| Based on variables such as geopotential height, wind, air temperature from NCEP-NCAR Reanalysis data; HadISST1 Sea surface temperature; OAFLUX heat flux from Woods Hole Oceanographic Institution; monthly NAO index from CPC; Ozone data from ERA-40, the impact of North Atlantic SST anomaly on circulation of NAO-like pattern is analyzed. A wn index of North Atlantic Horseshoe (NAH) SST pattern is first defined as the time series associated with the leading EOF components of the North Atlantic SST anomaly in September. Then, statistical techniques and ECHAM4 global atmospheric general circulation model simulations are used to analyze the relationship between NAH and atmospheric circulation anomalies before (after) September. Results show that:ⅰ) tropical Atlantic SST anomalies (convection anomalies) can invigorate teleconnection wave-trains toward mid-high latitudes; and, different from PNA and PJ wave trains in Pacific, the wave-train originated from tropical Atlantic show phase-lock characteristics in Fall; ⅱ) NAH is significantly correlated with NAO in late phases; ⅲ) Local forcing of NAH on atmosphere combined with the teleconnection wave-trains originated from tropical Atlantic can make a phase switch (e.g., positive to negative) of NAO in fall.Contrast with previous NAH index (defined on MCA analysis of winter NAO and its previous North Atlantic SST anomaly), NAH index in this study is constructed on a relatively independent SST anomaly in early fall. Statistical analysis shows that NAH is not significantly correlated with the triple SST anomaly in winter. Lag regression analyses between NAH and different atmospheric fields show that NAH could be driven by the NAO-like pattern; and, NAH is significantly correlated with atmospheric circulations of negative NAO in later stages. Take NAH as a predictand, ECHAM4 simulations show that the Iceland low of NAO in November can be explained 12% but none in December. ECHAM4 simulations confirm that local forcing of NAH on atmosphere can form a NAO-like anomaly patterns in a short period. The lag regression analyses also show phase shift of NAO in October, which suggests external signals from processes other than NAH at mid-high latitudes.Correlation and composite analyses are used to locate the external forcings, which turns out to be over the Caribbean regions. SST warm anomaly of Caribbean can enhance the local convection to excite Rossby wave propagating to mid-high latitudes, which counteracts to NAH local forcing, leading to the phase shift of NAO.North Atlantic SST anomaly affects NAO and further affects atmosphere over China. Composite analyses of NAH index show that NAH and NAO are negatively correlated. Corresponding to the negative NAO, the sub-polar high anomaly weakens the trough over East Asian and shifts it northward; meanwhile the cold air strength at high latitudes becoming weak, leading to lower temperatures over northeastern China. Both regression analyses and numerical simulations show that the pressure over Northeast China becomes higher when teleconnection wave train is excited over Caribbean and propagates above China in autumn.NAO can affect the distribution of Ozone, especially ozone over the northeast of China. In positive NAO years, less heat flux reach stratosphere leads to a cooler environment, which benefits Ozone depletion. According to Ozone amount over northeast China, sensitivity experiments based on SBDART radiation transfer model show that stratosphere absorption of solar radiation is stronger in summer. Contrast with normal years, in positive NAO years, annual and monthly absorption of solar radiation in 20-50 kilometers layer are stronger at all wavelength and with smaller interannual variability. In ultraviolet band, positive NAO years show dramatic ultraviolet radiation (0.051W/(m2·DU)) in winter due to changes of Ozone. In summer, at least 25.6DU changes of Ozone are required to have radiative forcing. These results can provide references for changes of stratosphere temperatures and design of aircraft. |
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