Exploring the Causes for Indian Ocean Decadal Climate Variability

印度洋(IO)海表面温度(SST)十年至十年的变化与世界范围内的各种气候波动有关，包括印度季风降雨量的变化、阿拉伯海气旋频率的变化以及美国、地中海和非洲部分地区干旱频率的变化。从盆地的平均来看，自1950年代以来，IO SST一直在增加，通常比其他热带海洋的SST增加得更快，甚至在最近全球变暖的间歇期仍在继续增加。这项研究的目的是了解产生IO SST年代际变化的机制，以及伴随着海平面气压(SLP)、地面风和其他气象条件的变化。一个特别的目标是确定来自太平洋部门的远程影响以及来自IO部门内部的海洋-大气耦合动力在产生IO SST年代际变化方面的单独作用。太平洋的影响与年代际太平洋振荡(IPO)有关，这是一种缓慢变化的海温异常模式，与厄尔尼诺/南方涛动事件中演变较快的海温有一些相似之处。以前的工作表明，IPO的积极阶段，当SST在赤道中部太平洋变暖时，可以在IO上产生均匀的变暖。PI假设变暖是由Walker环流增强引起的，Walker环流增强了IO上的下沉，并通过减少云量增加了地面日照。但最近的观测记录显示，即使IPO已过渡到负值阶段，平均IO SST仍在继续上升，因此IPO以外的因素肯定也在整个盆地的IO变暖和降温中发挥作用。虽然太平洋的影响与统一的SST异常有关，但IO盆地包含第二个显著的年代际变化模式，该模式在很大程度上独立于IPO。这种模式是一种东西向的模式，盆地西半部的暖海温伴随着印度尼西亚沿海的冷异常，被称为印度洋偶极子(IOD)。PI假设IOD是由海-气耦合动力驱动的，而次表层热量输送在其中起着关键作用。除了将IO变化归因于内部动力和来自太平洋的外部影响之外，该项目还试图了解外部强迫的作用，包括温室气体引起的变暖、太阳变化和人为气溶胶。在观测、再分析产品和气候模式模拟中，各种静态技术被应用于诊断IO变化的机制。此外，还进行了专门的气候模式实验，其中大气-海洋相互作用仅在IO部门进行，以确定IO部门的动态能在多大程度上产生年代际变化。使用补充实验来确定IO部门的年代际变化在多大程度上是由盆地外部的大气-海洋动力耦合造成的。进一步的气候模型实验通过施加温室气体、气溶胶和太阳辐射的变化来检验外部强迫的作用。鉴于印度洋的条件影响到邻近陆地地区的天气和气候，这项工作具有社会和科学意义，该地区可能是世界三分之一人口的家园，其中大部分位于脆弱程度更高的发展中国家。此外，如上所述，有证据表明，IO SST的年代际变化与世界范围内的各种气候影响有关，包括一些影响美国的影响。此外，该项目将支持和培训博士后，从而在这一研究领域提供劳动力发展。

Decade-to-decade changes in the sea surface temperature (SST) of the Indian Ocean (IO) have been implicated in a variety of worldwide climatic fluctuations, including variations in Indian monsoon rainfall, changes in the frequency of cyclones in the Arabian Sea, and changes in drought frequency in the US, the Mediterranean, and parts of Africa. Averaged over the basin, IO SSTs have been increasing since the 1950s, often at a faster rate than SSTs in other tropical oceans, and have continued to increase even during the recent global warming hiatus. The goal of this research is to understand the mechanisms that produce decadal variability of IO SSTs and accompanying variations of sea level pressure (SLP), surface winds, and other meteorological conditions. A particular goal is to identify the separate roles of remote influences from the Pacific sector, and coupled ocean-atmosphere dynamics internal to the IO sector, in generating decadal IO SST variability. The Pacific influence is associated with the interdecadal Pacific oscillation (IPO), a slowly varying pattern of SST anomalies with some resemblance to the more rapidly evolving SSTs of El Nino/Southern Oscillation events. Previous work suggests that the positive phase of the IPO, when SSTs warm over the central equatorial Pacific, can produce uniform warming over the IO. The PI hypothesizes that the warming is caused by strengthening of the Walker circulation, which enhances subsidence over the IO and increases surface insolation by reducing cloudiness. But the more recent observational record shows a continued increase in mean IO SSTs even as the IPO has transitioned to its negative phase, so factors other than the IPO must also play a role in basin-wide IO warming and cooling. While the Pacific influence is linked to uniform SST anomalies, the IO basin contains a second prominent decadal variability mode which is largely independent of the IPO. This mode is an east-west pattern in which warm SSTs in the western half of the basin are accompanied by cold anomalies off the coast of Indonesia, referred to as the Indian Ocean dipole (IOD). The PI hypothesizes that the IOD is driven by coupled atmosphere-ocean dynamics in which subsurface heat transport plays a key role. In addition to attribution of IO variability in terms of internal dynamics and external influences from the Pacific, the project also seeks to understand the role of external forcing, including greenhouse gas-induced warming, solar variability, and anthropogenic aerosols. A variety of statical techniques are applied to diagnose the mechanisms of IO variability in observations, reanalysis products, and climate model simulations. In addition, specialized climate model experiments are performed in which atmosphere-ocean interactions are enabled only in the IO sector to determine the extent to which IO-sector dynamics can produce decadal variability. Complementary experiments are used to determine the extent to which decadal variability in the IO sector is due to coupled atmosphere-ocean dynamics external to the basin. Further climate model experiments examine the role of external forcing by imposing changes in greenhouse gases, aerosols, and solar irradiance.The work is of societal as well as scientific interest given that that conditions in the Indian Ocean affect the weather and climate of the adjacent land region, which is home to perhaps a third of the world's population, much of it in developing countries with heightened vulnerability. Also, as noted above, there is evidence that the decadal variability of IO SSTs are linked to a variety of climatic impacts worldwide, including some that affect the US. In addition, the project would support and train a postdoc, thereby providing workforce development in this research area.