Dynamics of Tropically Forced Zonally Symmetric Climate Variability

最近的观测和模拟研究表明，在热带和温带地区普遍存在纬向对称和半球对称的气候异常，但人们对这些异常的认识并不充分。这些在年际至年代际时间尺度上发生的异常非常强烈，足以在中纬度地区的降水异常上留下印记，从而影响人口稠密地区的多年干旱和降水的年代际变化。提出了热带海温（SST）异常对相干、纬向对称气候异常的强迫作用。海温正强迫首先产生一个均匀的热带对流层暖带。后者引起纬向对称涡旋驱动环流的经向移动，导致中纬度地区变冷和异常上升。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-温带相互作用和温带气候异常分区的机制。这项工作的主要工具是动力学模型的层次结构，包括大气环流模型（GCMs）和线性三维和二维模型，有或没有瞬态涡旋，都是由热带强迫驱动的。将进行广泛的GCM模拟，以将纬向对称变率的统计显著信号归因于不同海洋盆地的热带海温。线性无瞬变模式将用于推导对热带强迫的直接纬向对称响应，而线性风暴路径模式将用于进一步阐明瞬变涡旋在产生和维持热带外纬向对称变率中的作用。综合这些模式的结果将揭示从局地热带海温强迫到温带纬向对称气候异常的路线图。知识价值：关于温带气候变率的热带影响的经典工作在强迫驻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的影响理论更加完整。为了实现这一目标，本研究将扩展和整合热带动力学、热带-温带相互作用和温带波纬向平均相互作用的现有知识，这些是pi在过去十年中一直在深入研究的主题。这项研究的预期科学成果和pi探索这些成果的意愿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温带水文气候变化联系起来的大气动力学。这一认识将为提前几个月到几十年预测旱涝期的努力奠定基础。通过这项资助，一名学生将得到培训和指导，一名年轻科学家将获得研究经验。

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.