Weather and climate are integral parts of the Earth system. Although they operate on different time scales, from short-term weather patterns to long-term climate trends, both are influenced by many of the same physical processes. Understanding the weather-climate continuum is essential for improving predictions across time scales. Jacob Bjerknes was a pioneer in understanding weather systems like fronts, cyclogenesis, and developing predictive models. He was the first to describe how changes in trade winds and SSTs could reinforce each other, leading to significant warming or cooling of the tropical Pacific. This feedback mechanism, known as Bjerknes instability, is crucial to explaining the development and amplification of large-scale climate systems, particularly the El Niño-Southern Oscillation (ENSO).

Air-sea exchanges of mass, heat, and momentum play a critical role in global weather and climate. Recent advancements in coupled atmosphere-wave-ocean models with higher resolution, such as ~O(1 km) grid spacing in regional models and O(10 km) in global models, have enabled more realistic simulations. These models can now test various new physical processes at the air-sea interface across time scales, from hours to subseasonal scales and beyond.

This lecture will provide an overview of the progress and challenges in atmosphere-ocean coupling, particularly in the context of high-impact weather events like tropical cyclones and subseasonal-to-seasonal variability, including the Madden-Julian Oscillation (MJO). The MJO’s influence on the onset of El Niño, and its broader implications for prediction across scales, will also be explored. The explicit coupling of surface waves is vital for improving predictions of hurricanes and winter storms, especially regarding their coastal impacts. The MJO precipitation and freshwater input to the ocean can induce eastward density currents, which contribute to the Warm Pool’s eastward extension over the tropical central Pacific, weakening the trade winds and initiating the Bjerknes feedback—a potential trigger for El Niño onset. These examples underscore the importance of developing a fully coupled atmosphere-wave-ocean Earth system modeling framework and the need for enhanced in situ and satellite observations to improve future predictions.