Unusual Surge in Typhoons Hits South China and Hong Kong in 2025
As Super Typhoon Ragasa is expected to hit Hong Kong this week, the Hong Kong Observatory has issued warnings that coastal water levels from the upcoming typhoon may rise to levels comparable to those seen during Typhoon Hato in 2017 and Typhoon Mangkhut in 2018, due to the threat of severe storm surges.
Fig. 1. Real time satellite image Typhoon Ragasa as of 22 September 2025. (Source: Zoom Earth)
The devastation caused by Typhoon Mangkhut in 2018 remains fresh in the public memory. That storm left approximately 4,000 households without electricity and 2,000 without water. It also led to the cancellation of 889 flights, injured 458 people, damaged over 850 properties, and toppled more than 60,000 trees across the city.
This year, South China—including Hong Kong—has faced an unusually high number of typhoons, far exceeding the seasonal norm. Historically, Hong Kong experiences around six typhoons annually. However, by mid-September 2025, the city has already been impacted by eight typhoons that triggered warning signals of level 1 or higher. The latest, Typhoon Ragasa, marks the ninth such system this year. Among the earlier storms, Typhoons Wipha and Tapah reached the highest warning level of Signal No. 8.
According to Prof. Jung-Eun Chu, a climate scientist in the School of Energy and Environment at City University of Hong Kong, large-scale climate patterns such as El Niño and La Niña play a significant role in determining the frequency of tropical cyclones in the South China Sea and surrounding regions. Typically, El Niño conditions tend to be associated with a later start to Hong Kong's tropical cyclone season, whereas La Niña events are generally associated with an increased number of tropical cyclones affecting Hong Kong.
Until August 2025, the El Niño–Southern Oscillation (ENSO) was in a neutral phase. However, recent observations show that sea surface temperatures (SSTs) across the central and eastern Pacific have dropped to near or below average levels. This cooling trend intensified in early September, signaling a shift toward La Niña-like conditions (Fig. 2). Consequently, warmer-than-average SSTs in the western Pacific are expected to fuel more tropical cyclone activity in the region from August through October.
Although it's still debated whether global warming affects the frequency of tropical cyclones, experts widely agree that it will lead to stronger storms with heavier rainfall in the future.
When a severe typhoon approaches, it's crucial to take appropriate safety measures. Typhoons typically bring three major types of hazards: powerful winds, intense rainfall, and storm surges. Each of these can pose serious risks to life and property. Hong Kong’s unique geography means that the effects of a typhoon can vary significantly depending on its position relative to the city. If the storm is more than 500 kilometers away, residents may experience clear skies due to strong descending air currents. However, as the typhoon draws closer, wind speeds increase, and bands of heavy rain begin to sweep through the region.
The most dangerous conditions often occur when the typhoon moves west of Hong Kong. In this scenario, the city falls within the storm’s “dangerous semicircle”—a zone where wind speeds intensify and southerly winds can drive high storm surges toward the coast. These surges pose a particular threat to low-lying areas, which should be especially vigilant and prepared for flooding.
Fig. 2. Global sea surface temperature anomalies as of 20 September 2025. (Source: NOAA)
To stay safe, it's important for residents to remain indoors or relocate to a secure shelter if necessary. Prepare enough food, drinking water, and basic supplies to last at least two to three days. This includes emergency medical kits, flashlights, batteries, and any essential medications. Having these items ready can make a big difference if services are disrupted. If your home is in a low-lying area or prone to flooding, take extra precautions. Clear out any objects from ground-level spaces that could block drains or trap water. This helps reduce the risk of water damage and ensures that drainage systems function properly during heavy rainfall.
Prof. Chu’s research team is actively exploring various aspects of typhoons, including the influence of climate change on their formation, the process of extratropical transition, interactions with tropical waves, and the development of AI-driven forecasting models.
The School of Energy and Environment (SEE) at City University of Hong Kong is dedicated to advancing research, education, and innovation in the fields of energy, environment, and sustainability. SEE brings together leading experts and interdisciplinary teams to address pressing global challenges such as climate change, renewable energy, air quality, and sustainable urban development. Through world-class research, impactful teaching, and strong industry and community partnerships, SEE aims to nurture the next generation of leaders and professionals who will drive positive change for a greener and more resilient future. Learn more at https://www.cityu.edu.hk/see/.
Appendix: Typhoon Ragasa Animations
Fig.3. Satellite image of Brightness Temperature
(Source: Tropical Tidbits)
Fig. 4. Satellite image of Reflectance
(Source: Tropical Tidbits)
Fig. 5. Predicted ocean wave heights
(Source: Tropical Tidbits)