Hong Kong could experience a less polluted winter than normal if a strong enough El Niño weather effect whips up in the Pacific Ocean towards the end of what is typically the smoggiest season of the year, findings from a new study suggest.
The climate pattern describes the slight warming of sea surface temperatures over the eastern equatorial Pacific Ocean every few years due to the slackening of the trade winds across it. A La Niña effect brings the opposite.
According to Chinese University researchers, Hong Kong’s air quality is usually higher under El Niño conditions as the associated weather – more rain, less frequent northerly winds and higher wind speeds – tend to disperse pollutants.
The opposite was true for La Niña events, in which pollutant concentrations, especially from mainland China, were found to increase substantially.
“Rainfall is typically higher during an El Niño period relative to a neutral phase [when no El Niño or La Niña is present], whereas during a La Niña, it will be relatively dry,” said study co-author Professor Steve Yim Hung-lam.
“The frequency of northerly and northeasterly winds during La Niña events is higher, favouring the transport of transboundary air pollution. These winds are also weaker – strong enough to bring pollutants here but not enough to disperse them.”
The largest increases were recorded during the intense La Niña effect in 2007-08, in which average annual concentrations of sulphur dioxide (SO2) rose by 69 per cent, fine suspended particulates (PM2.5) by 52 per cent, and respirable suspended particulates (PM10) by 15 per cent.
Hong Kong’s air quality tends to decline during the autumn and winter months as prevailing winds usually come from the north – and with these, pollutants from the mainland – while during the summer, winds tend to come from the sea in the south.
“Because of the expected El Niño, the frequency and probability of northerly winds this winter is relatively smaller so this year’s air quality should fare better,” Yim said.
Despite this, pollution levels are only affected significantly if the El Niño and La Niña events are strong. The last intense El Niño took place in 2015-16 and the last strong La Niña occurred between 2010-11.
The World Meteorological Organisation has reported a “75 to 80 per cent chance” of a full-fledged El Niño forming between December 2018 and February this year. But how intense it will be remains to be seen.
The Hong Kong Observatory said surface temperatures over the central and eastern equatorial Pacific had been above normal between November and December. Already this had contributed to a warmer-than-expected Christmas – last month was the fourth-hottest December in the city on record.
“The El Niño effect will develop, therefore it is expected there will be a normal-to-high amount of rain and normal-to-high temperature levels [in the city],” Observatory director Shun Chi-ming said in November. “The El Niño effect will sustain for a period of time, although we don’t see it being very strong.”
Yim’s research, which will be published in the scientific journal Atmospheric Researchin April, aimed to pinpoint the contributions of local and transboundary emissions during different weather patterns.
Among its other findings were that heatwaves, which are typically associated with incoming typhoons more than 1,000km from the city, were correlated with an increase in transboundary pollution, due to low wind speeds, less rainfall and favourable wind direction.
According to the Environmental Protection Department, concentrations of key air pollutants such as SO2 and PM2.5 and PM10 have declined by about 30 per cent over the past five years “as a result of a series of emission control measures”.
But Yim said local dominant pollutants such as nitrogen dioxide – from vehicle exhausts – remained high and any improvements would be offset by the increase in cars.
Meanwhile, a band of clouds and a northeast monsoon have been affecting the southern China coast for days, causing low visibility. Light rains are expected in the next few days.