The region in question, which the authors abbreviate as WPNECCSR, sits at a dynamical crossroads of tropical circulation. It is here that the North Equatorial Countercurrent, an eastward-flowing current sandwiched between the westward North and South Equatorial Currents, draws its waters from the confluence of western boundary currents, including the Mindanao Current and the Kuroshio. The area is also home to two persistent circulation features, the Mindanao Eddy and the Halmahera Eddy, and it feeds the Indonesian Throughflow, the only low-latitude pathway connecting the Pacific and Indian Oceans. Because of this geography, changes in the region’s eddy field can ripple far beyond the local waters, influencing heat and mass exchange between two ocean basins.

Mesoscale eddies are rotating bodies of water tens to hundreds of kilometers across that persist for weeks to months. Their kinetic energy, known as eddy kinetic energy or EKE, is a standard measure of how vigorous this small-scale turbulence is. Eddies matter because they transport heat, salt, carbon, and nutrients, and because they exchange momentum with the larger-scale currents around them. Understanding what controls EKE in the WPNECCSR therefore speaks to a broader question in climate science: how the ocean’s mean circulation and its turbulent eddies trade energy back and forth as the climate varies.

To attack the problem, Shang and Liu turned to OFES, the Ocean General Circulation Model for the Earth Simulator, a high-resolution, eddy-resolving simulation of the global ocean developed by the Japan Agency for Marine-Earth Science and Technology. They analyzed model output spanning 1990 to 2024, a window long enough to capture many ENSO cycles of both flavors. The simulation was supplemented by satellite altimeter measurements of sea level anomaly from the AVISO archive and by the Oceanic Niño Index maintained by NOAA’s Climate Prediction Center, which served as the benchmark for identifying El Niño and La Niña episodes. Statistical significance was assessed with methods designed for serially correlated climate data, including the Ebisuzaki technique and bootstrap resampling, guarding against the false positives that plague autocorrelated ocean time series.

The analysis revealed a striking spatial pattern. Eddy kinetic energy in the region concentrates in a pronounced high-value band aligned with the axis of the North Equatorial Countercurrent itself, confirming that the current is not merely a passive backdrop for eddies but the very engine that organizes them. When the authors composited EKE anomalies during ENSO events, they found an antisymmetric structure: anomalies of one sign on one side of the current axis and of the opposite sign on the other. This mirror-image pattern is the fingerprint of a current whose position and meandering change with the phase of ENSO, rather than a uniform brightening or dimming of turbulence across the whole region.

The two phases of ENSO act in opposite directions. During El Niño conditions, the North Equatorial Countercurrent shifts southward, its meandering weakens, and the regional circulation develops cyclonic anomalies, meaning the large-scale flow acquires a counterclockwise twist in the Northern Hemisphere sense. During La Niña, the picture reverses: the countercurrent migrates northward, its meandering intensifies, and anticyclonic circulation anomalies dominate. These shifts matter because the countercurrent’s lateral shear and curvature are precisely what generate eddies through instability. A current that wanders and wiggles more vigorously sheds more energy into eddies; a current that straightens and slides southward does the opposite, redistributing where the eddy energy appears.

To move beyond description into mechanism, the study performed a full eddy kinetic energy budget, decomposing the sources and sinks of EKE into physically distinct terms. Barotropic conversion, the process by which the energy of the mean current is transferred directly into eddy motion through horizontal shear and deformation of the flow, emerged as the most important and consistent energy pathway feeding EKE variability in the region. Baroclinic conversion, which draws on available potential energy stored in tilted density surfaces and releases it through buoyancy fluxes, contributed as well, but its importance varied substantially from place to place. The same was true of eddy advection, the redistribution of existing eddy energy by the flow itself, and of wind work, the direct injection or extraction of eddy energy by the overlying atmosphere as winds push against eddy-driven surface currents.

The key conclusion from this budget analysis is one of reorganization rather than amplification. ENSO does not act as a uniform throttle on eddy energy input. Instead, it changes the spatial distribution of each energy term and reshuffles their relative importance between El Niño and La Niña phases. In one subregion, barotropic conversion may dominate during La Niña while baroclinic processes and wind work take over during El Niño; elsewhere the hierarchy flips. This phase-dependent, spatially heterogeneous response helps explain why earlier studies, which often focused on single terms or single locations, reached apparently conflicting conclusions about how ENSO modulates eddy activity in the western Pacific.

The study also looked in the opposite direction, asking whether eddies feed back on the mean flow. Using an eddy potential vorticity forcing analysis, the authors diagnosed bidirectional eddy-mean flow interactions, finding that mesoscale eddies can contribute to variations in the North Equatorial Countercurrent circulation itself by redistributing potential vorticity, a conserved quantity that combines fluid rotation and stratification. This feedback was particularly pronounced under La Niña conditions, when the strengthened meandering of the countercurrent generates more vigorous eddies capable of reshaping the very current that spawned them. Such two-way coupling is a reminder that the boundary between mean flow and turbulence in the ocean is porous, with each side continuously sculpting the other.

The implications extend beyond regional oceanography. The WPNECCSR sits upstream of the Indonesian Throughflow, whose heat and freshwater transport helps set the properties of the Indian Ocean’s upper layers, and it anchors part of the Pacific’s shallow tropical circulation cell that returns water from the subtropics toward the equator. If ENSO reorganizes eddy energy pathways here, then the eddy field’s contribution to heat transport, mixing, and even the modulation of throughflow variability may shift systematically between El Niño and La Niña years. As climate models push toward eddy-resolving resolution, the study’s message is a caution and an opportunity: predicting how eddies respond to future climate change requires resolving not just how much energy they carry, but where the ocean and atmosphere deliver that energy, and how those delivery routes migrate with the phases of ENSO.

Subject of Research: ENSO-driven variability of mesoscale eddy kinetic energy and eddy-mean flow interactions in the western Pacific North Equatorial Countercurrent source region

Article Title: ENSO-related eddy kinetic energy in the western pacific north equatorial countercurrent source region: characteristics and dynamical mechanisms

Article References: Shang, J., & Liu, Q.-Y. (2026). ENSO-related eddy kinetic energy in the western pacific north equatorial countercurrent source region: characteristics and dynamical mechanisms. Climate Dynamics, 64(11), 466. https://doi.org/10.1007/s00382-026-08422-7

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08422-7

Keywords: ENSO, eddy kinetic energy, North Equatorial Countercurrent, western tropical Pacific, mesoscale eddies, barotropic conversion, baroclinic instability, eddy-mean flow interaction, OFES model, potential vorticity, wind work, Climate Dynamics