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The ENSO phenomenon is a large‐scale, global, coupled atmosphere–ocean phenomenon that results in major surface climate anomalies throughout much of the tropics (Timmermann et al. 2018). The extreme phases of ENSO can bring either warmer (an El Niño event) or colder (a La Niña event) than normal SSTs to the central and eastern tropical Pacific that then affect global atmospheric circulation and weather patterns on timescales ranging from weeks to decades. Most ENSO events occur on a timescale of three‐five years. The occurrence of droughts, floods, cyclones, and other weather phenomena across the planet have been associated with ENSO periods. The term ‘Southern Oscillation’ refers to the variability in the pressure difference across the Pacific Ocean (between Darwin and Tahiti), referred to previously as SOI which has been developed to measure the strength of El Niño.

The dynamics of ENSO are complex and variable from event to event (Timmermann et al. 2018), but a composite evolution of El Niño events captures the typical evolution of ocean and atmosphere conditions from the early spring initiation of El Niño, to its wintertime peak and transition to La Niña during the subsequent summer (Figure 2.9). Onset of an ENSO cycle begins with warm and deep SSTs in April with random forcing and downwelling Kelvin waves followed by growth by August and an eastward shift with nonlinear feedbacks (positive feedback along the equator in which a weakened equatorial SST gradient weakens trade winds which in turn further reduces the SST gradient). By December, the El Niño matures with warm SSTs in a narrow west–east band and a southward shift of wind anomalies. By April of the following year, the event decays as heat content in the deep ocean is discharged. A transition to colder SSTs in the eastern Pacific occurs by August followed by maturation of the La Niña (Figure 2.9) by December. Strong El Niño conditions typically last for one year, but La Niña events can persist for up to several years.

      The El Niño event of 2015/2016 was initiated in boreal spring by a series of westerly wind events. This wind forcing triggered downwelling oceanic Kelvin waves, thus reducing the upwelling of cold subsurface waters in the eastern Pacific cold tongue, leading to surface warming in the central and eastern Pacific. The positive SST anomaly shifted atmospheric convection from the western Pacific Warm Pool to the central equatorial Pacific, causing a reduction in equatorial trade winds, which in turn intensified surface warming through positive feedback. Termination of the 2015/2016 event was associated with ocean dynamics and the slow discharge of equatorial heat into off‐equatorial regions. The event started to decline in early 2016 and transitioned into a weak La Niña in mid‐2016.

      The most dramatic example of the impact of an El Niño event on oceanographic processes is the Peruvian upwelling system (Chapter 10, Section 10.9.2). During El Niño, the thermocline and nutricline deepen significantly during the passage of coastal‐trapped waves within the Peruvian upwelling system. While the upwelling‐favourable wind increases, the coastal upwelling is compensated by a shoreward geostrophic near‐surface current. The depth of upwelling source waters remains unchanged during El Niño, but their nutrient content decreases dramatically, which along with a mixed layer depth increase, impacting phytoplankton growth. Offshore of the coastal zone, enhanced eddy‐induced subduction during El Niño plays a potentially important role in nutrient loss.

      Another dramatic example of the effect of an El Niño event are mass coral bleaching events (Section 13.2). A relatively modern phenomenon first reported in the 1980s, bleaching of corals has been unequivocally linked to SSTs above the upper thermal tolerance limits of corals and is widespread typically during El Niño events.

Each ENSO event is different and such variability may arise from climate phenomena outside the tropical Pacific, including the North and South Pacific meridional modes, extra‐tropical atmospheric circulation patterns, and tropical Atlantic variability. The negative phase of the North Pacific Oscillation tends to favour the development of positive SST anomalies in the central Pacific by weakening the trade winds in the Northern Hemisphere, while the positive phase of the South Pacific Oscillation tends to weaken the trade winds in the Southern Hemisphere, thereby favouring the development of positive SST anomalies in the eastern Pacific. Western wind anomalies in the western equatorial Pacific tend to favour El Niño in the central Pacific, whereas westerly wind anomalies in the central and eastern Pacific tend to favour El Niño in the eastern Pacific. There is some evidence of an increasing trend in ENSO amplitude during the past century (Christensen et al. 2013).

FIGURE 2.9 The three phases of the El Niño‐Southern Oscillation (ENSO). During an El Niño event (upper left panel), trade winds weaken or may reverse, allowing the area of warmer than normal water to move into the central and eastern tropical Pacific Ocean with the thermocline shallowing eastwards. During normal (or neutral) conditions (upper right panel), trade winds blow east to west bringing warm moist air and warmer surface waters towards the western Pacific and keeping the central Pacific relatively cool with the thermocline much deeper in the west. During a La Niña event (lower panel), the Walker circulation intensifies with greater convection over the western Pacific and stronger trade winds.

      Source: Public domain image from https://www.pmel.noaa.gov/elnino (accessed November 13, 2020). © United States Department of Commerce.

Although ENSO is global in scope and effect, it is linked to a series of other climatological phenomena, such as the Indian Ocean Dipole (Fan et al. 2017). The equatorial Indian Ocean is warmer in the east with a deeper thermocline and mixed layer and supports a more convective atmosphere than in the west. During September–October, the eastern equatorial Indian Ocean becomes usually cold with equatorial winds blowing from east to west and from the SW off the coast of Sumatra, facilitating coastal upwelling. At the same time, the western equatorial Indian Ocean becomes warm and enhances atmospheric convection. Sea‐level decreases in the eastern equatorial Indian Ocean and rises in the central region. This coupled ocean–atmosphere phenomenon of interacting convection, winds, SSTs, and thermocline is known as the Indian Ocean Dipole (IOD). The state described above is referred to as a positive IOD and the reverse, a negative IOD is characterised by warmer STT anomalies, enhanced convection, higher sea level, a deeper thermocline in the east and cooler SSTs, lower sea level, a shallower thermocline, and suppressed convection in the west. A negative IOD event is an intensification of the normal state whereas positive IOD represents the opposite to the normal state.

      The IOD results in two large‐scale patterns in countries bordering the Indian Ocean: (i) anomalously high land temperature and rainfall in the western Indian Ocean and low land temperature and rainfall in the east and (ii) enhanced rainfall over the Asian monsoonal trough, extending from Pakistan up to southern China. During IOD events, biological productivity of the eastern Indian Ocean increases as does the frequency of coral deaths. The IOD also affects rainfall over Australia and eastern Africa. The IOD is characterised by (i) a Walker cell anomaly over the equator in the Indian Ocean, (ii) deep modulation of the monsoonal westerlies, and (iii) a Hadley cell anomaly over the Bay of Bengal (Fan et al. 2017). The IOD is important to global climate as about 50% of IOD events over the past century have co‐occurred with ENSO events.

      Both ENSO and the IOD interact with a phenomenon called the Madden‐Julian Oscillation or MJO (Zhang 2005). The MJO is a major source of intra‐annual variability in the tropical atmosphere and often results in breaks and bursts of monsoonal activity, helping to invigorate tropical cyclones (e.g. Cyclone Winston in 2016). The MJO also modulates cyclone development in the Caribbean. Even though it originates in the equatorial Indian and western Pacific Oceans, the MJO affects equatorial surface winds in the tropical Atlantic. The MJO interacts with the underlying ocean to influence weather and climate, especially over the Pacific Islands, monsoonal Asia and Australia, South America, and Africa. The interannual variability

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