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,cwchang@atmos.pccu.edu.tw, +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.