Pei-Yu Chueh 2010/7/1

 From 1948 to 2005 for DJF found decreases over the Arctic, Antarctic and North Pacific, an increase over the subtropical North Atlantic, southern Europe and North Africa  The strength of mid-latitude MSLP gradients and associated westerly circulation appears to have increased in both hemispheres, especially during DJF, since at least the late 1970s.

 Nakamura et al. (2002) found a weakening of the North Pacific winter jet since 1987, allowing efficient vertical coupling of upper-level disturbances with the surface temperature gradients.

 poleward shift in storm track location  increased storm intensity  decrease in total storm numbers  Significant decreases in cyclone numbers, and increases in mean cyclone radius and depth over the southern extratropics have been associated with the observed trend in the SAM.

 Blocking events, associated with persistent high- latitude ridging and a displacement of mid- latitude westerly winds lasting typically a week or two, are an important component of total circulation variability on intra-seasonal time scales.  Observations show that in the Euro-Atlantic sector, long-lasting (>10 day) blockings are clearly associated with the negative NAO phase.  Wiedenmann et al. (2002) found that blocking events, especially in the North Pacific region, were significantly weaker during El Niño years.

 The breaking of vertically propagating waves, originating from the troposphere, decelerates the stratospheric westerlies.  This sometimes triggers ‘sudden warmings’ when the westerly polar vortex breaks down with an accompanying warming of the polar stratosphere, which can quickly reverse the latitudinal temperature gradient (Kodera et al., 2000).

 On average, weak vortex conditions in the stratosphere tend to descend to the troposphere and are followed by negative NAM anomalies at the surface for more than two months. negative NAM anomalies

 Visual VOS observations of wind waves for more than a century, often measured as significant wave height(SWH).  Significantly positive almost everywhere in the North Pacific.