Wind changes above warm Agulhas Current eddies

Transcription

1 Wind changes above warm Agulhas Current eddies M. Rouault 1,2, P. Verley 1,3, B. Backeberg 2,4 1 Department of Oceanography, Mare Institute, University of Cape Town 2 Nansen-Tutu Center for Marine Environmental Research 3 Nansen Environmental and Remote Sensing Centre, Bergen, Norway 4 CSIR, Stellenbosch AMSR microwave sea surface temperature estimate

2 Inauguration: 20 May 2010 From the foundation ceremony of the Nansen-Tutu Centre for Marine Environmental Research in Cape Town in May First row from the left: Prof Berit Rokne, Achbishop Desmond Mpilo Tutu and Prof George Philander. Second row from the left: Ambassador Tor Chr. Hildan, Prof Johnny A. Johannessen, Prof Frank Shillington, Dr Neville Sweijd, Mr Hans Erstad, Prof John Field and Dr Lasse Pettersson.

3 Vision The vision of the Nansen-Tutu Centre for Marine Environmental Research is to serve Africa through advancing knowledge of the marine environment and climate system in the spirit of Nobel Peace Laureates Desmond Tutu and Fridtjof Nansen.

4 Research Groups The Centre s research activities are grouped into 4 overarching themes: 1. Ocean modelling and prediction. Group coordinator: Bjorn Backeberg (CSIR/NTC/NERSC), Co-investigators: Marcello Vichi (NTC/UCT), Issufo Halo (CPUT), Charine Collins (SAEON), Johnny A. Johannessen (NERSC), Francois Counillon (NERSC) and Annette Samuelsen (NERSC). 2. Ocean-atmosphere, climate and regional impact. Group coordinator: Mathieu Rouault (NTC/UCT) Co-investigators: Noel Keenlyside (UiB), Marek Ostrowski (IMR) 3. High resolution satellite remote sensing of the regional shelf seas. Group coordinator: Marjolaine Krug (CSIR) Co-investigators: Johnny A. Johannessen (NERSC), Morten Hansen (NERSC), Anton Korosov (NERSC) 4. Regional sea level variability and global change. Group coordinator: Frank Shillington (NTC/UCT) Co-investigators: Ola Johannessen (NERSC)

5 THE SAAMES 3 Cruise, June 1993 PI: Prof Lutjeharms SA AGULHAS

6

7 Ship track and stations during the SAAMES 3 cruise in June Eddy Lesley is found south of the Agulhas Current Rouault and Lutjeharms, 2000

8 Satellitte derived ERS 21 june 1993 wind speed and direction showing a 3 to 4 m/s wind speed increases above the eddy Rouault and Lutjeharms, 1995, 2000

9 Mean sea surface temperature in August 2003 south of Africa estimated with the Advanced Microwave Scanning Radiometer (AMSR) on board the satellite Aqua. Two warm eddies with temperature of about C can be seen centered on 40 S 15 E and 42 S 24 E. They create a strong sea surface temperature gradient with the surrounding ocean.

10 Mean weekly sea surface temperature (contourline) estimated with AMSR and merged seal level anomaly (color) for 6 selected eddies. Positive sea level anomalie indicate cyclonic eddies Time correspond to weekly mean centered at 10/07/2002 for Eddy 1 at 4/09/2002 for Eddy 2 at 19/03/2003 for Eddy 3 at 16/07/2003 for Eddy 4 at 3/09/2003 for Eddy 5 at 5/05/2004 for Eddy 6. Sea level anomaly is in color and sea surface temperature contour is every degree.

11 Mean weekly wind speeds in m.s -1 (color) and directions (arrows) and SST in o C (contours) above the six eddies. Times given correspond to weekly means centered at 10/07/2002 for Eddy 1, at 4/09/2002 for Eddy 2, at 19/03/2003 for Eddy 3, at 16/07/2003 for Eddy 4, at 3/09/2003 for Eddy 5 and at 5/05/2004 for Eddy 6

12 Clockwise from top left, Instantaneous estimate of QuikScat wind speed in m/s (color) and directions (arrows) and AMSR microwave sea surface temperature (contours) south of the Agulhas Current system on the (morning path) (morning path), (evening path) and (evening path) showing strong and homogeneous increase and decrease in wind speed collocated with increases and decrease of sea surface temperature.

13 Figure 9: scatter plot of SST perturbation (DSST) versus wind speed increase (DW) in m.s -1 for the 340 cases when the wind increased above warm core eddies and decrease downstream and for SST perturbation from the surrounding ocean > 1 o C. Solid line is a linear fit. The circle represent the means within each 1 o C bin and the error bars represent +/-1 standard deviation of the wind differences within each bin divided by the square root of the number of sample within each bin. The dashed line is a linear fit of the means within the bins.

14

15 Conclusion When sailing in the Roaring Forty beware of the Agulhas Eddy No linear relationship between DSST and DW for warm Agulhas eddies Increase of up to 4 m/s not uncommon Air sea interaction measuring system onboard SA Agulhas II