Mathijs W. Schouten

Eddies and variability in the Mozambique Channel

Abstract Between 1995 and 2000, on average 4 eddies per year are observed from satellite altimetry to propagate southward through the Mozambique Channel, into the upstream Agulhas region. Further south, these eddies have been found to control the timing and frequency of Agulhas ring shedding. Within the Mozambique Channel, anomalous SSH amplitudes rise to 30 cm , in agreement with in situ measured velocities. Comparison of an observed velocity section with GCM model results shows that the Mozambique Channel eddies in these models are too surface intensified. Also, the number of eddies formed in the models is in disagreement with our observational analysis. Moored current meter measurements observing the passage of three eddies in 2000 are extended to a 5-year time series by referencing the anomalous surface currents estimated from altimeter data to a synoptic LADCP velocity measurement. The results show intermittent eddy passage at the mooring location. A statistical analysis of SSH observations in different parts of the Mozambique Channel shows a southward decrease of the dominant frequency of the variability, going from 7 per year in the extension of the South Equatorial Current north of Madagascar to 4 per year south of Madagascar. The observations suggest that frequency reduction is related to the Rossby waves coming in from the east.

Upstream control of Agulhas Ring shedding

Rings shed in the Agulhas retroflection region play an important role in the global thermohaline circulation. The sheding of these rings has been considered very irregular. In this paper, we present evidence for remote control of the timing and frequency of these events. This turns out to be a far more regular process, at a frequency of 4-5 cycles per year. The movement of the Agulhas retroflection, and thereby the shedding of rings, is timed by incomming eddies from the upstream regions. Eddies from the Mozambique Channel, and from the East Madagascar current reach the Retroflection region at the frequency of 4-5 times per year. The existence of these eddies can be related to incomming Rossby waves that cross the Indian ocean and reach the Agulhas current system. These may in turn be part of a basin wide oscillation. The irregularity found in ring shedding statistics can be ascribed to processes occuring between the actual shedding and the first unamigouos observation of a seperated ring.

Variability of the southwest Indian Ocean.

The variability in the southwest Indian Ocean is connected to the basin–scale and global–scale ocean circulation. Two bands of enhanced variability stretch across the Southern Indian Ocean east of Madagascar around 12○ S and 25○ S, respectively. They mark the preferred routes along which anomalies, generated by varying forcing over the central basin, near the eastern boundary or in the equatorial region, propagate westward as baroclinic Rossby waves. Sea–surface height anomalies pass along the northern tip of Madagascar and are observed by satellite altimetry to propagate into the central Mozambique Channel. There, eddies are subsequently formed that propagate southward into the Agulhas retroflection region. The anomalies along the southern band trigger the formation of large dipolar vortex pairs in the separation region of the East Madagascar Current at the southern tip of the island. South of Africa these eddies and dipoles trigger the shedding of Agulhas Rings that feed the Atlantic meridional overturning circulation with warm, salty, Indian Ocean water. Interannual variability of the forcing over the Indian Ocean, such as that associated with the Indian Ocean Dipole/El Niño climate modes, propagates along these pathways and leads to associated modulations of the eddy transports into the South Atlantic.