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CHAPTER 3 Tropical Marine Hydrosphere
3.1 Introduction
The circulation patterns of the oceans are closely intertwined with the atmosphere. Most heat and precipitation occur in the tropics, and these factors are important drivers of tropical ocean circulation (Webster 2020). Indeed, sea surface temperatures (SSTs) are sufficiently high that deep atmospheric convection occurs over it. Small changes in SSTs result in movements of deep convection globally, underscoring the connection of the oceans to the atmosphere and vice versa. Circulation patterns in the tropics are complex, and much of this complexity occurs due to inherent thermal instability and mixing, which is a highly non‐linear function of the mean circulation such that it may vary considerably with seasonal and non‐seasonal circulation changes. There is evidence that global warming is weakening tropical ocean circulation (Vecchi and Soden 2007).
Tidal mixing and horizontal mixing are also non‐linear and research into them is still in its infancy. Below, we will describe both large‐scale oceanic and small‐scale coastal and estuarine circulation patterns and how the small‐scale processes fit into the scheme of oceanic processes which have major impacts on the ecology of the tropical ocean.
3.2 Large‐Scale Circulation Patterns
Equatorial flows within the Pacific Ocean are complex, driven mostly by equatorial heat influx (Johnson et al. 2001). NE trade winds north of the equator and SE trade winds south of the equator drive the North and South Equatorial Currents (NEC and SEC) westward at the surface, pushing warm water into the western Pacific (Figure 3.1). To counteract these currents, the Equatorial Under Current (EUC) is driven eastward by an along equatorial pressure gradient which develops over the upper 250 m to roughly balance wind stress. It is this current system and its linkage to ENSO that results in the Peruvian upwelling system (Chapter 11). The EUC thus shoals and upwells, supplying the bulk of the surface water that diverges from the equatorial east Pacific. The SEC is a broad shallow (upper 200 m) current extending from the subtropical south Pacific to 2–5°N and its width is set by patterns of wind curl which also generate the North Equatorial Counter Current (NECC) found north of 2°N in the west and 5°N in the east Pacific. These flows combine with surface Ekman flows and the equatorial western boundary currents seasonally through the ENSO cycle to carry mean heat and freshwater inputs out of the equatorial Pacific. Two other flows feed into the EUC: The Mindanao Current and the New Guinea Coastal Current. These western boundary flows are in turn fed by subduction in subtropical latitudes and are characterised by high oxygen and high salinity. The NECC encounters the Philippines where it forms the Mindanao Current that partly flows into the Celebes Sea; most flows northwards and becomes the warm Kuroshio Current. In the west, the SEC flows mostly southward along the coast of Australia to become the Eastern Australian Current, but it also partially flows into the shallow Arafura Sea north of Australia.
FIGURE 3.1 Main currents of the world ocean showing the main cold and warm flows.
Source: Image obtained from Alamy Australia Pty. Ltd., Brisbane and constructed by Rainer Lesniewski/Alamy stock vector and reproduced under royalty‐free license agreement (accessed 10 June 2021)
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Internal flows exist across both sides of the equator, that is, a surface poleward flow and mixing resulting in flow towards the equator below the thermocline. The equatorward flow is well defined in the south Pacific being between 17°S and 7°S, nearly all in the East and Central Pacific, and directly feeds the EUC (Johnson et al. 2001). The situation in the North Pacific is more complex and less well
