Collaborative Research: An Eddy-resolved Ensemble Approach to Pacific Ocean Decadal Variability

北太平洋海洋和大气在年际至年代际时间尺度上的低频波动严重影响了北美和欧亚大陆的天气和气候，并推动了在整个太平洋海洋生态系统中观察到的重要状态转变。热带太平洋气候变率主要由与厄尔尼诺南方涛动(ENSO)有关的海洋/大气耦合动力所主导。ENSO的传统特征是东太平洋明显变暖，信风减弱，西(东)热带太平洋海平面气压异常为正(负)。热带大气环流的这些变化通过大气遥相关改变了哈德利环流和温带外大气环流的大尺度模式。具体地说，已经证明了ENSO的极端通过一个众所周知的“大气桥”来激发阿留申低压的可变性。由ENSO产生的阿留申低压的变率被整合并经海洋低空传播，在北太平洋产生了太平洋年代际涛动(PDO)型。最近发现了一种特殊类型的ENSO(太平洋中部显著变暖)与北太平洋环流之间的一种新的动力联系，这为热带和热带外之间潜在的正反馈提供了基础。该项目将使用涡旋分解的集合模拟方法来诊断控制次表层太平洋年代际变化的机制及其在热带太平洋年代际变化中的作用。将制作1950-2012年期间六个长期太平洋涡旋解析海洋模式的集合，以实现两个目标：第一个目标是描述和诊断太平洋次表层平均和涡旋环流的年代际变化。第二个目标是了解产生热带温跃层年代际调制的次表层动力的作用。这将使用基于观测、再分析产品和模型集合模拟的线性逆建模框架来实现。学术价值：直到最近，北太平洋的年代际变化还被放在典型的东太平洋厄尔尼诺现象(EP-ENSO)及其年代际表现--太平洋年代际涛动(PDO)的背景下理解。PIs Di Lorenzo和Schneider(在他们之前的赠款中)通过认识到一种新的十年变率模式，称为北太平洋环流(NPGO)，扩展了这一观点。通过大尺度动力学诊断发现，与PDO相似，NPGO的年代际变化起源于热带，并受到不同的太平洋中部厄尔尼诺现象(CP-ENSO)的强迫。这表明热带太平洋是整个太平洋表面年代际变化的主要驱动因素。然而，控制这一年代代方差来源的动力在很大程度上仍然未知。虽然许多研究已经通过粗分辨率模式和观测探索了热带年代际变化的次表层路径，但尚未系统地探讨涡旋分辨动力学的作用。然而，在直接大气强迫较弱的次表层，涡旋尺度过程的随机强迫可以产生和/或输送水团性质的大幅度年代际异常。这项研究将以一种涡旋分解的集合模拟方法对激发太平洋年代际方差的机制进行新的审视，这种方法允许解决和隔离海洋变率的确定性和内在动力学。这种方法从未被用来研究海洋年代际动力学，尽管越来越多的科学证据表明，涡旋尺度过程对海洋气候起着重要的控制作用。更广泛的影响：加强我们对太平洋次表层气候变异性的了解，对十年气候预测以及生物地球化学和海洋生态系统科学具有重要影响。地下输运和水团性质(例如氧气和营养物质)的十年变化与美国西海岸戏剧性的海岸缺氧事件有关。Di Lorenzo和Schneider PI已经并将继续充当跨学科的沟通者，通过向海洋生态系统科学家提供建模数据、分析和来自该项目的新理解，在物理和生物海洋学社区之间架起桥梁，通过当地的环境项目和几个由首席调查人员共同主持的国际工作组。还将通过佐治亚理工学院的数据服务器提供涡旋分辨后向预报，并将为探索太平洋涡旋尺度动态和利用嵌套沿海海洋模式进行区域气候影响研究提供前所未有的数据档案。该项目还将培养一名女研究生。

Low-frequency fluctuations of the ocean and atmosphere over the North Pacific Ocean on interannual to decadal timescales significantly impact the weather and climate of North America and Eurasia, and drive important state transitions observed in marine ecosystems across the Pacific Ocean. Tropical Pacific climate variability is dominated by ocean/atmosphere coupled dynamics associated with the El Niño Southern Oscillation (ENSO). The traditional expression of ENSO is characterized by a pronounced eastern Pacific warming, a weakening of the trade winds, and positive (negative) Sea Level Pressure anomalies over the western (eastern) tropical Pacific. These changes in the tropical atmospheric circulation modify the large-scale Hadley Cell and extratropical atmospheric circulation patterns via atmosphericteleconnections. Specifically, it has been shown that ENSO extremes excite variability in the Aleutian Low through a well-known "atmospheric bridge". The ENSO-derived variability of the Aleutian Low is integrated and low-passed by the ocean to yield the Pacific Decadal Oscillation (PDO) pattern in the North Pacific. The recent discovery of a new dynamical link between a special type of ENSO (with a pronounced warming in the central Pacific) and the North Pacific Gyre provides the basis for a potential positive feedback between tropics and extra-tropics. This project will use an eddy-resolved ensemble modeling approach to diagnose the mechanisms controlling decadal-scale variations in the subsurface Pacific Ocean and their role in tropical Pacific decadal variability. An ensemble of six long-term Pacific eddy-resolving ocean model hindcasts for the period 1950-2012 will be generated to address two goals: The