La Nina conditions developing in the Eastern Pacific Ocean

  • Map of sea surface temperature anomalies for August 2017. Note the below average temperatures in the equatorial central and eastern Pacific Ocean. Image credit: NOAA

    Graph showing forecast sea surface temperature anomalies for the coming months. Most forecasts take the sea surface temperatures below the -0.5C La Nina threshold. Credit: NOAA

  • La Nina conditions developing in the Eastern Pacific Ocean
    16.09.2017 14:45

    Recent forecasts suggest that ‘La Niña’ conditions will develop in the central and eastern Pacific Ocean over the next month or so. But what is La Niña and how could it affect the weather over the UK?

    In simple terms, La Niña is the reverse of El Niño. A more detailed explanation is that La Niña is associated with cooler than average sea surface temperatures around the equator in the central and eastern Pacific Ocean. The most used threshold is that temperatures need to be 0.5C below average for 6 consecutive months. El Niño is the opposite of this and is associated with higher than average temperatures across the same area of the Pacific Ocean. The last El Niño occurred between 2014 and 2016.

    Whilst El Niño and La Niño tend to alternate, it is not necessarily true that one will immediately follow the other. However, the latest update from the National Oceanographic and Atmospheric Administration (NOAA) says that sea surface temperatures are currently a little below average, and forecasts a further decline, with a 60% chance of La Niña conditions developing this autumn and continuing through the winter.

    Whilst the La Niña tends to have the most influence on the weather across the Pacific Ocean, it is well known that the state of the Pacific Ocean can have an effect on the weather in other parts of the world. For example, La Niña tends to give wetter conditions over southern Africa but drier conditions across south-western parts of the USA, although this does depend on the time of year.

    But what effect does La Niña have on the weather over the UK? La Niña tends to be associated with colder weather patterns occurring across North-West Europe in the late autumn and early winter, rather than in the mid-to-late winter. However, there are other competing influences which could negate this influence - for example, the pressure pattern over the North Atlantic Ocean, or winds in the stratosphere. So whilst there are hints, it is too early to say whether this means we will have a colder or drier than average winter this year.

    By: George Goodfellow