Equatorial Annual Cycle Shang-Ping Xie IPRC/Met, University of Hawaii Ocean University of China...
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Transcript of Equatorial Annual Cycle Shang-Ping Xie IPRC/Met, University of Hawaii Ocean University of China...
Equatorial Annual Cycle
Shang-Ping XieIPRC/Met, University of Hawaii
Ocean University of China
PowerPoint file available at http://iprc.soest.hawaii.edu/~xie/ppt/annual.ppt
References
Mitchell, T.P. and J.M. Wallace, 1992: The annual cycle in equatorial convection and sea surface temperature. J. Climate, 5, 1140-1156.
Xie, S.-P., 1994: On the genesis of the equatorial annual cycle. J. Climate, 7, 2008-2013.
SST165W, 20N
90W, Eq
Galapagos SST and Precipitation
Calendar Month
Nino3 std dev
ENSO’s Seasonal Phase Locking
u vT
Equatorial Annual Cycle
Why annual? Why Strong in the east? Why propagate westward?
Lukas and Firing (1985, J. Phys. Oceanogr.)
x x
yy
= A(x) ei[ t - (x) ]
A(x) (x)
cf. Horel (1982, Mon. Wea. Rev.)
Mar-Apr
Aug-Sept
SST, Precipitation and Surface Winds
August-May Difference
Sea surface height (cm)
cf. Mitchell and Wallace (1992)
Sept-Mar SST & Wind Diff (COADS)
Buoy Measurements at 110W, Eq.
From Xie (1994, JC)
Why is the annual cycle in h small in the Eq Pacific?
h
TwρQ/c
tT ep
ρc
2TWe
p
3* Q
gh
mu
22* VUCu a
D
1D Ocean
x
TU
'
'
U2H
aμc εT'V'
H
Va'
t
T'2
x
Tc
(coupling)
Simple Theory of Equatorial Annual Cycle
Linearization 22'*
''
VU
VVUUCu a
D
εT')V'VU'U(H
a
t
T'2
How to make this coupled equation unstable? Hint: atmospheric model.
Northward displaced ITCZ ( >0) Annual frequency (V’);
Tilt of the thermocline H(x) Stronger annual cycle in the east;
Prevailing easterlies ( <0) Westward phase propagation.
V
U
(Xie 1994, J. Climate, p.2008)
cf: Liu & Xie (1994, JAS)
Evaporation: E= )(22asEa qqVUCL
Upwelling: ]2)1([1
22 yx
wbw
Xie 1998, J. Climate, Eq. (2.5), p. 191.
-1< <0
εT'V'H
Va'
t
T'2
x
Tc U2H
aμc
cf: Giese & Carton (1994, JC); Chang (1996, JC)
0
0V
|V| Annual
|V| Annual
Annual V’ in both cases
Temperature along equator
SST’ & u’ at Eq
Veq
-
+
Xie 1994, J. Climate
Model Results
1
nn nω
QT
Response to cross-equatorial winds
Philander & Pacanowski (1981, Tellus)
SSTWindCloud
SST: Mean & Annual Harmonic
Sensitivity to the length of year
SST
x
y
1 yr = 12 mon 1 yr = 18 mon
Giese and Carton (1994, JC)
Control
Flux corrected
Li and Hogan (1999, JC)
Control
Annual-meancorrection
Seasonal correction
Obs
Li and Hogan (1999, JC)
Improved the mean state (asymmetrical about the equator) Annual cycle on the equator
Gordon et al. (2000, JC)Yu and Mechoso (1999, JC)
Prescribed observed cloudiness in a CGCM
Improved the mean state (asymmetrical about the equator)
Seasonal forcing by cloud
0o, Eq
110oW, Eq
Pacific
Atlantic
Dep
th (
m)
Equatorial Annual Cycle in the Atlantic
Ocean dynamics play a more important role
Houghton (1983, JPO, p. 2070)
Annual-mean
March-April
July-August
I year I year
Annual cycle in the equatorial oceans
Mitchell and Wallace (1992)Role of Air-sea interaction
Seasonal cycle of equatorial zonal wind:
(1) Local air-sea interaction
Ueq (m/s)
April
June
Longitude
CTL run
APR run
CTL-APR
Surface wind & precip
Monsoon Effect June-April diff in APR run
with cold tongue removed
ITCZ
Eq.
0y
uv
Equatorward momentumadvection
Mean
Monsoon Cold tongue
Cold tongue effectCTL-APR anomalies in June
Surface wind (m/s) and precipitation (mm/day)
Monsoon effectJune-April diff in APR run
with cold tongue removed
Okumura and Xie (2004, J. Climate)
Northward displaced ITCZ Annual frequency (V’)
Tilt of the thermocline Stronger annual cycle in the east
Prevailing easterlies Westward phase propagation
While secondary in the eastern Pacific, ocean dynamics are important for equatorial annual cycle in the Atlantic.
Atlantic equatorial cycle is strongly influenced by continents and African monsoon in particular.
Summary
Eq IO seasonal cycle: uncoupled in the central basin
SST x
uo
Wyrtki jets
SST cloud: 1 yrZonal wind & current: 0.5 yr
Cane and Sarachik (1981, JMR); Cane and Moore (1981, JPO)
Cn= 163 cm/s, m = 1,
Cn = 82 cm/s, m =2,
Basin-mode resonance at the semi-annual periodJensen (1993, JGR, 22 533-); Han et al. (1999, JPO, 2191-)
T = 0.5 year (period)L = 5,6327 km (basin width)
K
K
R
wind
wind
COADS Zonal Wind (m/s)
Nov
AVHRR SST (C, 5-day, 85-99)
Nov easterly acceleration and SST response
COADS SST (C)TOPEX/Poseidon SSH (cm)
Nov
Thermocline depth control of SST variability
Rms SST (1982-2003)
Nov
0
Jun
T/P SSH (cm)
20W40W
Yuko Okumura, U of Hawaii