How El Niño and La Niña drive seasonal predictability

ENSO is the largest source of seasonal predictability. How Pacific sea-surface temperatures shape the coming season, where the effect is strong and where it is weak.

The largest source of seasonal signal

The El Niño–Southern Oscillation, or ENSO, is a coupled swing of the tropical Pacific between warm (El Niño), cold (La Niña) and neutral states. Because it involves the upper ocean, it evolves over months and is predictable one or two seasons ahead. Its footprint on the atmosphere is global, which makes it the single largest source of seasonal predictability for most of the world.

How the ocean reaches the atmosphere

Tropical convection follows the warmest water. When El Niño shifts warm water eastward, the main region of rising air moves with it, and the change in tropical heating launches wave trains through the atmosphere that alter the jet streams over both hemispheres. These remote responses are called teleconnections. They are strongest in the winter hemisphere and show up in seasonal models as anomalies in mean sea-level pressure, which is why this site includes a pressure field alongside temperature and precipitation.

Where the effect is strong

The tropical Pacific itself, the maritime continent, Australia, parts of southern Africa, eastern Africa, India and the Americas have well-documented ENSO responses. In North America, El Niño winters tend to be wetter across the southern United States and milder across the north. Seasonal models resolve these relationships well, and the sea-surface temperature maps on this site make the state of the Pacific visible directly.

Where the effect is weak

Europe sits at the far end of the teleconnection chain. ENSO has a measurable but modest and inconsistent effect on the North Atlantic circulation, and other drivers such as the stratospheric polar vortex and North Atlantic sea-surface temperatures compete with it. That is one reason European seasonal forecasts carry less skill; see why skill is high in the tropics and low over Europe.

Reading ENSO on the maps

Select the sea-surface temperature field and the global region. A band of positive anomalies along the equator east of the dateline is El Niño; a band of negative anomalies is La Niña. Comparing the five models shows whether they agree on the strength and timing of the event, and the run comparison shows whether the outlook is strengthening or fading from month to month. Because SST anomalies are the most predictable field, they are also where model agreement is usually highest.