Even though the coastal sea of Northern Vietnam is located entirely in the tropics, it has an unusually cold period when the air temperature drops below 18°C. It has been not commonly seen in other tropical regions on Earth. Applying the traditional climate classification method, selecting dominant factors of heat (radiation, sunshine, annual temperature range), humidity (rainy season type, the three most rainy months) and adding the tropical marine dynamic factor (frequency of tropical cyclones — TCs); with a multi-year climate database (31 meteorological stations, including 8 stations at islands); the coastal sea climate of Vietnam (stretching from about 8–23 geographical latitudes) is divided into 2 subzones and 7 different climate regions. (1) The Northern coastal sea climate subzone with winter — BB (north of latitude 16°): includes 3 climate regions: BB1 — Tropical monsoon climate (TMC) with cold winter, summer rain, very frequent TCs; BB2 — TMC with slightly cold winter, summer rain, very frequent TCs; BB3 — TMC with warm winter, autumn-winter rain, frequent TCs. (2) The Southern coastal sea climate subzone is hot all year round — BN (south of latitude 16°) including 4 climate regions: BN1 — TMC is hot all year round, autumn-winter rain, infrequent TCs; BN2 — TMC is hot all year round, summer rain, occasional TCs; BN3 — TMC is hot all year round, summer rain, rare TCs; and BN4 — TMC is hot all year round, summer rain, extreme rare TCs. The diversity of coastal marine climates in Vietnam is the result of the interaction between zonal factors (tropical geographical location), non-zonal factors (cold winter monsoon circulation) and the geographical location of the eastern edge of the Asian continent (autumn-winter rain regime).

Nguyen Khanh Van
Professor, PhD
Senior Researcher
Vietnam Academy of Science and Technology (VAST)
Lecturer, Faculty of Tourism
Nguyen Tat Thanh University
300A Nguyen Tat Thanh, District 4, Ho Chi Minh city,
email: ngkhvan@gmail.com

Nguyen Duc Ngu
Professor, PhD
Senior Researcher, Former General Director
Vietnam Meteorological and Hydrological Administration (VNMHA)
8 Lang Ha, Dong Da District, Hanoi city, Vietnam
email: nguyenducnguvn@gmail.com

Tran Van Truong
Associate Professor, PhD
Lecturer, Faculty of Geography
University of Natural Science, Vietnam National University VNU – HUS
334 Nguyen Trai, Thanh Xuân District, Hanoi city, Vietnam
email: tranvantruong@hus.edu.vn

Vuong Van Vu
Master of Geography
Researcher
Institute of Geography, VAST
18 Hoang Quoc Viet, Cau Giay District, Hanoi city, Vietnam
email: v3thaonguyen@yahoo.com

Hoang Thi Kieu Oanh
PhD in Geography
Lecturer, Faculty of Social Sciences and Art
Saigon University
273 An Duong Vuong, District 5, Ho Chi Minh city, Vietnam
email: htkoanh@sgu.edu.vn

