1. Taher A. Sharif Department of Atmospheric science, University of AlFateh, Tripoli, Libya Taher.sharif@gmail.com

2. Outline North Atlantic oscillation Overview Correlation of NAO index with seasonal rainfall over Libya Summary 2 MedCLIVAR Wokshop Trieste Oct 2008

3. Definition of North Atlantic Oscillation The North Atlantic Oscillation (NAO) establishes a link between the intensity of the Icelandic low pressure and Azores high pressure. It is traditionally quantified by an index defined as the difference between normalized SLP anomalies in the Azores and Icelandic regions, respectively. 3 MedCLIVAR Wokshop Trieste Oct 2008

4. NAO Index 4 MedCLIVAR Wokshop Trieste Oct 2008

5. North Atlantic Variability Climate variability in the North Atlantic sector, comprise three primary interrelated phenomena: Tropical Atlantic Variability (TAV) North Atlantic Oscillation (NAO) Atlantic Meridional Overturning Circulation (MOC) 5 MedCLIVAR Wokshop Trieste Oct 2008

6. Other Important Actors Arctic Oscillation (AO) Blocking High (a persistent anomalous high pressure over Northern Europe) Atlantic Ridge (Northward extension of Azores High pressure) 6 MedCLIVAR Wokshop Trieste Oct 2008

7. Normal Sea-level pressure for January 7 MedCLIVAR Wokshop Trieste Oct 2008

8. Normal sea-level pressure for July 8 MedCLIVAR Wokshop Trieste Oct 2008

9. NAO exhibits two main anomalies of pressure cores of opposite signs, Extending respectively from Iceland to Spitzberg and from center of the Atlantic to the Iberian peninsula. 9 MedCLIVAR Wokshop Trieste Oct 2008 Sea Level Pressure

10. 10 MedCLIVAR Wokshop Trieste Oct 2008

11. Positive NAO Deepening of the Icelandic Low (negative SLP anomalies over the Icelandic region throughout the Arctic). Strengthening and eastward extension of the Azores High (positive SLP anomalies over the Azores region) 11 MedCLIVAR Wokshop Trieste Oct 2008

12. Positive NAO 12 MedCLIVAR Wokshop Trieste Oct 2008

13. Features Associated with Positive NAO Stronger than average westerlies across middle latitudes in the Atlantic and western Europe. Colder winter conditions dominate the northwest Atlantic. Warmer weather over Europe. Storm tracks are shifted to the north with wetter conditions from Iceland through Scandinavia. Dryer conditions over south-western Europe and the Mediterranean Basin. 13 MedCLIVAR Wokshop Trieste Oct 2008

14. Weakening of the Icelandic Low ( positive SLP anomalies over the Icelandic region throughout the Arctic). Weakening the Azores High (negative SLP anomalies over the Azores region). MedCLIVAR Wokshop Trieste Oct 2008 14 Negative NAO

15. 15 MedCLIVAR Wokshop Trieste Oct 2008

16. Weaker than average westerlies across middle latitudes in the Atlantic and western Europe. Warmer winter conditions dominate the northwest Atlantic. cooler weather over Europe. Storm tracks are shifted to the south with dryer conditions from Iceland through Scandinavia. Wetter conditions over south-western Europe and the Mediterranean Basin. Features Associated with Negative NAO 16 MedCLIVAR Wokshop Trieste Oct 2008

17. Standardized seasonal Mean (JFM) NAO index (1950-2008) 17 MedCLIVAR Wokshop Trieste Oct 2008

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19. Correlation between monthly NAO index and October to November Total Rainfall for Tripoli 19 MedCLIVAR Wokshop Trieste Oct 2008

20. Fig 1: The Correlation Between August NAO Index and October to February Rainfall 20 MedCLIVAR Wokshop Trieste Oct 2008

21. Fig 2: The Correlation Between August NAO Index and November to February Rainfall 21 MedCLIVAR Wokshop Trieste Oct 2008

22. Fig 3: The Correlation Between August NAO Index and December to February Rainfall 22 MedCLIVAR Wokshop Trieste Oct 2008

23. Fig 4: The Correlation Between August NAO Index and January to February Rainfall 23 MedCLIVAR Wokshop Trieste Oct 2008

24. On-going work Nowadays scientists are involved in a major research program focusing on Atlantic climate variability in the context of CLIVAR. The main objectives of the program are: To describe and model coupled atmosphere-ocean-land interactions in the Atlantic sector, quantify there influence on the regional and global climate system, and determine their predictability. To assemble quantitative historical and real time data sets that may be used to test, improve and initialize models of coupled Atlantic climate variability. To investigate the sensitivity of the MOC to changes in the surface forcing and assess the likelihood of abrupt climate change. 24 MedCLIVAR Wokshop Trieste Oct 2008

25. Summary NAO is a very important synoptic feature in predicting climate over Europe and Mediterranean regions. Rainfall over Libya is significantly correlated with NAO index. It increases when the NAO is in negative phase. NAO variability seems to be a possible predictor for statistical prediction of climate over Europe and Mediterranean regions. The Standing mean of Icelandic low, Azores high and Siberian high pressure are natural synoptic controller systems That should be preserved for any climate prediction modelling over Europe and Mediterranean. 25 MedCLIVAR Wokshop Trieste Oct 2008

26. Thank you MedCLIVAR Wokshop Trieste Oct 2008 26