Is El Niño on the way?

  • Figure 1: Temperature anomalies in the 0-300m layer of the equatorial Pacific Ocean over the past two months. Source: NOAA

    Figure 2: Sea Surface temperature (SST) anomalies across the four regions of the tropical Pacific. Anomalies in the Niño 3.4 region are used to determine the El Niño status. Source: NOAA

    Figure 3: A satellite image of floods in northern California during December 2002, which may have been linked to a moderate El Nino at the time.

  • Is El Niño on the way?
    24.04.2014 14:19

    An El Niño event looks increasingly likely to develop across the tropical Pacific through the coming months, with potential impacts on global weather patterns. In recent weeks, the World Meteorological Organisation, Australia’s Bureau of Meteorology and the NOAA Climate Prediction Centre have all pointed to a model consensus for El Niño conditions to develop this year, with a possible onset during the summer months.

    El Niño conditions are characterised by persistently above average sea surface temperatures (SSTs) in the central and eastern Pacific. During normal conditions, easterly trade winds pile up warmer water in the western Pacific, with an ‘equatorial cold tongue’ extending from the coast of South America into the central Pacific. As a result, the sea is around 50cm higher and 8C warmer around Indonesia than it is around Ecuador. A weakening or reversal of the trade winds can trigger large-scale waves (called Kelvin waves), which propagate eastwards and can transfer warm water across the Pacific towards South America, giving rise to anomalously high SSTs. El Niño conditions are said to exist when SSTs in a defined region of the equatorial central Pacific are at least 0.5C above average for five consecutive overlapping 3-month periods. This typically occurs every 2-7 years, with the last El Niño event observed during 2009-10.

    The eastward shift of the warmest water also leads to an eastward displacement of pressure and rainfall patterns across the equatorial Pacific, leading to flooding in western South America, and drought, bushfires and high pollution levels across Australia and Indonesia. Additionally, the upwelling of nutrient-poor warm water along the coast of South America leads to a decline in fish stocks, adversely affecting fisheries. Further afield, El Niño can disrupt India’s monsoon rains, causing crops to fail, while heavy rains lash the southern USA and Mexico as a result of a more southerly jet stream. Tropical Cyclone frequency is also affected during El Niño years: Hurricane numbers are often reduced across the Atlantic and Gulf of Mexico due to high wind shear, while activity increases slightly in the eastern Pacific. El Niño also has a warming effect on global temperatures, with two of the three warmest years on record classed as “El Niño years” (1998 and 2010).

    During the past two months, experts have been monitoring an eastward shift of some very warm water beneath the surface of the Pacific Ocean (see Figure 1), which has been triggered in response to weakening trade winds. Temperature anomalies in excess of 6C were recently observed at a depth of 150m, with March ocean heat content values at their highest since records began in 1979. This warm water has started emerging at the surface in the central and eastern Pacific in recent weeks, causing a rise in SSTs (see Figure 2). Large sub-surface temperature anomalies have been a historical precursor to El Niño events. A comparable sub-surface warming preceded the 1997/8 “super El Niño”, which was attributed to 23,000 deaths and $34 billion in damage.

    But despite the signatures and model predictions, there remains uncertainty in the extent to which El Niño will develop. Models perform less well in the spring, and the very warm sub-surface waters could simply dissipate. However, areas prone to El Niño’s impacts will no doubt be monitoring the forecast through the coming months.

    By: Billy Payne