Why are El Nio and La Nia strongest during December-April?
El Nio and La Nia are typically strongest during December-April because the equatorial Pacific sea-surface temperatures are normally warmest at this time of the year. Consequently, a slight warming of the waters due to El Nio can result in a major redistribution of tropical convective rainfall, whereas a slight cooling due to La Nia can restrict the tropical convection to Indonesia. The El Nio and La Nia-related sea-surface temperature and tropical rainfall anomalies also affect the wind patterns, which in turn further amplify the sea-surface temperature anomalies. This coupling between the ocean and atmosphere is a critical aspect of the El Nio and La Nia phenomena. In a typical December-April the Pacific warm pool is most extensive, water temperatures in the central and east-central equatorial Pacific are at their warmest levels, and tropical convection extends from Indonesia to the International Date Line. During El Nio the Pacific warm pool and associated area of deep tropical conv
