LITMIR.BIZ

CHAPTER 1 Introduction

      1.1 Definition of the Tropics

      There is no standard definition of the tropics. It has been defined in so many ways, as a reflection of its complexity, that only an operational definition can suffice; there have been notable climatological and oceanographic exceptions to all definitions. No one definition meets with universal approval, and there have been many attempts to define it, first most simply, by the patterns of the trade winds of the “torrid zone” (Dampier 1699) to a rigid definition of the region between the Tropic of Capricorn and the Tropic of Cancer (Townsend 2012), that is, the most northerly and southerly position at which the sun may appear directly overhead at its zenith. In fact, the word ‘tropical’ comes from the Greek tropikos, meaning ‘turn’ referring to the fact that these latitudes mark where the sun appears to turn annually in its motion across the sky. Recent evidence indicates that the tropics have expanded 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).