Research Topics

The Quasi-Biennial Oscillation (QBO) is the dominant mode of variability in the equatorial stratosphere, characterized by alternating easterly and westerly zonal winds with a period of approximately 28 months. In the 2015/16 winter, the QBO experienced an unexpected disruption, and this occurred again in the 2019/20 winter (Figure on the left). While several hypotheses have been proposed based on reanalysis data, the disruption mechanism has yet to be understood from a modeling perspective. My research aims to develop a holistic understanding of physical mechanisms that cause disruption using a combination of theory and a hierarchy of numerical model simulations. Based on the theory of Plumb's model (1977), I investigate the QBO in simulations ranging from idealized radiative-convective equilibrium (RCE) models to comprehensive climate models. By bridging theory, observations, and numerical models across different complexity levels, I aim to develop a scaling theory that can be used to predict the factors controlling future QBO disruptions.

The Madden-Julian Oscillation (MJO) is the dominant mode of intraseasonal variability in the tropics, characterized by eastward-propagating convective anomalies with a period of 30–60 days. While the MJO has traditionally been viewed as a single phenomenon, recent studies have classified MJO propagation into four distinct types: standing, jumping, slow-propagating, and fast-propagating. My research investigates this MJO diversity across multiple scales, from seasonal to long-term timescales (Figure on the left). I aim to understand the underlying dynamics of these different propagation types based on the moisture mode theory, which highlights the role of column-integrated moisture in the tropical troposphere. I also evaluate the ability of CMIP6 climate models to simulate MJO diversity, providing insights for improving model performance.

Recent studies have revealed that the QBO modulates MJO activity, with enhanced MJO amplitude observed during the easterly phase of the QBO (EQBO). However, the mechanism behind this connection remains poorly understood, and no climate model has successfully reproduced this relationship. My research aims to bridge this gap by reproducing the QBO-MJO connection using high-resolution simulations with the Weather Research and Forecasting (WRF) model. By prescribing different stratospheric background states, I successfully demonstrated that stratospheric conditions can influence MJO amplitude (Figure on the left). I identified cloud-longwave radiation feedback as a key mechanism linking the QBO to MJO variability. This work connects my research on stratospheric and tropospheric dynamics, providing a more complete picture of tropical atmosphere interactions across vertical layers.