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