the western North Pacific, typhoons are more frequent than in other ocean basins (seasonal average of 26), with peak activity from June to October. More intense typhoons are found in the Central Pacific compared to eastern Pacific El Niño years. The higher occurrence of intense typhoons in the Central Pacific is related to longer typhoon lifespan, maximum potential intensity, ocean heat content (OHC), vertical shear of the zonal wind, outgoing long‐wave radiation and moist static energy (Zhang et al. 2015). A longer typhoon lifespan is caused by the westward shift of the subtropical high, which tends to steer typhoons to the west and NW, usually towards the Philippines, Pacific Islands, Taiwan, and southern China. Typhoons tend to obtain more energy from the warm SSTs along such tracks. The evolution of SST anomalies over the north Indian Ocean and tropical Pacific plays a crucial role in typhoon formation following strong El Niño events.
In the eastern Pacific, hurricanes occur mostly from May to November with a seasonal average of 16.6, making this region the second most active cyclone region. Oceanic control through meridional redistribution of subsurface heat is the main driver of tropical cyclone activity following eastern Pacific El Niño events (Boucharel et al. 2016a). Equatorial Kelvin waves control the sub‐annual and intra‐seasonal variability of thermocline depth in the eastern Pacific region (Boucharel et al. 2016b), affecting ocean subsurface temperature which in turn fuels hurricane intensification. (An equatorial Kelvin wave is a wave in the ocean that balances the Coriolis force.)
The Indian Ocean accounts for about 20% of global cyclonic activity. Cyclones in the northern Indian Ocean (seasonal average of 4.8) mainly occur in the western and central part of the Bay of Bengal. The strong vertical shear during the SW monsoon inhibits cyclones, leading to a bimodal seasonal distribution with peak activity during the pre‐ and post‐monsoon (Lengaigne et al. 2019). Cyclones that form in the northern Indian Ocean are among the world's deadliest, inundating coastal north and NE India, Bangladesh, and Myanmar. In the southern Indian Ocean, cyclones (seasonal average of 9.3) occur over an elongated band centred on 15°S from November to April, with enhanced cyclone occurrence over the SW Indian Ocean around Mauritius, La Réunion and Madagascar. Considerably different upper ocean thermohaline structures in the southern and northern Indian Ocean may result in a different sensitivity of cyclones to ocean–atmosphere coupling (Lengaigne et al. 2019). The Bay of Bengal is characterised by strong stratification that may limit cooling promoting cyclone intensification.
In contrast, the SE Indian Ocean is one of the rare oceanic regions where warm SSTs coexist with a shallow thermocline favouring an enhanced cooling below the storm and a strengthened negative feedback at the early stages of storm intensification. The south Indian Ocean warms following a strong El Niño affecting Indo‐Pacific climate in early summer. Warming of this region induces an anomalous meridional circulation with descending motion over the NE Indian Ocean. The SE Indian Ocean warming lags the SW Indian Ocean warming by one season. South of the equator, El Niño‐forced Rossby waves are reflected as eastward‐propagating Kelvin waves along the equator on the western boundary. The Kelvin waves subsequently depress the thermocline and develop the warming of the southeast tropical Indian Ocean (Chen 2019).
Like cyclones in the northern Indian Ocean, tropical cyclones in the South China Sea are prone to landfall resulting in huge losses in human life and infrastructure along the southeast coast of China. Cyclones in the South China Sea form in summer (May–August) and in winter (September–December) with significant interannual variations in the two seasons and relatively clear inter‐decadal variability in summer (Wang et al. 2019). In winter, vertical circulation is obvious due to a strong south Indian Ocean Dipole (Section 2.6) that induces intensified ascending air and high SSTs over the northern South China Sea with enough moisture to favour tropical cyclone formation. The impact of the south IOD is weaker in the summer, but a convergent zone with upwards motion also can be found over the NE South China Sea. The south IOD and ENSO are two primary factors impacting on cyclone formation in winter and numbers of cyclones increase when La Niña and positive IOD events occur simultaneously. However, the IOD plays a dominant role in tropical cyclone formation compared with the influence of ENSO (Wang et al. 2019).
