How an Intensified Kuroshio Extension Warms the North Pacific: Insights from a High-Resolution Ocean Model
2026.04.16
Overview
This study examines how the representation of upper ocean temperature in the North Pacific depends on the resolution of ocean models. In conventional models that do not resolve oceanic eddies, the subtropical North Pacific tends to be simulated as colder than observed. In contrast, high-resolution models that explicitly represent eddies simulate a strengthened Kuroshio Extension, which efficiently transports warm water and substantially reduces this cold bias.
Details
Numerical models used to study the ocean and future climate divide the ocean into a grid of small cells and calculate temperature and currents at each location. The size of these grid cells is referred to as spatial resolution: models with larger grid cells have lower resolution, while those with smaller grid cells have higher resolution.
When predicting future climate over long periods, models with relatively low resolution are generally used because of computational limitations. These low-resolution models are well suited for studying large-scale, global changes, but they may not adequately represent the finer structures of ocean currents. In contrast, high-resolution models use much smaller grid cells and can reproduce ocean circulation more realistically, but they require far greater computational resources. As a result, their use in long-term climate projections is limited.
In this study, we examined how differences in model resolution affect the simulated upper-ocean temperature in the subtropical North Pacific. We found that low-resolution models tend to reproduce temperatures that are lower than observed over broad areas of the subtropics. In high-resolution models, however, this temperature bias is substantially reduced.
In both types of models, warm water is transported into the subtropical region from the Kuroshio Current. After being carried from the Kuroshio, this water circulates within the subtropical gyre while being cooled at the sea surface along its pathway. Within this common circulation framework, the key difference between the two models lies in the strength of the Kuroshio Extension east of Japan.
In low-resolution models, the Kuroshio Extension is relatively weak, so water transported from the Kuroshio takes longer to reach the interior of the subtropical gyre. During this longer transit time, the water is exposed to surface cooling, resulting in lower upper-ocean temperatures in the subtropical interior.
In contrast, high-resolution models represent a stronger, more realistic Kuroshio Extension. As a result, warm water is transported eastward and into the interior of the subtropical region more rapidly, even while undergoing surface cooling. This leads to the supply of relatively warmer water over a wide area of the subtropics and produces an upper-ocean temperature distribution that is closer to reality.
This study demonstrates that in low-resolution models commonly used for climate projections, insufficient representation of the Kuroshio Extension can lead to biases in subtropical upper-ocean temperature. Our results highlight the importance of realistically representing ocean circulation in order to improve our understanding of future oceanic conditions and climate change.
Publication Information
Journal: Journal of Climate
Title: Upper-ocean warming in the subtropical North Pacific Ocean due to the intensified Kuroshio Extension in the eddying model compared to the non-eddy-resolving model
Authors: Yusuke Ushijima, Hideyuki Nakano, Kei Sakamoto, Ryo Mizuta, Hiroyuki Tsujino
DOI: 10.1175/JCLI-D-25-0060.1
URL: https://doi.org/10.1175/JCLI-D-25-0060.1
Research Support
This work was supported by MEXT-Program for the advanced studies of climate change projection (SENTAN) Grant Number JPMXD0722680734.
Contact Information
Name: Yusuke Ushijima
Phone:+81899279833
E-mail: ushijima.yusuke.fp@ehime-u.ac.jp