The Upscaling of Tropical Pacific Ocean Rain Layers to Convection and Modes of Pacific Climate Variability

热带太平洋雨层对流的升级和太平洋气候变率的模式

• 批准号: 2333171
• 负责人: Charlotte Demott
• 金额: $ 71.18万
• 依托单位: Colorado State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2024
• 资助国家: 美国
• 起止时间: 2024-04-15 至 2027-03-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2333171&HistoricalAwards=false
• 关键词: Upscaling; Tropical; Pacific; Ocean; Rain

Air-sea interactions are the key to the weather and climate of the tropics, as energy and moisture extracted from the ocean fuels the deep convective clouds that in turn produce the large-scale atmospheric circulation and drive the hydrological cycle. The critical dependence of weather and climate on heat and moisture fluxes from the ocean means that any restriction on those fluxes can have an outsized influence on the convective clouds and their large-scale effects. Previous work by the Principal Investigators (PIs) has shown that rainwater falling on the ocean can form a "cold lid", also called a rain layer, that restricts air-sea fluxes. Fresh rainwater is less dense than salty sea water, thus rain falling on the ocean stays at the surface and forms a shallow stable layer typically less than a meter thick and slightly colder than the underlying ocean. The lower temperature of the cold lid can inhibit the onset of convection, which in turn can allow more sunlight to reach the ocean, most of which passes through the lid to warm the ocean below it. But when convection develops it can generate strong surface winds that break up the lid and expose the warmer ocean, thus intensifying convection. Work performed here explores the implications of this delay-then-intensify effect for the aggregation of individual convective clouds into broad areas of organized convection, and for the development of El Nino events. In both cases the work focuses on the Madden-Julian Oscillation (MJO), a broad area of organized convection that forms in the Indian Ocean and propagates slowly eastward across the Pacific. The MJO generates westerly wind bursts (WWBs), periods of strong west-to-east surface winds along the equator, which promote the development of El Nino events. Thus the intensification of MJO convection through the cold lid mechanism could affect El Nino by intensifying WWBs. The research is carried out through comparison of model simulations in which cold lid effects are either active or disabled, some involving a regional atmospheric model (the Weather Research and Forecasting model) coupled to a regional ocean model with very high vertical resolution near the surface so that rain layers can be simulated. Other simulations use the Community Earth System Model (CESM), a global model which simulates El Nino events but lacks the fine resolution needed to explicitly represent rain layers and the cold lid mechanism. Instead the PIs implement a new ocean surface flux parameterization which indirectly captures the flux restriction caused by the cold lid. Some simulations are performed with higher levels of carbon dioxide to investigate the effects of the cold lid in a warmer climate.The work is of societal as well as scientific interest given the substantial weather and climate impacts of the MJO and El Nino, which include changes in the frequency of landfalling hurricanes and atmospheric rivers in the US. Models used for weather prediction and climate projections typically produce MJO events which are too weak and propagate too fast, thus the delay and intensification produced by the cold lid mechanisms could be a missing ingredient in these models. The project has educational value as it provides support and training to a graduate student. The PIs also serve as mentors to summer interns at the CSU Research Experiences for Undergraduates (REU) site.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.

海气相互作用是热带地区天气和气候的关键，因为从海洋中提取的能量和水分为深对流云提供了燃料，而深对流云又产生了大规模的大气环流，并推动了水文循环。 天气和气候对来自海洋的热量和水分通量的关键依赖性意味着对这些通量的任何限制都可能对对流云及其大尺度效应产生巨大影响。 主要研究人员（PI）先前的工作表明，落在海洋上的雨水可以形成一个“冷盖”，也称为雨层，限制海气通量。 新鲜雨水的密度低于含盐的海水，因此落在海洋上的雨水停留在表面，形成一个浅的稳定层，通常不到一米厚，比下面的海洋略冷。 冷冰盖的低温可以抑制对流的发生，这反过来又可以让更多的阳光到达海洋，其中大部分通过冰盖来温暖下面的海洋。但是当对流发展起来时，它可以产生强大的表面风，打破冰盖，暴露出温暖的海洋，从而加强对流。在这里进行的工作探讨了这种延迟，然后加强个别对流云聚集成广泛的有组织的对流领域的影响，并为厄尔尼诺事件的发展。 在这两种情况下，工作的重点是马登-朱利安振荡（MJO），一个广泛的有组织的对流，在印度洋形成，并缓慢向东传播跨越太平洋。 MJO产生西风爆发（WWBs），即沿赤道沿着由西向东的强烈地表风，这促进了厄尔尼诺事件的发展。 因此，通过冷盖机制加强MJO对流可以通过加强WWB来影响厄尔尼诺。 这项研究是通过比较冷盖效应是活跃的或禁用的模型模拟进行的，其中一些涉及区域大气模型（天气研究和预报模型）耦合到区域海洋模型，具有非常高的垂直分辨率近表面，以便可以模拟降雨层。 其他模拟使用社区地球系统模型（CESM），这是一个模拟厄尔尼诺事件的全球模型，但缺乏明确表示雨层和冷盖机制所需的精细分辨率。 相反，PI实现了一个新的海洋表面通量参数化，间接捕获冷盖造成的通量限制。 一些模拟是在更高的二氧化碳水平下进行的，以研究冷盖在更温暖的气候中的影响。考虑到MJO和厄尔尼诺的重大天气和气候影响，包括美国登陆飓风和大气河流频率的变化，这项工作具有社会和科学意义。 用于天气预报和气候预测的模型通常会产生太弱且传播太快的MJO事件，因此冷盖机制产生的延迟和强化可能是这些模型中缺少的成分。 该项目具有教育价值，因为它为研究生提供了支持和培训。 该奖项反映了NSF的法定使命，并被认为值得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来支持。