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  • What if Usagi returns at the end of the century ?

  • Monday, 7th December 2015

The article "A narrow escape from repeating history" has pointed out the alarming situation that would have occurred if Super Typhoon Usagi had adopted a slightly westward track passing over Hong Kong or just to the south in coincidence with the astronomical high tide after the Mid-Autumn Festival in September 2013.

After a detailed analysis of the intensity and wind field of Usagi during its passage over the South China Sea, the storm surge scenario assuming that Usagi took on a track passing 80 km southwest of Hong Kong (Figure 1) was re-examined through computer simulation. The simulation results suggested that the storm surge could reach about 5 m and more than 6 m above Chart Datum in Victoria Harbour and Tai Po Kau respectively (Figures 2 and 3) should it occur at the time of astronomical high tide. Such scenarios are not implausible, as it would only require slight changes in the steering flow for Usagi to adopt such a track upon entering the northern part of the South China Sea. Also we have assumed that Usagi had already weakened into a severe typhoon by the time it was closest to Hong Kong, with an intensity similar to Typhoon Wanda in 1962.

Figure 1

Figure 1      Actual track of Usagi (in black) and the simulated track should Usagi pass about 80 km (in green) to the southwest of Hong Kong.



Figure 2

Figure 2      Simulated storm surges and sea levels (above Chart Datum) at Quarry Bay tide gauge station in Victoria Harbour should Usagi pass 80 km southwest of Hong Kong and couple with the astronomical high tide.



Figure 3

Figure 3      Simulated storm surges and sea levels (above Chart Datum) at Tai Po Kau tide gauge station should Usagi pass 80 km southwest of Hong Kong and couple with the astronomical high tide.


Recently, using data obtained from the climate models under the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, projections of the mean sea level changes in Hong Kong and the surrounding waters in the 21st century were made [1]. The results showed that under the high greenhouse gas concentration scenario (RCP8.5), the annual mean sea level in Hong Kong and its adjacent waters in 2081-2100 is expected to rise by 0.63 - 1.07 m relative to the 1986-2005 mean (http://www.hko.gov.hk/climate_change/proj_hk_msl_e.htm). In other words, should Usagi recur and meet up with astronomical high tide again at the end of this century, there is a chance that the elevated mean sea level rise due to climate change would bring the water level up to over 6 m above Chart Datum in Victoria Harbour and even exceeding 7 m above Chart Datum in Tai Po Kau. The havoc caused by sea flooding then would be quite unimaginable!

Over the media recently, there were also mentions of the storm surge that would be brought about by so-called "perfect storm" like Super Typhoon Haiyan which devastated the town Tacloban in the Philippines in 2013. While there is no historical record that Hong Kong had been visited by such an intense super typhoon in the past, super typhoons like Rammasun in 2014 did appear over the northern part of the South China Sea. This is why we need to remain vigilant in preparing for future tropical cyclones and to get our city climate ready to be resilient against extreme weather brought by climate change.



H.Y. Mok


Reference:

[1] Y. H. He, H.Y. Mok and Edwin S.T. Lai, 2015: Projection of sea-level change in the vicinity of Hong Kong in the 21st century, Int. J. Climatol.



Last revision date: <04 Feb 2016>