The Agulhas Current system: its dynamics and climatic importance BJØRN BACKEBERG 1 NANSEN-TUTU CENTRE FOR MARINE ENVIRONMENTAL RESEARCH 2 DEPTARTMENT OF OCEANOGRAPHY, UNIVERSITY OF CAPE TOWN Nansen Centers 25 years Colloquium in honour of Fridtjov Nansens 150 year anniversary 25 YEARS COLLOQUIUM, 18 TH NOVEMBER
Outline Introducing the Agulhas Current system Subtropical Indian Ocean Aspects of its dynamics Agulhas leakage debate Why study the Agulhas system? Recent changes observed from the satellite altimeter record. What are the drivers of these changes? In the context of previous studies? Main conclusions.
The Agulhas Current system van Sebille et al., 2009 Penven et al., Boebel et al., 2003 Westward propagating retroflection loop & Agulhas Ring occlusion Eddies in its source regions Lutjeharms, 2006 Sources: Recirculation SEC (incl ITF) via MZC & EMC Intense oceanic turbulence the Cape Cauldron Debate over leakage: (1) Weak AC 70 transport Sv Ł stronger leakage (2) Increased winds Ł increased Variability AC predominantly transport Ł increased leakage associated with eddies & South of Africa meanders. current retroflects Lutjeharms and van Ballegooyen, 1988b
Importance of the Agulhas leakage Salt injection into South Atlantic via Agulhas rings is five times larger than Mediterranean Outflow. Salinity increase into North Atlantic from 1960-present Suggested that increased warm & salty flux may compensate a slowdown of MOC due to increased freshening in North Atlantic Biastoch et al., 2009 Trajectories of virtual floats released along the GoodHope section with T 10ºC
Impact on the global climate Agulhas leakage now recognised as key component in global climate. Paleo-climatic record indicates that: 1. Agulhas leakage shutdown associated with extreme glacial events 2. Vigorous increase preceded shifts towards interglacials. Thought that Agulhas leakage closely associated with changes in MOC Beal et al.,
Intensification of the subtropical gyre? Decadal sea surface height trend (1993-2009) SSH increase in subtropical gyre, with decrease north of Madagascar Zonal average highlights the general increase of SSH in subtropical gyre Indicates that from 1993-2009 no southward shift of the gyre occurs Enhanced pressure gradients associated with trends imply intensification of the gyre
Increased trade winds and westerlies Decadal ERA-interim wind stress trend (1993-2009) Intensification of extra-tropical westerly winds and subtropical trade winds. For the period 1993-2009, there does not appear to a southward shift of the westerlies. Increased winds may be related to intensification of Hadley circulation associated with increased SSTs & rainfall in Indo-Pacific warm pool causing increased pressure gradients.
Can winds account for SSH pattern? Sverdrup SSH derived decadal decadal trend : trend 1993 :- 1993 2009-2009 positive trend Zonal mean wind stress curl has positive trend between 8º and 30ºS. SSH trend derived from Sverdup relation and wind stress curl in good agreement with altimetry observed SSH trend pattern. Evidence that SSH trends are determined by the long term wind changes. 25 YEARS COLLOQUIUM, 18 TH NOVEMBER
Impact on circulation and mesoscale variability Increased large scale pressure gradients reflected in mean surface geostrophic flow and mesoscale variability. Changes predominantly in WBC. Large positive EKE trend in MZC, SEMC, and AR. Trends are statistically significant. 25 YEARS COLLOQUIUM, 18 TH NOVEMBER
Quantifying changes in mesoscale variability Implement eddy-tracking scheme. Combines the Okubo-Weiss and closed-contour methods Provide information on: o Position o Amplitude o Radius o U and V component drift velocities Using these information we can study trends Detected eddies in MZC
Impacts on eddies in the Agulhas system Eddy kinetic energy decadal trend (cm 2 s- 2 / decade) Mozambique Channel Slight increased cyclonic eddy diameters. Significant acceleration of poleward moving anticyclones (110% of the mean) 25 YEARS COLLOQUIUM, 18 TH NOVEMBER
Impacts on eddies in the Agulhas system Eddy kinetic energy decadal trend (cm 2 s- 2 / decade) South of Madagascar Increased amplitudes and radii of anticyclonic & cyclonic eddies. Accelerated westward propagation of anticyclonic eddies. 25 YEARS COLLOQUIUM, 18 TH NOVEMBER
Impacts on eddies in the Agulhas system Mean Eddy kinetic energy decadal trend (cm 2 s- 2 / decade) Agulhas Current Increasing EKE trend with faster southwestward propagating eddies. Concurrent decreasing MKE implies energy shift. 25 YEARS COLLOQUIUM, 18 TH NOVEMBER
Impacts on eddies in the Agulhas system Eddy kinetic energy decadal trend (cm 2 s- 2 / decade) Agulhas retroflection Increased northward propagation velocity of anticyclonic eddies (130% of the mean). Enhanced intrusion of Agulhas rings into South Atlantic 25 YEARS COLLOQUIUM, 18 TH NOVEMBER
Modelled increase of Agulhas leakage in recent years Altimetry observation are consistent with previous modelling studies. Increased leakage attributed to poleward shift of Southern Hemisphere westerlies. Not observed between 1993-2009. Subtropical Indian Ocean gyre needs to be considered together with Southern Hemisphere westerlies. Studies of the leakage have relied heavily on ocean models. Biastoch et al., 2009 Increased westward transport in Agulhas leakage
Sensitivity of numerical models Model simulations are sensitive to numerical choices. In the Agulhas this is due to the high levels of oceanic turbulence. Models are not able to accurately represent the retroflection. Snapshot of model vorticity Eddy kinetic energy in different models Backeberg et al., 2009 Barnier et al., 2006
Summary and conclusions Agulhas system is a highly turbulent region. Through the Agulhas leakage it is recognised to be a key component in the global climate. Models and now observations indicate that the leakage has increased in recent years. A changing leakage has been shown to have implications for glaciations associated with the MOC. It has now been shown that changes in the subtropical Indian Ocean need to be considered in addition to the Southern Hemisphere westerlies when studying the Agulhas leakage and the MOC. Models and observations need to be combined in order to advance our understanding of this complicated system. The Agulhas is the perfect natural laboratory in which to test state-of-the-art advancements in modelling and observations
Acknowledgements Natural laboratory Large signals High velocities Co-authors: Pierrick Penven and Mathieu Rouault Marine-Research Institute at the University of Cape Town for travel support. This work has been supported by the Nansen-Tutu Centre for Marine Environmental Research and the International Centre for Education, Marine Strong gradients and Atmospheric Sciences over Africa. Challenging to model Eddy rich The altimeter data products were produced by Ssalto/Duacs and distributed by Aviso, with support from Cnes (http://www.aviso.oceanobs.com/duacs/). The ERA-interim data was obtained from http://www.ecmwf.int