arXiv subjects

Tsubasa Kohyama

Publications and source records attributed to Tsubasa Kohyama.

3 recordsLinked to original sources

arXiv2026

Gulf Stream contribution to recent North Pacific warming

Recent unprecedented ocean warming has produced coherent sea surface temperature (SST) anomalies across the Northern Hemisphere extratropics. While the tropical Pacific is a natural source of North Pacific variability, the influence of the midlatitude North Atlantic has remained poorly understood. Here we show that Gulf Stream SST variability remotely modulates Kuroshio variability, explaining 13% of internal Kuroshio SST variance in climate model simulations, with no significant reverse influence. Positive Gulf Stream SST anomalies excite a Northern Annular Mode (NAM)-like atmospheric circulation response, weakening the Aleutian Low over the North Pacific. The resulting northward shift of the Kuroshio Extension enhances northward warm-water transport and favors positive SST anomalies in the western midlatitude North Pacific. We also show that a high-resolution ocean model is required to properly capture this SST-NAM coupling, suggesting the importance of representations of western boundary currents and associated SST fronts for this interbasin pathway. These findings reveal the Gulf Stream-Kuroshio linkage through which North Atlantic variability has measurably contributed to the recent exceptional North Pacific warming, implying a previously underrecognized source of decadal climate predictability.

physics.ao-ph↗

arXiv2026

A Stochastic-Thermodynamic Constraint on the Seasonal Phase Locking of the El Niño-Southern Oscillation

We investigate the seasonal phase locking of the El Niño-Southern Oscillation (ENSO) in a linear stochastic recharge oscillator (SRO), a damped oscillator with additive noise and a time-dependent growth rate. Phase locking is reflected in the seasonality of the variance of the sea surface temperature anomaly (SSTA). In general, energy drives such a change, whereas entropy governs whether it occurs; phase locking is thus subject to both an energy- and an entropy-based constraint. We quantify this entropy-based constraint using a thermodynamic uncertainty relation (TUR), a fundamental inequality in stochastic thermodynamics. The TUR constrains the tendency of the SSTA variance by the partial entropy production rate, which is dominated by the ratio of forward and backward transition probabilities and quantifies the irreversibility of SSTA transitions. The growth rate governs this irreversibility: its extrema occur in boreal autumn and late winter, and the entropy production rate peaks at both times. These peaks relax the TUR constraint on the tendency of the SSTA variance, so that the variance itself can peak in boreal winter, consistent with observed ENSO phase locking. Conversely, when irreversibility is insufficient, ENSO cannot grow or decay. If this irreversibility were interpreted as dissipated energy, the constraint on ENSO growth and decay would require this dissipation to be exported from the equatorial Pacific. A more realistic model is needed to test this hypothesis and to further explore the physical connection between entropy and dissipated energy.

physics.ao-ph↗

arXiv2025

A Mechanism of Stochastic Synchronization in the Climate System: an Interpretation of the Boundary Current Synchronization as a Maxwell's Demon

This study has applied information thermodynamics to a bivariate linear stochastic differential equation (SDE) that describes a synchronization phenomenon of sea surface temperatures (SSTs) between the Gulf Stream and the Kuroshio Current, which is referred to as the boundary current synchronization (BCS). Information thermodynamics divides the entire system fluctuating with stochastic noise into subsystems and describes the interactions between these subsystems from the perspective of information transfer. The SDE coefficients have been estimated through regression analysis using observational and numerical simulation data. In the absence of stochastic noise, the solution of the estimated SDE shows that the SSTs relax toward zero without oscillation. The estimated SDE can be interpreted as a Maxwell's demon system, with the Gulf Stream playing the role of the "Particle" and the Kuroshio Current playing the role of the "Demon." The Gulf Stream forces the SST of the Kuroshio Current to be in phase. By contrast, the Kuroshio Current maintains the phase by interfering with the relaxation of the Gulf Stream SST. In the framework of Maxwell's demon, the Gulf Stream is measured by the Kuroshio Current, whereas the Kuroshio Current performs feedback control on the Gulf Stream. When the Gulf Stream and the Kuroshio Current are coupled in an appropriate parameter regime, synchronization is realized with atmospheric and oceanic fluctuations as the driving source. This new mechanism, "stochastic synchronization," suggests that such fluctuations can be converted into directional variations in a subsystem of the climate system through synchronization by utilizing information (i.e., information-to-energy conversion).

physics.ao-ph↗