The Australian region is unique as about 50% of all tropical cyclones form within approximately 300 km of the north coast. The region is climatologically active for tropical cyclones, with a typical season average of 12.5 cyclones, with six forming in the eastern Indian Ocean, three forming in the Timor and Arafura Seas and the remaining three and one‐ half cyclones developing in the Coral Sea. About five cyclones cross the coastline each season doing considerable damage. These cyclones are embedded within the monsoonal trough, although there is considerable variability in areas of low pressure during the summer. Heavy rains and flash floods are common during summer to the extent that many rivers that are reduced to dust in the winter dry season can burst their banks in the wet season. The strength of the summer monsoon is strongly associated with variations in cyclonic activity. Tropical cyclones form closer to the north coast during El Niño years than La Niña years, owing to an equatorward shift of the monsoon trough, warmer SSTs, and weaker vertical wind shear. Tropical cyclones are more likely to make landfall in Western Australia and the Northern Territory during El Niño years. There is a significant correlation between the number of tropical cyclones and the Southern Oscillation Index (SOI), which is defined as the difference between the standardised sea‐level pressure anomalies of Tahiti and Darwin in the Northern Territory. Increases in tropical cyclone frequency and in landfall impacts have been noted for strongly positive SOI values, indicating anomalously low sea‐level pressures over northern Australia.
Hurricanes in the North Atlantic develop from African easterly waves which originate along the southern and northern flanks of the African easterly jet. Hurricanes develop from June to November with peak activity in August–September and seasonally average 12.1 systems per season (Burn and Palmer 2015). These systems occur within the Atlantic warm pool, a region of warm water (> 28.5 °C) encompassing the Caribbean, the Gulf of Mexico, and the western tropical North Atlantic and varying spatially on interannual and interdecadal timescales. Atlantic cyclonic activity is influenced primarily by ENSO and the Atlantic Multidecadal Oscillation (AMO), which combine to modulate SSTs and vertical wind shear. ENSO affects Atlantic climate interannually, and its amplitude modulates on interdecadal (50–90 years) timescales (Burn and Palmer 2015). ENSO influences climate in the tropical Atlantic by controlling variations in sea‐level pressure across the equatorial Pacific, which modulate the strength of the Pacific Walker Circulation and upper‐level westerly wind flow into the region where Atlantic hurricanes develop. Corresponding changes in vertical wind shear in the tropical Atlantic parallel changes in the strength of the Atlantic NE trade winds caused by fluctuations in the surface pressure gradient between the North Atlantic and equatorial Pacific. El Niño (La Niña) events are typically associated with stronger (weaker) vertical wind shear, which suppresses (enhances) rainfall and the formation of hurricanes in the tropical Atlantic. The thermodynamic environment that underpins hurricane development is controlled by the AMO which is an index of Atlantic SST anomalies varying periodically on interdecadal timescales. However, in the Caribbean Sea, much more cyclonic activity occurs with La Niña conditions than with El Niño events.
2.6 The El Niño‐ Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), the Madden‐Julian Oscillation (MJO), and the Pacific Decadal Oscillation (PDO)
Seasonal variations in climate are modulated on intra‐annual, interannual, and decadal time scales by climate phenomena such as ENSO, the IOD, the MJO, and the PDO. How they affect each other is less well‐known than how they impact Earth's climate, but it is reasonably clear that most or all of Earth's climate systems are somehow interlinked, especially within subtropical and tropical latitudes. These phenomena are accompanied by changes in atmospheric and oceanic circulation, affecting global climate, marine and terrestrial ecosystems, and human activities.
The ENSO phenomenon is a large‐scale, global, coupled atmosphere–ocean phenomenon that results in major
