The researchers found that summer rainfall in Hong Kong is more influenced by mid-latitude weather patterns and sea surface temperatures than by tropical factors like El Niño. In negative El Niño years, warm sea temperatures in the North Atlantic lead to more rain in Hong Kong due to changes in wind patterns. In positive El Niño years, rainfall is affected by winds from the Indian Ocean and Bay of Bengal. Understanding these mid-latitude influences is crucial for predicting summer rainfall on the South China coast.