In 2007 Fedorov explained that ocean-atmosphere coupling is one the most important concepts of climate dynamics. He said that in order to predict changes in global temperature patterns, the understanding of the ocean-atmosphere coupling is critical. Within the ocean-atmosphere coupling there are ocean-atmosphere oscillations. These oscillations are natural climate fluctuations that take place on a cyclical basis. One of the most topical ocean-atmosphere oscillations is the El Nino effect and this essay will explore the El Nino Southern Oscillation (ENSO) and how it affects Australasia.
ENSO is a combination of the El Nino affect and the Southern Oscillation and its two components are sea surface temperature and atmospheric pressure. El Nino is responsible for the fluctuation in sea surface temperature which occurs every few years. El Nino is used when the sea surface temperature is warmer than average, and La Nina (El Nino’s opposite phase) is used when the sea surface temperature is cooler than average. The Southern Oscillation is responsible for the variation in air pressure of the overlying atmosphere. This variation is observed from two points in the Pacific using the Southern Oscillation Index; Darwin (Australia) and Tahiti (“El Nino/Southern Oscillation (ENSO) Technical Discussion | Teleconnections | National Centers for Environmental Information (NCEI)”, 2018). Typically, when the pressure between the two points weakens (conveniently during an El Nino event) parts of western Pacific, including Australia, experience severe drought whilst eastern Pacific experiences increased precipitation and ultimately flooding. The oscillation has three phases which can last between a few months or sometimes a few decades; the phases are either positive, neutral and negative. The neutral phase is considered to be ‘normal’ conditions as this is the phase that is mainly in control of the climate. The positive phase is the El Nino and the negative phase is La Nina.
The neutral phase (average conditions) consists of the trade winds blowing from east to west across the Pacific Ocean. This brings warm, moist air and warmer surface waters to the western Pacific whilst keeping the central Pacific Ocean cooler. During this phase the thermocline is deeper in the west than it is in the east. During a positive phase, an El Nino event, the easterly trade winds weaken and sometimes even reverse. This allows the area of warmer water to move towards the central and eastern areas of the Pacific Ocean. Due to the weakening of the trade winds, the ocean current that draws the surface water away from the western coast of South America slows down. This in turn reduces the upwelling of the cold water from the deeper ocean, and as a result, the thermocline flattens out, which allows for the warm surface water to build up in the eastern part of the basin. During a negative phase, a La Nina event, the Walker Circulation (“longitudinal circulation across the equatorial Pacific” (“The Walker Cell and ENSO”, 2018)) intensifies which means that the trade winds are stronger, and the warmer water is confined to the far western area of the Pacific.
ENSO affects many areas around the world, but in particular, countries located around the equator. As aforementioned this essay is going to be looking at the effects of ENSO on Australasia. In 2009 Joshi stated that Australia’s climate is very variable, especially in terms of rainfall, and ENSO has a large role to play in the matter. During an El Nino event rainfall is lower than average, especially in the winter months between June and August after the peak of ENSO. El Nino also means that there are drier conditions over the country (Joshi, 2009). When a La Nina event occurs, the opposite happens; wetter conditions.
During the positive phase, the area of warmer water moves away from Australia leaving the sea surface temperatures cooler than normal around northern Australia (“The three phases of ENSO”, 2018). El Nino typically results in reduced rainfall for northern and eastern Australia and this is because the area of increased convection is now towards the east of the Pacific, near the western coast of South America. This in turn can result in increased risk of drought in the country, particularly on the eastern side, as El Nino events are associated with increased dry conditions across the country (El Nino, La Nina and Australia’s Climate, 2018). Australia experienced an El Nino event during the year 2015-16. It was categorised on the Southern Oscillation Index as moderate to strong with the sea surface temperatures being classed as very strong. In the lead up to the El Nino event, “dry conditions were already in place” in parts of the country, such as Queensland and south-eastern Australia. This had implications for the event as it would prolong the impacts of the El Nino; i.e. the period of drought would remain for longer than it should have. The event was caused by the oceans “warming up rapidly” as a result of the westerly winds from January to March, and the wind bursts “consolidated the event in May and June”. From April to August 2015, eastern Australia saw decreased rainfall, and this became “widespread during September and October” (see figure 1) (“El Niño – Detailed Australian Analysis”, 2018). The prolonged event caused the fire season in the country to begin earlier than normal, resulting in prolonged heatwaves during the Autumn of 2016. This coupled with the cloudless skies and extremely warm waters ended up causing devastating effects to the marine life of the oceans. The Australian Government revealed that the El Nino event “resulted in the worst coral bleaching event on record for the Great Barrier Reef” (“El Niño – Detailed Australian Analysis”, 2018).
Figure 1: September 2015 – October 2015
During a negative phase, a La Nina event, the trade winds strengthen which causes the sea surface temperatures to rise in the northern region of Australia. The stronger winds “increase the convection and cloud cover over the northern region of Australia” due to the increased moisture in the atmosphere above. This also “intensifies the Walker Circulation” which means that there is increased humidity and rainfall over inland Australia (“The three phases of ENSO”, 2018). Australia experienced a La Nina event between 2010-2012 which was classed as strong on the Southern Oscillation Index and moderate to strong on the sea-surface temperature index. This event was one of Australia’s “strongest on record” (“La Niña – Detailed Australian Analysis”, 2018). The event caused 2010 and 2011 to be Australia’s “third-wettest and second-wettest years on record” with rainfall reaching “703mm and 708mm respectively” (see figure 2) (“La Niña – Detailed Australian Analysis”, 2018). This prolonged period of rain lasted for 24 months and caused widespread flooding across the country with the most severe cases being reported in southeast Queensland. Another impact of this La Nina event was the increase in the occurrence of tropical storms during the summers. Tropical cyclone Yasi was the most significant to hit the county as a category 5 tropical cyclone. Yasi maintained strong, “damaging winds and heavy rain” as it progressed westwards across the country. Yasi became one of the strongest and “most powerful tropical cyclones” to have hit Queensland “since records began” (“Severe Tropical Cyclone Yasi”, 2018).