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North Atlantic SSTs as a link between wintertime NAO and the following spring climate

North Atlantic SSTs as a link between wintertime NAO and the following spring climate. IVANA HERCEG-BULIĆ. Andrija Mohorovičić Geophysical Institute, University of Zagreb, Croatia Fred Kucharski

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North Atlantic SSTs as a link between wintertime NAO and the following spring climate

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  1. North Atlantic SSTs as a link between wintertime NAO • and the following spring climate • IVANA HERCEG-BULIĆ • Andrija Mohorovičić Geophysical Institute, University of Zagreb, Croatia • Fred Kucharski • ICTP Presentation from paper: North Atlantic SSTs as a link between wintertime NAO and the following spring climate, Herceg Bulic and Kucharski J. Climate, 2013, doi: http://dx.doi.org/10.1175/JCLI-D-12-00273.1, in press.

  2. Some basic (philosophical considerations) What is the North Atlantic Oscillation? (personal opinion) - Certainly NOT an Oscillation Power spectrum (Source: http://www.mit.edu/people/goodmanj/NAOI/index.html) “Peaks” at 2-3 years, 7-10 years, 60-80 years But not very clear and significant, More of a red noise spectrum

  3. It is definitely a pressure sea-saw between Iceland and the Azores (more or less) in Winter

  4. NAO as defined by a principal component analysis applied to DJFM SLP (source: http://www.cgd.ucar.edu/cas/jhurrell/indices.info.html) NAO is therefore the most powerful pattern In the North Atlantic region, And could therefore be referred to as statistical ‘mode’

  5. Is the NAO a ‘regime’? Perhaps….. Probability density functions of NAO From Coppola et al., GRL, 2005

  6. And there is a physical mechanism behind is that relates to transient eddy feedbacks….e.g. Is the North Atlantic Oscillation a Breaking Wave? C. Franzke, S. Lee, S. Feldstein, J. Atmos. Sci., 2004 The simple message of this presentation is that North Atlantic mixed-layer feedback help NAO to persist into following spring, could stop here…

  7. Observed NAO composite based on primciple comonent of first EOF ( absolute value of PC larger than 0.5; data: HadSLP) DJF MAM All Figs. black/white because of J Climate…..

  8. There are many reasons why NAO could be persistent to spring: Atmospheric memory Land-surface interaction Sea-ice interaction Ocean Mixed-layer interaction Probably more...... We will concentrate mainly on d), because its easiest to investigate

  9. Observed (CRU) composites based on NAO first PC (as before) DJF MAM PREC ST

  10. There is vast literature on air-sea interaction in the North Atlantic and Its role in modifying NAO persistence: Bjerknes (1964 Frankignoul 1985; Frankignoul et al. 1998; Kushnir et al. 2002 Mosedale et al. (2006) Bladé 1997; Cayan 1992a, b; Deser and Blackmon 1993; Kushnir 1994;Barsugli and Battisti 1998; Peng and Whitaker 1999; Bretherton and Battisti 2000;Czaja and Marshall 2000; Watanabe and Kimoto 2000; Rodwell and Folland 2002; Kucharski and Molteni 2003; Cassou et al. 2007; Eden and Jung 2000; Eden and Willebrand 2001,Greatbatch and Jung 2007; Visbeck et al. 2003, Herceg-Bulić and Kucharski 2011. Sorry if I forgot your papers in the list…..

  11. Numerical modelling (low-performance computing!): ATL-MXL: ICTP AGCM coupled to a simple slab ocean model ATL-CLIM: ICTP AGCM forced with climatological SSTs ICTP AGCM is an intermediate complexity model with 8 vertical levels and is run here at T30 horizontal resolution (about 3.75x3.75 degrees!!!). The simulations are 150 years long each. Web-page: http://users.ictp.it/~kucharsk/speedy-net.html

  12. Observed SST (HadISST) composited based on NAO PCs DJF MAM

  13. ATL-MXL model SST composited based on NAO PCs DJF MAM

  14. SST index defined as difference in the two boxes: 30°-45°N, 20°-60°W minus 50°-65°N, 20°-60°W NAO in DJF SSTI in respective month ATL-MXL 2 things to note: OBS show some correlation of DJF NAO with leading SST index, ATL-MXL does not b) Largest correlations when NAO leads for OBS and ATL-MXL.

  15. ATL-MXL model responses DJF MAM SLP PRECIP ST

  16. ATL-CLIM model responses DJF MAM SLP PRECIP ST

  17. DIFF in SLP between ATL-MXL and ATL-CLIM Response is not exactly NAO, but projects on it…. but is clearly strongest over continental Europe

  18. Correlation with obs pattern in Europe only in ATL-MXL

  19. Mixed-layer run shows increased NAO persistence, although response is not exactly an NAO DJF NAO with JFM NAO (=1) FMA NAO (=2) MAM NAO(=3), etc.

  20. DJF MAM Variance of Z850 Climatology Possible influence On storm tracks DJF NAO composite of Z850 variance MAM

  21. Conclusions: • - Mixed-layer interactions may help to increase winter NAO persistence • to influence European spring climate. • Should try to use more sophisticated techniques (e.g. cluster, SOM), to check • if the could be interesting subtle changes in spring regime frequency due • to mixed-layer effects?

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