1. INSTITUTE OF METEOROLOGY AND HYDROLOGY AND ENVIRONMENT (IMHEN)
Southeast Asia summer monsoon and rainfall seasons over climatic zones of Vietnam
Presented by Nguyen Thi Hien Thuan
Melbourne, 9-14 Dec 2012

2. Outline Overview on summer monsoon (focusing on Vietnam region)
Relationship between summer monsoon and rainfall seasons of Vietnam
Summer monsoon and ENSO
Remarks

3. Overview on summer monsoon
Pham and Phan before 1990s, showed the basic feature of two distinct monsoon subsystems of the Asian monsoon: the south Asian monsoon and the east Asian monsoon
Nguyen and Nguyen (2004) analyzed the influence of Asia monsoon on climate
Nguyen and Thuan, 2007: define a new monsoon circulation index for studies of interannual variability of South Vietnam summer monsoon
Pham et al., 2010 focused on predictability of monsoon onset based on relationship of rainfall and monsoon indices

4. Overview on summer monsoon
The summer monsoon brings major amount of rainfall to south of Vietnam:
Major source of rainfall: wet season brings ~90% of total annual
The summer monsoon – Southwest monsoon season: May-Sep, traditional summer/rainy season: May-Oct (May-Nov)
Critical issues: the onset of the rainy season/ dry spells

5. Overview on summer monsoon
Traditionally: rainfall characteristics: rainfall amount (monthly, seasonal, yearly) or their anomalies – rainfall variation
But: rainfall data: influenced by local conditions, other factors, not always reliable
Worldwide and in the region: various monsoon indices – objective – representative of a large scale circulations of the monsoon (circulation index – dymamic index, convection index, …

6. Overview on summer monsoon
Mean summer monsoon onset date based on rainfall
(Wang and Linho, 2001)

7. Overview on summer monsoon
monsoon onset pentad
monsoon withdrawal pentad 50 72 20 46
Summer monsoon onset and withdraw date over Vietnam change region to region
(Lanh.N.V et. al., 2007)

8. Overview on summer monsoon
Summer monsoon onset date over Vietnam change year to year over southern Vietnam, (Pham et. al., 2010)

9. summer monsoon and rainfall seasons
Relationship between
summer monsoon and rainfall seasons

10. Rainfall seasons of 7-sub climatic region
Double-peaks

11. Corr. coefficient between monthly rainfall with..

Monsoon index: U850 (2.5N-12.5N, 95E-110E)-(20.0N-27.5N, 105E-120E), V-IX. Nguyen and Thuan (2006)

12. Corr. coefficient between monthly rainfall with..
Lien.T.V (2010)
SCSSM index
over east sea
CI index
Over south of VN

13. summer monsoon and ENSO
Relationship between
summer monsoon and ENSO

14. Rainfall data: Two data sets:
The Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) data.
Pentad and monthly CMAP rainfall averaged over 8o – 15oN, 104o – 110oE, for Jan Dec 2006, on 2.5o x 2.5o.
Station rainfall data over South Vietnam

15. Summer monsoon index for SVN
Circulation index: CI
Map of correlation coefficients of U850 hPa & seasonal CMAP SVN (8o-15oN, 104o-110oE).
The areas with the highest correl. coeff. are selected
RCMAP&U850 (V-IX)
CI = U850 ( oN, oE and oN, oE)

16. Anomalies of Circulation index over SVN
Summers
Light yellow color: Non_ENSO, red: El Nino, blue: La Nina,

17. Average CI anomalies EL Nino developing (E0) EL Nino decaying (E1)
La Nina developing (L0)
La Nina decaying (L1)
Non-ENSO (Non)
(1)
(2)
(3)
(4)
(5)
(6)
Ave
0.11
-2.03
-0.59
0.17
0.64
Min
-1.2
-3.2
-2.5
-0.8
-1.5
Max
1.3
0.8
1.4
2.1
El Nino : (m/s)
La Nina: (m/s)
Non-ENSO 0.64

18. Remarks The influence of ENSO on monsoon circulations
Different impact according to ENSO development
El Nino summers: weak monsoon.
La Nina summers: strong monsoon.
But: stronger impact in the decaying summers, especially in El Nino decaying summers (‘83, ‘87, ‘98)

19. Conclusions
Monsoon plays a crucial role on Vietnam climate
Rainfall over southern Vietnam has a close relationship with summer monsoon activities
There is no universal monsoon index for all seven climatic zones of Vietnam
Impacts of ENSO on the summer monsoon are different according to ENSO phases: development or decaying: stronger impacts are in ENSO decaying year, especially in El Nino cases.

20. Thank you for your attention!