arXiv subjects

Sooman Han

Publications and source records attributed to Sooman Han.

3 recordsLinked to original sources

arXiv2026

Detectability of Forced ENSO Changes under Global Warming: Insights from the Recharge Oscillator

We use perfect-model, large-ensemble nonlinear Recharge Oscillator (RO) simulations to quantify ENSO internal variability and the detectability of forced changes in ENSO characteristics. Fixed-parameter simulations show that linear trends in ENSO standard deviation, period, and skewness as large as those observed since 1955 can arise from internal variability in 10% of simulations. RO parameters estimated from single realizations exhibit spurious drifts even without forcing, underscoring the need for ensembles. Using 100-member ensembles, comparable to the largest climate-model ensembles, we identify detectable parameter trends. For ENSO amplitude, the detectability thresholds for forced changes in underlying processes are 15% per century for stochastic forcing, 25% per century for basin adjustment, and 50% per century for the Bjerknes feedback. For ENSO period, the detectability threshold for forced changes in the underlying recharge--discharge processes and delayed oceanic feedback is 15% per century. These results provide a testbed for interpreting RO-diagnosed ENSO changes in climate-model ensembles.

physics.ao-ph↗

arXiv2025

Thermal Inertia Controls on Titan's Surface Temperature and Planetary Boundary Layer Structure

Understanding Titan's planetary boundary layer (PBL)-the lowest region of the atmosphere influenced by surface conditions-remains challenging due to Titan's thick atmosphere and limited observations. Previous modeling studies have produced inconsistent estimates of surface temperature variability, a critical determinant of PBL behavior, often without clear explanations grounded in surface energy balance. Here, we develop a theoretical framework and apply a three-dimensional dry general circulation model (GCM) to investigate how surface thermal inertia influences surface energy balance and temperature variability across diurnal and seasonal timescales. At diurnal timescales, lower thermal inertia surfaces experience larger temperature fluctuations and enhanced daytime sensible heat fluxes due to less efficient subsurface heat conduction. In contrast, at seasonal timescales, surface temperature variations show weak sensitivity to thermal inertia, as atmospheric damping tends to dominate over subsurface conduction. The PBL depth ranges from a few hundred meters to 1,000m on diurnal timescales, while seasonal maxima reach 2,000m, supporting the interpretation from a previous study that the Huygens probe captured the two PBL structures. Simulated seasonal winds at the Huygens landing site successfully reproduce key observed features, including near-surface retrograde winds and meridional wind reversals within the lowest few kilometers, consistent with Titan's cross-equatorial Hadley circulation. Simulations for the planned Dragonfly landing site predict shallower thermal PBLs with smaller fluctuation amplitudes, while maintaining similar wind patterns. This work establishes a physically grounded framework for understanding Titan's surface temperature and boundary layer variability, and offers a unified explanation of Titan's PBL behavior that provides improved guidance for future missions.

astro-ph.EP↗

arXiv2025

Realistic ENSO Dynamics Requires a Damped Nonlinear Recharge Oscillator

The dynamics of the El Ni\~no-Southern Oscillation (ENSO) are succinctly captured by the Recharge Oscillator (RO) framework. However, to simulate ENSO realistically, careful choices must be made regarding the RO's key parameters. In particular, nonlinear parameters govern how well the model reproduces ENSO asymmetries-El Ni\~no events tend to be stronger but relatively short, often transitioning into La Ni\~na, whereas La Ni\~na events are typically weaker but may last longer. While amplitude asymmetry has been studied within the RO framework, duration and transition asymmetries remain less explored and their causes are debated. In this study, by systematically exploring the RO parameter space-rather than relying on commonly used fitting methods-we identify optimal parameter values that successfully capture key linear and nonlinear ENSO characteristics. In doing so, we revisit several foundational elements of the RO framework. First, we analytically derive the phase relationship between temperature and heat content anomalies, showing that it depends on the signs of the Bjerknes feedback and the ocean damping timescale. We show that self-sustained oscillations fail to reproduce the observed kurtosis of Ni\~no indices. We further derive an analytical expression for the power spectrum and argue that incorporating red noise forcing, rather than white noise, introduces unnecessary complexity. The most realistic yet simplest RO configuration is a strongly damped oscillator, with a decay timescale shorter than the dominant period, forced by multiplicative white noise and influenced by weak deterministic nonlinearities. Identifying these minimal components preserves the RO framework's clarity and isolates the core physical processes underlying ENSO behavior.

physics.ao-ph↗