A Global Reanalysis of Nodal Modulation on High Tidal levels from Hourly Tide Gauge Records
Abstract
The lunar nodal cycle, produced by the varying declination of the Moon over a period of 18.61 years, drives changes in tidal amplitude globally. Nodal modulation on high tidal levels is an important contributor to extreme sea levels on decadal timescales. Constraining the range of changes in tidal amplitude that can be expected over a nodal cycle from real observations is therefore valuable for hazard planning. In this study, we use hourly tide-gauge observations with record lengths >19 years from 574 stations distributed worldwide to examine the contribution of nodal modulation to monthly mean high waters (MMHW), monthly mean higher high waters (MMHHW), and the monthly 99th percentile water level.
Spatial variation of nodal modulation is related to both the form of tides and tidal range. Our results show that the range of nodal modulation increases with tidal range; it is 6% and 2% of tidal range at the locations in diurnal and semidiurnal regions, respectively. The influence of nodal modulation on the selected high tidal levels (MMHW, MMHHW and the 99th percentile) is largest at locations in the Gulf of Tonkin, English Channel and Bristol Channel, amounting up to 36 cm in range. From a coastal flooding perspective, the phase of nodal modulation is also important. We calculate the phase of nodal modulation from the MMHW time-series at each site, and show that the estimated phases exhibit two clusters: one cluster (111° ± 10°) corresponds with the locations having a diurnal form of tides, where nodal modulation last peaked in 2006 and correlates with maximum lunar declination; whereas the other cluster (-59° ± 11°) corresponds with the locations exhibiting a semidiurnal form of tides, where the nodal modulation last peaked in 2015 and correlates with minimum lunar declination. Nodal modulation in the diurnal and semidiurnal regions will peak again in 2025 and 2034, respectively, with the potential for high levels of coastal hazard in the respective regions.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMOS51E1292P
- Keywords:
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- 1225 Global change from geodesy;
- GEODESY AND GRAVITYDE: 1641 Sea level change;
- GLOBAL CHANGEDE: 4217 Coastal processes;
- OCEANOGRAPHY: GENERALDE: 4556 Sea level: variations and mean;
- OCEANOGRAPHY: PHYSICAL