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Impact of upstream anomalies in the Agulhas Current system on the inter-ocean exchange and primary production around Southern Africa

Final Activity Report Summary - IAC IOCEX (Impact of upstream anomalies in the Agulhas Current system on the inter-ocean exchange and primary production around Southern Africa)

The inter-ocean exchange south of South Africa is recognised to be a key link in the maintenance of the global overturning circulation. The Agulhas Current system plays a critical role in this process through the large Agulhas rings that are spawned at the Agulhas retroflection. Perturbations of the Agulhas Current behaviour are triggered by flow disturbances coming from the source regions of the current known as the Mozambique Channel, the East Madagascar Current and the recirculation gyre in the southwest Indian Ocean. This latter source is supposed to have a large influence on the dynamic of the Agulhas Current and so, on the variability of its retroflection farther downstream. It is therefore important to analyse the dynamical behaviour of the southwestern part of the subtropical gyre in the Indian Ocean.

Our proposal aims to investigate the spatial and temporal variability of the recirculation gyre in the southwest Indian Ocean and to determine the causes responsible for such behaviour. Using multi-remote sensing data sets of sea level anomalies (T/P-ERS2-Jason-GFO-Envisat), sea surface temperature (AVHRR) and Ocean Colour (SeaWiFS) provided by CTOH / LEGOS in Toulouse, we monitored the upstream region of the Agulhas retroflection to clearly identify the nature of the tight recirculation cell at around 30E.

We show that propagating eddies seem to cluster in the upstream region of the Agulhas retroflection between the Agulhas Plateau and the Mozambique Plateau where they subsequently dissipate or merge into larger features. Eddies trapped in this area trigger a tight recirculation cell characterized by a mean flow of around 0.1-0.3 m.s-1. Tracking of drifter trajectories simulated by a Langrangian drifter model for a six-year period confirmed this result. A 1D wavelet analysis has been performed on the time series of multi-sensors satellite data sets in order to examine the dominant periods of the observed variability in strategic areas such as the recirculation cell area, the Agulhas retroflection, the Cape Cauldron region downstream.

We found that the recirculation gyre in south-western Indian Ocean presents a significant variability on a timescale of 90-100 days (3 months) and 200-230 days (7-8 months). Scale averaged variance shows that the highest variability in the upstream region occurs in 1995, 2001, 2003 and 2005 respectively suggesting a connection between the climate modes (Indian Ocean Dipole / El Niño) and the interannual variability of the south-western Indian Ocean.

Then, using a multichannel singular spectrum analysis, we mainly focused on the propagating modes of sea surface height anomalies variability. Coherent propagating structures both in time and space are identified and the connection between the various source regions of the Agulhas Current and the Agulhas retroflection far downstream is clearly visible.

Finally, we have shown the existence of a large-scale coherent signal at subannual timescale in the southern Indian Ocean. We confirmed that the dominant variability observed in the Agulhas retroflection is tightly connected to impulses coming from the Mozambique Channel and from the southeast of Madagascar at periods between 70-90 days (approximately 4-5 times/yr).

Our investigation reveals for the first time the presence of a dominant variability with a characteristic period of 135 days (approximately 2.5 times/yr). We found that westward propagating baroclinic instabilities seem to generate the sub-annual variability at both 135 and 90 days periods in the south-western Indian Ocean whereas the significant low-frequency variability, with a period of 7-8 months, is associated to the ocean basin mode.