La Niña set to develop this summer

  • Average sea surface temperature (SST) anomalies over the tropical Pacific between 8th May and 4th June 2016. Source: NOAA.

    SST departures over the past 12 months Pacific across the four geographical monitoring regions. Anomalies in the Niño 3.4 region are used to determine the El Niño status, with latest values near neutral. Source: NOAA.

    Satellite imagery of Tropical Storm Colin as it approached Florida on Monday 6th June 2016. Source: NASA

  • La Niña set to develop this summer
    09.06.2016 15:53


    2015 saw one of the strongest El Niño events on record, which likely contributed to the active Pacific hurricane season and the record global temperatures observed earlier this year. Since early spring, however, a change has been underway with cooling taking place across the tropical Pacific. So how are conditions set to evolve, and what could the implications be for global weather conditions over the summer?

    Over the coming months, models indicate the central and eastern Pacific will continue to cool, with la Niña expected to emerge during the summer. A La Niña watch has been issued by the NOAA Climate Prediction Centre, who have forecast a 75% chance of conditions persisting through the autumn and winter months. During La Niña conditions, stronger than normal easterly trade winds lead to increased equatorial upwelling, leading to below average temperatures across the Central and Eastern Pacific. Meanwhile, the westward displacement of warm surface waters fuels wetter than normal conditions across Indonesia and northern and eastern Australia. By definition, La Niña conditions are present when the monthly sea surface temperature (SST) is at least 0.5C below normal in a region of the Pacific known as Niño 3.4. Furthermore, to qualify as a fully-fledged episode, this condition must be met for at least five consecutive overlapping 3 month periods.

    One impact of La Niña is its tendency to increase hurricane activity in the North Atlantic by reducing vertical wind shear, making conditions more favourable for their development. Should La Niña conditions emerge over the next month or two, hurricane activity during the peak months of August to October could be enhanced. A similar swing from El Niño to La Niña during 2010 contributed to one of the most active hurricane seasons on record. At present, however, the National Hurricane Centre is anticipating a near-normal season with 10-16 named systems and 4-8 hurricanes. The evolution of Atlantic SSTs has been cited as a complicating factor, with speculation that the cool waters over the far north Atlantic and eastern subtropics may expand into the main development region later in the season. Three Atlantic systems have developed so far this year. The most recent, Tropical Storm Colin, caused minor disruption in western Florida this week and was the earliest third storm to have developed since accurate records were kept in the 1950s. 

    A further possible impact of La Niña could be seen on the Indian South-west Monsoon, which is already underway. During spring, heating of the Indian subcontinent leads to the development of low pressure, drawing in moisture laden air and leading to intense showers and thunderstorms from June to September. La Niña can boost the monsoon rains by enhancing rising motion over the Indian Subcontinent, and two-thirds of La Niña years since 1901 have yielded above normal seasons. Monsoon rains have been forecast at an above-average 106% by the Indian Meteorological Department, with speculation that the monsoon could be extended beyond its normal withdrawal date. The monsoon is of great importance to Indian agriculture and the economy, with last year’s El Niño contributing to a 14% deficit during 2015.

    By: Billy Payne