North Atlantic Oscillation (NAO)

1. North Atlantic Oscillation (NAO) Ercan Kahya Istanbul Technical University Istanbul Technical University / Civil Engineering Department

2. NAO REFERENCES ► Hydroclimatology: Perspectives and Applications, M.L. SHELTON, Cambridge University Press ► http://www.pnas.org/ ► ► ► Istanbul Technical University / Civil Engineering Department

3. NAO Characteristics ► Dominant mode of winter climate variability in the North Atlantic region ranging from central North America to Europe and much into Northern Asia. ► Large-scale seesaw in atmospheric mass (north–south atmospheric mass oscillations) between the subtropical high and the polar low ► The corresponding index varies from year to year, but also exhibits a tendency to remain in one phase for intervals lasting several years. Istanbul Technical University / Civil Engineering Department

4. NAO Characteristics • This meridional oscillation of atmospheric pressure and winds is considered less pervasive than ENSO • NAO is recognized as a factor influencing weather patterns throughout the Northern Hemisphere • More influential weather factor than ENSO in regions surrounding the North Atlantic Ocean • The atmospheric pressure and wind variations associated with NAO alterheat and moisture transport between the Atlantic Ocean and surrounding continents by influencing the number and path of winter storms Istanbul Technical University / Civil Engineering Department

5. NAO Characteristics • most pronounced during the winter • exhibits a tendency to remain in one phase for intervals lasting less than 15 years • generally positive phase • from about 1900 to 1930 • over the last 30 years of the • 20th century, the NAO abruptly • changed to a highly positive • phase Istanbul Technical University / Civil Engineering Department

6. NAO Characteristics • A SIMPLE WAY OF DESCRIBING MECHANISIMS FOR THE NAO INFLUENCES: • Most directly associated with changes in the surface westerlies across the Atlantic Ocean into Europe • But plays a role in the Northern Hemisphere planetary wave system structure & a factor in defining global atmospheric circulation • The planetary waves  important in defining temperature patterns, storm tracks, and time and space characteristics of precipitation Istanbul Technical University / Civil Engineering Department

7. NAO Characteristics • TYPICAL CLIMATE & HYDROLOGICAL SIGNALS OF THE NAO OVER THE GLOBE: • In addition to Europe, precipitation patterns and winter temperatures in North America and North Africa are attributed to NAO phases • NAO has been related to monsoon rainfall in India • A significant part of the winter variability in NH mid-latitude precipitation is accounted for by NAO • Seasonal NAO variability is evident in streamflow variability in the eastern United States, Europe, and tropical South America and Africa Istanbul Technical University / Civil Engineering Department

8. NAO Characteristics • TYPICAL CLIMATE & HYDROLOGICAL SIGNALS OF THE NAO OVER THE GLOBE: • A positive correlation between the NAO and the discharge of northern Eurasian rivers • Aweak relationship between NAO and Dec–Feb river discharge in north western Europe • A strong relationship between annual runoff and NAO since the early 1960s in Scotland and maritime areas of western England and Wales. • January–March streamflow in the Iberian Peninsula highly correlatedwith the December–February NAO index. Istanbul Technical University / Civil Engineering Department

9. What are the mechanisms that govern NAO variability? ♦ Although the NAO is a natural mode of variability of the atmosphere; Surface, stratospheric or even anthropogenic processes may influence its phase and amplitude. ♦ At present there is no consensus on the process or processes that are responsible for observed low frequency variations in the NAO, including its unprecedented upward trend over past 30 years. ♦ There is ample evidence, which shows that much of the atmospheric circulation variability in the form of the NAO arises from internal atmospheric processes. Istanbul Technical University / Civil Engineering Department

10. NAO index defined as the anomalous difference between the polar low and the subtropical high during the winter season. Istanbul Technical University / Civil Engineering Department

11. Istanbul Technical University / Civil Engineering Department

12. Positive NAO Index • Shows a stronger than usual subtropical high pressure center and a deeper than normal Icelandic low. • The increased pressure difference • more and stronger winter storms • crossing the Atlantic Ocean • on a more northerly track. Istanbul Technical University / Civil Engineering Department

13. Positive NAO Index • Results: • Warm and wet winters in Europe & cold and dry winters in northern Canada and Greenland. • Eastern US experiences mild and wet winter conditions. Istanbul Technical University / Civil Engineering Department

14. Positive NAO Index ► The high index winter/springs of 1989, 1990, and 1995, were caused by a net displacement of air from over the Arcticand Icelandic regionstowards the subtropic belt near the Azores and the Iberian Peninsula, and had strengthened westerlies over the North Atlantic ocean. ► Stronger westerlies bring more warm moist air over the European continent and gives rise to milder maritime winters. Istanbul Technical University / Civil Engineering Department

15. Negative NAO Index • Shows a weak subtropical high and a weakIcelandic low. • The reduced pressure gradient • fewer and weaker winter storms • crossing on a more west-east pathway Istanbul Technical University / Civil Engineering Department

16. Negative NAO Index • Results: • Moist air into the Mediterranean & cold air to northern Europe • The US east coast experiences more snowy weather conditions • Greenland, however, will have milder winter temperatures Istanbul Technical University / Civil Engineering Department

17. Negative NAO Index ► The low index winter/springs of 1917, 1936, 1963, and 1969: weaker mean westerlies over the North Atlantic Ocean with corresponding colder than normal European winters. ► The strengthened or weakened westerlies over the North Atlantic are also known to have major impacts on oceanic ecosystems and ultimately North Atlantic fish stocks. Istanbul Technical University / Civil Engineering Department

18. Istanbul Technical University / Civil Engineering Department

19. Global Temperature and Precipitation Correlations with NAO Index NAO vs. Winter Temperature Istanbul Technical University / Civil Engineering Department

20. Global Temperature and Precipitation Correlations with NAO Index NAO vs. Winter Precipitation Istanbul Technical University / Civil Engineering Department

21. NAO Istanbul Technical University / Civil Engineering Department