low latitudes is a feature known as the Hadley Circulation (Nguyen et al. 2013), a thermally driven atmospheric circulation that features ascent of equatorial air to a height of about 15 km, with transportation aloft towards the poles, descent at the subtropics and a return flow near the surface. This circulation pattern explains the persistence and extent of the trade winds and the subtropical high‐pressure belt that dominates the climate at low latitudes. There is an overall strengthening trend in the trade winds in the western equatorial Pacific and an overall weakening trend in the eastern equatorial Pacific (Li et al. 2019). This trend can be primarily attributed to a cold tongue mode, an out‐of‐phase relationship in SST anomaly variability between the Pacific cold tongue region in the east and elsewhere in the Pacific. The cold tongue region in the eastern equatorial Pacific (Liu et al. 2019) is characterized by a strong atmospheric subsidence that exerts powerful controls on global circulation patterns and the position of the intertropical convergence zone.
In the tropics, the air mass is barotropic (horizontally homogenous) so these large horizontal differences are unable to drive large‐scale weather systems. Instead, winds and their associated weather are driven by vertical motion, which is a result of both heating by the sun and the northerly and southerly convergence of air. Over the Atlantic and most of the Pacific, these winds are westerlies and are often relatively light at upper levels but are highly variable seasonally at the lowest layer of the earth's atmosphere, the trophosphere. Over the Indian Ocean, Africa, and the western Pacific, winds are easterly close to the equator. These patterns are the trade winds that originate in the subtropical high‐pressure systems centred near 30 °N and 25 °S. They persist to such an extent that early explorers such as William Dampier (Dampier 1699) were able to delimit the global trade wind patterns in the late‐seventeenth century (Figure 2.4). In the Northern Hemisphere, northeasterlies converge near the equator with southeasterlies converging in the Southern Hemisphere. The trade winds provide enough forcing for deep convection to form a belt of convective cloud about the equator called the inter‐tropical convergence zone or ITCZ.
FIGURE 2.3 Annual mean net surface heat flux (W m−2) into the global ocean. White contours show the mean dynamic sea surface height. Boxes are western boundary current regions. Ocean Climate Station moorings of the Pacific Marine Environmental Laboratory, NOAA, are indicated by stars. Abbreviations: KOE = Kuroshio Current; ARC = Agulhas Return Current; EAC = Eastern Australia Current; GS = Gulf Stream; BMC = Brazil‐Malvinas Confluence.
Source: Public access at https://www.pmel.noaa.gov/ocs/air‐sea‐fluxes (accessed 15 April 2019). © United States Department of Commerce.
The ITCZ is a narrow band of rising air and intense precipitation. The latter in the ITCZ is driven by moisture convergence associated with the northerly and southerly trade winds that collide at the equator. The ITCZ accounts for 32% of global precipitation (Kang et al. 2018) and moves north and south across the equator following the seasonal cycle of solar insolation and is intimately connected to seasonal monsoon circulations. On an annual average, the ITCZ lies a few degrees north of the equator. The location of the ITCZ has not changed significantly over the past three decades, but there has been a narrowing and strengthening of precipitation in the ITCZ over recent decades in both the Atlantic and Pacific Oceans (Byrne et al. 2018). Climate models project further narrowing and a weakening of the average ascent within the ITCZ as the climate continues to warm.
Wind speed and direction over the global tropical ocean are thus the result of a balance of forces that vary with distance from the equator. As the Coriolis force is weak at the equator this balance breaks down, although there is reasonable balance until about 6° latitude where some momentum usually carries the wind in the direction it is moving when in near‐geostrophic balance, that is, wind in equilibrium between the pressure gradient and Coriolis forces thus blowing parallel to isobars or contours of height. Thus, there is a strong effect of latitude on the wind field patterns. Vertical wind motion takes place at a range of scales throughout the tropics. This vertical motion is facilitated by the Hadley Circulation, the main means by which the atmosphere tries to move energy from the equator to the poles (Nguyen et al. 2013)
FIGURE 2.4 Late‐seventeenth century chart of global trade winds. (top) Atlantic and Indian Oceans and (bottom) Pacific Ocean.
Source: Modified from Dampier (1699), figures b2 and table 1, p 134 and 156. © John Wiley & Sons.
2.3 Tropical Rainfall and Temperature Patterns
Precipitation peaks about the equator and heavy rainfall are associated with the ITCZ where the trade winds converge. Moisture‐laden air near the earth's surface flows towards the equator from both hemispheres and converges about the equator where it is released. Evaporation varies more smoothly than precipitation with a broad maximum in the tropics (Figure 2.5); precipitation exceeds evaporation in the belt from 15°–40° latitude. The runoff (‘P‐E’ in Figure 2.5) implies transport of water vapour from the subtropics to the equatorial and high latitude zones. A return flow in oceans and rivers carries the water back towards the subtropics.
FIGURE 2.5 Latitudinal distribution of the annually averaged surface water balance, showing evaporation, E, precipitation, P, and P‐E (runoff), 1979–2009.
Source: Hartmann (2016), figure 5.2, p. 134. © Elsevier.
These patterns are reflected in the seasonal distribution of global precipitation (Figure 2.6), which shows a heavy band of precipitation about the equator (excluding the Arabian Sea region), but especially in the regions of northern South America and Southeast Asia where convective instability and moisture content of the air are high. These high rates of precipitation result in high rates of river discharge to the coastal ocean. These global averages mask great variability. For example, there are tropical coastal areas of high aridity, such as the Red Sea and Arabian Sea as well as the NW coast of Australia. In these areas, rates of evaporation exceed precipitation and temperatures often exceed the global average, having devastating effects on coral growth and reproduction and lower biodiversity, as fewer species can tolerate these conditions.
In the tropical Atlantic, atmospheric circulation anomalies interact with ocean circulation to produce anomalous SST and precipitation patterns (Figures 1.1 and 2.6).
