| Harmonic analysis has long been an important tool for tidal level analysis and prediction,in which Least Square Estimation(LSE)plays a major role.However,the importance of this role may now be challenged by the Inaction method(IM),which is based on normal time-frequency transform(NTFT),because this provides a new way to analyze and predict the tidal constituents by extracting the tidal constituents from the tidal level directly and obtaining the time-varying harmonic constants simultaneously.In this study IM is combined with LSE(LSEIM)to predict long-term tidal levels.Nineteen tide gauge records with long-term(more than 6 years)and continuous records were chosen for prediction tests.The results showed that,at eight tide gauges,the tidal level predictions using the LSEIM were improved in accuracy by 9-15mm with a lead time of two months,with respect to those using purely LSE.Variations in ocean tides reflect the response of ocean to planet,atmosphere and earth.The seasonal variations and secular changes in tides have been found around the world.However,the variations of tides in the 2-7-year time-scale,which could reveal the response of tides to ENSO event,are unknown.Here we show that the ENSO event has significant effects on the largest tidal constituent-M2.We found that significant ENSO signals are modulated on the M2 amplitudes,especially at three tide gauges(Honi,Kapi and Yap)located in west tropical Pacific Ocean.The M2 amplitudes variations taking place on the 2-7-year time-scale range in～± 10 mm,which should not be ignored in future applications including navigation and coastal management.The amplitude and phase of individual ocean tidal constituent have been reported changing.However,the combined changes of these constituents,which could help us understand the total changes of ocean tides,remained unclear.Here we give a quasi-global analysis on the secular changes of MHAT(mean highest astronomical tides)and MLAT(mean lowest astronomical tides),which are the annual average of daily highest tides and lowest tides,respectively.We found that the secular changes in MHAT and MLAT mainly range from-0.5 mm/yr to+0.5mm/yr,accounting for～5-30%variations of MHHW and MLLW.Nuisance flooding(NF)is defined as minor,nondestructive flooding that causes substantial,accumulating socioeconomic impacts to coastal communities.While sea-level rise is the main driver for the observed increase in NF events in the United States,we show here that secular changes in tides also contribute.An analysis of 40 tidal gauge records from U.S.coasts finds that,at 18 locations,NF increased due to tidal amplification,while decreases in tidal range suppressed NF at 11 locations.Estuaries show the largest changes in NF attributable to tide changes,and these can often be traced to anthropogenic alterations.Limited long-term measurements from estuaries suggest that the effects of evolving tides are more widespread than the locations considered here.The total number of NF days caused by tidal changes has increased at an exponential rate since 1950,adding-27%to the total number of NF events observed in 2019 across locations with tidal amplification.In addition to tides,the contributions of other components composing the water level are also calculated here.We found that tides and residuals contribute the most to water level on top of SLR.The number of contributors needed to cross NF threshold is becoming less and less(1-2 in recent 20 years)in the U.S.East coast,while in the past it has to be 3-4.There is an urgency to predict coastal flooding in the coming future since coastal flooding has been becoming a serious problem in many coastal communities.Applying statistical model to coastal flooding prediction,we found that New York,D.C.and Key West,will experience NF every other day by 2050 under intermediate SLR scenario.Coastal cities in Middle East and South coast will be the first class to go through chronic NF progress and‘every other day’ NF progress in the coming 10-100 years. |
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