Southeast Asia summer monsoon and rainfall seasons over climatic zones of Vietnam

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. 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 Overview on summer monsoon

4. 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 Overview on summer monsoon

5. 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, … Overview on summer monsoon

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

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

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

9. 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. 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 1979 - Dec 2006, on 2.5o x 2.5o. Station rainfall data over South Vietnam

15. Circulation index: CI Summer monsoon index for SVN 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 (2.5-12.5oN, 95-115oE and 20.0 - 27.5oN, 105- 120oE)

16. Anomalies of Circulation index over SVN Summers 1961-2004 Light yellow color: Non_ENSO, red: El Nino, blue: La Nina,

17. Average CI anomalies

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. 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. Conclusions

20. Thank you for your attention!