Hong Kong surpasses July rainfall in just 20 days – why?

Hong Kong’s rainfall so far this month has already surpassed typical levels for July by 23 per cent, a review of Observatory data shows, with a meteorologist warning the city will experience more extreme downpours due to global warming.

The Observatory also said on Tuesday that Hong Kong would be hit by rainstorms again over the weekend following weeks of heavy rain as a tropical cyclone neared the city.

A South China Morning Post reporter reviewed the forecaster’s data and found the city recorded 474.7mm (18.69 inches) of rainfall in the first 20 days of July – exceeding the typical monthly level of 385.8mm by 23 per cent.

The city also recorded 600.5mm of rainfall in June, 22 per cent higher than normal.

Last month, heavy showers prompted the forecaster to issue three black rainstorm warnings – the highest in Hong Kong’s three-tier alert system.

Since June, the Observatory has issued 48 rainstorm warnings, nearly double the 25 alerts sent out during the same period last year.

Kwok Yu-ting, an Observatory scientific officer, said that Hong Kong was hit by multiple rainstorms in June under the influence of a southwestern monsoon and a low-pressure trough.

“In the first half of July, Hong Kong was continually influenced by multiple low-pressure systems, including wide low-pressure troughs, low-pressure zones and tropical cyclone Maysak,” she said.

Leung Wing-mo, a former assistant director at the Observatory, said that multiple weather systems – including tropical cyclones and widespread low-pressure zones – contributed to Hong Kong’s recent heavy rainfall.

He pointed to the outer rainbands brought by Typhoon Maysak earlier this month and the low-pressure area associated with Super Typhoon Bavi.

“There are multiple factors behind rainfall in a particular month or year, but looking from the perspective of a long-term trend, we are not surprised at all,” Leung said, warning that extreme downpours would become more common due to global warming.

“In the context of climate change, rainfall will be heavier when conditions are suitable for rain, while droughts would be more severe when there are no such conditions.”

Leung explained that as temperatures rose due to global warming, more moisture would evaporate from rivers and seas, while hotter air could also hold more water, leading to heavier rainfall.

In early July, Maysak made landfall in the Guangxi Zhuang autonomous region in southern mainland China, causing a dam to collapse. The resulting floods killed at least 39 people and displaced another 4,500.

Bavi made landfall in the eastern province of Zhejiang on July 11 after weakening to a typhoon, bringing heavy rainfall and flooding to large parts of the country.

On Tuesday, the Observatory warned that an area of low pressure was expected to gradually develop into a tropical cyclone east of the Philippines over the next few days.

It said the storm would move in a general direction that would bring it close to the coast of Guangdong province over the weekend.

“Its intensity will gradually strengthen, but the landfall position remains uncertain,” scientific officer Kwok said.

“According to our forecast, from the second half of Saturday to Sunday, Hong Kong will see squally showers and thunderstorms. The wind will be strong, and there will be swells.”

The forecaster earlier warned that the planet could experience a “strong to very strong” El Nino event, lasting from late summer until at least early next year.

El Nino refers to the abnormal warming of sea surface temperatures over the central and eastern equatorial Pacific Ocean, which generally leads to wetter winters and springs in Hong Kong.

“Under the joint influence of global warming and El Nino, it is expected that Hong Kong will be hotter than normal these two years, and the annual mean temperature will be higher, with a chance of breaking the high temperature record,” the forecaster wrote in a blog post late last month.

It added that El Nino also provided more conducive conditions for intensive rainstorms.

While Leung cautioned that a strong El Nino event did not necessarily guarantee more rainfall, he noted that four of the five wettest years on record in Hong Kong, excluding 2005, occurred during such weather events.

He also warned that the heavy rainfall would increase the risk of landslides.

“While the government has stepped up regulation of slopes, many of the high-risk slopes are managed by private owners. But many of them may not realise they have an obligation to maintain the structures in good condition,” he said, calling for property owners to pay more attention. -- SOUTH CHINA MORNING POST