Convection Activities Over the Bay of Bengal and Severe Flooding Events Over the Southern China i

n June

Haiming XuNanjing Institute of Meteorology, NUIST

Collaborators: Xiaolin Xu, and Dong Si

Oct. 31~Nov. 3, Boulder, USA

1. Motivation

A severe flooding event occurred over the southern China in June 2005, and brought large economic losses. It was reported that this flooding event killed 75 people, and caused 5.97 billion RMB yuan economic losses (National Climate Center).

Objective:Objective:

●Any large-scale circulation anomalies over the tropical and subtropical regions?

●What cause these circulation anomalies ?

●Any common features for flooding events in the southern China?

Total rainfall (mm) in June, 2005

(From National Climate Center)

Percentage (%) of rainfall anomalies in June, 2005.

(From National Climate Center)

Total rainfall during the heavy rainfall event from 17 to 25 June, 2005

A heavy rainfall event occurs over the South China from June 17 to June 25, 2005

Long Men (1300.2 mm)、 He Yuan (780.5 mm)、 Shan Wei (471.7 mm)，Tai Ning (571.9 mm)、 Bing Nan (559.5 mm)、 Jian Ou (552.1mm)

2. Data

Datasets used here include:

(1) Monthly and daily mean NCEP/NCAR reanalysis, on a 2.5×2.5 grid , available since 1958.

(2) Daily and monthly mean outgoing longwave radiation (OLR), on a 2.5×2.5 grid, available since 1975.

(3) Monthly Tropical Rainfall Measuring Mission (TRMM) product available from January 1998 to December 2005 on a 0.25º grid (Wentz et al. 2000).

(4) Observed daily precipitation over the Southern China from 1976 to 2005 (45 weather stations)

3. Large-Scale Circulation Anomalies

June, 2005 Climatology ( 1958~2005)

Geopotential Height at 500hPa

The western North Pacific subtropical high in June 2005 is much strong and shifts westward

H H

Monthly Mean OLR in June, 2005

OLR Anomalies in June, 2005. （ 05 － 7505 ）

Anomaly Convection Activities Over the Bay of Bengal in June, 2005.

Effects of an anomaly heating over the bay of Bengal on precipitation over the South China in June: A

numerical study

(Xu et al., 1997 ， Journal of Tropical Meteorology)

Rainfall anomalies (EXPA-CTL)

Wind anomalies at 300 hPa

(EXPA-CTL)

A CA

+–+

+

Effects of SSTA in Tropical Atlantic

TRMM SSTA （ ºC ） TRMM Precipitation Anomalies （ mm/d ）

May

June

May

June

Wind anomalies at 500hPa in June, 2005

A

C

AC

A C

A

C

AA

C

The anomaly tropical circulation over the Indo-Pacific is possible related to this La Nina-like event, the latter probably affects the former via a two-dimension Rossby wavetrain forced by the La Nina-related SST anomalies in the tropical Atlantic.

CA

4. Short-term Variation for the June 17~25 Case

Section of geopotential height at 500hPa along 10~20ºN

Time-longitude section of OLR along 10~15ºN

Strong convection occurs over the Bay of Bengal, followed by an enhancement and westward extension of the western Pacific subtropical high

BOB SCS

Time

OLR over the Bay of Bengal (10ºN-20ºN,85ºE-100ºE, solid) , and over the western Pacific (10ºN-20ºN,110ºE-

125ºE, dashed)

Convection activities over the Bay of Bengal lead the western subtropical Pacific high enhancement by 2~3 days

－ －－ western Pacific

———— Bay of Bengal

Zonal vertical circulation, averaged between 10º N and 20ºN

June 13~14 June 18~20

June 21~22 June 23~24

Convection activities over the Bay of Bengal interact with the western Pacific subtropical high through a zonally vertical cell

5. Composite Analysis

Nine severe flooding events are selected from 1976 to 2005, based on the following criteria:

(1) At least 1 out 45 stations in the southern China have daily rainfall > 50 mm and at least last for 5 days;

(2) Total stations with daily rainfall >50 mm are greater than 20, during the period of flooding event.

Years Period Years Period Years Period

1983 June 5~June 23

1991 June 9~June 14

1996 June 19~June 26

1984 June14~June 22

1993 May 31~June 18

1998 June 12~June 26

1990 June 07~June 11

1994 June 08~June 20

2005 June 17~June 23

9 severe flooding events from 1976 to 2005

Composite geopotential height at 500 hPa （ 10gpm ）

Years for severe flooding events

Years without severe flooding events

H

H

Composite OLR for years with flooding events

OLR Anomalies (years with flooding events minus years

without flooding events)

Anomaly convection over the Bay of Bengal for years with flooding events over the Southern China in June.

Composite geopotential height and winds at 500hPa

－8

－4

0

－ 6d

－ 2d

＋ 2d

H H

HH H

H H

Composite OLR

Day

-4

-2

0

Day

-3

-1

+1

Time-longitude cross section of composite 500hPa geopotential height at 15~20ºN

Time-longitude section of composite OLR at 12.5~17.5ºN

After

Before

After

Before

Days

Convection activities over the Bay of Bengal are associated with Convection activities over the Bay of Bengal are associated with the western Pacific high via a zonal vertical cellthe western Pacific high via a zonal vertical cell

Zonally vertical anomaly cell along 10°N~20°N

－ 4d

0d

－ 2d

＋ 2d

5. Summary

★ The development of flooding events in the Southern China is associated with abnormal tropical circulations over the Indo-Pacific which are characterized by an enhanced western Pacific subtropical high with its position shifting westward, and anomaly convection activities over the Bay of Bengal.

★ Strong convection activities over the Bay of Bengal interact with the western Pacific subtropical high through a zonally vertical cell, and thus leading to the formation of this severe precipitation event.

★ For the Case in 2005. In May and June 2005, a La Nina-like event also occurred in the eastern Atlantic, The anomaly tropical circulations over the Indo-Pacific is possibly related to the La Nina-like event in the tropical Atlantic, the latter probably affects the former via a two-dimension Rossby wavetrain forced by the La Nina-related SST anomalies in the tropical Atlantic.

Thank you for your attention!Thank you for your attention!

Delayed Effects of SSTA in Indian OceanTRMM SSTA （ ºC ） TRMM Precipitation Anomalies (mm/d)

May

June

May

June

Time- latitude section of winds at 850hPa along 115ºE

With the western Pacific high increasing in its intensity and shifting westward, a LLJ occurs over the South China Sea

and migrates northward

South China South China Sea

A

CA

C

AC

Wind anomalies at 500 hPa

Geopotential height difference at 50Geopotential height difference at 500 hPa0 hPa

Geopotential high difference at 500 hPa between the years with and without heavy flooding events. Shade denotes regions where the differences pass the two-side T student test at 0.05 significant level.