Tropical Marine Ecology. Daniel M. Alongi

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CHAPTER 2 Weather and Climate

      2.1 Tropical Heat Engine

      Due to the inequitable distribution of solar insolation, the tropical ocean absorbs most of the incoming solar energy and is the heat engine of Earth's climate (Webster 2020). The oceans receive more than half of the energy (mostly in the upper 100 m) absorbed by the planet and balanced by evaporative cooling, making the ocean the primary source of water vapour and heat for the atmosphere. As the oceans have great capacity to store heat energy, seasonal cycles in surface temperatures tend to be small in the tropics compared to higher latitudes. The mixed layer of the upper ocean tends to be thinner in the tropics, where the ocean is being heated and thicker at higher latitudes where the ocean gives up its energy via a complex series of atmospheric and oceanographic processes (Webster 2020).

As the sun's apparent position migrates north and south through the year, the zone of maximum tropical heating also migrates to either side of the equator (Figure 2.1). In January, most solar insolation is received south of the equator when it is summer in the Southern Hemisphere (Figure 2.1 top); in April, maximum tropical heating shifts north of the equator during summer in the Northern Hemisphere (Figure 2.1 bottom). The low latitudes are the focus of most heat as they receive the sun's rays at the least angle compared with the high latitudes where the angle of the earth's axis to the sun lengthens summer days and shortens them in winter. Net radiation is greatest over the tropical ocean where the surface albedo is low, and the surface temperature is moderate compared with the land masses.

Absorbed solar radiation exceeds outgoing long‐wave radiation in the tropics so that the net radiation balance is positive (Figure 2.2). This latitudinal gradient in annual mean net radiation is balanced by a poleward flux of energy in the atmosphere and ocean. This flux is about twice as great in the Southern Hemisphere due to the greater area of ocean and smaller land areas mass compared to the Northern Hemisphere.

Evaporation driven by solar insolation is the boiler driving the circulation of the atmosphere and earth's hydrological cycle. The net surface heat flux (i.e. flux of heat energy from the atmosphere into the ocean) is large and negative over the western boundary currents of the ocean basins; it is positive along the equator where the air is heating the water, and along the eastern margins of the oceans where upwelling brings cold, nutrient rich water to the surface (Figure 2.3). In upwelling systems, such as the summer upwelling in the South China Sea, there are prominent ocean–atmosphere interactions with the ocean driving the atmosphere, with wind with steady direction sustaining these interactions (Yu et al. 2020). Interannual variabilities in air–sea coupling are largely impacted by El Niño events changing the regional wind. The influence of a strong El Niño event on summer upwelling is characterised by warmer sea surface temperatures (SSTs) and declining winds, which are associated with weakened SST gradients and mesoscale air–sea coupling.

FIGURE 2.1 Mean daily solar insolation (kWh m⁻² d⁻¹) in the global ocean (top) from January 1984 to 1993 (bottom) from April 1984 to 1993.

      Source: Image in the public domain courtesy of Roberta DiPasquale, Surface Meteorology and Solar Energy Project, NASA Langley Research Center and the ISCCP Project. http://eoimages.gsfc.nasa.gov/images/imagerecords/1000/1355/insolation.gif (accessed 12 February 2018). © NASA.

FIGURE 2.2 Annual mean absorbed solar radiation, outgoing longwave radiation and net radiation (W m⁻²) as a function of latitude, 2000–2013.

      Source: Hartmann (2016), figure 2.12, p. 44. © Elsevier.

2.2 Tropical Winds and the Intertropical Convergence Zone (ITCZ)

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