Analysis On Climatic Characteristics Of Sea Fog On The Coast Of South China And Air Sea Temperature Difference In The South China Sea

The appropriate air sea temperature difference(ASTD) is a significant factor which related to the formation of sea fog closely. The climatic characteristics of sea fog were analyzed through statistical analysis and composite analysis by using the fog day data of 21 stations on the coast of South China and the ERA-Interim data. Besides, the spatial distribution characteristics of ASTD in the South China Sea(SCS) from 1979 to 2013 were compared by using the ERA-Interim and ICOADS data, the variation characteristics of spatial and temporal distribution of ASTD in the SCS were discussed by using the empirical orthogonal function(EOF) decomposition and wavelet analysis methods. The results obtained as follows:(1) Sea fog on the coast of South China appeared frequently in the spring and winter.During these two seasons, accumulated fog day from 1967 to 2014 in different month on the coast of Western Guangdong, was commonly higher than that on the coast of Pearl River Delta or Eastern Guangdong, not including May. The fog day during winter was generally more than spring on the coast of Western Guangdong. It became opposite on the coast of Pearl River Delta and Eastern Guangdong. Sea fog was more easily appeared in Leizhou and Zhanjiang than the others during spring and winter. The characteristic of standardized accumulated fog day from January to April mainly presented as March greater than February, and February more than January. Sea fog on the coast of South China rarely appeared in the summer and autumn. It was worth noting that the sea fog season for Suixi was not only including spring and winter, but also autumn included. For most stations on the coast of South China, the accumulated fog day went through the twist that from more to less during the end of 1980 s, or the early and middle 1990 s, and the tendency keeps declining till now. Besides, the trend being increasing from the 21 st century in Zhanjiang,Wuchuan, and Shangchuan Station.(2) In the more year of fog day on the coast of Western or Eastern South China or Peal River Delta, the SST was warmer and the air temperature was colder than climate state in the adjacent area of coast of South China, it resulted in the ASTD been lower than climatic state about 0-1 degree. When the ASTD became lower, sea fog appears more easily. The relative humidity was positive anomaly from lower levels(from 1000 h Pa to 950 h Pa) tomiddle levels(from 975 h Pa to 850 h Pa) on the coast of Western South China, it’s positive in lower levels and became negative in middle levels on the coast of Eastern South China and Peal Rive Delta. On the coast of Western South China, the humidity condition equaled a thick wet layer, and it would contributed to forming sea fog and increasing fog days in this area, but on the coast of Eastern South China and Peal Rive Delta, the humidity condition equaled a thin wet layer, it would contributed to forming sea fog, but the effect to increasing the fog days would not been significant. Similarly, the flux of water vapor transport mainly appeared in southern direction and higher value on the coast of Western or Eastern South China, this resulted in the water vapor in tropical marine being more easily to transport to the coast of South China, it would lead to the sea fog process being persistent more easily. For the Peal Rive Delta sea area, the direction of the flux of water vapor transport mainly appeared in southern, too, but the value was so small that nearly zero, so it would not being so helpful for the persistence of sea fog and the rising of fog days.(3) Both ERA-Interim and ICOADS data could reflect the actual distribution characteristics of ASTD in the SCS, but the value of ASTD with the ERA-Interim data was lower than that of the ICOADS data in the same region. In addition, the characteristics of ASTD were not obvious inshore with the ERA-Interim data.(4) The distribution of ASTD in the SCS appeared a negative peak in the south and a positive peak in the north in April and May, while a positive peak in the south and a negative peak in the north in November. There was a seesaw type distribution of ASTD between the south of Beibu Bay and the bulk part of the SCS in summer. And the ASTD reached the peak distribution during 1995, 1998, 2010 in summer. The ASTD reached the peak distribution during 1998, 2010 in autumn. Moreover, there were two high value centers among the SCS. The primary center was situated in the Nansha Island area in autumn, the secondary center located in the Taiwan island adjacent area. The ASTD in the SCS was under significant quasi-3a oscillation period in all seasons, and it also had a significant quasi-5a oscillation period in spring. The ASTD positively correlated to the Ni?o3.4 index in summer and autumn, but negative correlated to it in spring and winter.