Pang-chi Hsu, Chih-Hua Tsou

Department of Earth Science,

NTNU, Taiwan

Motivation

Data and Model

Description

Numerical Experiments

Simulated Results

Summary

Outline

Which one plays the active role ? ENSO-related warm SST anomaly, the enhancement a

nd eastward-extension of low-level westerly, monsoon

trough beneficial for TS formation and development al

l occur simultaneously.

Eddy energetics

Numerical experiment

Motivation

Eddy energy budget equations

Zonal mean Eddy Less study

Time mean Transient Eddy

General circulationExtratropical cyclones

Lorenz (1955)

Kung (1966)

﹕

Hu et al. (2004)

General circulation

Oort (1964)

No calculation

Synoptic disturbanceLau and Lau (1992)

MJO-TD type eddyMaloney and Dickson (2003)

By 3-D EKE budget eq.

& 2-D EAPE budget eq.

3-D EKE/EAPEdiagnosis

Walker Circulation

Data

NCEP / DOE (NCEP R2) : 144 × 73 × 17

NCEP monthly SST : 180 × 90

JTWC 6-hourly best track

Based on Wang and Chan (2002), the TS activity over

the WNP is closely related to Nino3.4 SSTA in

summer.

The classifications of ENSO are stratified by JAS

SSTA in Nino3.4 region into

Warm years (SSTA ≥ 0.8 S.D.): 1982, 1987, 1991, 1997

Cold years (SSTA ≤ -0.8 S.D.): 1981, 1985, 1988, 1998

ECHAM4.6 : MPI, Germany (Roecknet et al. 1996)

High resolution: T106 (320×160) L19

6-hourly output for TS identification and trajectory

Forced with observed monthly SST and sea-ice (AMIP run) f

or 1979-1999

Model Description

Vitart et al. (1997) and Knutson et al. (2007)

ECHAM 4.6 T106 control run

Numerical Experiments

Expt SST boundary condition

CTL Globally observed SST

noTPO Observed SST everywhere except Central-Eastern

Pacific (100ºE-80ºW, 15ºS-15ºN)

Two numerical experiments are designed to clarify whether the ENSO-related SST anomaly

plays an active role to the interanuual variations of large-scale circulations and TS activity.

Motivation

Data and Model

Description

Numerical Experiments

Simulated Results

Summary

Outline

Overestimate (underestimate) the response of heating (cooling ) effect

TS frequency has eastward bias

Camargo et al. (2005)

Obs

CTL

<Warm minus Cold>

No warming and cooling

no westerly jet

A-C wavetrain still exists

but shifts northeastward

ENSO-related SST anomalies

Strong westerly

Cyclonic shear (monsoon trough)

favorable environment for TS activity

insignificant interannual TS variation

Ratio (Warm years) TS activity

indices JTWC CTL noTPO

Number 1.10 ( 54 ) 1.80 ( 54 ) 1.15 ( 39 )

ACE (104 m2/s2) 2.17 (55.14) 1.51 (15.61) 0.93 (9.93)

Lifespan (day) 1.48 (9.75) 0.93 (8.01) 1.03 (8.72)

Max wind (m/s) 1.27 (47.57) 0.98 (21.30) 0.98 (21.46)

The TS number and life span of TS during warm years are similar to JTWC,

but the intensity of simulated TS is too weak (Bengtsson et al. 2007).

ENSO-related SST anomalies play an active role in modulating the TS activit

y over the WNP.

WHY?

Eddy energetics along the TS track (obs)

MKE to EKE EAPE to EKE

Enhanced eddy barotropic and baroclinic energy conversion might be benefici

al for the southeastward extension of the TS formation region and the intensificat

ion of the TS during warm events.

The eddy baroclinic energy conversion plays a significant role in maintaining th

e subsequent development of eddies.

Less EKE converted from EAPE

Insufficient EAPE generation and eddy baroclinic energy conversion

Summary

The results of numerical experiments support that the El Nino-r

elated anomalous SST forcing plays an active role in setting the

favorable large-scale environments for the formation and early-

development of transient eddies including TS over the WNP.

The inadequate simulation of eddy diabatic heating and eddy b

aroclinic processes which might be resulted from insufficient res

olution would lead to the underestimate of TS intensity in ECH

AM4 T106.

Thanks for your attention