SPATIAL AND TEMPORAL VARIATIONS OF INTERNAL WAVE SEA SURFACE SIGNATURES IN THE NORTHERN SOUTH CHINA SEA STUDIED BY SPACEBORNE SAR IMAGERY

摘要

In this paper the spatial and temporal variationsrnof sea surface signatures of the internal waves inrnthe northern South China Sea and theirrnrelationships with thermocline and wind arerndiscussed. A total of 344 synthetic aperture radarrn(SAR) images from ERS-1/2, ENVISAT andrnRADARSAT-1 taken over 13 years betweenrn1995 and 2007 are used for the observations ofrnsea surface signatures of the internal waves. Thernthermocline calculated from the Letvitus datarnand the wind speed estimated from thernBlendQscat data are utilized for the study of thernvariations of the internal waves with thernthermocline and wind speed. The results showrnthat the internal waves are mainly distributedrnbetween 114°E to 120°E with a high percentagernof 73. About 26% of internal waves arernconcentrated within a range from 110°E torn114°E while the other 1% of internal wavesrnappear within a range from 120°E to 123°E. Thernoccurrence of the internal waves reaches itsrnseasonal maximum with a percentage of 70 inrnsummer and has its seasonal minimum with arnpercentage of 1 in winter. Spring and autumnrnappear to be transit periods with percentages ofrn18 and 11 respectively. The thermocline exits allrnyear round indicating that it is possible to formrninternal waves all the time. The strong decreasernof the thermocline depth from 119°E to 115°Ernwhere there is a high occurrence probability ofrninternal wave sea surface signatures mightrnsuggest that the strong shallowing of thernthermocline provides the boundary condition forrnthe internal wave generation. About 96% of therninternal waves are observed by SAR under windrnspeeds of 10 m/s while only 4% of internalrnwaves are imaged above wind speeds of 11 m/s.rnThe higher wind speeds are in autumn andrnwinter, the lower probability of the internalrnwaves observed by SAR, suggesting that thernlower occurrence of internal waves in autumnrnand winter is partly due to the instrument used tornobserve the internal waves.