The impact of El Niño on the Hadley Circulation (HC) has been a topic of previous studies, but the results have been inconclusive. We study how El Niño affects the HC during different stages of its cycle. In development years, the HC anomaly shows an equatorial quasi‐symmetric pattern, while in decay years, it shows an asymmetric pattern. This resolves previous discrepancies among studies about El Niño's impacts on the HC. The differences in tropical sea surface temperature (SST) during different stages of El Niño cause differences in SST meridional gradients, which determine the location of convergence. This explains why the HC anomalies have different spatial structures during El Niño development and decay years. Our results show that the meridional distribution of SST during different El Niño stages has significantly distinct impacts on meridional circulation and clarify the differences in El Niño's effects on climate.

This study examines the comparative atmospheric circulation and tropical sea surface temperature (SST) relationships during the developing and decaying stages of El Niño from a meridional structure standpoint. Results indicate a transition in the variability of the first two modes of the Hadley circulation (HC) during these stages, with the first mode exhibiting a larger explained variance in the decaying stage. The regime change in HC variability corresponds to underlying anomalous SST distributions, as confirmed by sensitive experiments. Quantitative assessment reveals the HC-SST response amplitudes are approximately two times stronger during the decaying stage compared to the developing stage. Employing the Kuo–Eliassen (KE) equation, diabatic heating anomalies during the decaying stage explain the difference in air-sea response intensity between the two stages. Diabatic heating variations are identified as the primary contributor to amplification or reduction of air-sea response intensity during the respective El Niño stages, providing insights into the different air-sea processes throughout the El Niño lifespan.

El Niño significantly impacts the global Hadley circulation (HC), however, the modulation of different decay rates of El Niño on the HC is overlooked. Our research reveals that rapid decay (RD) events cause the global HC to exhibit an equatorially asymmetric configuration, in contrast to the quasisymmetric structure seen during slow decay (SD) events. This is primarily driven by HC anomalies in the central-eastern Pacific, influenced by anomalous sea surface temperatures that impact the conversion of atmospheric perturbation potential energy to kinetic energy. The structure of regional HC anomalies in the Indo-Pacific Warm Pool (IPWP) exhibits contrasting patterns within different events. During RD (SD) events, the intensified (weakened) regional HC over the IPWP results in increased (decreased) terrestrial precipitation over its coastal countries. Therefore, our findings have important implications for understanding and predicting regional climate impacts associated with different El Niño decay patterns.

Since 2007, green tides have recurrently occurred in the Yellow Sea during spring and summer, with a massive outbreak recorded in 2021. Given the critical significance of green tide monitoring and prediction for marine ecological security and sustainable development, this study developed a satellite remote sensing-validated coupled simulation system for green tide drift and growth, by integrating multi-source satellite remote sensing data and oceanographic reanalysis datasets. Leveraging this system, we systematically analyzed the spatiotemporal evolution characteristics and underlying driving mechanisms of both routine green tide processes in 2014–2015 and the extreme 2021 event. Satellite images with low cloud cover and extensive green tide distribution were screened to confirm the accuracy of green tide drift trajectories and distribution ranges for validating the model’s reliability, and the results demonstrated the spatial consistency between simulation results and satellite observations. The validated model was used to track the drift and growth–decline processes of green tides and investigate the underlying cause of high-biomass appearance in 2021. Combined with environmental parameters, our analyses revealed that variations in attachment substrates alter wind resistance coefficients, thereby potentially accelerating the northward drift velocity of green tides. Furthermore, substrate properties may exert a significant regulatory effect on the attachment, germination, and biomass accumulation of Ulva prolifera spores, which could be a leading factor driving the massive green tide outbreak.

Based on observations and reanalyses, the ability of 24 models from the Coupled Model Inter-comparison Project Phase 6 (CMIP6) in simulating the Hadley circulation’s (HC) long-term variability was evaluated, and possible reasons for the simulation uncertainties were analyzed. The results showed that each model utilized in this study can reproduce the HC climatological pattern well, but the simulated HC variability shows large inter-model uncertainties. The first principle mode of the HC’s long-term variability (EOF1) exhibited an equatorial asymmetric structure (i.e., asymmetric mode, AM) in observations. The CMIP6 models were divided into two groups according to their abilities to reproduce the AM: models that could simulate the AM well were classified as Type I, and models that could not simulate the AM and exhibited an approximately equatorial symmetric structure of the EOF1 were classified as Type II. The results showed that the EOF1 spatial structure is determined by variations in the meridional gradient of the tropical ocean sea surface temperature (SST). Type I models can significantly reproduce the variation relationship between EOF1 and the meridional gradient of the tropical SST, so as to reproduce the HC’s long-term variability characteristics. In Type II models, the EOF1’s variation is tightly related to SST distribution, similar to the El Niño-Southern Oscillation (ENSO). Type II models exaggerate the variation of ENSO, which is associated with the equatorial symmetric structure of SST anomalies, resulting in enhanced variability in the HC of the equatorial symmetric part. This is the reason why EOF1 of the HC in Type II models exhibits an equatorial symmetric structure. The analysis results in this study point out the possible reasons for the different simulations of CMIP6 models in the HC variability and provide a reference for future climate models to be developed in this area.