first goal is to characterize and diagnose the decadal variability of the subsurface mean and eddy circulations of the Pacific Ocean. The second goal is to understand the role of the subsurface dynamics that generate decadal modulations of the tropical thermocline. This will be accomplished using a linear inverse modeling framework based on observations, reanalysis products, and the model ensemble simulations. Intellectual Merit: Until recently, the decadal variability of the North Pacific was understood in the context of the canonical eastern Pacific El Niño (EP-ENSO) and its decadal expression -- the Pacific Decadal Oscillation (PDO). The PIs Di Lorenzo and Schneider (in their previous grant) expanded this view by recognizing a new decadal pattern of variability termed the North Pacific Gyre Oscillation (NPGO). By diagnosing the large-scale dynamics of the NPGO it was found that similar to the PDO the decadal variance of the NPGO originates from the tropics and is forced by a different flavor of central Pacific El Niño (CP-ENSO). This suggests that the tropical Pacific acts as a primary driver of Pacific-wide surface decadal variability. However, the dynamics controlling this source of decadal variance remain largely unknown. While many studies have explored subsurface pathways to tropical decadal variability with coarse resolution models and observations, the role of eddy-resolved dynamics has not been systematically explored. Yet in the subsurface where direct atmospheric forcing is weak, stochastic forcing by eddy-scale processes can generate and/or transport large-amplitude decadal anomalies in water mass properties. This study will take a fresh look at the mechanisms energizing the Pacific decadal variance in an eddy-resolved ensemble modeling approach that allows to resolve and isolate deterministic and intrinsic dynamics of ocean variability. This approach has never been used to study ocean decadal dynamics even though there is growing scientific evidence that eddy-scale processes exert an important control on ocean climate. Broader Impacts: Improving our understanding of subsurface climate variability of the Pacific Ocean carries important implications for decadal climate prediction, and for biogeochemical and marine ecosystem sciences. Decadal changes in subsurface transport and water mass properties (e.g. oxygen & nutrients) are linked to dramatic coastal hypoxia events along the US west coast. The PIs Di Lorenzo and Schneider have acted and will continue to act as interdisciplinary communicators to bridge the physical and biological oceanography communities by making the modeling data, analyses and the new understandings derived from this project available to marine ecosystem scientists through a local environmental program and several international working groups that the lead investigators co-chair. The eddy-resolving hindcasts will also be made available through the Georgia Tech Data server and will provide an unprecedented data archive for exploring eddy-scale dynamics in the Pacific and for conducting regional climate impacts studies with nested coastal ocean models. The project will also train a female graduate student.