Topic Area

LCDR Marc Touchton, USN

Natal pulses are large solitary meanders that form in the region of the Natal Bight.  Generation of pulses is caused by the shallowing of the shelf slope near Durban.  An increase of the Agulhas Current flow allows barotropic instability and the formation of the cyclonic meander.  Cross-correlation with ring shedding events shows strong evidence that Natal Pulses are responsible for the Agulhas rings.  Evidence also shows Natal Pulses causing an early retroflection of the Agulhas Current.

Important aspects of the Agulhas Current and Natal Pulses:

• Agulhas is the WBC of the Southern Indian Ocean.
• Agulhas is ~100 km wide.
• Agulhas is ~1500 m deep.
• Agulhas surface velocity is 1.6 m/s and exceeds 2.5 m/s in most months.
• Agulhas tranport is 70 Sv increasing to 95-135 Sv over the Agulhas Bank.
• Agulhas is very stable and only varies by +/- 15 km over much of its path.
• Agulhas retroflection propagates westward at 7-15 km per day.
• Agulhas rings are 200-280 km wide, 1100 m deep, swirl velocity of 0.6 m/s, transport 3-15 Sv and drift NW at 5-8 km per day.
• ~6 rings are shed from the Agulhas each year.
• Natal Pulses grow laterally from 30 km to 200 km.
• Natal Pulses propagate at 10-20 km per day, slowing to ~5 km per day over the Agulhas Bank.
• They can be tracked via satellite altimetry and infrared imagery.
• Strong correlation between Natal Pulses and Agulhas Ring shedding occurs at 165 days.
• Natal Pulses will sometime interact with a Rossby-wave like meander of the Agulhas Return Current and cause an early retroflection.

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Figure 1. Infrared imagery of  a Natal Pulse. (from Leeuwen et. al.)

Figure 2. Cross-correlation diagram between Natal Pulses and ring shedding.  (from Leeuwan et. al.)