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1. Exceptional multi-year prediction skill of the Kuroshio Extension in the CESM high-resolution decadal prediction system

   https://doi.org/10.1038/s41612-023-00444-w  

   Kim, Who M.; Yeager, Stephen G.; Danabasoglu, Gokhan; Chang, Ping (December 2023, npj Climate and Atmospheric Science)

   Abstract The Kuroshio Extension (KE) has far-reaching influences on climate as well as on local marine ecosystems. Thus, skillful multi-year to decadal prediction of the KE state and understanding sources of skill are valuable. Retrospective forecasts using the high-resolution Community Earth System Model (CESM) show exceptional skill in predicting KE variability up to lead year 4, substantially higher than the skill found in a similarly configured low-resolution CESM. The higher skill is attained because the high-resolution system can more realistically simulate the westward Rossby wave propagation of initialized ocean anomalies in the central North Pacific and their expression within the sharp KE front, and does not suffer from spurious variability near Japan present in the low-resolution CESM that interferes with the incoming wave propagation. These results argue for the use of high-resolution models for future studies that aim to predict changes in western boundary current systems and associated biological fields. 

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2. Interdecadal Modulation of the Relationship between Decadal Variability of the Kuroshio Extension and the Central Tropical Pacific in an Eddy-Resolving Coupled Model

   https://doi.org/10.1175/JCLI-D-24-0305.1  

   Tamura, Yukito; Tozuka, Tomoki; Chang, Ping (October 2025, Journal of Climate)

   Abstract The Kuroshio Extension (KE) exhibits significant decadal variations, particularly following the 1976/77 Pacific climate regime shift. Robust ocean–atmosphere interactions over the KE imply a potential key role in basin-scale climate variability. Recent studies suggest that North Pacific Oscillation (NPO)-like atmospheric teleconnections from the central tropical Pacific dominantly influence KE decadal variability through oceanic Rossby waves. However, this relationship varies on interdecadal time scales and only achieves statistical significance after the regime shift. This study utilizes outputs from an unprecedented 500-yr preindustrial control simulation conducted with an eddy-resolving coupled general circulation model to explore the potential and mechanisms of natural variability–induced interdecadal modulation. When the entire simulation period is divided into five segments, a relationship between the KE and the central tropical Pacific, resembling that after the regime shift, is found during the segment marked by the strongest decadal variability in the central tropical Pacific and the NPO. These variabilities enhance each other by positive feedback on decadal time scale via the Pacific meridional mode triggered by air–sea interaction. As the westerly jet migrates southward over the eastern North Pacific during this period, the southern lobe of the NPO also expands southward, likely providing a favorable background condition for its active interaction with the tropics. In contrast, the relationship is absent in other periods, likely due to weaker and less persistent tropical variability, differences in the location of atmospheric teleconnections, and/or influences of the Kuroshio large meander. These findings suggest that the interdecadal modulation of the KE may occur independently of anthropogenic forcing. Significance StatementAtmospheric teleconnections from the central tropical Pacific have been suggested to drive the decadal variability of the Kuroshio Extension, particularly after the 1976/77 climate regime shift. This study analyzes a 500-yr preindustrial simulation from a high-resolution coupled model, revealing a possible interdecadal modulation of this relationship due to natural variability. The observed relationship emerges during a simulation period characterized by strong decadal variability in the central tropical Pacific and the North Pacific atmosphere. They enhance each other via air–sea interaction, suggesting a positive feedback operating on decadal time scales. During this period, anomalous atmospheric circulation over the eastern North Pacific expands southward, associated with a southward-shifted westerly jet, providing a favorable background condition. 

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3. NAO predictability from external forcing in the late 20th century

   https://doi.org/10.1038/s41612-021-00177-8  

   Klavans, Jeremy M.; Cane, Mark A.; Clement, Amy C.; Murphy, Lisa N. (December 2021, npj Climate and Atmospheric Science)

   null (Ed.)

