Climate Change Research Centre

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1 2S On Agulhas rings and leakage (and whether it all matters) All trajectories of observational Agulhas buoys 3S 4S Erik van Sebille 1E 2E 3E 4E With input from Lisa Beal, Arne Biastoch, Matthew England, Bill Johns, Peter Jan van Leeuwen, Will de Ruijter and Wilbert Weijer Climate Change Research Centre

2 Why is measuring Agulhas leakage so difficult? Schematic of the current systems around the Agulhas region Time-mean velocity across the GoodHope section in a model Depth [m] From PhD thesis Van Sebille, Offshore distance [km] 5 Van Sebille et al., 21, JGR

3 Tracking the water: Lagrangian analysis in an ocean model Some of the most recent ocean models have a decent representation of Agulhas dynamics However, the data from these models is 1s of TeraBytes In its most simple form, Lagrangian analysis is the integration of the velocity field x(t) =x() + Z t u( )d Typically solved using a 4th order Runge-Kutta method for the integration Most appropriate way to analyse Agulhas leakage in that case in in the Lagrangian framework Lagrangian floats in an idealised Agulhas ring Follow the path of the water parcels Also very apt to study the role of Agulhas rings in Agulhas leakage

4 Releasing particles in a numerical model Using the 1/1 degree NEMO model A subset of all particles released Total of 5.6 million particles are released between 1968 and 24 (37 years) Particles are released every five days according to transport distribution in Agulhas Current Particles are released throughout entire water column Particles are tracked for five years or until outside domain Movie available at

5 Obtaining a time series of Agulhas leakage Separating particles by end basin 3S 4S Agulhas leakage F AL [Sv] E 2E 3E 4E The time series of transport across the GoodHope line Year [AD] Van Sebille et al., 21, Ocean Sci 1E 2E 3E 4E Latitude [degrees] 35S 4S Van Sebille et al., 21, JGR Pathway of particles in upper 5 m 15E 2E 25E Longitude [degrees] Van Sebille et al., 21, JGR

6 Upstream control of Agulhas leakage water Float destiny at 19E 2 F Beal et al [26]. 5. A cartoon of the source regions and pathways of water masses that converge into the 4 A zipper mechanism? Depth [m] Atlantic Ocean Indian Ocean 39S 38S 37S 36S 35S Latitude [degrees] Van Sebille et al., 21, JGR

7 ,., What happens to Agulhas rings in the Cape Basin?.8 I I I I I I Byrne et al, I I I I I I time from shedding (months) Schouten et al, 2 ¼ Dencausse et al, 21 Froyland et al, 212

8 How much Agulhas leakage is carried within rings? Latitude [degrees] 32S 34S 36S 38S 4S 42S 44S 6E 8E 1E 12E 14E 16E 18E 2E 22E 24E 26E Longitude [degrees] Fraction of floats within vorticity range [%] Snapshot of drifter density Van Sebille et al., 21, JGR Sv s] o E Floats at GoodHope line Time after crossing 19E [days] Temporal evolution of relative vorticity Anticyclonic water Cyclonic water Nonrotating water Van Sebille et al., 21, JGR s ] Distribution of relative vorticity Van Sebille et al., 21, JGR

9 Agulhas rings and the flow of NADW 15.9 ±1 12.1±9 1.7 ±7 4 ±2 2.7 ±5 6.5 ±3 9.7 ±5 4.8 ±3? 7.5 ± ±4 3 7 Stommel [1958] Arhan et al [23]

10 Releasing particles in the DWBC off Brazil Advecting floats Using the deep velocities in the Japanese OFES model Horizontal resolution of.1, 54 vertical layers Model integrated for 27 years Data available each 3 days 1, floats released in DWBC between 1 m and 35 m depth, using the RSMAS CMS code Floats advected for 2 years, cycling through velocity fields Trajectories of 1% of floats for 25% of integration time Movie shows first 45 years of 1% of floats

11 The deep zonal jet from particles Transport by floats through each grid cell 6.4 latitude [degrees] 5S 1S 15S 2S 25S 3S 35S 4S S 5W 4W 3W 2W 1W 1E longitude [degrees] Van Sebille et al., 212, JGR

12 Relating the pathway to Agulhas ring decay The mid-latitude NADW pathway and the Agulhas ring corridor SSH variability (colors, in cm 2 ) and connectivity (lines) 5 5S 1S 4 latitude [degrees] 15S 2S 25S 3S S 4S 45S 5W 4W 3W 2W 1W 1E longitude [degrees] 1 Van Sebille et al., 212, JGR

13 A relation between Agulhas leakage and AMOC? From paleoceanographic proxies, we observe relations between sudden changes in Agulhas leakage and sudden changes in AMOC strength * * * Conversion Surface currents Deep currents of surface to deep waters However, it is unclear whether the two are related outside of such strong events In high-resolution models, signals seem to propagate but quickly get damped Furthermore, these high-res models are often ocean-only So how about the latest generation of coupled climate models? Warm surface water Cold, deep water Gulf Stream Subsurface pressure waves Salt leakage Agulhas current Subtropical front Subtropical gyre Zahn [29] Together with Wilbert Weijer, we therefore set out to study the relation between Agulhas leakage and AMOC strength in the control run of the CCSM4 coupled climate model

14 Understanding Cape Basin salinity variability Anomaly in Cape Basin salinity 1 ref S 1 Coherence analysis of time series vs. V ag (red), and F S (blue) Anomaly in Lagrangian salt flux K Frequency (cpy)

15 Coherence of the Cape basin salinity signal 1 ref S 1 Coherence between Cape Basin salinity and AMOC vs. AMOC at 15N (black), 15S (red), and 3S (blue) K Frequency (cpy) 1 ref Coherence between Nino3.4 and other time series Nino 3.4 vs. S 1 (black), AMOC at 26N (red), and AMOC at 3S (blue) K Frequency (cpy)

16 A connected optimum pathway? Optimum pathways using salinity correlations Mean correlation to each gridpoint with optimum path for upper 1m signal 6N 5N 4N 3N 2N 1N 1S 2S Correlation along path b) Mean correlation along path = Correlation along path Lag [years] 3S 4S 6W 4W 2W 2E.1

17 The unrealistic Agulhas leakage in CCSM4 Time series of Agulhas leakage in CCSM4 Upper 1 m salinity in obs and model Agulhas Current transport [Sv] Leakage across 21S [Sv] Year [AD] Weijer et al., 212 GoodHope leakage [Sv] latitude latitude a) CARS b) CCSM Longitude (psu) Path density of Agulhas leakage in CCSM4 latitude [degrees] Density of numerical float trajectories [Sv] 25S 3S 35S 4S 45S 6W 5W 4W 3W 2W 1W 1E 2E 3E 4E longitude [degrees]

18 Conclusions Agulhas leakage can most aptly be assessed in a Lagrangian framework, where Indian Ocean water parcels are tracked into the South Atlantic Fraction of floats within vorticity range [%] Time after crossing 19E [days] Anticyclonic water Cyclonic water Nonrotating water Agulhas rings decay fast, so most Agulhas leakage is not within rings beyond the Cape Basin. This decay, however, does control the deep flow of NADW Agulhas rings are probably not a good proxy of Agulhas leakage, as the rings decay quickly and don t carry all leakage N N N N N N S S In one of the most widely-used coupled climate models, the CCSM4, internal variability of Agulhas leakage has no clear impact on AMOC strength S S For more information, visit