Seasonal forecasting research and development

Specific environmental phenomena can have an influence on the weather months ahead. This means they can be used as indicators as part of the seasonal forecasting process. Both our seasonal forecasting and climate experts are continually researching these phenomena and their effects on global weather and climate.

El Niño

El Niño (Spanish for 'the boy') is the warm phase of a natural oscillation in the Earth's climate system. It involves both the ocean and the atmosphere and occurs when the tropical surface waters of the Pacific Ocean warm, the westward trade winds slacken, and the region of strongest rainfall moves eastwards, from Indonesia out into the Pacific Ocean. It is not totally predictable but on average occurs about once every four years and lasts between 12 and 18 months.

La Niña

La Niña (Spanish for 'the girl') is the cold phase of a natural oscillation in the Earth's climate system. It also involves both the ocean and the atmosphere but tends to be weaker than El Niño. It occurs when the tropical surface waters of the Pacific Ocean cool, the westward trade winds strengthen, and the region of strongest rainfall is confined to South-East Asia and the west Pacific. A La Niña event will typically last between 12 and 18 months and often follows an El Niño event.

Both El Niño and La Niña affect the weather globally. Global temperatures tend to be slightly lower during La Niña events and slightly higher during El Niño events. Their effect on the tropical atmosphere also sends out a ripple of influences that can have an effect on global weather patterns.

El Niño tends to produce warm and dry conditions over Indonesia and warm conditions over parts of southern and eastern Asia; parts of tropical, eastern South America and north-western parts of North America. There tend to be opposite effects during La Niña.

North Atlantic Oscillation (NAO)

The winter North Atlantic Oscillation (NAO) was discovered in the 1920s by Sir Gilbert Walker; it is one of the most important drivers of climate fluctuations in the North Atlantic and surrounding regions. 

The NAO is a natural climate phenomenon associated with fluctuations in temperature, rainfall and storminess over much of Europe. When the NAO is in its positive phase, eastward winds are stronger over the North Atlantic, the storm track moves north and northern Europe tends to be warmer and wetter than average, while southern Europe tends to be colder and drier. When the NAO is in its negative phase, westward winds are more common and northern Europe tends to be cold and dry while southern Europe tends to be warmer and wetter than average.

Particularly during the months of November to April, the NAO is responsible for much of the variability of weather in the North Atlantic region, affecting winds, temperature, precipitation and the intensity, number and track of storms.