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

海气相互作用是热带天气和气候的关键，因为从海洋中提取的能量和水分为深对流云提供燃料，后者反过来产生大范围的大气环流并驱动水循环。天气和气候对海洋热量和水汽通量的严重依赖意味着，对这些通量的任何限制都可能对对流云及其大范围影响产生过大的影响。首席调查员以前的工作表明，落在海洋上的雨水可以形成一个“冷盖”，也被称为雨层，它限制了海-气通量。淡水的密度低于咸水，因此落在海洋上的雨水停留在表面，形成一个通常不到一米厚的稳定的浅层，比下面的海洋略冷。寒冷冰盖的较低温度可以抑制对流的开始，对流反过来又可以让更多的阳光进入海洋，其中大部分阳光会穿过冰盖，使其下方的海洋变暖。但当对流发展时，它会产生强烈的地表风，打破冰盖，暴露更温暖的海洋，从而加强对流。这里开展的工作探索了这种延迟然后加强的效应对单个对流云聚集成大范围的有组织对流的影响，以及对厄尔尼诺事件发展的影响。在这两种情况下，工作的重点都是马登-朱利安振荡(MJO)，这是一个广泛的有组织的对流区域，在印度洋形成，并缓慢向东传播穿过太平洋。MJO产生西风爆发(WWB)，即沿赤道由西向东吹强的地面风，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来影响厄尔尼诺现象。这项研究是通过比较冷盖效应活跃或无效的模式模拟进行的，其中一些模式模拟涉及一个区域大气模式(天气研究和预报模式)与一个在地表附近具有非常高垂直分辨率的区域海洋模式，以便可以模拟雨层。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，它间接地捕捉到了由冷盖造成的通量限制。一些模拟是在二氧化碳水平较高的情况下进行的，以研究寒冷的冰盖在气候变暖时的影响。鉴于MJO和厄尔尼诺对天气和气候的重大影响，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式通常产生太弱和传播太快的MJO事件，因此冷盖机制产生的延迟和加剧可能是这些模式中缺失的成分。该项目具有教育价值，因为它为研究生提供支持和培训。PIs还担任CSU本科生研究体验(REU)网站暑期实习生的导师。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.