Tropical Marine Ecology. Daniel M. Alongi

FIGURE 3.2 Pathways of the Indonesian Throughflow between the Pacific and Indian Oceans and linkage to other major ocean currents.

      Source: Feng et al. (2018), figure 1, p.3. Licensed under CC BY 4.0. © Springer Nature Switzerland AG.

The ITF varies both seasonally and annually (Tillinger 2011) as about 60–90% of sea‐level variability, and 70% of thermocline variability can be understood in terms of free Kelvin waves and Rossby waves generated by remote zonal winds along the equator in the Indian and Pacific Oceans. Variations in zonal Pacific equatorial winds force a response along the Arafura Sea/Australia shelf break through Pacific equatorial Rossby waves exiting coastally trapped waves off the western end of New Guinea which propagate poleward along the Australian west coast as the Leeuwin Current. The regional circulation off west Australia is thought to be embedded in a subtropical ‘super gyre’ that connects the Indo‐Pacific via south Australia (‘the Tasman Gateway’) and the passages of the Indonesian archipelago (Lambert et al. 2016). There is also an effect of the ITF felt by energy radiating westward across the Banda Sea and into the southern Indian Ocean. Wind energy across the equatorial Indian Ocean propagates along the south coasts of the islands of Sumatra, Java, and Nusa Tenggara to penetrate the Savu Sea, the western Banda Sea, and Makassar Strait. Thus, the ITF affects nearly the entire ocean field of the Indonesian Archipelago, as well as coastal New Guinea and Australia.

      The New Guinea Coastal Current (NGCC) likely sets up a strong shear flow in austral summer when the surface flow of the NGCC is towards the southeast against the mean NW flow. Tidal mixing may also play a role in producing vertical eddies and coastal upwellings throughout the Indonesian Archipelago, implying that surface heat fluxes are carried through the mixed layer. It is possible that tidally enhanced eddies are widely distributed throughout the west Pacific especially near reef complexes. As noted in Chapter 2, the MJO, ENSO, as well as small‐scale seasonal cycles play a strong role in large‐scale water circulation in the equatorial Pacific.

      Once the ITF passes through the many islands of the Indonesian archipelago, it circulates through the Indian Ocean back into the Pacific south of Australia (Lambert et al. 2016). Somewhat reminiscent of the equatorial Pacific, although the wind system is greatly different, the South Indian Ocean Counter Current (SICC) flows from west to east across the Indian Ocean against the wind‐driven circulation.

      Circulation in the Indian Ocean is driven not only by the SICC but also by ENSO, IOD, and the MJO (Chapter 2). Wind‐driven upwelling occurs mainly in the seasonally reversing, western boundary currents rather than in the eastern equatorial region; a completely different set of mechanisms drives heat and freshwater absorption (Hood et al. 2017). In the north, the Indian Ocean has two large water bodies west and east of India: the Arabian Sea and the Bay of Bengal. In the Arabian Sea, there is upwelling of cold, nutrient‐rich water during the SW monsoon (SWM) along Somalia, Oman, and the west coast of India. In the western Bay of Bengal, upwelling occurs during the NE monsoon (NEM), whereas south of Sri Lanka, there is coastal upwelling where upwelling blooms are swept into the Bay of Bengal by the SW Monsoon Current (McCreary et al. 2009). In the tropical south Indian Ocean, there is a weak surface plankton bloom during boreal summer when new phytoplankton production is enhanced by nutrient entrainment. In boreal winter, the mixed layer is thinner resulting in less plankton production as the thermocline is deeper and nutrient entrainment is weaker. ENSO/IOD events can cause plankton blooms south of the islands of Sumatra and Java, while upwelling further east is driven by entrainment and mixing of the ITF with other currents such as the Java Current.

In the Atlantic, the average circulation bears some resemblance to the equatorial Pacific (Figure 3.1). An Equatorial Under Current (EUC) in the equatorial thermocline is surrounded by westward currents, the Southern Equatorial Current (SEC). North of 5°N, a seasonal surface‐trapped Northern Equatorial Counter Current (NECC) occurs. At the equator, the EUC usually overlies a westward current bounded by eastward currents at 4°N and 3–4°S. The current structure is more variable than in the Pacific with some suggestion of eastward currents near 2–3°S (South Intermediate Counter Current) and 2–3°N (North Intermediate Counter Current). Both the EUC and SEC derive their physical properties from the Southern Hemisphere via the North Brazil Under Current (NBUC) with some seasonal input from the Northern Hemisphere. Currents carry low oxygen water to the western boundary, whereas the eastward currents of the Antarctic Intermediate Water (AAIW) often carry oxygen‐rich water.

      In the western tropical Atlantic Ocean, fresh surface waters from the Amazon may induce a strong halocline in the 3–30 m depth range, which in turn induces a pycnocline that acts as a barrier layer for mixing between the surface and subsurface waters. Following maximum Amazon discharge, the river plume and resultant barrier layer extends over a large part of the equatorial basin north of the equator in boreal summer and autumn (Varona et al. 2019). This anomaly due to the river discharge is powerful enough to contribute to a northward shift in the ITCZ during this period. The Amazon plume is great enough to drive spatial and temporal variations in oceanic primary productivity (Gouveia et al. 2019).

      Even in the open sea in the tropics, high rates of evaporation and precipitation and upwelling can destroy the permanently stratified thermocline, unlike in temperate and polar oceans where water masses turnover by cooling in autumn and winter. North of the equator, the eastern Pacific and eastern Atlantic Oceans are eddy‐dominated, with counter currents impinging upon inshore waters and estuaries fed by major rivers and wide shelf areas.

3.3 Coastal Circulation

      Tides and wind‐generated waves play in important role in the circulation of water close to shore, although the large‐scale circulation patterns set the characteristic signatures of nearshore water masses (Masselink and Hughes 2014). Water circulation in embayments, bays, and other nearshore water bodies are greatly influenced by daily tidal cycles and wind waves and are also affected by long‐shore currents that are in turn influenced at the macro‐ and meso‐scale. Coastal circulation is ultimately driven by energy derived from solar heating or gravity, barometric pressure, and the density of oceanic waters that impinge on the coastal zone. Mixing results from tides and waves and buoyancy effects from river runoff, if any. Water mixing and circulation are greatly affected by geometry and bathymetry of the coastal zone.

      Regional variability of precipitation and high solar insolation produces very sharp gradients in temperature, salinity, and other properties, such as dissolved nutrient concentrations, in tropical coastal waters. Sharp thermoclines and haloclines coincide with strong vertical discontinuity maintained throughout most of the year, except where equatorial upwellings force cooler water to the surface, or where waters from central oceanic gyres intrude into humid regions to become warmer and more dilute. Lower salinities are characteristic of surface waters of the wet tropics, and conversely, surface waters in arid tropical regions are hypersaline. Great variability in salinity and its ability to adjust rapidly to changes in wind‐induced motion and temperature characterises tropical surface coastal waters (Webster 2020).

Three main types of coastal circulation are recognised: (i) gravitational (due to river runoff), (ii) tidal, and (iii) wind‐driven. Tidal circulation is usually the most prevalent with interaction by coastal boundaries generating turbulence, advective mixing, and longitudinal mixing and trapping, the latter setting