Investigating coastal upwelling during Natal Pulses of the Agulhas Current

By Dr Wayne Goschen (SAEON Egagasini Node), Dr Tommy Bornman and Dr Shaun Deyzel (SAEON Elwandle Node) Tweet

The Agulhas Current is a warm, fast ocean current that flows towards the earth’s south pole along the east and south coasts of South Africa.

At first it is constrained at the continental shelf break, which is close to the coastline south of Durban, but moves further offshore as the shelf widens onto the Agulhas Bank.

The linearity of its flow is sometimes broken by small (about 30 km wide) inshore cold-water eddies moving downstream from the KwaZulu-Natal coast, around which the Agulhas Current flows in what are termed meanders. Cold water (upwelling or upliftment) is also found along the inshore edge of the Agulhas Current, caused by a number of mechanisms.

Natal Pulses

On occasion, perhaps one or more times a year (but none during some years), the Agulhas Current moves far offshore (of the order of 150-300 km from shore), forming a large meander. These large solitary episodic meanders in the Agulhas Current are termed Natal Pulses (Figure 1).

Figure 1. Sea surface temperature of a large solitary meander (Natal Pulse) in the Agulhas Current off Algoa Bay (Image: MODIS, NASA, MRSU)

Since the wide-spread use of satellites for oceanographic research, which are able to measure sea surface temperature (SST) and surface chlorophyll, among other variables, it has been easy to track surface features of the Agulhas Current, such as shown by Natal Pulses. Since the 1970s it was observed, in passing, that cold water occurred close inshore and in the bays of the Eastern Cape during Natal Pulses.

Recent studies have shown that Natal Pulses influence the coastal waters on the Agulhas Bank for around 65 days (Krug et al., 2014), which implies that cold water should also be found close inshore for around this amount of time (Figure 2).

Figure 2. A schematic of a Natal Pulse off Algoa Bay, also showing different types of upwelling (after Goschen et al., 2015)

SAEON data underpin study

The authors used data obtained from SAEON’s Algoa Bay Sentinel Site moorings over a number of years to further investigate this Agulhas Current-driven upwelling (the results were recently published by Goschen et al., 2015).

The study found that upwelling in the nearshore and bays of the Eastern Cape did indeed occur during Natal Pulses. Active upwelling (the time over which isotherms rise toward the surface) takes place over one to three weeks, while cold water remained near the coast for a further two to three weeks, on average (Figure 3).

Figure 3. Sea temperatures in Algoa Bay during a Natal Pulse (June 2010)

The cold water thus influenced the coastal waters for a considerably longer period than coastal wind-driven upwelling, which lasts only for around one to seven days. In addition, the cold water was widespread, covering the entire study area, not like wind-driven upwelling which was mostly anchored to the capes.

Coastal sea temperatures during Natal Pulses dropped by as much as 9^oC (average 10-12^oC) and isotherms rose at an average rate of ~2 m/day (much slower than wind-driven upwelling, estimated to be ~ 12 m/day at the shoreline, but less further offshore).

Important discovery

An important discovery was that the rise of the isotherms at the Algoa Bay moorings (upwelling) began opposite the leading edge (crest) of the meander. Coastal currents were also affected, changing from northeastward to southwestward at the start of upwelling.

The southwestward currents reached a maximum speed of 80 cm/s (comparatively fast) at this time but gradually decreased in strength (with considerable variability). This could be equated to a large offshore transport of coastal water, which Porri et al. (2014) have shown leads to a permanent loss of larvae from the coastal waters opposite a Natal Pulse.

What causes this upwelling?

The mechanisms behind this upwelling are still under investigation. One way this could happen is due to currents and pressure gradients that are set up at the crest of a Natal Pulse as it encroaches upon the coastline. Another way could be due to cold water moving onshore caused by the divergence of the Agulhas Current from the coastline as it meanders offshore.

Figure 4. Sea temperatures recorded at three depths at SAEON’s Algoa Bay moorings during a Natal Pulse, showing widespread coastal upwelling (after Goschen et al., 2015)

The onshore movement of cold water could be in the bottom boundary layer (a band of water above the sea floor) or as upwelling along the inshore edge of the Agulhas Current that is forced onshore at the crest of the meander.

There could be other mechanisms, yet to be discovered. There are just not enough measurements to support any theory and there are no consistent analytical theories against which to test the observations.

Numerical models are being developed which should shed some light on the subject (e.g. Neil Malan’s PhD). However, for real understanding, a robust analytical model needs to be developed.

Physics or applied mathematics students who are interested in finding a solution are encouraged to contact the authors.

References

• Goschen W. S., T. G. Bornman, S. H. P Deyzel & E. H. Schumann (2015). Coastal upwelling on the far eastern Agulhas Bank associated with large meanders in the Agulhas Current. Continental Shelf Research 101: 34-46.
• Krug, M., J. Tournadre and F. Dufois (2014). Interactions between the Agulhas Current and the eastern margin of the Agulhas Bank. Continental Shelf Research 81, 67-79.
• Porri, F., Jackson, J.M., Von der Meden, C.E.O., Weidberg, N., McQuaid, C.D. (2014). The effect of mesoscale oceanographic features on the distribution of mussel larvae along the south coast of South Africa. Journal of Marine Systems 132, 163-173.