The relationship between time series sea surface slope and El Nino event from satellite altimetry: a potential indicator

China suffered a serious flooding in the summer of 1998, which is regarded as initiated by the 1997/1998 El Nino. From previous investigators we know that the sea water transportation across Takara Strait is closely related with the occurrence of El Nino event. As the geostrophic current Kuroshio flows almost perpendicularly across the Takara Strait, the net volume transportation at this site may be well described by the sea surface slope. In the present study, decadal monthly averaged sea surface slope across Takara Strait are derived and presented combining the ERS-1,2/ALT measurements of 35-day repeat and the TOPEX/Posidon data of 10 day repeat. It is found that an abnormally high peak takes place in the October of 1996, i.e., one year prior to the 1997/1998 El Nino. To further test the feasibility of the method, the time series sea surface slope across PN section were also examined through which the Kuroshio in the East China Sea goes perpendicularly as well. It is revealed that an abnormally low peak appears exactly in the October of 1996. This is just the opposite to what happens in Takara Strait during the same period. However, the geostrophic transport at PN section is out-of-phase with that at Takara Strait on the basis of field investigations, which shows the two results are virtually in support of each other. Our preliminary results demonstrate the sea surface slope at Takara Strait may be regarded as an index for the upcoming El Nino event. Relevant dynamic mechanisms are discussed