arXiv subjects

Xichen Li

Publications and source records attributed to Xichen Li.

3 recordsLinked to original sources

arXiv2026

A Regime Shift in Atlantic Surface Currents Reveals a Step-like Decline of the Meridional Overturning Circulation

The Atlantic surface currents associated with the Atlantic Meridional Overturning Circulation (AMOC) play a central role in regulating Earth's climate, yet their large scale dynamical response to climate variability remains poorly understood. Here we identify a previously unrecognized basin scale phase of Atlantic surface circulation, termed the Atlantic Convergence Divergence Mode (ACDM), characterized by a convergence divergence pattern in the North Atlantic and coherent meridional flows in the South Atlantic. We show that the ACDM experienced a pronounced regime shift in 2009, marked by weakened vertical water exchange and reduced meridional transport. This transition closely coincides with direct RAPID MOCHA AMOC observations and is driven by AMOC modulated multicale forcing: a low frequency oceanic thermal reorganization that preconditions the system, and episodic atmospheric shocks that trigger the shift. By identifying the ACDM variability as a sensitive and physically grounded proxy for interannual AMOC fluctuations, we reveal that the observed 2009 shift signifies a nonlinear, step like weakening of AMOC that triggered a fundamental basin scale reorganization of Atlantic surface currents. Our results offer a dynamical explanation for the AMOC's recent decline and demonstrate its inherently nonlinear nature, highlighting the need to account for step like transitions in assessing its stability and future evolution.

physics.ao-ph↗

arXiv2025

Generative assimilation and prediction for weather and climate

Machine learning models have shown great success in predicting weather up to two weeks ahead, outperforming process-based benchmarks. However, existing approaches mostly focus on the prediction task, and do not incorporate the necessary data assimilation. Moreover, these models suffer from error accumulation in long roll-outs, limiting their applicability to seasonal predictions or climate projections. Here, we introduce Generative Assimilation and Prediction (GAP), a unified deep generative framework for assimilation and prediction of both weather and climate. By learning to quantify the probabilistic distribution of atmospheric states under observational, predictive, and external forcing constraints, GAP excels in a broad range of weather-climate related tasks, including data assimilation, seamless prediction, and climate simulation. In particular, GAP is competitive with state-of-the-art ensemble assimilation, probabilistic weather forecast and seasonal prediction, yields stable millennial simulations, and reproduces climate variability from daily to decadal time scales.

cs.LG↗

arXiv2019

The effects of urbanization on the temperature over the Beijing-Tianjin-Hebei region under changing climate

Urbanization has a large impact on human society. Besides, global warming has become an increasingly popular topic since global warming will not only play a significant role in human beings daily life but also have a huge impact on the global climate. Under the background of global warming, the impacts of urbanization may have differences, which is the topic this project mainly discusses. In this study, the monthly air temperature data from 2 meteorological stations in Beijing under the different underlying surface: urban and suburban for a period of 1960-2014 were analyzed. Besides, two years, 1993 and 2010, selected with the different circulation conditions have been simulated by weather forecasting models. We conducted experiments using the Weather Research and Forecasting (WRF) model to investigate the impacts of urbanization under different circulation conditions on the summer temperature over the Beijing-Tianjin-Hebei Region (BTHR). The results indicate that the effect of urbanization under low-pressure system is greater than that under high-pressure system. That is because the difference of total cloud cover between the urban area and the suburban area is greater under low-pressure system in 1993. This research will not only increase our understanding of the effect of urbanization but also provide a scientific basis to enhance the ability to prevent disasters and reduce damages under the global warming background.

physics.ao-ph↗