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 up coastal boundary layers or fronts. All three circulation patterns may, however, operate simultaneously in a water body. While bays and lagoons are tidal‐ or wave‐dominated systems, some coastal systems such as small, restricted inlets and large, semi‐enclosed bays, defy simple classification. Depending on several factors noted above, coastal embayments and bays and open coastal waters may or may not be stratified. Some waters are seasonally stratified.
Coastal waters are greatly affected by the larger oceanic currents. Off East Africa, seasonal circulation patterns are generated by the behaviour of the ITCZ, which creates two distinct seasons, the NE and SE monsoons. During the southeast monsoon, coastal waters are characterised by cool water, a deep thermocline, high water column mixing and wave energy, and fast currents and low salinity due to high precipitation. These characteristics are reversed during the NE monsoon. The 1998 ENSO event produced heavy rains with resulting large sand bars deposited off river mouths, with a persistent decrease in salinity and temperature in inshore waters indicating a coastal boundary layer. ENSO rains also produced semi‐permanent flood channels serving as tidal inlets leading to tidal flooding of low‐lying areas.
Inshore waters bathe, nurture, and are critical to the development and persistence of mangrove forests, coral reefs, and seagrass meadows. For example, in Gazi Bay, Kenya, a shallow, coastal bay, the main forcing function for water circulation are semi‐diurnal tides which generate strong reversing currents in the deep, narrow channels in the mangrove zone, but not in the seagrass and coral reef zones (Kitheka 1997). The peak ebb and flood currents are symmetrical in the seagrass and coral zones with equal duration and magnitude, unlike in the mangrove zone where tidal asymmetry results in ebb currents being slightly stronger than flood tides. Current speeds are slower in the seagrass and coral reef areas of the bay, but the tidal asymmetry in the mangrove zone promotes export of mangrove detritus to the seagrass zone. There is spatial variation in salinity due to evaporation, freshwater, and ocean water inflow. Ocean water is driven out of the bay during ebb tide. The mixing of the different water masses is especially noticeable in the dry season when the freshwater outflow is negligible. The influx of oceanic water into the bay often leads to a slight lowering of water temperatures. The coral zone is dominated by cooler temperatures, higher oxygen concentrations, and higher salinity due to turbulent mixing promoted by wave breaking. Overall, due to the orientation of the bay with respect to dominant tidal water circulation patterns, the lack of sills, and an open entrance, the rate of exchange between inshore and offshore waters is high, about 60–90% of the volume per tidal cycle (Kitheka 1997).
While each bay and embayment are unique, findings as in Gazi Bay have been found in other wet tropical regions where it is common to find a clear gradient from mangrove‐lined creeks to seagrass meadows to coral reefs with some mixing between estuarine and offshore waters (e.g. Sawi Bay in southern Thailand, Ayukai et al. 2000). Such physical gradients are even more distinct in the dry tropics where strong salinity gradients persist year‐round, especially in hypersaline lagoons. Good examples of such lagoons lie along the east coasts of Mexico, Brazil, and Sri Lanka. The northern Yucatán coastal zone of Mexico is a complex environment characterised by extreme evaporation and high seasonal rainfall (Enriquez et al. 2013). It has water exchange with numerous coastal lagoons ranging from brackish to hypersaline and receives intense groundwater discharges. Two main water masses persist along the coast: (i) the Caribbean Subtropical Underwater mass (CSUW) which is upwelled from the Caribbean and hugs the bottom along the coast and (ii) the Yucatan Sea Water mass (YSW) which is a mass of warm hypersaline water which originates in Yucatán due to high temperature and evaporation rates. Permeable karstic geology prevents the surface discharge of riverine water and instead water permeates directly to the aquifer and travels to the ocean via caves and fractures. Similarly, the Lagoa de Arauama is a hypersaline lagoon in coastal Brazil and persists due to semi‐arid conditions, but with a small drainage basin and a choked entrance channel (Kjerfve et al. 1996). For at least 450 a, the lagoon has been hypersaline, although the average salinity has varied in response to the difference between evaporation and precipitation. There is a long‐term trend of decreasing salinity due to constant pumping of freshwater from an adjacent watershed. A salt budget indicates that there is a delicate balance between the import of salt from the coastal ocean and eddy diffusive export of salt to the ocean.
The hypersaline conditions of coastal lagoons can also be affected by climate change and by reduced river discharge due to anthropogenic alterations in water flow (Kennish and Paerl 2010). In the Puttalam Lagoon, a large, shallow water body on the west coast of Sri Lanka, salinity averages 37 with maximum values exceeding 50 during drought periods. The salinity regime is seasonal with rapidly decreasing salinities during the rainy season and increasing salinities during drought. Salinities were lower than oceanic water in 1960–1961 due to high freshwater discharge prior to human‐induced changes in river flow.
Open coastal waters can also be hypersaline in the dry season. Salinities of 37 have been recorded in the Great Barrier Reef lagoon due to evaporation exceeding precipitation. These hypersaline waters are not flushed out by salinity‐driven baroclinic currents because lagoon waters are vertically well‐mixed and are transported by a longshore residual current, thus forming a coastal boundary layer (Andutta et al. 2011) exhibiting both longshore and cross‐shelf characteristics. The dynamics of the coastal boundary layer reaches steady‐state in about 100 days which is the average length of the dry season and differs from other coastal boundary layers that often are one‐dimensional with a dominant along‐channel salinity gradient. Although distinctive in its two‐dimensional nature, coastal waters of the tropics often have similar types of boundary layers with clear salinity gradients. These boundary conditions often disappear and break down completely during the wet season when heavy rains and storms help to mix inshore and offshore coastal waters.
At the opposite extreme, continental shelf waters in the tropics commonly undergo ‘estuarisation’, especially near rivers during the wet season (Longhurst and Pauly 1987). This phenomenon occurs on the inner and middle portions of continental shelves and consists of low‐salinity waters usually exhibited as discrete plumes of discharged river water. The transport of river plumes onto continental shelves is a prime example of such ‘estuarisation’ and the exemplar is the Amazon plume, the low salinity (32–34) of which
