westwards. Indeed, precipitation is low in these regions, and SSTs are correspondingly high year‐round. A large area of the tropical southeast Atlantic is low in rainfall, south of the equator to about 30 °S, like the eastern boundary of South America. The El Niño‐Southern Oscillation (ENSO) phenomenon is the primary source of short‐term climate variability and is associated with distinct and different atmospheric and oceanic climate anomalies that affect rainfall, river flow, temperature, tropical cyclone activity, and shifts in the position of the major convergence zones (see Section 2.6).
SSTs (Figure 1.1) vary across tropical seas, but one of the most unique features is the existence and persistence of the Indo‐Pacific Warm Pool (IPWP), a large area (>30 × 106 km2) of the western Pacific Ocean that is characterised by permanent SSTs >28 °C and is therefore called the ‘heat engine’ of the planet (De Deckker 2016), playing an important role in the seasonal monsoon and interannual variability such as ENSO. The IPWP is called the ‘steam engine’ of the globe because of high convective clouds which can reach altitudes up to 15 km, generating much latent heat in the process of convection. Broad seasonal change in surface salinities is caused by seasonal and contrasting monsoonal activity throughout the region. This area lies along the path of the Great Ocean Conveyor Belt and is a ‘dilution’ basin due to high incidence of tropical rain; away from the equator tropical cyclones contribute to a significant decline in ocean salinity. The IPWP has been expanding on average by 2.3 × 105 km2 a−1 during 1900–2018 and at an accelerated average rate of 4 × 105 km a−1 during 1981–2018. Most of this expansion has occurred in the Indian Ocean rather than in the western Pacific Ocean and has been attributed to increasing concentrations of greenhouse gases and natural fluctuations associated with the Pacific Decadal Oscillation (Roxy et al. 2019)
FIGURE 2.6 Annual mean global precipitation (mm per year) from the IMERG climatology dataset, NASA, June 2000‐May 2019. Colours grading to yellow ‐red indicate greater precipitation.
Source: Image in the public domain courtesy of the NASA Scientific Visualization Studio. https://sus.gsfc.nasa.gov/4760 (accessed 10 June 2021). © NASA.
2.4 Monsoons
Monsoons are a central component of global climate and are critical to the global transport of atmospheric energy and water vapour. More than 70% of the world's population lives in monsoonal regions and these systems have a profound effect on society and the global economy. Zhisheng et al. (2015) proposed the following definition of the global monsoon:
The global monsoon is the significant seasonal variation of three‐dimensional planetary‐scale atmospheric circulations forced by seasonal pressure system shifts driven jointly by the annual cycle of solar radiative forcing and land‐sea interactions, and the associated surface climate is characterised by a seasonal reversal of prevailing wind direction and a seasonal alternation of dry and wet conditions.
FIGURE 2.7 Global distribution of monsoon domains and their local components, including by differences of 850 hPa wind and precipitation between the June‐July‐August and December‐January‐February mean.
Source: Zhou et al. (2016), figure 1, p. 3590. Licensed under CC BY 4.0. © Copernicus Publications.
The tropical monsoon primarily lies between the seasonal migration boundaries of the ITCZ (Figure 2.7). The seasonal migration of the ITCZ, caused by cross‐equatorial pressure gradients, produces a strong tropical monsoon under the annual cycle of solar radiation. The global monsoon is distributed primarily across several major subtropical and tropical regions: tropical Asia, Indonesia‐Australia, Africa, and South America (Figure 2.7). Although West Africa, northern Australia and parts of South America depend on much of their annual rainfall from seasonal monsoons, the archetypal monsoon system is the tropical Asian‐Australian monsoon which consists of several main subsystems: the tropical Australian monsoon, the Maritime Continent monsoon, the South China Sea monsoon, the Indochina Peninsula, and western North Pacific monsoon and the Indian or South Asian monsoon.
In the Southern and Northern Hemispheres, subtropical monsoons (Figure 2.8) are caused by the seasonal shift of the subtropical high and land‐sea distribution and are closely related to several features: large‐scale topography, the Rossby radius of deformation, the jet stream, and the interaction between the jet stream and large‐scale topography. (The Rossby radius of deformation is the length scale at which rotational effects become as important as buoyancy or gravity wave effects in the evolution of the flow about some disturbance.) The Southern Hemisphere subtropical monsoon includes the Southern Australian, South African, and subtropical South Pacific monsoons. The Northern Hemisphere subtropical monsoon consists of the East Asian, North American, North African, Tibetan Plateau, subtropical North Atlantic, and North Pacific monsoons.
2.4.1 The Asian Monsoon
The Asian monsoon system reaches from the western Arabian Sea through East Asia and North Australia (Figure 2.8). This system is composed of the Indian (or South Asian) and East Asian subsystems, roughly divided at about 105°E. Both subsystems are linked to varying degrees by regions of strong sensible heating (Indo‐Asian landmass) and strong latent heat export (the western Pacific Warm Pool and the southern subtropical Indian Ocean). However, they also have significant differences determined by the contrasting sea‐land distributions.
FIGURE 2.8 Global tracks and intensity of all tropical storms, 1856–2006.
Source: Image retrieved via public access from NASA Earth Observatory. http://earthobservatory.nasa.gov/IOTD/view.php?id=7079. (accessed 25 June 2019). © NASA.
The Indian system is characterised by land in the north and ocean in the south, whereas the East Asian system has land in the north and south, a maritime continent in the west and open ocean to the east. Summer heating of the Asian continent, especially around the Tibetan Plateau, generates low‐pressure cells and thus summer rains in South and East Asia. In the winter, a reversed high‐pressure system is established, with dry, cool to cold winds blowing out of Asia. Waters of the northern Indian Ocean and South China Sea are subjected to seasonal changes in SSTs and salinity patterns and current strength and direction.
These dynamics lead to seasonal upwelling in the NW Arabian Sea. The monsoon
