Dynamics of Tropically Forced Zonally Symmetric Climate Variability

最近的观测和模拟研究表明，在热带和热带外地区普遍存在着纬向对称和半球对称的气候异常，这些异常还没有得到很好的理解。这些异常现象发生在年际至年代际的时间尺度上，其强度足以在中纬度地区的降水异常上留下印记，影响到人口稠密地区的多年干旱和降水的年代际变化。提出了一个假设，连贯的，纬向对称的，气候异常主要是由热带海表温度（SST）异常强迫。一个正的SST强迫首先产生一个均匀的热带对流层暖带。后者导致纬向对称涡驱动环流的纬向移动，导致中纬度地区的冷却和异常上升。通过数值模拟和理论研究，本项目的目标是验证和完善这一假设，并阐明促进热带-热带外气候异常的相互作用和分带的机制。这项工作的主要工具是一系列动力学模式，包括大气环流模式和有无瞬变涡旋的线性三维和二维模式，所有这些模式都由热带强迫驱动。将进行广泛的GCM模拟，以将纬向对称变率中统计上显著的信号归因于不同海洋盆地的热带SST。将使用线性无瞬变模型来推导对热带强迫的直接纬向对称响应，并使用线性风暴路径模型来进一步澄清瞬变涡旋在产生和维持热带外纬向对称变率方面的作用。综合这些模式的结果，将揭示一个路线图，从局部热带SST强迫的纬向对称气候异常。智力优点：关于热带外气候变率的热带影响的经典研究在强迫定常波方面取得了实质性进展，但忽略了同样重要的纬向对称气候信号。该项目的结果将促进我们对后者的理解，使热带对中纬度气候变率影响的理论更加完整。为了实现这一目标，这项研究将扩大和整合目前的知识，热带动力学，热带外相互作用，和热带外波纬向平均相互作用，主题的PI一直在集中调查在过去的十年。研究的预期科学收益和PI愿意探索它们突出了这项工作的智力价值。 更广泛的影响：这项工作将有助于全面了解将热带海洋温度与热带以外地区水文气候变化联系起来的大气动力学。这一认识将成为提前数月至数十年预测干旱和湿润期的基础。一名学生将接受培训和指导，一名年轻科学家将通过这笔赠款获得研究经验。

Recent observational and modeling studies have revealed the ubiquitous presence of zonally symmetric and hemispherically symmetric climate anomalies in the tropics and extratropics that are not well-understood. These anomalies, occurring on interannual to interdecadal time scales, are strong enough to leave an imprint on the precipitation anomalies in midlatitudes, influencing multi-year droughts and interdecadal shifts in precipitation over densely populated land areas. A hypothesis is put forth that the coherent, zonally symmetric, climate anomalies are forced primarily by tropical sea surface temperature (SST) anomalies. A positive SST forcing first produces a homogenized band of tropical tropospheric warmth. The latter then induces a meridional shift of the zonally symmetric eddy-driven circulation, causing cooling and anomalous ascent in midlatitudes. Through numerical modeling and theoretical investigations, the goals of this project are to verify and refine this hypothesis and to clarify the mechanisms that facilitate the tropics-extratropics interaction and zonalization of extratropical climate anomalies. The principal tools of this work are a hierarchy of dynamical models including atmospheric general circulation models (GCMs) and linear 3-D and 2-D models with and without transient eddies, all driven by tropical forcings. Extensive GCM simulations will be performed to attribute the statistically significant signals in the zonally symmetric variability to tropical SSTs in different ocean basins. Linear transient-free models will be used to deduce the direct zonally symmetric response to tropical forcing, and linear storm track models used to further clarify the role of transient eddies in creating and maintaining the extratropical zonally symmetric variability. Synthesizing the results from these models will reveal a road map from a localized tropical SST forcing to extratropical zonally symmetric climate anomalies. Intellectual merit: Classical work on the tropical influence of extratropical climate variability has made substantial progresses on the aspect of forced stationary waves but overlooked the equally important zonally symmetric climate signals. The results from this project will advance our understanding of the latter, making the theory of the tropical influences on midlatitude climate variability complete. To achieve that goal, this study will expand and integrate the current knowledge on tropical dynamics, tropics-extratropics interaction, and extratropical wave-zonal mean interaction, subjects that the PIs have been intensively investigating in the last decade. The expected scientific gains of the research and the PIs' readiness to explore them highlight the intellectual merit of this work. Broader impacts: The work will help provide a full understanding of the atmospheric dynamics that link tropical ocean temperatures to hydroclimatic changes in the extratropics. This understanding will form the basis for efforts to predict droughts and wet periods months to decades in advance. A student will be trained and mentored and a young scientist will gain research experience through this grant.