Читать книгу Tropical Marine Ecology - Daniel M. Alongi - Страница 32

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 understood, but it is unlikely that the EUC is fed as strongly as in the south Pacific. Convergence towards the equator occurs below the surface mixed layer on the trough between the SEC and the NECC. An excess of heat energy that is needed to supply the EUC is presumably warmed in the westward‐flowing SEC, subducted in the east or central Pacific, and then upwelled again to flow out in the surface mixed layer further west. This phenomenon is referred to as the ‘Tropical Cell’ (Godfrey et al. 2001).

The complex oceanographic features of the equatorial Pacific are the foundation for a rich yellowfin and skipjack tuna fishery, which accounts for roughly 40% of the world's annual tuna catch (Chapter 9). There is a paradoxical link between the tuna catch and a strong divergent equatorial upwelling in the central Pacific called the ‘cold tongue’, which is favourable to the development of high phytoplankton production (Lehodey 2001). This ‘cold tongue’ is contiguous with the Indo‐Pacific Warm Pool (IPWP), which is characterised by lower rates of primary production. Consistent with the observed movements of tuna, there is a clear out‐of‐phase pattern linked to ENSO between the western Pacific region and the ‘cold tongue’.

These equatorial Pacific currents are linked to the Indonesian Throughflow (ITF) (Figure 3.2) but precisely how is uncertain (Feng et al. 2018). The ITF is a unique feature at the crossroads between the Indian and Pacific Oceans, carrying warm and fresh Pacific waters through the Indonesian Archipelago into the Indian Ocean. It is the only series of channels in the tropics through which water passes from one ocean to another. The ITF contributes to circulation and thermal structure around northern and eastern Australia and the southern Indian Ocean (Tillinger 2011). Blockage of the ITF weakens the Indian Ocean South Equatorial Current and Agulhas Current and strengthens the Eastern Australian Current. It also maintains a stronger New Guinea Coastal Undercurrent, enhancing tropical‐subtropical exchange in the south. Blockage or weakening of the ITF reduces thermocline fluctuations and increases SSTs in the central and equatorial Pacific and raises the mean thermocline of the Indian Ocean and decreases SSTs in the southern Indian Ocean.

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.