The Agulhas Current is the strongest western boundary current in the Southern Hemisphere and plays an important role in the exchange of heat and salt between the Indian and South Atlantic Ocean basins, thereby affecting global climate. The variability in the northern Agulhas Current is influenced by both cyclonic and anticyclonic mesoscale eddies, originating from the Mozambique Channel and south of Madagascar (known as source region eddies) and which propagate toward the offshore edge of the Agulhas Current. Using a combination of an eddy-tracking data set with in-situ surface drifter observations and altimetry-derived geostrophic currents, it is shown that source region eddies dissipate upon approaching the Agulhas Current. Their entrainment into the Agulhas Current affects its mean velocity and offshore position through a transfer of momentum, with anti-cyclonic eddies consistently increasing the Agulhas Current’s velocity by 0.16 ± 0.17 m.s -1 . In contrast, entrainment of cyclonic eddies results in a decrease in velocity by 0.13 ± 0.16 m.s-1 and shifting the current up to 144 ± 85 km offshore. These velocity anomalies propagate downstream at rates of 44 km.d-1 (anti-cyclonic eddies) and 23 km.d-1 (cyclonic eddies). Whilst existing numerical models are successfully able to capture many aspects of the Agulhas Current, many models are unable to accurately represent the observed eddy dissipation and interaction processes, affecting our understanding of mesoscale variability within in the current. In this study, we compare two simulation experiments in a regional Hybrid Coordinate Ocean Model (HYCOM), where we change the wind forcing, and using an eddy tracking algorithm assess the local effect of the changed wind stress on source region eddies and their interaction with the northern Agulhas Current. There is an overall reduction in eddy kinetic energy (EKE) of 33% over the Agulhas Current domain. Changes in eddy pathways, properties and energy conversion terms, resulting from the change in forcing from absolute to relative winds (the wind speed relative to the current speed) have resulted in significantly different mesoscale eddies in the regional HYCOM. The effects of the change in wind forcing on the variability within the Agulhas Current were examined and the differences between the two simulations were found to be very small. Finally, the evolution of meanders in the Agulhas Current, including the properties and dissipation of smaller meanders as well as mesoscale Natal Pulses type meanders, were assessed using both HYCOM experiments and compared to satellite observations. The representation of smaller meanders (under 50km in size) improved with the changed in wind forcing. However, larger Agulhas Current meanders (greater than or equal to 50km) which previously occurred too frequently in the regional HYCOM, are now too infrequent in the regional HYCOM, with an average of 1.1 meanders occurring each year. A decrease in the frequency of larger meanders was observed from the location offshore of Port Edward (30.22° E, 31.05° S) to the region of the ACT array (27.48° E, 33.35° S), in the satellite data as well as both model experiments, indicating that some of the meanders have dissipated and that both regional HYCOM models are able to resolve this.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/31213 |
| Date | 20 February 2020 |
| Creators | Braby, Laura |
| Contributors | Backeberg Björn, Krug, Marjolaine, Reason, Christopher |
| Publisher | Faculty of Science, Department of Oceanography |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Doctoral Thesis, Doctoral, PhD |
| Format | application/pdf |
Page generated in 0.0022 seconds