Intra-seasonal and Annual variability of the Agulhas Current from satellite observations

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1 Intra-seasonal and Annual variability of the Agulhas Current from satellite observations Marjolaine Krug Ecosystem Earth Observation (CSIR NRE) Pierrick Penven Laboratoire de Physique des Océans (IRD) Jean Tournadre Laboratoire d'océanographie Spatiale (IFREMER) --> Thanks to US Office of Naval Research for funding registration

2 The Agulhas Current system The northern Agulhas Current is connected to the wind driven circulation of the South Indian Ocean through its source regions in the Mozambique Channel and south of Madagascar. Large anticyclonic eddies from the source regions can lead to the formation of Natal Pulses (De Ruijter 1999, Tsugawa 2010). These Natal Pulses have been thought to drive most of the variability north of the Retroflection. The seasonality of the Agulhas Current has been a matter of debate: numerical ocean model show an annual cycle which has not been confirmed in satellite or in-situ observations. 2

3 Approach The Agulhas Current is a strong and narrow flow which is well captured in satellite observations of Sea Surface Temperature (SST) and geostrophic currents (derived from the altimeters Sea Surface Height (SSH) measurement). We use the distinct thermal and SSH signature of the Agulhas Current to: 1) Track the Agulhas Current s path 2) Assess changes in the strength of the Agulhas Current flow 3

4 Data Altimetry Close to 20 years of data available 10 to 35 day repeat cycle along particular track Only large (>50km) and fairly persistent (>1 week) features of the circulation can be mapped with altimetry. Altimeters not good within 50km off the coast due to land contamination and large uncertainties in geophysical corrections Microwave SST and Tracks from the Jason 1, Jason 2 and Envisat altimeters over a 1 week period ( to ) 4

5 Data Infra Red SST Observations of SST from space have been available for close to 30 years, making SST one of the preferred remote sensing variable to study climate change. High frequency acquisitions from MSG-2 SEVIRI significantly improve our imaging capacity in the Agulhas Current Use OSI-SAF hourly SST dataset from CERSAT Available since June 2006 at a 5km resolution But cloud contamination of IR-SST images is a major problem in the region. Over the Agulhas Current core, about 5 times as much water vapor is transferred to the atmosphere in comparisons to neighboring waters (Rouault et al. 2000). The Agulhas Retroflection also constitutes one of the most significant region of heat flux loss globally (Lutjeharms, 2006). Climatology of the number of monthly SST observations available from the Pathfinfer 5.2 (a) and the hourly Seviri dataset (b). (Picture from F. Dufois) 5

6 Tracking the Agulhas Current with SST Compute 3 day moving averages derived from the Seviri hourly dataset Use local peak maximum to detect core Identify position of inshore front as maximum SST gradient between core position and coast Define Natal Pulse as offshore deviation in the path of the Agulhas Current > 30 km from mean position within 10-day low-passed time-series. Example of algorithm output for Agulhas Current path detection. Green dots show position northern wall on 18 March

7 Tracking the Agulhas Current with SST No Annual cycle seen in position of Agulhas Current. Variability in the position of the Agulhas Current inshore front is not the same everywhere. There is an increase in the number of Natal Pulses from northern Natal Bight to Port Edward. There is a decrease in the number of Natal Pulses south of Port Edward. Only 1.6 Natal Pulses per year reach the southern Agulhas region (Port Elizabeth transect). This is in contradiction to estimates of 4 to 6 Natal Pulses per year derived from numerical models. Why do so few pulses reach the southern Agulhas Current? 7

8 Tracking the Agulhas Current with SST Upstream instabilities are triggered during the passage of Natal Pulses. These instabilities show as meanders upstream of Natal Pulses. During their southward progression, adjacent meanders (including Natal Pulses) can evolve rapidly to 1) dissipate, 2) merge or in some rare occasion, 3) detach from the Agulhas Current 1) Dissipation Natal Pulse interacts with topography to generate upstream instability at the inshore Agulhas Current front. Initial instability dissipates away from expanding upstream instability Note the potential influence of offshore anti-cyclonic eddy on the evolution of Agulhas Current meander 8

9 Tracking the Agulhas Current with SST 1b) Dissipation 9

10 Tracking the Agulhas Current with SST 2) Merging Natal Pulse interacts with topography to generate upstream instability at the inshore Agulhas Current front. Upstream and initial instabilities merge Again we see the potential influence of offshore anticyclonic eddy on the evolution of Natal Pulse 10

11 Tracking the Agulhas Current with SST 3) Detachment Natal Pulse interacts with topography to generate upstream instability at the inshore Agulhas Current front. Initial instabilities becomes occluded eddy and moves offshore Again potential influence of offshore anticyclonic eddy on the evolution of Agulhas Current meander 11

12 Tracking the Agulhas Current with Altimetry In the southern Agulhas Current (south of 34S), altimetry can be used to track the position of the Agulhas Current quite successfully. Analysis conducted with SST and altimetry both show increased number of meanders south of Port Elizabeth. This makes region of Port Elizabeth well suited to track Agulhas Current. Using cross-stream coordinate approach we look at the intrinsic variability of the geostrophic current flow within the Agulhas Current offshore Port Elizabeth. 12

13 Variations in the strength of the Agulhas Current s flow The absolute geostrophic velocities derived from the AVISO merged product at the position of the Agulhas Current core (varying) have a distinct annual cycle Geostrophic current speeds within Agulhas Current core (grey). Black line shows current speeds low-passed with a cut-off at 300 days. Wavelet transform for geostrophic current speed within Agulhas Current core 13

14 Variations in the strength of the Agulhas Current s flow Climatology shows peak in Feb-March and through in Jul-Aug in agreement with oceanic numerical model results. Monthly climatology for geostrophic current speed within the Agulhas Current core Spectra for geostrophic current speed within Agulhas Current core 14

15 Variations in the strength of the Agulhas Current s flow Analysis conducted on a Gausian Amplitude fit to along-track (1Hz) geostrophic velocities (a) produces similar results. 15

16 Conclusions Variations in the position of the Agulhas Current are dominated by the intermittent passage of large offshore meanders with no visible annual cycle. The number of observed large offshore meanders decreases from 28S to 34S and then increases again south of Port Elizabeth, making the area offshore Port Elizabeth an attractive region to monitor the variability of the Agulhas Current. The non-linear evolution of large meanders in the northern Agulhas Current is a possible indication of a direct cascade of energy from the westward propagating ocean eddies towards the smaller scales, in agreement with the findings of Xiaoming et al (2010). Natal Pulses are much more complex than previously anticipated and more research is required to better understand how energy is lost from Natal Pulse to surrounding flow and what the influence of offshore eddies on Natal Pulses are. Do ocean models accurately represent the dynamics of Natal Pulses? The pre-supposed links between Natal Pulses and Agulhas Rings formation must be re-examined. Within the Agulhas Current proper, it is the annual cycle that dominates the variability with a stronger flow in austral summer. More research / data is needed to link the altimetry signal to volume transports in the Agulhas Current. For more info look at Rouault and Penven, JGR-Oceans (2011) and Krug and Tournadre, GRL (2012) 16