Diagnostic Analysis On The Formation Of The Abnormal NAM/AO Index During The Winters Of 2007-2008 And 2009-2010

The Northern Hemisphere Annular Mode (NAM), which is also referred to as the Arctic Oscillation (AO), is the leading mode of wintertime variability of sea level pressure over the extropical region of the Northern Hemisphere. The NAM/AO owes its existence entirely to internal atmospheric dynamics, and the wave-flow interaction theory can explain the formations of the NAM/AO and its three regional centers of action. In the troposphere the NAM/AO fluctuates on timescale of 10²0 days and is associated with the interaction of the baroclinic wave and zonal flow, while the fluctuation of the timescale of 30⁶0 days is associated with the interaction of the quasi-stationary planetary wave and zonal flow in the stratosphere. The tropospheric centers of wave-flow interaction are situated at two jets over the Atlantic and the Pacific and represented by the NAO and the PNA. Another stratospheric center is polar vortex that is affected by upwards propagating quasi-stationary waves. The local wave-flow interactions at these three intensive centers together can explain variations of the NAM/AO index to a large extent.According to the results of predecessor research, this paper focused on analysing the abnormal NAM/AO indices during the winters of 2007²008 and 2009²010, when exceptionally serious snow calamities occurred in the south and north part of China respectively. The conclusions are as follows:(1) During the period of 2007-2008 winter, there are both anticyclonic and cyclonic synoptic wave breakings in the North Atlantic. Synoptic-scale waves develop from North America and migrate across the North Atlantic very fast. As a result, there are anomalous vibrations of the NAM/AO index on the timescale of 10²0 days. We highlight a positive-phase event whose zero-lag day is 24^th Jan 2008. On the other hand, the NAM/AO index on the timescale of 30⁶0 days is consistent with the polar vortex index of the same timescale in the stratosphere. And the upward and poleward refraction of planetary waves into the stratosphere over high latitudes of the Northern Hemisphere is weak so that the stratospheric polar vortex begins to strengthen in December 2007 and stay strong in Jan and Feb 2008. In the 2007-2008 winter, La Ni?a occurs in the tropical Pacific, leading to a negative PNA over the North Pacific Ocean and the North American continent, and the Pacific jet exhibiting a pronounced north-shift. This cause the polar waveguide to be weakened, thus, the polar vortex is strong. Consequently, the winter mean NAM/AO index is positive.(2) During the period of 2009-2010 winter, cyclonic synoptic wave breaking is prominent in the North Atlantic and the flow characteristics are strikingly similar to the blocks, which cause the NAM/AO index to stay negative during the winter. We highlight a negative-phase event whose zero-lag day is 3^rd Jan 2010. On the other hand, the NAM/AO index on the timescale of 30⁶0 days is consistent with the polar vortex index of the same timescale in the stratosphere. And the upward and poleward refraction of planetary waves into the stratosphere over high latitudes of the Northern Hemisphere is strong so that the stratospheric polar vortex is weaker. In the 2009-2010 winter, El Ni?o occurs in the tropical Pacific, leading to a negative PNA over the North Pacific Ocean and the North American continent, and the Pacific jet exhibiting a pronounced south-shift. This cause the polar waveguide to be strengthened, thus, the polar vortex is very weak. Consequently, the winter mean NAM/AO index is negative, whose absolute value is great.