1 Istanbul Technical University / Civil Engineering Department Ercan Kahya Istanbul Technical University

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

3 Istanbul Technical University / Civil Engineering Department ► 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.

4 Istanbul Technical University / Civil Engineering Department  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 alter heat and moisture transport between the Atlantic Ocean and surrounding continents by influencing the number and path of winter storms

5 Istanbul Technical University / Civil Engineering Department  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 20 th century, the NAO abruptly changed to a highly positive phase

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

7 Istanbul Technical University / Civil Engineering Department 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 theeastern United States, Europe, and tropical South America and Africa

8 Istanbul Technical University / Civil Engineering Department TYPICAL CLIMATE & HYDROLOGICAL SIGNALS OF THE NAO OVER THE GLOBE:  A positive correlation between the NAO and the discharge of northern Eurasian rivers  A weak 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 correlated with the December–February NAO index.

9 Istanbul Technical University / Civil Engineering Department

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

11 Istanbul Technical University / Civil Engineering Department

12 Istanbul Technical University / Civil Engineering Department 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.

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

14 Istanbul Technical University / Civil Engineering Department ► The high index winter/springs of 1989, 1990, and 1995, were caused by a net displacement of air from over the Arctic and Icelandic regions towards 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.

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

16 Istanbul Technical University / Civil Engineering Department 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

17 Istanbul Technical University / Civil Engineering Department ► 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.

18 Istanbul Technical University / Civil Engineering Department

19 Istanbul Technical University / Civil Engineering Department NAO vs. Winter Temperature

20 Istanbul Technical University / Civil Engineering Department NAO vs. Winter Precipitation

21 Istanbul Technical University / Civil Engineering Department