The North Atlantic Oscillation (NAO) is one of the most prominent and recurrent patterns of atmospheric circulation variability. It dictates climate variability from the eastern seaboard of the United States to Siberia and from the Arctic to the subtropical Atlantic, especially during boreal winter, so variations in the NAO are important to society and for the environment.
On average we find a surface pressure dipole over the North Atlantic Ocean. A low pressure is centered between Greenland and the United Kingdom (known as the Icelandic Low), and high pressure is centered to the southwest of Europe (known as the Azores High). The NAO index measures the departure from this normal pressure pattern between the two pressure centres.
Neutral Phase :-
The average NAO conditions result in prevailing westerly flow into western Europe, which has a moderating effect on surface temperatures. The Atlantic ocean is relatively cool compared to continental Europe in summer, and so a westerly wind has a cooling influence and prevents excessive heat events. In winter, the Atlantic ocean is relatively warm compared to continental Europe, and so a westerly wind has a warming influence and keeps the colder weather at bay.
Positive phase :-
When the pressure difference between the Icelandic Low and the Azores high is greater than normal, the North Atlantic Oscillation is said to be positive. A positive NAO leads to a stronger westerly flow into Western Europe that is the more positive for moderating effect.
They are also associated with above-normal precipitation over northern Europe and Scandinavia and below-normal precipitation over southern and central Europe. There is also a dipole evident of the Central US, with the North dry and the South wet
Negative Phase :–
Conversely, when the pressure difference between the Icelandic Low and the Azores high is smaller than normal, the North Atlantic Oscillation is said to be negative. A negative NAO leads to a weaker westerly flow into Western Europe, that is the less moderating effect.
The weak westerly flow over the Atlantic sector prevents mild oceanic airmasses being driven into North America, Europe, and much of Russia, and thus these areas are, on average, colder than normal. Northern Europe is typically dry and calm, while Southern Europe is typically wet and windy. There is also a dipole over the Midwestern US where the North is wet and the South is dry.
NAO and Stratospheric Polar Vortex (SPV) :-
High above the tropospheric polar vortex, a vortex also forms in the thin stratospheric air each winter. The Stratospheric Polar Vortex (SPV) develops in the fall and dissipates in the spring, but otherwise behaves much like the tropospheric vortex in the sense that spins from west to east (counter-clockwise) when looking down on the North Pole. The SPV is important because it can influence the North Atlantic Oscillation below.
When the SPV is stronger than normal it tends to support a positive North Atlantic Oscillation. The reverse is also true; a weaker than normal SPV is more likely to coincide with a negative North Atlantic Oscillation. Around every two years, the SPV temporarily collapses in a process known as sudden stratospheric warming. When this happens, the North Atlantic Oscillation often shifts more negative for anywhere between a few weeks and a few months.
REFERENCE :-
- https://www.worldclimateservice.com/2021/08/26/north-atlantic-oscillation/
- https://www.ncei.noaa.gov/access/monitoring/nao/
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