   Abstract The North Atlantic Oscillation (NAO) is predictable in climate models at near-decadal timescales. Predictive skill derives from ocean initialization, which can capture variability internal to the climate system, and from external radiative forcing. Herein, we show that predictive skill for the NAO in a very large uninitialized multi-model ensemble is commensurate with previously reported skill from a state-of-the-art initialized prediction system. The uninitialized ensemble and initialized prediction system produce similar levels of skill for northern European precipitation and North Atlantic SSTs. Identifying these predictable components becomes possible in a very large ensemble, confirming the erroneously low signal-to-noise ratio previously identified in both initialized and uninitialized climate models. Though the results here imply that external radiative forcing is a major source of predictive skill for the NAO, they also indicate that ocean initialization may be important for particular NAO events (the mid-1990s strong positive NAO), and, as previously suggested, in certain ocean regions such as the subpolar North Atlantic ocean. Overall, we suggest that improving climate models’ response to external radiative forcing may help resolve the known signal-to-noise error in climate models. 

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4. GEFSv12 High- and Low-Skill Day-10 Tornado Forecasts

   https://doi.org/10.1175/WAF-D-22-0122.1  

   Miller, Douglas E.; Gensini, Vittorio A. (July 2023, Weather and Forecasting)

   On average, modern numerical weather prediction forecasts for daily tornado frequency exhibit no skill beyond day 10. However, in this extended-range lead window, there are particular model cycles that have exceptionally high forecast skill for tornadoes because of their ability to correctly simulate the future synoptic pattern. Here, model initial conditions that produced a more skillful forecast for tornadoes over the United States were exploited while also highlighting potential causes for low-skill cycles within the Global Ensemble Forecasting System, version 12 (GEFSv12). There were 88 high-skill and 91 low-skill forecasts in which the verifying day-10 synoptic pattern for tornado conditions revealed a western U.S. thermal trough and an eastern U.S. thermal ridge, a favorable configuration for tornadic storm occurrence. Initial conditions for high skill forecasts tended to exhibit warmer sea surface temperatures throughout the tropical Pacific Ocean and Gulf of Mexico, an active Madden–Julian oscillation, and significant modulation of Earth-relative atmospheric angular momentum. Low-skill forecasts were often initialized during La Niña and negative Pacific decadal oscillation conditions. Significant atmospheric blocking over eastern Russia—in which the GEFSv12 overforecast the duration and characteristics of the downstream flow—was a common physical process associated with low-skill forecasts. This work helps to increase our understanding of the common causes of high- or low-skill extended-range tornado forecasts and could serve as a helpful tool for operational forecasters. 

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5. The abyssal origins of North Atlantic decadal predictability

   https://doi.org/10.1007/s00382-020-05382-4  

   Yeager, Stephen (July 2020, Climate Dynamics)

   The fundamental mechanisms that explain high subpolar North Atlantic (SPNA) decadal predictability within a particular modeling framework are described. The focus is on the Community Earth System Model (CESM), run in both a historical forced-ocean configuration as well as in a fully coupled configuration initialized from the former. The initialized prediction experiments comprise the CESM Decadal Prediction Large Ensemble (CESM-DPLE)—a 40-member set of retrospective hindcasts documented in Yeager et al. (Bull Am Meteorol Soc 99:1867–1886. https://doi.org/10.1175/bams-d-17-0098.1, 2018). Heat budget analysis confirms the driving role of advective heat convergence in skillful prediction of SPNA upper ocean heat content out to decadal lead times. The key ocean dynamics are topographically-coupled overturning/gyre fluctuations that are geographically centered over the mid-Atlantic ridge (MAR). Long-lasting predictive skill for ocean heat transport can be related to predictable barotropic gyre and sigma-coordinate AMOC circulations, but depth-coordinate AMOC is far less predictable except in the deepest layers. The foundation of ocean memory (and circulation predictive skill) in CESM-DPLE is Labrador Sea Water thickness, which propagates predictably through interior pathways towards the MAR where large anomalies accumulate and persist. Abyssal thickness anomalies drive predictable decadal changes in the gyre circulation, including changes in sea level gradient and near surface flow, that account for the high predictability of SPNA upper ocean heat content. 

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