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Equatorial Annual Cycle. Shang-Ping Xie IPRC/Met, University of Hawaii Ocean University of China PowerPoint file available at http://iprc.soest.hawaii.edu/~xie/ppt/annual.ppt. References

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Equatorial Annual Cycle


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slide1

Equatorial Annual Cycle

Shang-Ping Xie

IPRC/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.

slide2

90W, Eq

SST

165W, 20N

Galapagos SST and Precipitation

slide3

ENSO’s Seasonal Phase Locking

Nino3 std dev

Calendar Month

slide4

Equatorial Annual Cycle

T

u

v

  • Why annual?
  • Why Strong in the east?
  • Why propagate westward?
slide5

i[ wt - f(x) ]

t = A(x)e

ty

ty

tx

tx

A(x)

f(x)

Lukas and Firing (1985, J. Phys. Oceanogr.)

cf. Horel (1982, Mon. Wea. Rev.)

slide7

Sept-Mar SST & Wind Diff (COADS)

August-May Difference

Sea surface height (cm)

cf. Mitchell and Wallace (1992)

slide8

Buoy Measurements at 110W, Eq.

From Xie (1994, JC)

Why is the annual cycle in h small in the Eq Pacific?

slide9

Linearization

(coupling)

Simple Theory of Equatorial Annual Cycle

1D Ocean

 How to make this coupled equation unstable?

Hint: atmospheric model.

slide10

Evaporation: E=

Upwelling:

Xie 1998, J. Climate, Eq. (2.5), p. 191.

-1< <0

cf: Giese & Carton (1994, JC); Chang (1996, JC)

  • 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.

(Xie 1994, J. Climate, p.2008)

cf: Liu & Xie (1994, JAS)

slide11

|V|  Annual

|V|  Annual

0

Annual V’ in both cases

slide12

Temperature along equator

SST’ & u’ at Eq

-

+

Veq

slide13

Model Results

Xie 1994, J. Climate

slide14

Response to

cross-equatorial winds

Philander & Pacanowski (1981, Tellus)

slide15

SST

Wind

Cloud

SST: Mean & Annual Harmonic

slide16

Sensitivity to the length of year

1 yr = 12 mon

1 yr = 18 mon

SST

tx

ty

Giese and Carton (1994, JC)

slide17

Control

Flux corrected

Li and Hogan (1999, JC)

slide18

Improved the mean state (asymmetrical about the equator)  Annual cycle on the equator

Control

Annual-mean

correction

Obs

Seasonal correction

Li and Hogan (1999, JC)

slide19

Prescribed observed cloudiness in a CGCM

  • Improved the mean state (asymmetrical about the equator)
  • Seasonal forcing by cloud

Gordon et al. (2000, JC)

Yu and Mechoso (1999, JC)

slide20

Pacific

110oW, Eq

Atlantic

0o, Eq

slide21

Equatorial Annual Cycle

in the Atlantic

Ocean dynamics

play a more important role

Depth (m)

Houghton

(1983, JPO, p. 2070)

slide22

Annual-mean

March-April

July-August

annual cycle in the equatorial oceans

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

CTL-APR

CTL run

APR run

June

April

Longitude

Seasonal cycle of equatorial zonal wind:(1) Local air-sea interaction

Ueq (m/s)

slide25

ITCZ

Eq.

Monsoon Effect

June-April diff in APR run

with cold tongue removed

Surface wind & precip

Equatorward momentum

advection

Mean

slide26

Monsoon  Cold tongue

Cold tongue effect

CTL-APR anomalies in June

Surface wind (m/s) and precipitation (mm/day)

Monsoon effect

June-April diff in APR run

with cold tongue removed

Okumura and Xie (2004, J. Climate)

slide27

Summary

  • 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.
slide28

Eq IO seasonal cycle: uncoupledin the central basin

tx

SST

uo

SST  cloud: 1 yr

Zonal wind & current: 0.5 yr

Wyrtki jets

slide29

T = 0.5 year (period)

L = 5,6327 km (basin width)

K

R

K

wind

wind

Basin-mode resonance at the semi-annual period

Jensen (1993, JGR,22 533-); Han et al. (1999, JPO, 2191-)

Cn=163 cm/s, m = 1,

Cn = 82 cm/s, m =2,

Cane and Sarachik (1981, JMR); Cane and Moore (1981, JPO)

nov easterly acceleration and sst response

AVHRR SST (C, 5-day, 85-99)

COADS Zonal Wind (m/s)

Nov

Nov

TOPEX/Poseidon SSH (cm)

COADS SST (C)

Nov easterly acceleration and SST response
thermocline depth control of sst variability

Nov

Jun

Thermocline depth control of SST variability

Rms SST (1982-2003)

T/P SSH (cm)

20W

0

40W

Yuko Okumura, U of Hawaii