18-year moon cycle can cause 30 cm rise in sea level in the South China Sea with consequences for extreme tides and storm surge
Hundreds of millions of people in Southeast Asia, live in low-lying coastal societies that are vulnerable to extreme sea level events. The impact of events such as mean sea level rise, storm surges and extreme tides on densely populated and/or highly developed coastlines can be devastating and also very costly. However, our ability to predict and prepare for such events is improving. Recently a group of researchers from the ASE and EOS published an article on how the effect of moon cycles that span several years, can alone cause as much as 30 cm change in sea level, with the next peak expected in 2025. This needs to be taken into account when predicting and preparing for sea level rise. The study was led by Dr Dongju Peng (Research Fellow), and the other authors are Assoc Prof Emma Hill, Asst Prof Aron Meltzner, and Assoc Prof Adam Switzer.
Most of us know that tides are caused by the gravitational power of the moon (and sun), causing spring tide at full or new moon and neap tide at half moon. These tidal variations result from the moons rotation around the Earth every month. But the moon also has interannual cycles, spanning many years. The nodal tidal cycle, or nodal modulation, causes a slow variation of the amplitude of diurnal (one high and low tide per day) or semidiurnal (two high and low tides per day) ocean tides. It is associated with the movements and relative positions of the Earth, Moon and Sun over a period of 18.6 years. It is an important contributor to extreme sea levels and can increase the risk of coastal flooding at specific, forecastable times. The manuscript presents a new assessment of the impact of the 18.6-year nodal tidal cycle on high water levels from global tide-gauge records, highlighting locations where the 18.6-year nodal cycle most influences high water levels, and providing predictions of periods where there is enhanced risk of flooding to coastal infrastructure.
The study shows that nodal modulation has the largest influence on the monthly highest water levels at locations in the Gulf of Tonkin (off the coast of northern Vietnam and southern China), in the Bristol Channel, and in the English Channel (both in the UK), amounting up to 30 cm in range. It suggests that in the coming decades the impact of the nodal cycle on high water levels in those regions could be greater than that of global mean sea-level rise, which is up to 17 cm by 2030, according to the Intergovernmental Panel on Climate Change fifth assessment report projections. Peng and co-authors also demonstrate that nodal modulation in diurnal and semi-diurnal locations will peak again in 2025 and 2034, respectively, resulting in enhanced potential for coastal hazards in the respective regions.