Based on the Mann-Kendall test and Sen’s slope method, this study investigates the monthly, seasonal, and annual sea surface temperature (SST) trends in the coastal area of Hai Phong (West of Tonkin Gulf) based on the measurements at Hon Dau Station from 1995 to 2020. The results show a sea surface warming trend of 0.02°C/year for the period 1995-2020 (significant level α = 0.1) and of 0.093°C/year for the period 2008-2020 (significant level α = 0.05). The monthly SSTs in June and September increased by 0.027°C/year and 0.036°C/year, respectively, for the period 1995-2020, and by 0.080°C/year and 0.047°C/year, respectively, for the period 2008-2020. SST trends in winter, summer, and other months were either different for the two periods or not significant enough. This may be due to the impact of ENSO, which caused interannual SST variability in the Hai Phong coastal with two intrinsic mode functions (IMF) signals a period of ~2 (IMF3) and ~5.2 years cycle (IMF4). A combination of these signals had a maximum correlation of 0.22 with ONI (Oceanic Niño Index) delayed by 8 months. ENSO events took ~8 months to affect SST at Hai Phong coastal area for 1995-2020 and caused a variation of SST within 1.2°C.

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The impact of climate change on sea level has attracted much attention from scientists worldwide. The analysis of sea-level change at global and regional scales is based on altimetry and/or tide gauge data. The main objective’s study relied on statistical tests to highlight the significant long-term changes in sea level in Ha Long coastal area through the 47 years (1974–2020) historical tide gauge record of the Bai Chay station. The average water level in this area was 207.9 cm, which tended to be higher in recent years, especially from 2006 to the present (above 210 cm). Moreover, sea level trends were carried out separately for the whole period (1974–2020) and the last 19 years (2002–2020) by the Mann-Kendall test and Sen’s slope estimator. The results showed a significant sea-level rise (SLR) trend with a 95–100% confidence level. The average annual rates of sea-level were 3.14 mm/year and 5.83 mm/year in 1974–2020 and 2002–2020, respectively, indicating an SLR of about 14.76 cm over 47 years and significant acceleration in SLR recently (11.01 cm over the last 19 years).