North Atlantic Oscillation (NAO)

1Istanbul Technical University / Civil Engineering Department

North Atlantic Oscillation(NAO)

Ercan KahyaIstanbul Technical University

2Istanbul Technical University / Civil Engineering Department

NAOREFERENCES

  ► Hydroclimatology: Perspectives and Applications, M.L. SHELTON,

   Cambridge University Press ► http://www.pnas.org/

►►►

3Istanbul Technical University / Civil Engineering Department

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. 

4Istanbul 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 

NAO Characteristics

5Istanbul 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 20th century, the NAO abruptly changed to a highly positive phase 

NAO Characteristics

6Istanbul Technical University / Civil Engineering Department

NAO Characteristics 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

7Istanbul Technical University / Civil Engineering Department

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 

8Istanbul Technical University / Civil Engineering Department

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

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. 

9Istanbul Technical University / Civil Engineering Department

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.

10Istanbul Technical University / Civil Engineering Department

NAO indexdefined as the anomalous difference between the polar low and the subtropical high during the winter season. 

11Istanbul Technical University / Civil Engineering Department

12Istanbul Technical University / Civil Engineering Department

Positive NAO IndexShows 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.

13Istanbul Technical University / Civil Engineering Department

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.

14Istanbul Technical University / Civil Engineering Department

Positive NAO Index ► 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.

15Istanbul Technical University / Civil Engineering Department

Negative NAO IndexShows 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 

16Istanbul Technical University / Civil Engineering Department

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

17Istanbul Technical University / Civil Engineering Department

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.

18Istanbul Technical University / Civil Engineering Department

19Istanbul Technical University / Civil Engineering Department

Global Temperature and Precipitation Correlations with NAO Index 

NAO vs. Winter Temperature

20Istanbul Technical University / Civil Engineering Department

Global Temperature and Precipitation Correlations with NAO Index 

NAO vs. Winter Precipitation

21Istanbul Technical University / Civil Engineering Department

NAO