Weakening of La Niña Impact on Negative Indian Ocean Dipole Under Global Warming.

As global warming intensifies, the coupling relationship between negative Indian Ocean Dipole (nIOD) and La Niña has substantially changed. However, the characteristics and mechanisms of these changes are not yet fully understood. Here, we find that the impact of La Niña on nIOD has considerably weakened since 1999, with the frequency of nIOD occurrences during La Niña years plummeting to a mere one‐third of the pre‐1999 levels. This is primarily attributed to the early onset of Indian summer monsoon and the decrease in La Niña intensity, while the effect of variations in Bjerknes feedback is relatively minor. Model simulations suggest that the influence of La Niña on nIOD will continue to weaken under future global warming through similar mechanisms as in the observations, increasing the complexity of air‐sea coupling in the Indian Ocean. Plain Language Summary: The negative Indian Ocean Dipole (nIOD) has significant impacts on both local and global climate change. La Niña is a major trigger for nIOD. However, as the globe warms up, the connection between nIOD and La Niña has changed a lot. This study analyze observation and reanalysis data from the 1950s onwards to see how nIOD and La Niña are linked. We find that La Niña has had less impact on nIOD since 1999. This is because the summer monsoon starts earlier, causing changes in wind patterns over the Indian Ocean. La Niña normally brings westerly wind anomalies in the Indian Ocean, but these winds are weakened because of the early monsoon. This makes it harder for nIOD to form. The feedback loop known as the Bjerknes feedback doesn't play a big role in maintaining nIOD. Furthermore, the reduction in La Niña intensity also contribute to weakening the relationship between La Niña and nIOD. Models predict that La Niña's impact on nIOD will continue to weaken under future global warming, increasing the complexity of air‐sea coupling in the Indian Ocean and leading to more challenges in forecasting. Key Points: The relationship between La Niña and negative Indian Ocean Dipole has weakened significantly in this centuryThe spring‐summer easterly anomaly induced by the premature summer monsoon weakens the La Niña‐induced westerly anomalyThe impact of La Niña on negative Indian Ocean Dipole will continue to weaken under future global warming [ABSTRACT FROM AUTHOR]