Doctoral Dissertation Research: Paleohurricane Proxy Records from the Pacific Coast of Mexico

为了推断热带气旋活动的长期模式，必须收集和解释历史观测期以外的数据。古风暴学是一门相对年轻的科学，主要通过地质代理技术研究过去的热带气旋活动。古风暴学研究已经在美国墨西哥湾和大西洋沿岸以及泛加勒比海地区的许多地点成功地进行了，但墨西哥太平洋沿岸的代用记录明显缺乏。博士生Thomas Bianchette将在Liu教授的指导下，利用沿海湖泊沉积物代用物，建立古飓风袭击墨西哥太平洋沿岸哈利斯科州和格雷罗州的百年至千年记录。研究地点的地貌环境和该地区热带气旋的典型路径和强度表明，在这些沉积物中可以识别飓风袭击。除了使用冲积沙层作为风暴潮的代理外，这项研究还将开创一种新的方法，使用坡面沉积物和化石花粉作为补充代理来检测由海岸平行风暴引起的间接飓风袭击，这些风暴带来强降水，但不登陆（即有限的风暴潮）。该方法将基于这样一个假设：在百年时间尺度上的活跃期，风暴活动的增加将导致气候更加湿润，从而导致可以在花粉记录中检测到的长期植被反应。造成地质和/或生物印记的近期风暴将被用作现代类比，以帮助解释沉积记录中较早的风暴事件。将开发一个地理信息系统，使历史风暴路径与气候条件（特别是厄尔尼诺/南方涛动）之间的关系可视化，而遥感技术将用于确定最近飓风的地貌和生物物理影响。根据数千年的数据，我们假设，与美国墨西哥湾和大西洋沿岸的热带气旋相比，墨西哥太平洋沿岸的北部地区（哈利斯科州）比南部地区（格雷罗州）显示出更高的风暴频率。这项研究有望产生沿墨西哥太平洋海岸热带气旋活动的第一个代理记录。由于揭示全球热带气旋活动取决于盆地间的相关性，因此北太平洋东部盆地长期活动机制的代理记录至关重要。利用冲坡沉积物和化石花粉作为补充指标的新方法可以通过允许使用内陆地点和探测带来过量降雨但不引起明显过冲过程的古风暴事件来推进古风暴学研究。此外，这项研究将具有重大的社会意义，因为历史上沿墨西哥太平洋沿岸的热带气旋袭击对社会和基础设施造成了巨大的破坏，包括重大的公共卫生问题。墨西哥对这些风暴的仪器记录非常短（自1949年以来），这使得长期风险评估变得困难。根据百年代用记录或千年代用记录建立更可靠的飓风回归期估计，有助于为决策者提供信息，从而改进规划和减灾政策。

To infer long-term patterns of tropical cyclone activity, it is imperative to collect and interpret data beyond the historical observational period. Paleotempestology is a relatively young science that studies past tropical cyclone activity mainly by means of geological proxy techniques. Paleotempestological research has been successfully conducted at many locations along the U.S. Gulf and Atlantic coasts and the pan-Caribbean region, but proxy records are notably lacking from the Pacific coast of Mexico. Doctoral student Thomas Bianchette under the supervision of Professor Kam-biu Liu will establish a centennial-to-millennial record of paleohurricane strikes from Jalisco and Guerrero on Mexico's Pacific coast using coastal lake-sediment proxies. The geomorphic setting of the study sites and the typical tracks and strengths of tropical cyclones in this region suggest that hurricane strikes will be identifiable in these sediments. In addition to using overwash sand layers as a storm surge proxy, this study will pioneer a new methodology using slopewash deposits and fossil pollen as supplementary proxies to detect indirect hurricane strikes resulting from coast-parallel storms that bring heavy precipitation but do not make landfall (i.e., limited storm surge). The methodology will be based on the hypothesis that increased storm activity during an active period on centennial timescales will result in a wetter climate, leading to a long-term vegetation response that can be detected in the pollen record. Recent storms that have caused a geological and/or biological imprint will be used as modern analogs to aid in interpreting older storm events in the sedimentary record. A geographic information system will be developed to visualize the relationship between historical storm tracks and climate conditions (especially El-Nino/Southern Oscillation), while remote sensing technologies will be used to determine the geomorphological and biophysical effects of recent hurricanes. We hypothesize that a longer return period will be found for tropical cyclones along this coast than for the U.S. Gulf and Atlantic coasts based on multi-millennial data, with northern areas (Jalisco) on Mexico's Pacific coast exhibiting a higher storm frequency than southern locations (Guerrero).This research promises to produce the first proxy records of tropical cyclone activity along the Pacific coast of Mexico. Because unraveling global tropical cyclone activity depends upon inter-basin correlations, proxy records of long-term activity regimes for the Eastern North Pacific basin are vital. The new methodology of using slopewash deposits and fossil pollen as supplementary proxies can advance paleotempestological studies by permitting the use of inland sites and detecting paleo-storm events that brought excessive rain but did not cause significant overwash processes. Furthermore, this study will have great societal relevance as historical tropical cyclone strikes along Mexico's Pacific coast have caused tremendous damage to society and infrastructure, including significant public health problems. Mexico's instrumental record of these storms is very short (since 1949), making long-term risk assessment difficult. Establishing more reliable estimates of hurricane return periods based on multi-centennial or millennial proxy records can help inform decision makers, thereby improving planning and mitigation policy.