Intrinsic versus Forced Decadal Variability in the Extratropical North Pacific

黑潮海流的年代际变化既是海洋急流内在动力学的结果，也是大气-海洋反馈过程的结果。黑潮在日本以南的独特路径构型与急流内部不同的内在稳定性状态、黑潮体积和热量输送的变化以及日本下游黑潮延伸部(KE)的N-S位置有关。这一位置影响着海-气相互作用的性质和强度，而海-气相互作用反过来又反馈到黑潮的变化。这项研究将研究控制路径和稳定状态之间变化的机制，以及内在海洋动力和影响变率的海-气反馈之间的相互作用。该提议的科学目标是：(1)阐明日本南部固有的黑潮路径变率改变KE动力状态的过程，以及它如何与盆地尺度风强迫控制的KE变率竞争；(2)阐明最近一次黑潮大弯曲(LM)事件背后的动力学；(3)量化最近KE动力状态变化对改变大气环流的海表面温度(SST)、SST锋面和中尺度涡旋的相对贡献。该项目将使用面向过程的海洋和大气数值模拟来解决系统的可变性问题。黑潮和KE是气候变异性的重要组成部分，影响北太平洋生态系统和盆地规模的天气模式，两者都具有重要的社会意义。过去几十年来，高质量卫星/现场观测的积累和最先进的海洋/大气环流模式的改进，大大提高了对KE在北太平洋十年气候变化中所起作用的理解。具体的反馈环路涉及风强迫的海洋内部温跃层的调整及其对KE动力状态的改变，以及KE对风暴路径和地面风场的影响。这种在过去30年一直在发挥作用的风力反馈回路，在2017年末被中断，当时日本以南的黑潮变成了一条巨大的蜿蜒路径，这是西方边界流内在变异性的表现。已经确定了三个可检验的假设：假设1：2017年KE指数逆转导致了Lm的发生；假设2：2017年Lm是由增强的副热带逆流涡旋活动触发的；以及假设3：曲折导致的KE动态逆转影响了大气。前两个假设将用修改为三层结构的Hallberg等周期模型进行检验，该模型(A)适应陡峭的海底地形对层界面的拦截，(B)允许等厚层的海面露头，(C)具有等周期坐标，以避免非物理的日周期混合。假设3将通过与日本东北大学的杉本秀作教授合作，使用日本气象厅的非静力中尺度模式来解决。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。

Decadal variability of the Kuroshio Current results from both the intrinsic dynamics of oceanic jets and from atmosphere-ocean feedback processes. The Kuroshio’s distinct path configurations south of Japan are associated with differing states of intrinsic stability within the jet, with variations in volume and heat transport by the Kuroshio, and with the N-S position of the Kuroshio Extension (KE) downstream from Japan. This position affects the character and intensity of air-sea interactions, which in turn feed-back upon Kuroshio variability. This study will examine the mechanisms controlling shifts between paths and stability states, and the interplay between intrinsic ocean dynamics and air-sea feedbacks affecting variability. The scientific goals of the proposal are to (1) clarify the processes by which the intrinsic Kuroshio path variability south of Japan alters the KE dynamic state and how it competes with the KE variability controlled by the basin-scale wind forcing, (2) elucidate the dynamics underlying a recent Kuroshio large meander (LM) event, and (3) quantify the relative contributions of sea surface temperature (SST), SST fronts and mesoscale eddies due to the recent KE dynamic state change in modifying the atmospheric circulation. The project will use process-oriented oceanic and atmospheric numerical modeling to address the system variability. The Kuroshio and KE are important components of climate variability, influencing North Pacific ecosystems and basin-scale weather patterns, both of important societal relevance. Student involvement and international collaboration are planned.Accumulation of high-quality satellite/in-situ observations and improvement in state-of-the-art ocean/atmosphere general circulation models over the past decades have significantly improved understanding of the roles played by the KE in the decadal North Pacific climate variability. The specific feedback loop involves the wind-forced interior ocean thermocline adjustment, its subsequent alteration to the KE dynamic state, and the KE’s influence on the stormtracks and the surface wind field. This wind- forced feedback loop, which had been at work in the past three decades, was disrupted in late 2017 when the Kuroshio south of Japan changed to a large meander path, a manifestation of intrinsic western boundary current variability. Three testable hypotheses have been identified: Hypothesis 1: The 2017 KE index reversal caused the LM occurrence; Hypothesis 2: The 2017 LM was triggered by enhanced Subtropical Counter Current eddy activity; and Hypothesis 3: Meander-induced KE dynamic state reversal impacts the atmosphere. The first two hypotheses will be examined with the Hallberg Isopycnal Model, modified to a three-layer configuration, which (a) accommodates the interception of layer interfaces by steep bottom topography, (b) allows sea-surface outcropping of isopycnic layers, and (c) has isopycnal coordinates to avoid unphysical diapycnal mixing. Hypothesis 3 will be addressed using the Japan Meteorological Agency’s non-hydrostatic mesoscale model, through a collaboration with Prof. Shusaku Sugimoto of Tohoku University, Japan.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.