Abstract
The strongest observed global climate variation on the interannual time scale is the El Nino - Southern Oscillation (ENSO) phenomenon, which is an irregular interannual oscillation resulting primarily from a coupled ocean-atmosphere interaction in the tropical Pacific. Since 70's the El Nino events have been stronger than the La Nina both in frequency and amplitude. Perhaps it links with the change in the global climate basic state. Three El Nino events happened in 1990's are different to these of 1980's on this account the forecast skill dropped down greatly. What are the characteristics of 90's ENSO? In this paper, sea surface temperature (SST), wind stress and OLR data are diagnosed to find the reasons of 90's ENSO anomalous development.
1. There are three El Nino events and one La Nina event from 1990 up to 1996. Having filtered from the raw SST data out 2-7 years ENSO cycle and lower frequency components, one can see the dominant anomalous occurrence on the lower frequency band. The SST anomaly persisted for long time in tropical central Pacific rather than the ENSO time scale component.
2. Having investigated the behavior of Nino3 and Nino4 indexes by wavelet analysis, it has been found that the period of ENSO cycle gets shorter during 1990's and the amplitude is weaker than that of 80's. The long-time linear trends of climate change happened in about recently 50 years. There is a significant climate change at 1976 from cold state to warm state. Interdecadal climate variation becomes more intensive during 90's. With the linear trend and the decadal time-scale changes together interacted, the climate condition may shefted to a warmer state with a turning point near 1990.
3. The 90's average climate state in the tropic pacific is different from the 80's. The main features of the wind pattern is the westerly anomaly at low level increasing the convection occurring near the date line and long-time positive SSTA appearing in the tropical central Pacific. This background is one of the reasons why the 90's ENSO anomalous development in significant contrast to the 80's