them so that they propagate along a physical boundary such as a mountain range in the atmosphere or a coastline in the ocean.
• In the tropics, each hemisphere can act as the barrier for a Kelvin wave in the opposite atmosphere, resulting in "equatorially-trapped" Kelvin waves.
• Kelvin waves are thought to be important for initiation of the El Niño Southern Oscillation (ENSO) phenomenon and for maintenance of the MJO.
5.1. Introduction• Convectively-coupled atmospheric Kelvin waves have a
typical period of 6-7 days when measured at a fixed point and phase speeds of 12-25 m s-1.
• Dry Kelvin waves in the lower stratosphere have phase speed of 30-60 m s-1.
• Kelvin waves over the Indian Ocean generally propagate more slowly (12–15 m s-1) than other regions.
• They are also slower, more frequent, and have higher amplitude when they occur in the active convective stage of the MJO.
5.2. Theory
• Shallow water model • Matsuno (1966)
v
Equatorial β-plan
1cos ay /sin
yf
he
z
xEq.
h
f is the coriolis parameter β is the Rossby parameter
u
y
Linearized Shallow Water Equations for perturbations on a motionless
basic state of mean depth he
0
0
0
yv
xugh
t
yuy
tv
xvy
tu
e
(1.1)
(1.2)
(1.3)
hg is the geopotential disturbancewhere
Momentum:
Continuity:
Seek solutions in form of zonally propagating waves, i.e., assume wevalike solution but retain y-variation:
tkxiyyvyuvu exp)(ˆ),(ˆ),(ˆRe,,
Substituting this into (1.1-1.3) gives:
0
0
0
yvuikghi
yuyvi
ikvyui
e
(2.1)
(2.2)
(2.3)
Eliminating u’ from (2.1) and (2.2) gives:
0)( 222
y
ykivy
Elimination of Φ between (3) and (4) and assuming ω2 ≠ ghek2 gives:
0ˆˆ 222
2
2
2
v
ghykk
ghdyvd
ee
Requires v̂ to decay to zero at large |y| (motion near the equator)
and from (2.1) and (2.3) gives:
0)( 22
vykyvghikgh ee
(3)
(4)
(5)
Schrödinger equation with simple harmonic potential energy, solutions are:
0ˆ v
Other solutions exist only for given k if ω takes particular value.
Non-dimensionalize and set
y
ghY
eYFv
ghkkgh
e
Y
e
e
4/1
2/1
2/
22
2
)(ˆ
21
2
(5) can be re-written as Hermite polynomial equation
022 FFYF
Solutions that satisfy the boundary conditions are:
cHF where n for ,...2,1,0n
and )( yHn Is a Hermite polynomial
Horizontal dispersion relation:
...2,1,0,1222
nnkk
ghgh
e
e
ω is cubic 3 roots for ω when k and n are specified.At low frequencies: equatorial Rossby waveAt high frequencies: Inertio-gravity waveFor n = 0 : eastward inertio-gravity waves and Yanai wave.
(6)
Theoretical Dispersion Relationships for Shallow Water Modes on Eq. Plane
Freq
uenc
y ω
Zonal Wavenumber k
Matsuno, 1966
Theoretical Dispersion Relationships for Shallow Water Modes on Eq. Plane
Freq
uenc
y ω
Zonal Wavenumber k
Westward Eastward
Matsuno, 1966
Theoretical Dispersion Relationships for Shallow Water Modes on Eq. Plane
Kelvin
Eastward Inertio-Gravity
Equatorial Rossby
Freq
uenc
y ω
Zonal Wavenumber k
Mixed Rossby-gravity (Yanai)
n =
-1
n = 0
n = 1n = 3
n = 1
n = 2
n = 3
n = 4
Westward Inertio-Gravity
Matsuno, 1966
For the Kelvin wave case, v’ = 0 (2.1-2.3) become:
dispersion relation given by (7.1) and (7.3):
kgheKelvin With meridional structure of zonal wind:
eghyuu
2expˆ
2
0
Zonal velocity and geopotential perturbations vary in latitude as Gaussian functions centered on the equator
0
0
0
uikghiy
uy
ikui
e
(7.1)
(7.2)
(7.3)
(8)
(9)
Kelvin Wave Theoretical Structure
Wind, Pressure (contours), Divergence, blue negative
Zonal phase speed
kcp
Zonal component of group velocity
kcg
Kelvin waves are non-dispersive with phase propagating relatively quickly to east with same speed as their group:10-50 m/s in troposphere correspond to he =10-250 m.0.5-3 m/s in ocean along the thermocline correspond to he =0.025-1 m.
The horizontal scale of waves is given by equatorial Rossby radius
2/1
eghL
for he = 10-250 m in troposphere, L = 6-13o latitude.for he = 0.025-1 m in ocean, L = 1.3-3.3o latitude.
Model experiment: Gill modelMultilevel primitive atmospheric model forced by latent heating in organized convection over 2 days.
imposed heating
Vectors: 200 hPa horizontal wind anomalies
Contours: surface temperature perturbations