Secular Changes in Pacific Tides

知识价值：全球海平面（GSL）上升是有充分记录的。然而，极端高高潮（HHW）的变化，包括潮汐贡献，与GSL上升同样相关。对潮汐记录的分析表明，东太平洋（18 ~ 60ºN）的日、半日振幅均以每百年2.2%的速度增长。总潮幅的平均增幅（0.59 mm /年）小于GSL的平均增幅（1.7 mm /年）。然而，东太平洋的空间平均海平面上升幅度较小，因此潮汐演变对高风浪的变化起主要作用。潮汐演变的空间格局暗示了大尺度过程的影响，似乎排除了频率依赖性强的机制。可能产生的影响的重要性和变化的大规模性质要求对多个潮汐双形点进行分析。这将允许在潮汐振幅增加和减少的地区检查过程，并便于调查原因。具体而言，我们将根据整个太平洋潮汐振幅的数据变化进行评估，并通过数值模拟确定可能的原因。研究人员将开展以下工作：1)方法、数据来源和初步分析：从构成复合振幅的时间序列中提取长期趋势（和可能的趋势变化）的最有效方法是什么？很少有台站的记录长度（LOR）超过50年，大约9年和18.6年的潮汐特性变化并不完全符合天文强迫，这使分析变得复杂。优化分析方法以充分利用稀缺数据是至关重要的。从潮汐数据和潮汐势中提取成分的调和方法，然后对潮汐导纳进行回归分析，将用于涵盖所有主要日和半日成分的分析。复解调将用于在选定的岛屿和沿海站分析几个潮汐带的相干和非相干能量。2)时空模式：潮汐性质在时空中是如何变化的？我们将客观地绘制（使用经验正交函数或EOF）潮汐演变的空间格局，并确定潮汐振幅变化是否具有类似于全球海平面上升的加速度。有些台站也表现出明显的、大幅度的气候响应，例如对ENSO （El Niño-Southern涛动）事件的响应，这很容易检测到。另一些则显示每年的准入周期。这些反应是了解长期趋势因果关系的重要线索。3)因果关系：潮汐振幅长期变化的原因是什么？正压和三维模型将用于测试观测到的大尺度变化是否可能由以下因素引起：a)由于风的变化而影响潮汐的背景涡度变化，或b)与海洋分层变化的相互作用导致斜压和正压潮汐能量随时间变化的平衡。我们还将研究变化的陆架分层在改变海岸困波、内部和表面潮汐的相互作用中的作用，因为这可能是导致两海尺度变化之上空间变化的重要因素。4)综合：导致潮汐快速演变的实际机制是什么？来自任务3的概念性见解必须与任务1和任务2的分析结果相结合，才能确定这一点。更广泛的影响：潮汐振幅的增加可能影响海洋混合、营养供应、初级生产、渔业、沿海基础设施和海岸侵蚀。潮汐特性的变化模式也可能是全球气候变化的一个重要症状，但迄今为止尚未被认识到。波特兰州立大学（PSU）认识到学生、教师和员工之间的多样性是其最宝贵的资源之一。多元化行动招聘和保留团队（DAHRT）将协助招聘研究生和博士后助理。此外，拟议的工作将促进早期职业科学家的成长。

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.