Modèle en boîte

Monsoon intraseasonal variability
Gilles Bellon
Centre National de Recherches Météorologiques
Toulouse, France
cnrs
The Monsoon Intraseasonal oscillation (MISO)
Northward propagation of convective disturbances
Brightness Temperature over the BoB
(CLAUS data, 70E-90E)
1 - 1.5 m/s
30 - 40 days
30 - 40 days
Monsoon onset
The Monsoon Intraseasonal oscillation (MISO)
Northward propagation of convective disturbances
NOAA OLR + ERA Interim winds
Anomalies of
850hPa wind and OLR
for the 8 phases of the
oscillation
OLR (Wm-2)
Why does it matter?
Intraseasonal variability can be used for medium range forecast
Ex: MJO and convectively-coupled equatorial wave used in Australia
Models do not simulate it very well.
Why does it matter?
NOAA OLR + ERA Interim winds
CNRM-CM5
Vertical structure of the MISO
Vorticity
Divergence
Specific humidity
Pressure (hPa)
v'
S
N
S
N
S
Latitude with respect to the convection maximum
N
NCEP composites from Jiang et al. (2004)
Convection
maximum
Known and unknown mechanisms of the MISO
Vorticity
Divergence
Specific humidity
Pressure (hPa)
v'
S
N
S
N
S
N
NCEP composites fromJiang et al. (2004)
Convection
maximum
Latitude with respect to the convection maximum
Moisture convergence:
Free-tropospheric
divergence via Coriolis,
+ mass conservation
and / or
boundary-layer
Ekman pumping
  y (q v' ) 
Boundary-layer
convergence
 q  y v'  v'  y q
Humidity
build-up
Barotropic
vorticity
Consitional
instability
?
Deep convection
?
Boundary-layer
convergence,
upper-tropospheric
divergence
Known and unknown mechanisms of the MISO
from DeMott et al. (2013)
Moisture convergence:
Free-tropospheric
divergence via Coriolis,
+ mass conservation
and / or
boundary-layer
Ekman pumping
  y (q v' ) 
Boundary-layer
convergence
 q  y v'  v'  y q
Humidity
build-up
Barotropic
vorticity
Consitional
instability
?
Deep convection
?
Boundary-layer
convergence,
upper-tropospheric
divergence
Many theories, no consensus
Theories relying on the interaction between mean state and intraseasonal anomalies
1. Gradient of anomalous vertical advection of the mean vertical shear in the zonal direction
= ∂y v’i
Vorticity: ζ’i = -∂y u’i
Divergence:
∂t ζ’0 > 0
D’i
i = 0 : barotropic
i = 1 : baroclinic
BL top
Eq
N
∂t ζ’0 = … + ∂y (ω∂pu) = … + U1 ∂y D’1
U1 < 0
0
∂y D’1
Jiang et al. (2004)
Many theories, no consensus
Theories relying on the interaction between mean state and intraseasonal anomalies
2. Horizontal advection of the anomalous baroclinic vorticity by the baroclinic mean flow
= ∂y v’i
Vorticity: ζ’i = -∂y u’i
Divergence:
∂t ζ’0 > 0
D’i
i = 0 : barotropic
i = 1 : baroclinic
BL top
Eq
0
N
∂y ζ’1
∂t ζ’0 = … - V1 ∂y ζ’1
V1 < 0
Bellon and Sobel (2008)
Many theories, no consensus
Theories relying on the interaction between mean state and intraseasonal anomalies
Mean circulation permits both mechanisms over the Indian Ocean,
But only the first one over the Western Pacific
Chou et al. (2009)
Both mechanisms at play over the BoB, with unclear respective roles:
from DeMott et al. (2013)
Many theories, no consensus
Theory relying on non-linear effects (anomaly on anomaly)
Self-advection of the vortex by beta-drift
Cloud-resolving model results: low-level vorticity (shadings) and precipitation (contours)
from Boos and Kuang (2010)
Particularly interesting for the first propagation of the year (monsoon onset and bogus onset?)
Tentative validation of the beta-drift mechanism (preliminary results)
The May 1998 event: Local Mode Analysis (LMA)
Convective activity (-OLR)
W m-2
850hPa vorticity
s-1
Courtesy of Meriem Chakroun
Tentative validation of the beta-drift mechanism (preliminary results)
The May 1998 event: Composite with respect to the maximum of vorticity
Same + (rotationally )asymmetric wind
Courtesy of Meriem Chakroun
Latitude / vorticity maximum
Convective activity and vorticity
Longitude
OLR
/ vorticity maximum
W m-2
10-7 s-1
-16 -12 -8 -4 0 4 8 12 16 20
850hPa vorticity
850hPa asymmetric wind
Beta drift does not work in this case, but is it representative?
Perspectives
 Identification of the main mechanisms of the MISO
 from observation/reanalysis: under way;
 from high-resolution models (cloud-resolving);
 Physically-based evaluation of General Circulation Models:
 Dynamics;
 Rain, cloud, and humidity signature of ISOs.
 Better understand/validate the interaction with other components of the
global climate:
 Surface (oceanic and continental) ;
 Extratropics.
Thanks