Assessing and Understanding Oceanic Climate Forcing on Decadal Climate Variability from Surface Heat Flux

评估和理解海洋气候对地表热通量十年间气候变化的影响

• 批准号: 2321042
• 负责人: Zhengyu Liu
• 金额: $ 65.24万
• 依托单位: Ohio State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2023
• 资助国家: 美国
• 起止时间: 2023-08-15 至 2026-07-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2321042&HistoricalAwards=false
• 关键词: Assessing; Understanding; Oceanic; Climate; Forcing

The North Atlantic and North Pacific are both home to large-scale sea surface temperature (SST) variations that last for years and even decades. An example is the Atlantic Multidecadal Oscillation (AMO), in which a large portion of the Atlantic north of the equator warms and cools over periods of perhaps 20 to 40 years. The low-frequency variability of North Pacific and North Atlantic SST must be driven by some combination of oceanic and atmospheric forcing, but their roles are not well understood and there is still some debate as to which is dominant. An important consideration is that low-frequency SST variability can be generated by changes in evaporation and surface heat exchange accompanying the passage of weather systems even though the movement of weather systems is much faster than the SST variability. If the slow variations of SSTs are driven by the "weather noise" of fast-moving systems the prospects for long-term SST prediction are somewhat limited, while a strong role for ocean dynamics, perhaps involving slow fluctuations of the global overturning circulation, could mean that SST anomalies can be predicted years in advance.Work under this award seeks to quantify the contributions of atmospheric and oceanic forcing to low-frequency SST variability using a simple stochastic model. The gist of the model is that atmospheric and oceanic forcing can be distinguished by looking at the timing of SST and surface heat flux anomalies, where the surface heat flux refers to both the evaporation and heat exchange occurring at the ocean surface and changes in surface sunlight and infrared radiation caused by changes in cloud cover. If an SST change is driven by the atmosphere it should be preceded by the surface heat flux anomaly that caused it. On the other hand an SST change driven by the ocean is likely to produce a change in surface heat flux that acts to damp the SST anomaly, for instance an ocean-driven warm anomaly would likely produce a surface heat flux that has a cooling effect. In that case the heat flux anomaly would be roughly synchronous with the SST anomaly but with opposite sign. The model used here includes explicit representations of both atmospheric and oceanic damping and treats the oceanic forcing as a red noise process. The model is used to analyze SST variability in observational datasets and output from climate models using standard and enhanced horizontal resolution. Simulations with modified versions of the Community Earth System Model (CESM) are then used to identify mechanisms of SST variability.The work has societal value as it addresses the question of long-range SST prediction. The slow variations of SST have a number of human impacts, for instance the number of severe Atlantic hurricanes roughly doubles from the cold phase of the AMO to the warm phase, and the AMO is implicated in the great Sahel drought of the mid-20th century. The extent to which the AMO and other forms of low-frequency SST are predictable is not known, and an understanding of the driving mechanisms is essential for an assessment of predictability and to provide useful guidance as to how predictive models might be developed. In addition, the project provides support and training for two graduate students, thereby providing for the future workforce in this research area.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

北大西洋和北太平洋都是大规模海面温度（SST）变化的所在地，这种变化持续数年甚至数十年。 大西洋多年代际振荡 (AMO) 就是一个例子，赤道以北的大西洋大部分地区在大约 20 到 40 年的时间内变暖和变冷。 北太平洋和北大西洋海温的低频变化必定是由海洋和大气强迫的某种组合驱动的，但它们的作用尚未得到很好的理解，并且对于哪一个占主导地位仍然存在一些争论。 一个重要的考虑因素是，低频海温变化可能是由于天气系统经过时蒸发和地表热交换的变化而产生的，尽管天气系统的运动比海温变化快得多。 如果海表温度的缓慢变化是由快速移动系统的“天气噪音”驱动的，那么长期海表温度预测的前景会受到一定程度的限制，而海洋动力学的强大作用（可能涉及全球翻转环流的缓慢波动）可能会产生巨大的影响。这意味着可以提前数年预测海表温度异常。该奖项的工作旨在使用简单的随机模型量化大气和海洋强迫对低频海表温度变化的贡献。该模型的要点是，可以通过观察海温和地表热通量异常的时间来区分大气和海洋强迫，其中地表热通量是指海洋表面发生的蒸发和热交换以及地表阳光的变化以及云量变化引起的红外辐射。 如果海温变化是由大气驱动的，则应先于引起海温变化的地表热通量异常发生。另一方面，由海洋驱动的海温变化可能会产生地表热通量的变化，从而抑制海温异常，例如海洋驱动的温暖异常可能会产生具有冷却作用的地表热通量。 在这种情况下，热通量异常将与海温异常大致同步，但符号相反。 这里使用的模型包括大气和海洋阻尼的明确表示，并将海洋强迫视为红噪声过程。 该模型用于使用标准和增强水平分辨率分析观测数据集中的海温变化和气候模型的输出。 然后使用社区地球系统模型（CESM）的修改版本进行模拟来识别海表温度变化的机制。这项工作具有社会价值，因为它解决了长期海表温度预测的问题。海表温度的缓慢变化对人类产生了许多影响，例如，从 AMO 的寒冷阶段到温暖阶段，大西洋严重飓风的数量大约增加了一倍，而 AMO 与 20 世纪中叶萨赫勒地区的大干旱有关。 AMO 和其他形式的低频海表温度的可预测程度尚不清楚，了解驱动机制对于评估可预测性并为如何开发预测模型提供有用的指导至关重要。 此外，该项目还为两名研究生提供支持和培训，从而为该研究领域的未来劳动力提供支持。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，认为值得支持。