The El Niño Southern Oscillation (ENSO) is one of the world’s most dominant climatological oscillations and is our planet’s single largest source of natural year-to-year variability. Caused by sea surface temperatures in the equatorial Pacific, warmer temperatures lead to an El Niño, and cooler temperatures to a La Niña. Over the last year, the world has been gripped by an El Niño, contributing to the record-breaking high global sea surface and air temperatures, precipitation extremes (high or low depending on location) and a rise in global number of wildfires (NOAA, 2024). However, as we close out Summer, ENSO has returned to neutral and is very likely to swing into its other extreme phase, a La Niña, as we head into Autumn. Having similar far-reaching impacts across the globe, how does this oscillation of sea surface temperatures over the Pacific impact the UK, and what could this mean for our weather over the coming autumn and winter?
Due to its proximity, ENSO has strong influences on surface pressure over the Northern Pacific, which in turn, impacts the weather over the USA. This is initially conveyed by the Pacific Jet stream, and during most winters with a La Niña, high-pressure builds in the Northern Pacific, often bringing warmer, drier weather to the southern States and Mexico, and cooler, stormier weather to the northern USA and into Canada. This then intensifies the North Atlantic Jet Stream, leading to more low-pressure systems (storms) being present in the North Atlantic.
This low-pressure anomaly in the North Atlantic is a sign of another climatic oscillation known as the North Atlantic Oscillation (NAO). Calculated by the difference in surface pressure from the Azores (high) to Iceland (low), a positive NAO occurs when this difference is greater than normal, which is common for La Niña winters. A positive NAO means westerly winds are normally stronger, leading to fiercer more frequent storms that track across northwestern Europe. This means that whilst milder and wetter air from the Atlantic keeps temperatures in the UK from dropping too low, precipitation (rain or snow) is more localised (Met Office, 2024) and is heavily influenced by the position of the Jet Stream.
In our previous post, we spoke about how important the Jet Stream is in controlling how much rain we get during the summer months. Well, the same applies to all seasons, with a northerly shifted Jet Stream normally bringing warmer and more settled weather to areas south of it and vice versa. During most La Niña autumns and winters, alongside a positive NAO, the Jet Stream progressively increases in strength, meaning it heads towards a zonal (straight) flow, as opposed to meridional (meandering) flow by late winter. Because there is less of this meandering, the Jet Stream can regularly split the UK, leading to drier conditions across Wales and central and southern England, and wetter conditions for the northern England and across Scotland.
The ability for the ENSO to impact our weather, despite being half a world away (almost literally) places it in a group of weather patterns called teleconnections. Teleconnections all share the ability to impact weather systems far away from their source, and as mentioned above, they are normally interlinked with one another, perpetually changing weather patterns around the world. If you would like to read more about these Teleconnections and their impacts worldwide, have a read of this fantastic article from NOAA.
