Читать книгу Tropical Marine Ecology - Daniel M. Alongi - Страница 16

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). These anomalies originate off the African coast and expand 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.