A gunner adjusts the eyepiece of a telescope: one more click, and the blurry blob becomes a crisp image. Climate scientists at the Institute of Atmospheric Physics, Chinese Academy of Sciences, made a similar adjustment to their model ICM. They doubled the resolution of the atmospheric grid, and the model began to see climate processes in East Asia and the western North Pacific with new clarity.
The previous version, ICM.V1, viewed the world through a coarse grid with cells of 3.75 degrees. It was like looking at a landscape through a fogged-up window: the general outlines are visible, but details blur together. In the new version, ICM.V2, the cells are half the size, 1.875 degrees. The ocean remains the same, but the atmosphere now conveys its state to the ocean in greater detail, and together they reproduce the climate more accurately.
El Niño is a periodic warming of the equatorial Pacific Ocean that acts like a stone thrown into a climatic pond, sending ripples across the globe. For East Asia, two mechanisms are particularly important. The first is the Indian Ocean capacitor effect: after El Niño, the Indian Ocean retains heat for a long time, like a heated stone holding warmth after sunset, and this heat modifies summer rainfall. The second is a local air-sea feedback in the western North Pacific, where wind and ocean temperature anomalies reinforce each other, like two people pushing a swing in sync.
The previous model reproduced these mechanisms weakly, making its regional forecasts unreliable. ICM.V2 corrected the major errors: the cold tongue in the tropical Pacific became warmer, the mei-yu summer rain belt no longer shifts too far north, and El Niño anomalies no longer extend excessively westward. Now the model sees the picture almost as observations do, with only one new error: in summer, it shows negative ocean temperature anomalies in the western North Pacific, whereas in reality they are positive.
It is like after focusing the eyepiece: the image is sharp, but a slight distortion appears at the edge of the frame. The scientists are aware of it and will investigate the cause. The main achievement is that ICM.V2 realistically reproduces both key mechanisms of El Niño's influence, meaning seasonal forecasts for East Asia will become more accurate. The eyepiece is focused, and now we can discern climatic details that were previously out of reach.
The ICM.V2 model was integrated for 1000 model years, equivalent to a thousand-year climate history, yet it uses an atmospheric resolution of T63, which corresponds to a grid of about 210 km at the equator, enough to distinguish individual monsoon systems but still too coarse to reproduce mesoscale phenomena such as tropical cyclones.