The earliest rainy season of The earliest rainy season of the Asian Summer Monsoon; the Asian Summer Monsoon;

the Bay of Bengal Onsetthe Bay of Bengal Onset

UAW5, JapanSpeaker : Yuwei Lin

01Jul 2008

1.The Asian summer monsoon evolution1.The Asian summer monsoon evolution

BOB monsoon onset is important for the formation of a large scale circulation pattern that favors the SCS monsoon onset. (Ding and Liu,2001 ;Wu et al.,2005)

LinHo and Wang, 2002

BOB onset

SCS onsetIndian onset

2.What triggers the BOB onset?2.What triggers the BOB onset?• The first and strongest

northward-propagating ISO in summer.

• The tightest phase locks with calendar as seen from CISO (Climatological ISO) .

A wet phase ISO triggers the BOB onsetFrom climatology, a northward-propagating ISO in early May is found associated with the BOB onset.

Northward ISOThe warmest SST of the BOB & northward-

propagating rain in early May

Time-lat. cross sections of 80E-100E

3.BOB Onset Compositefind min OLR_KZ

80E-100EEQ-10N

80E~105E15N~20N

1998 13May 20May

1999 11Apr 14Apr

2000 23Apr 27Apr

2001 27Apr 05May

2002 02May 16May

2003 10May 13May

2004 02May 17May

2005 05May 15May

2006 22Apr 25Apr

2007 02May 12May

Ave Date 29Apr 07May

SD 9.5 days 11.8 days

compositecomposite Day 0Day 0 Day 8Day 8

Two crucial events to stage BoB onset (need high-resolution data)

Find the date of the regional minimum OLRKZ( 20-80days filter) => Onset date !Composite period in 1998-2007 Check by 200mb H

H >12470

Continue more than 2 PTD.

=>South Asian High development 80E~100E,EQ~10N

80E~105E,15N~20N

OLRkz & SSTkz in BOB

SSTkz warming occurs before the northward ISO.

Charge stageCharge stage|||

Trigger stage Trigger stage |||

Discharge stageDischarge stage

Contours show OLRkz anomalies at 5 w/m2 and arrows show anomalies 850hPa winds. Shading denote areas with statistically significant wet (dry) singularity at 95% confidence level.

4. BOB onset is 4. BOB onset is Classified into 3 stagesClassified into 3 stages

Day-20 ~Day -10Day-20 ~Day -10

Day-8 ~Day -3Day-8 ~Day -3

Day-2 ~ Day 2Day-2 ~ Day 2

Day 3 ~Day 8Day 3 ~Day 8

Day 10~Day 20

5.The three stages (I)5.The three stages (I)

Charge stageCharge stage||||||

Trigger stage Trigger stage ||||||

Discharge stageDischarge stage

The strongest convection induces a Gill response. Then an explosion of Rossby dipole leads to BoB onset.

1. A north-eastward propagating ISO brings strong low level south-westward flow.

2. India-Burma trough is formed.

1. The dry phase ISO Inhibits convection over the BOB.

2. The wet phase ISO moves eastwards along the equator.

OLRkz(shaded), wnd85kz 500mb H(contour)

vort85kz, wnd85(stream)

.

6. Pre-onset conditionSmall warm pool in northern Small warm pool in northern Indian Ocean serves as heat batteryIndian Ocean serves as heat battery

For duration of ISO time scale For duration of ISO time scale

Effect of Hot Peninsula in springEffect of Hot Peninsula in spring the effect of Tibetan Plateau the effect of Tibetan Plateau

08Apr~23Apr

28Apr~03May

Low level trough

The reversed meridional SST gradient.

hCQ

tSST net

0

y

SST

Qnet: Net heat fluxC: the specific heat of sea

waterρ: density of sea water h: mixed layer depth (m)

Charge stageCharge stage||||||

Trigger stage Trigger stage ||||||

Discharge stageDischarge stage

1. Max moisture convergence is in south BOB.

2. The meridional SST gradient reverses.

1. Northward-propagating ISO discharges the warm SST and low-level moist potential energy.

2. The moist SW flow supports the convection in BOB.

1. SST warms up and the water vapor increases.

2. BOB provides moisture flux to eastern equatorial Indian ocean.

3. The low-level trough flow is decease in BOB by equatorial wet ISO.

In BOBIn BOB8.The three stages (II)8.The three stages (II) SSTkz (shaded), Vaporkz

Low-level moisture convergence (shaded), moisture fluxkz

7. Air-sea interaction of the northward ISO7. Air-sea interaction of the northward ISO

SSTkz & LHkz

SSTkz & SWkzOLRkz & Qconvkz

OLRkz & SSTkz

EQ

EQ

Composite ISO anomaly data, time-latitude cross sections of 85E-97.5E

Day 0Kemball-cook and Wang,2000

8.The large scale circulation change8.The large scale circulation change

The release of latent heat of condensation is the primary heat source driving the Asian summer monsoon.

Krishnamurti et al. 1976

9. Summary9. Summary1.The northward-propagating ISO in early May induces the BOB onset.2.The evolution of BOB onset can be classified into three stages:

1.1.Charge stage:Charge stage:The moisture convergence induced by the eastward-propagating ISO neutralized pre-onset

wind field in BoB as part of a low-level hot trough of Peninsula, leading to weak (total) wind stress in BoB lasting around 20 days.

SST increases rapidly around 13N。2.2.Trigger stage:Trigger stage:

• Eastward-propagating wet phase ISO reaches Sumatra and excites an n=1 Rossby wave. • Only the northward component thrives under the favorable basic state, which starts the BOB

Onset.

3.3.Discharge stage:Discharge stage:• The Rossby wave moves northwards by consuming warm SST on its northern flank. • India-Burma trough is formed, further enhances moisture convergence in North rim of BoB.• South Asian High develops rapidly.

Thanks for your attention!