Secular Changes in Pacific Tides

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Intellectual Merit: Global sea level (GSL) rise is well documented. However, changes in extreme Higher high water (HHW), including the tidal contribution, are just as relevant as GSL rise. Analysis of tidal records shows that the diurnal and semi-diurnal amplitudes are both growing at 2.2% per hundred years in the NE Pacific (between 18 to 60ºN). The mean increase in total tidal amplitude (0.59 mm per year) is less than GSL rise (1.7 mm per year). However, spatially averaged mean sea level (MSL) rise is small in the NE Pacific, so tidal evolution plays a major role in changes in HHW. The spatial pattern of tidal evolution suggests the influence of large-scale processes, and seems to exclude mechanisms with strong frequency dependence. The importance of the possible impacts and the large-scale nature of the changes demand an analysis that encompasses multiple tidal amphidromes. This will allow examination of processes in areas with both increasing and decreasing tidal amplitudes, and facilitate investigation of causes. Specifically, we will evaluate from data changes in tidal amplitudes for the entire Pacific Ocean and determine likely causes through numerical modeling. The investigators will carry out the following tasks: 1) Methods, data sources and preliminary analyses: What is the most efficient method to extract secular trends (and possible changes in trends) from time series of the constituent complex amplitude? There are few stations with length of record (LOR) greater than 50 years, and roughly 9 and 18.6 year variations in tidal properties do not exactly follow astronomical forcing, complicating analysis. Optimizing analysis methods to make the best use of scarce data is vital. A harmonic method of constituent extraction from both the tidal data and tidal potential, followed by a regression analysis of the tidal admittance, will be used in an analysis that covers all major diurnal and semidiurnal constituents. Complex demodulation will be used to analyze coherent and incoherent energy in several tidal bands at selected island and coastal stations. 2) Space-time patterns: How do tidal properties vary in space and time? We will objectively map (using empirical orthogonal functions or EOF) spatial patterns of tidal evolution and deter-mine whether tidal amplitude changes show acceleration analogous to GSL rise. Some stations also exhibit apparent, large-amplitude climate responses, e.g. to ENSO (El Niño-Southern Oscillation) events, that are easy to detect. Others show annual cycles of admittance. These responses are important clues regarding causation of secular trends. 3) Causation: What are the causes of secular changes in tidal amplitudes? Barotropic and three-dimensional models will be used to test whether the observed large-scale changes could be caused by: a) changes in background vorticity affecting the tides due to changes in winds, or b) interactions with changing ocean stratification resulting in a time-varying balance of baroclinic and barotropic tidal energy. We will also investigate the role of changing shelf stratification in altering the interaction of coastal trapped waves, and internal and surface tides, because this is likely a significant factor causing spatial variations on top of the amphidromic-scale changes. 4) Synthesis: What are the actual mechanisms causing tides to evolve rapidly? Conceptual in-sights from Task 3 must be combined with analysis results from Tasks 1 and 2 to determine this. Broader Impacts: Increasing tidal amplitudes may impact ocean mixing, nutrient supply, primary production, fisheries, coastal infrastructure, and coastal erosion. Changing patterns of tidal properties may also be a significant, but heretofore unrecognized, symptom of global climate change. Portland State University (PSU) recognizes diversity among its students, faculty, and staff as one of its most precious resources. The Diversity Action Hiring and Retention Team (DAHRT) will assist in recruiting a graduate student and Post-Doctoral Associate. In addition, the proposed work will foster the growth of an early-career scientist.