南沙海区夏季风期间西南大风的统计分析

Statistic Analysis of Southwest Gale in Summer in the Nansha Sea Area

通过对南沙永暑礁夏季观测资料的统计分析，发现了南沙海区西南大风产生的基本规律，通过分析南沙西南大风与南海和西太平洋热带气旋、辐合带、赤道高压、西南低压、孟加拉湾季风槽、越赤道气流、东风急流等天气系统的关系，得出了几点结论：( 1 ) 10°N 以北南海有热带气旋活动或南海 ITCZ 上的扰动不断加强，作为西南入流气流的通道，南沙海区一般可出现西南大风；( 2 ) 西太平洋台风西移到 125°E 以西，纬度在 15°N～25°N 之间，继续靠近南海且有台风槽向南海伸展时，南沙海区将出现西南大风；( 3 ) 西南低槽的南压加强再结合赤道高压的加强可使南沙海区产生大风；( 4 ) 孟加拉湾低槽前的西南风向东扩展且纬度较低时，南半球低空越赤道气流逐渐加强时以及南海南部高空东风明显增强时，南海可出现西南大风；( 5 ) 只要有以上 4 种形势中的任一种形势出现，再加上观测到云图上南沙海区附近有明显成片的对流云发展时，即可预报南沙有西南大风，其平均风力一般 6 ～ 7 级，阵风 8 ～ 10 级。

Estival meteorologic observational data from Yongshu Reef in the Nansha Islands are statistically analyzed, and then a basic order of the cause to Southwest gale in the Nansha Islands is deduced in this paper. From the ample relationship analysis between Southwest gale in the Nansha Islands and different synoptic systems in SCS ( South China Sea ), such as West Pacific tropic cyclone, belt of convergency, equatorial high, depression in southwest area, the Bay of Bengal monsoon trough, cross-equatorial flow, easterly jet airstream and so on, five invaluable conclusions concerning the birth of Southwest gale in the Nansha Islands are drawn: (1) As a channel of SW inflow, the Southwest gale will commonly arise in the Nansha Islands sea while the tropic cyclone is actue or the disturbance of ITCZ increases in the north of 10°N in SCS. (2) The Southwest gale in the Nansha Islands sea area may arise too when West Pacific typhoon moves to the west of 125°E and lies in the latitude range of 15°N to 25°N and continues getting near to SCS and when the typhoon trough is extending to the SCS. (3) The combinational effects of two different high-pressure systems increasing, as the SW trough and equatorial high, will cause the naissance of Southwest gale in the Nansha Islands sea area. (4) Southwest gale may occur when SW wind in the foreside of the Bay of Bengal trough moves eastwards along low latitude, and the low-altitude cross-equatorial flow in the southern hemisphere comes to be stronger and the east high-altitude wind in the south of SCS increases obviously. (5) So long as one of those four circumstances occurs and when it is clear in the cloud map that there the convection cloud greatly develops over the Nansha Islands area, Southwest gale there may be predicted with a mean wind force 6 to 7 and flurry level 8 to 10.