By
Mohammad Raza Khan
Chairman APM 

It is defined as at the normalized sea level atmospheric pressure difference between Iceland (65 degree N-23 degree W) and Arctic (38 degree N- 26 degree W).

For positive NAO Index, a stronger than usual Subtropical high pressure center and deeper than normal Icelandic low. Consequently,  north-eastern strong winter winds rush to pole ward and engulfed the polar chill and northern Europe experiences higher temperatures than normal.

Positive Index prevails for 2 to 5 years and peaking at the scale of 12 years that could directly be co-related to the period of the Sunspot Cycle.

During weak Subtropical high pressures and a weak Icelandic low weaker winter cyclones cross the Atlantic and this allows polar chill to leak downward to Europe and Northern USA. It brings cold and comparatively drier air for Scandinavia  and Moist one for the Mediterranean.
 

Sunspot Cycle

Sunspots have a diameter of about 37,000 km and appear as dark spots within the photosphere, the outermost layer of the Sun. The photosphere is about 400 km deep, and provides most of our solar radiation. The layer is about 6,000 degrees Kelvin at the inner boundary and 4,200 K on the outside. The temperature within sunspots is about 4,600 K. The number of sunspots peaks every 11.1 years.

Sunspots minimum refers to the minimum count or even zero count for the darker areas on the surface of the sun.

There have been several periods during which sunspots were rare or absent, most notably the Maunder minimum (1645-1715), Which brought Little Ice Age for the Europe as some scientists believe.

On the other hand, some Scientists are confident from 31 years of accurate, direct measurements of total solar radiation by satellites that there has been no overall increase in the amount of sunlight coming to Earth. Total solar irradiance, as it is called, has stayed remarkably constant and so cannot be held responsible for the warming of the past half century.

Again, as the solar winds induce current in the mesosphere and the resulting fluctuations can cause changes in tropospheric dynamics, so, NAO might be affected.

The wintertime stratospheric mean-flow variations inducing circulations that penetrate lower atmosphere. This downward propagation takes between 15 and 50 days and is most common from November to March.