Mechanisms controlling ENSO:
A simple hybrid coupled model study
Cheng-Wei Chang1* and Jia-Yuh Yu2
1. Institute of Geography, Chinese Culture University, Taipei, Taiwan,[email protected], +886-2-28610511 ext25705
2. Department of Atmospheric Sciences, Chinese Culture University
Contents
1. Introduction 2. Data Sources 3. Mechanisms:
3-1、Westerly Wind Bursts
3-2、 Non-homogeneous Air-sea Feedback
4. Concluding Remarks
Introduction
During the cold epoch (1960s and 1970s), the ENSO scenario (viewed as SSTA) starts in the east and propagates westward along the equator.
During the warm epoch (1980s and 1990s), eastward propagating SSTA associated with westerly wind burst in the central Pacific strengthen ENSO ( Rasmusson and Carpenter 1982; Zhang and Busalacchi 1999).
The surface winds over the tropical Pacific Ocean show energetic large-scale variability on timescales ranging from a few days to decades (Luther et al. 1983; Luther and Harrison 1984; Harrison and Luther 1990).
The instability involves feedbacks between SST, which affects the atmospheric circulation, and the dynamics of the ocean circulation must adjust to the changes in wind.
Air-sea interaction over deep convection region
net
Tpt F
P
gTqTqVTq )()(
HERRRSSSSF sslSttnet
VM)Tq(p
Quasi-equilibrium convective constraints assumption (Betts and Miller, 1986)(Yu and Neelin,1997)
The vertically-integrated moist static energy
Over deep convection region
HEVMTqt
HEVM
's00 T)r1(VM
(Yu and Neelin,1997)
Long time average
'1sTM
V
Gross moist stability (M)
Data Sources
NCEP/NCAR re-analysis grid data of atmosphere
Reconstruction OI SST Domain:90°N~ 90°S/0°E~ 0°W Horizontal resolution: 2.5°×2.5° Study period: 1949~2000
Simple Hybrid Coupled Model Ocean Component
– Cane-Zebiak (CZ) model with Niller-Kraus vertical mixing scheme
Atmospheric Component
– Empirical atmospheric model based on SVD
projections of the first 7 modes
Domain: 0°E~ 0°W/30°N~ 30°S Horizontal resolution: 2°×1°
The Obs. ENSO
ENSO started in the eastern basin
The Obs. ENSO
ENSO started in the eastern and central basin
The OBS. westerly wind bursts Hartten(1996)
1000-hPa zonal winds anomaly exceeded 5 m/s with a zonal extent over 10° and lasting 10days
Feb. ½~Apr. ½
120°E 180°E
15°N
15°S
westerly wind bursts(Max 10m/s)
Hybrid coupled model
WWB effect
Belamarl,2003
Feb. ½~Apr. ½
WWB can excite the ENSO-like pattern
The hybrid couple model simulates ENSO
Ideal exp. +WWB
Feb. ½~Apr. ½
What cause WWB? how to maintain it in
long time?
Stand run+ WWBStand run
Feb. ½~Apr. ½
Gross moist stability (M) climatology
2.1W
W M
M
'1sTM
V
Hybrid coupled model
non-homogeneous air-sea feedback
120°E 180°E
15°N
15°S
%120W
Stand run
non-homogeneous air-sea feedback
Stand run+ non-homStand run+ WWB
1years
1.5years
~1years
Air-sea interaction increases interannual period
1.5years
Stand run+ non-homStand runHalf period ~1 year Half period ~1.5 years
It makes the WWBs effectively self-sustained in the tropical region
ENSO decadal change
The decadal change of WWB
WWB’ strength and period in warm epoch are stronger and longer than in cold epoch
The decadal change of M
2.1M
M
WW 5.1
M
M
WW
warm epoch climatology
04.1M
M
W
Decadal effect
Stand run Stand run+warmStand run+non-hom
Concluding Remarks The westerly wind burst occurring in the western/ce
ntral Pacific is able to excite or strengthen the El Nino event.
Non-homogeneous air-sea feedback (strong in western Pacific while weaker elsewhere) can produce a series of El Nino events with longer period (~3 years) compared to homogeneous case (~2 years).
Decadal change of the observed ENSOs (i.e., longer period and stronger strength in warm epoch) might be due to the change of air-sea feedback.
Thanks for listening
Gross moist stability (M)
Conclusion.
Westerly Wind Bursts modulate the development of the ENSO.
Conclusion.
WWBs modulated by the Non-homogeneous air-sea feedback over deep convection region.
To increase the air-sea feedback strength over deep convection region, which makes the WWBs effectively self-sustained in the tropical region.
Conclusion.
The decadal oscillation modulates ENSOs by intensifying air-sea feedback over western pacific ocean in warm epoch
Similar to Obs.
WPC Obs.