1. Tran Van Truong (Project leader) et al. Research on tropical monsoon marine landscapes in the coastal sea of Vietnam to serve sustainable blue marine economic development. 2024 ((code: DTDL.CN-91/21) (in Vietnamese).
2. Köppen climate classification. In: Wikipedia. The Free Encyclopedia. Available at: https://en.wikipedia.org/wiki/K%C3%B6ppen_climate_classification (accessed 17.02.2025).
3. Alisov V.P. Geograficheskie tipy klimatov [Geographical types of climates]. Meteorologiya i gidrologiya. 1936;(6):16–25 (in Russian).
4. Peel M.C., Finlayson B.L., McMahon T.A. Updated world map of the Köppen-Geiger climate classification. Hydrology and Earth System Sciences. 2007;11(5):1633–1644. DOI: 10.5194/hess-11-1633-2007.
5. Crosbie R.S., Pollock D.W., Mpelasoka F.S., Barron O.V., Charles S.P., Donn M.J. Changes in Köppen-Geiger climate types under a future climate for Australia: hydrological implications. Hydrology and Earth System Sciences. 2012;16(9):3341–3349. DOI:10.5194/hess-16-3341-2012.
6. Nguyen Xien, Pham Ngoc Toan, Phan Tat Dac. Climate characteristics of Northern Vietnam. Hanoi: Science and Technology Publishing House; 1968. 265 p. (in Vietnamese).
7. Do Dinh Cuong. Vietnam’s Climate. Saigon: Bookstore Khai Tri; 1968. 350 p. (in Vietnamese).
8. Mai Van Khiem, Nguyen Dang Mau, Nguyen Duc Ngu et al. Construction of heat distribution map in Vietnam territory. Journal of Meteorology and Hydrology. 2014;644(8):13–15. (in Vietnamese).
9. Tran Thuc, Nguyen Van Thang, Mai Van Khiem et al. Building a map of rainfall distribution in Vietnam. Vietnam Journal of Science and Technology. 2014;16(2024):43–47. (in Vietnamese).
10. Nguyen Duc Ngu. Climatic conditions of the Tay Nguyen Plateau and possibilities of their economic use. Lublin: Maria Curie-Skłodowska University, Faculty of Biology and Earth Sciences; 1989 (in Polish).
11. Nguyen Khanh Van. Climate zoning of the Northern mountainous provinces and Western Thanh Nghe. Journal of Earth Sciences. 2015;37(3):204–212 (in Vietnamese).
12. Nguyen Van Chien (editor-in-chief) et al. Vietnam National Atlas, Cartography. –Hanoi : General Department of Land Administration Publishing House, 1996. 163 p. (in Vietnamese).
13. Nguyen Duc Ngu. Vietnam climate zoning. Hanoi: Meteorological Department; 1976 . 235 p. (in Vietnamese).
14. Pham Ngoc Toan, Phan Tat Dac. Climate of Vietnam. Hanoi : Science and Technics Publishing House, Part 3 – Climates of Regions, Chapter IV: East Sea Climate Zone; 1993. pp. 227–232 (in Vietnamese).
15. Phan Tat Dac, Pham Ngoc Toan. Climate and life (Basic issues of bioclimatology). –Hanoi : Science and Technics Publishing House, 1980. 116  p. (in Vietnamese).
16. Nguyen Khanh Van. Tipologiya bioklimatov V’etnama [Typology of bioclimates in Vietnam]. Izvestiya Rossiiskoi Akademii nauk. Seriya geograficheskaya. 1993;(6):114–123 (in Russian).
17. Nguyen Duc Ngu, Nguyen Trong Hieu. Climate and climate resources of Vietnam. Hanoi: Agriculture Publishing House; 2004. 296 p. (in Vietnamese).
18. Nguyen Duc Ngu, Nguyen Trong Hieu, Nguyen Van Thang et al. Delimiting climate sub-regions in the climate zoning diagram of Vietnam. Journal of Climate Change Science. 2017;(2):31–41 (in Vietnamese).
19. Diagram of monitoring and forecasting areas for tropical cyclones : Appendix 1 to the QD-TTg Regulations on forecasting, warning, transmitting information about natural disasters and disaster risk levels : approved by Decision of Prime Minister No. 18 of 2021 issued on April 22, 2021 (in Vietnamese).
20. Vu Tu Lap. Natural Geography of Vietnam. 5th edition. University of Education Publishing House, Hanoi. 2010. 351 p. (in Vietnamese).
21. Nguyen Van Thang, Nguyen Trong Hieu, Tran Thuc. Climate change and its impact in Vietnam. Hanoi: Science and Technics Publishing House; 2010. 160 p. (in Vietnamese).
22. Nguyen Duc Ngu. Effects of ENSO on cold – air activities and tropical cyclones in Vietnam. Vietnam Journal of Science, Technology and Engineering. 2017;59(2) (in Vietnamese).
23. Nguyen Duc Ngu. Uwilgotnienie a Cyrkulacja atmosferyczna na obszarze Azji Południowej/ Folia Societatis Scientiarum Lublinesis: Geografia; 1976;18. 86 p. (in Polish)
24. Nguyen Khanh Van. Heavy rain and heavy rain with terrain in the coastal strip of Central Vietnam. Natural Sciences and Technology Publishing House, 2015. 477 p. (in Vietnamese).
25. Vietnam Natural Disaster Atlas: State Key Science and Technology Program (Code: KH 08/11-15), Project KC 08.28/11-15 / Institute of Geology, VAST. Vietnam Resources – Environment and Cartogaphy Publishing House; 2015, 153 p. (in Vietnamese).