El Niño - can we tell how it will shape winter?

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  • Global sea surface temperatures anomalies on September 15th 2015, clearly showing the tropical Pacific warmth of El Niño, the warm eastern North Pacific, and the large cold pool in the Atlantic - all likely to contribute to the shape of winter's atmospheric circulations. Source: World Climate Service.

    Australia's precipitation anomalies during the El Niño of 1997-98. Source: Australian Bureau of Meteorology.

    Flooding along California's Russian River in March 1998 after torrential rains, during the strong El Niño event of 1997-98. Source: Dave Gatley/FEMA.

  • El Niño - can we tell how it will shape winter?
    17.09.2015 17:22

     

    As El Niño continues to strengthen the chatter about how it might affect this winter’s weather becomes louder.

    There have been extravagant claims made already, and we are barely into the northern hemisphere’s autumn; and while a strong El Niño certainly is a powerful influence on atmospheric circulations, it is unwise to make confident claims based on this alone.

    Some broad patterns are associated with El Niño far more often than not, including above-normal winter precipitation in California and drier-than-normal conditions across Indonesia and eastern Australian.

    But even these well-recognized effects are not guaranteed, and seasonal forecasts can only talk in terms of probabilities. Other climate influences act as modifiers: some amplify the outcome of El Niño, such as the very warm Eastern Pacific being likely to bolster an unseasonably mild winter in the northwest USA and western Canada; but others disrupt.

    For example, the waters of the southern Indian Ocean are at record-high temperatures while they are rather cool around Indonesia. This is a positive phase of what is known as the Indian Ocean Dipole (IOD), and while this tends to reinforce dry conditions over eastern Australia it can make some central and southern areas wetter than would be expected with an El Niño alone.

    Teleconnections

    There are many of these influential climate anomalies, known as teleconnections, intertwining around the globe. The difficulty for forecasters is finding years in which similar combinations occurred. Oftentimes, and especially this year, analogues are few and far between because we have rather a short reliable history of these phenomena. 

    For Europe and the British Isles, the connections are more tenuous than in other parts of the world anyway, and the current changing state of the Atlantic sea-surface temperature anomaly distributions is unhelpful to say the least, introducing a high degree of uncertainty. Not only that, no two El Niños are quite the same, and this year’s event is itself is a rarity: it is already close to the strength of that in 1997-98 and could be as strong as any since 1950. 

    On the face of it this could be a reasonable comparison – but the way that it developed, the associated sub-surface sea temperatures (not just surface temperatures), and the position of the warmest anomalies a little farther west mean that the comparison is not precise, never mind the confusion of other teleconnections. The El Niños of 1982 and 1987 may prove to be more comparable, if less strong, but outcomes are also different depending on just when the El Niño hits peak strength.

    The closer we get to the season the more assumptions can be made; but a forecast for an entire season is still only going to be probabilistic. Prolonged colder-than-normal outbreaks at the moment seem unlikely to be a feature of early winter in the British Isles.

    By: Stephen Davenport