Investigating internal waves in Algoa Bay

Internal waves are important for marine ecosystems as nutrients, phytoplankton, copepods and larvae are uplifted nearer to the surface at the crests of these waves (Photo © Jesús Pineda and WHOI )

By Dr Wayne Goschen (SAEON Egagasini Node), Dr Marjolaine Krug (CSIR, UCT), Mr Xolisa Dlomo (SAEON-NMMU PhD student), Dr Tommy Bornman and Dr Shaun Deyzel (SAEON Elwandle Node)

Internal waves in the ocean occur at surfaces of different densities (interface) in the water column and are found in layered or continuously stratified seas.

The most common interface to identify internal waves is the thermocline, where there is a sharp vertical temperature gradient, but they may also occur at haloclines such as is common in coastal waters. Internal waves may be generated at the air/sea interface (atmospheric pressure and wind systems), at bottom obstacles or lateral boundaries (e.g. capes) and from the interior of the ocean at the shelf break, for example the Agulhas Current.

Role in local food chain

Internal waves are important for marine ecosystems because nutrients, phytoplankton, copepods and larvae are uplifted nearer to the surface at the crests of the internal waves. This will also be a place of high chlorophyll concentrations where zooplankton grazing is most effective. Thus high concentrations of biomass will be associated with the crests of internal waves and low biomass concentrations with the troughs. Internal waves are likely to play a role in the local food chain, but their influence is yet to be determined in the Algoa Bay region.

Signature of internal waves observed

Oscillations in temperature records at periods of between 2.5 hours and 21.4 hours (the inertial period), which appear to have the signature of internal waves, have been observed in the deeper (bottom depth > 60 m) SAEON underwater temperature records (UTRs) array in Algoa Bay. All these events have not been fully investigated since this project is in its infant stage, but a start has been made in identifying periods of internal wave activity in some records.

Figure 1 shows one such event on 27 March 2010. An internal wave with period of 2.8 hours was recorded by the Algoa Bay Mouth UTR string (Figure 2) moored near the mouth of Algoa Bay (between Cape Recife and Cape Pardone) in 80 m bottom depth. During that period the thermocline lay at a depth of about 30 m, below a well-mixed upper layer at a temperature of 19^°C, and oscillated with a vertical displacement of 5-10 m during the morning of 27 March.

The waves were also recorded at a mooring in the centre of Algoa Bay (60 m bottom depth), but not at the shallower moorings (30 m bottom depth) along the shoreline of Algoa Bay.

Figure 1: Isotherms at 0.5^° intervals from the Algoa Bay Mouth mooring (80 m bottom depth) on 17 March 2011 showing internal waves with period of 2.75 hours

On the radar

Orbital motions associated with internal wave propagation cause regions of converging and diverging flow at the ocean's surface. These short-scale modulations in the surface current affect the properties of the Bragg waves observed with Synthetic Aperture Radars (SAR).

In SAR images, internal waves manifest themselves on the sea surface as alternating bands of rough and smooth water, which appear as light and dark strips. Figure 2 shows the Normalized Radar Cross Section (NRCS) derived from a wide-swath image acquired from the Advanced Synthetic Aperture Radar (ASAR) satellite on 18 November 2007.

Figure 2: A SAR image taken on 18 November 2007 of a possible internal wave train that appears to be generated at the shelf edge offshore Algoa Bay (see southeast corner of the image, from about 34.23^°S/26.0^°E towards the northeast). The Algoa Bay Mouth (ABM) thermistor string mooring is marked on the image. Image and text in SAR courtesy of Dr Marjolaine Krug (CSIR).

The selected SAR image shows low values of backscatter over most of the sampled region due to weak winds and associated smooth sea surfaces. A band of increased backscattering intensity aligned in the south-west/north-east direction and east of 26^°E, suggests the presence of an internal wave. This internal wave is thought to have been generated at the shelf edge by the Agulhas Current.

Although no measurements were made during this time, it is likely that the wave train propagated north-westwards towards Algoa Bay. Several images showed similar surface structures near the shelf break off Algoa Bay.

Previous oceanographic studies in Algoa Bay, using current meter measurements, have mentioned in passing that there are reasonable energies in oscillations matching internal wave frequencies, in particular internal tides oscillating at about the M2 tidal period (12.42h). However, those investigations could not find much internal wave activity in the shallow reaches of the bay, although the moorings were not specifically designed to search for internal waves.

Gaining a deeper understanding of internal waves in the region

The knowledge of internal waves in the bays of the Eastern Cape, off the coastline and over the inner shelf over the eastern Agulhas Bank remains poor. This project aims to gain a deeper understanding of internal waves and internal tides in this region and then, in another parallel biological oriented project, to link the results to the spatial distribution of biological parameters.