due to climate change (Seidel et al. 2008).
Other definitions have recognised that the boundaries of the tropics sensu lato do not equate with rigid zones and have classified the tropics on the basis of terrestrial vegetation (the Kӧppen‐Geiger system) or seasonal patterns in rainfall, where the zonation is identified as ‘humid,’ ‘wet and dry,’ and ‘dry.’ Such definitions are functional, but none fit our requirement for an ocean climate‐based scheme.
The marine tropics is defined here as the area of ocean and coastline included within the annual isotherms of sea surface temperature (SST) of 25 °C (Figure 1.1). This area encompasses (i) most of the Indian Ocean including most of the east coast of Africa to Mozambique and the southern tip of Madagascar, (ii) the Red Sea and the Gulf of Aden, (iii) the Arabian Sea, (iv) the Bay of Bengal, (v) the waters of Southeast Asia, New Guinea, and northern Australia (the South China Sea, Java Sea, Coral Sea, Philippine Sea, Timor and Arafura Seas, and the Gulf of Carpentaria), (vi) most of the small island arcs of the northern and southern Pacific Ocean to the west coast of Mexico and down the Central American coast to Ecuador, (vii) most of the Caribbean Sea and the Gulf of Mexico and the coasts of Central and South America down to central Brazil, and (viii) a large portion of the West African coastline from Guinea‐Bissau to Gabon (Gulf of Guinea). The marine tropics is thus not a uniform or fixed region. There is a considerable degree of plasticity to these boundaries considering differences between the extremes of winter and summer which foster biological plasticity. The West African coast from Gabon to the Congo and down to the north coast of Angola, for instance, has an essentially tropical benthic biota (Longhurst 1959). Such variations are caused in part by the asymmetrical form and unequal size of the ocean margins, which strongly influences sea surface temperatures and current and nutrient regimes (Webster 2020).
FIGURE 1.1 Annual mean sea surface temperatures in the global ocean, 2005–2017.
Source: Image retrieved via public access from the NASA Scientific Visualization Studio. https://sus.gsfc.nasa.gov/3652 (accessed 7 June 2020). © John Wiley & Sons.
1.2 What Makes the Tropics Different?
What makes the marine tropics unique compared to seas of higher latitude? Tables 1.1 and 1.2 summarise many of the characteristics that this book will cover; clearly, there are many environmental attributes that are either unique to or are more common in the tropics. Several habitats attain peak luxuriance in the tropics, namely, mangrove forests, seagrass meadows, and coral reefs. Both ‘wet’ (or ‘humid’) and ‘dry’ tropical regions occur as do areas that undergo distinct ‘wet’ and ‘dry’ seasons. More research has tended to focus on what at first glance appears to be richer, wetter ecosystems, but areas and periods of aridity are more common than are reflected in the literature.
Spatial and temporal variations in rainfall and temperature are large in the tropics; daily thermal and precipitation changes increase away from the equator. The western boundaries of the tropical oceans are warmer, wetter, and more stable climatically than the eastern boundaries, caused by the asymmetrical form and unequal size of the ocean margins, which in turn strongly affect sea surface temperatures, currents, and nutrient regimes (Webster 2020) These geographic differences are of considerable ecological importance, influencing the distribution and abundance of shallow water habitats.
TABLE 1.1 Major hydrological and climatological characteristics unique to or dominant in the tropical oceans. Summary from Chapters 2 and 3.
| Hydrology | Climatology |
|---|---|
| 37% of world ocean area | High and stable solar radiation |
| 69.1% of freshwater discharge to the world ocean | Absorbed solar radiation exceeds long‐wave radiation so net radiation balance is positive |
| Lower mean tidal amplitudes | High and stable temperatures |
| Small Coriolis parameter in proximity to the equator | Lowest and highest rates of evaporation and precipitation |
| Large Rossby radius | Trade winds (easterlies and westerlies) |
| Weak rotational constraint on bottom boundary layer | Absent/uncommon frontal storms within 5° of equator |
| Large buoyancy flux | Interannual variation > seasonal variation |
| Wind‐produced homogenous layer deepest in equatorial waters | Monsoons (dry–wet or arid): Asian, African, Indo‐Australian, and South American systems |
| DCRITICAL DEPTH > DWATER DEPTH | Tropical ocean absorbs most incoming solar energy |
| Seasonal upwelling | Tropical ocean‐atmospheric system is the heat engine of the global climate system |
| Permanently stratified thermoclines and haloclines; oxygen minimum layers | Hadley Circulation distributes equatorial winds in the low latitudes |
| Salinity and pH highly variable; acidic and hypersaline conditions common | Intertropical Convergence Zone, a belt of convective cloud about the equator. Zone of rising air and intense precipitation (accounts for 32% of global precipitation) |
| Estuarization of shelves by river plumes | Indo‐Pacific Warm Pool, an oceanographic/climatological phenomenon in the western Pacific Ocean; heat engine of the planet |
| Strong tidal fronts | Formation of tropical cyclones (typhoons, hurricanes) |
| Lutoclines (a front between two layers of comparatively high and low suspended sediment concentration) and high‐salinity plugs in estuaries and nearshore waters in dry season/arid regions | El Niño‐Southern Oscillation, large‐scale, global, coupled atmosphere–ocean system resulting in major surface climate anomalies throughout tropics |
| Tidal mixing, trapping, and complex small‐scale circulation in mangrove tidal waters | Indian Ocean Dipole, coupled ocean–atmosphere differences in convection, winds, sea surface temperatures, and thermocline causing large‐scale differences in rainfall patterns |
| Highly complex, small‐scale circulation on coral reefs and in hypersaline lagoons | Madden‐Julian Oscillation, a phenomenon that is a major source of intra‐annual variability in the tropical atmosphere, affecting monsoonal and cyclonic patterns |
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Indonesian Throughflow, unique feature passing warm and fresh Pacific waters into the Indian Ocean via the Indonesian Archipelago
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