Response of Pacific Subtropical-Tropical Thermocline Water Pathways and Transports to Global Warming and its Implications for ENSO

知识优势：亚热带-热带温跃层水路是副热带细胞（STCs）的地下分支。STCs是浅层经向环流单元，其中水在表层从热带流出，在副热带俯冲，在温跃层内向赤道流动，并在东赤道海洋向上流动。太平洋海温输运的变化通过影响与厄尔尼诺-南方涛动（ENSO）相关的热带温跃层变率和海温（SST）异常，对全球气候产生深远影响。ENSO是一种影响全球大部分地区天气、生态系统和社会的气候现象。在过去几十年里，对ENSO的观测和理解取得了迅速进展；然而，其相当大的时间不规则性尚未得到很好的理解。最近的研究表明，赤道上的温跃层变化可能是造成这种不规则现象的主要原因。观测表明，从20世纪70年代到90年代，太平洋从副热带到热带的温跃层水输送明显减少，导致热带海温明显增加。耦合气候模式将观测到的变暖归因于人为强迫。在未来的人为强迫下，全球变暖还可能对太平洋亚热带-热带温跃层的水通道和输送产生重大影响，从而对ENSO产生重大影响。本项目的目的是研究在未来人为强迫或全球变暖的影响下，太平洋副热带和热带大尺度环流的变化，并探讨这些变化在副热带-热带温跃层水通道和输送中对ENSO的调节作用。假设：(1)在与STCs相关的大尺度海洋环流变化中，浮力的作用与风的作用一样重要；(2)从亚热带到热带的水路和输送的变化通过改变赤道上的温跃层结构来调节ENSO性质。更广泛的影响：了解ENSO的时间不规则性仍然是气候变率和可预测性中的一个突出问题，在许多科学学科中具有广泛的社会意义。本研究将确定一种将太平洋STCs与ENSO不规则性联系起来的方法，并为理解全球变暖如何影响太平洋亚热带-热带环流环流提供基础。这些结果将应用于未来的模式改进，并与IPCC气候模式界共享。研究结果将通过在科学会议/讲习班上的演讲和在科学期刊上的出版物来传播。

Intellectual Merit:The subtropical-tropical thermocline water pathways are the subsurface branches of the Subtropical Cells (STCs). The STCs are shallow meridional circulation cells in which water flows out of the tropics within the surface layer, subducts in the subtropics, flows equatorward within the thermocline, and upwells in the eastern equatorial ocean. The changes in the Pacific STC transports have profound implications on global climate through its impact on tropical thermocline variability and sea surface temperature (SST) anomalies associated with the El Nio-Southern Oscillation (ENSO). The ENSO is a climate phenomenon that affects weather, ecosystems, and societies across much of the globe. Observations and understanding of ENSO have rapidly progressed over the last few decades; however its considerably temporal irregularity is not well understood. Recent studies have demonstrated that thermocline variability along the equator can be a leading candidate responsible for this irregularity. Observations show that the thermocline water transport from the subtropics to the tropics in the Pacific has considerably been reduced from the 1970s to the 1990s, causing an appreciable SST increase in the tropics. Coupled climate models attribute the observed warming to the anthropogenic forcing. Under the future anthropogenic forcing, global warming may also have significant influence on the Pacific subtropical-tropical thermocline water pathways and transports, and thereby on ENSO. The objectives of this project are to investigate the changes in the large-scale circulation around the subtropics and the tropics in the Pacific under future anthropogenic forcing or global warming, and to explore the role of these changes in the subtropical-tropical thermocline water pathways and transports for modulating ENSO. The hypotheses are that (1) buoyancy forcing plays as important a role as wind forcing in the changes in the large-scale ocean circulation associated with the STCs, and (2) changes in the water pathways and transports from the subtropics to the tropics modulate ENSO properties through shifting the thermocline structure along the equator. Broader Impacts:Understanding ENSO temporal irregularity remains an outstanding issue in the climate variability and predictability with broad societal implications across many scientific disciplines. This research will identify a way of connecting the Pacific STCs with ENSO irregularity and provide a basis for understanding how does global warming affect subtropical-tropical inter-gyre ocean circulation in the Pacific. The results will be applied to future model improvements and be shared with the IPCC climate model community. The results will be disseminated through presentations at scientific meetings/workshops and publications in scientific journals.