资源环境科技发展态势分析平台

The 1925 El Nino (EN) event was the third strongest in the twentieth century according to its impacts in the far-eastern Pacific (FEP) associated with severe rainfall and flooding in coastal northern Peru and Ecuador in February-April 1925. In this study we gathered and synthesised a large diversity of in situ observations to provide a new assessment of this event from a modern perspective. In contrast to the extreme 1982-1983 and 1997-1998 events, this very strong coastal El Nino in early 1925 was characterised by warm conditions in the FEP, but cool conditions elsewhere in the central Pacific. Hydrographic and tide-gauge data indicate that downwelling equatorial Kelvin waves had little role in its initiation. Instead, ship data indicate anabrupt onset of strong northerly winds across the equator and the strengthening/weakening of the intertropical convergence zones (ITCZ) south/north of the equator. Observations indicate lack of external atmospheric forcing by the Panama gap jet and the south Pacific anticyclone and suggest that the coupled ocean-atmosphere feedback dynamics associated with the ITCZs, northerly winds, and the north-south SST asymmetry in the FEP lead to the enhancement of the seasonal cycle that produced this EN event. We propose that the cold conditions in the western-central equatorial Pacific, through its teleconnection effects on the FEP, helped destabilize the ITCZ and enhanced the meridional ocean-atmosphere feedback, as well as helping produce the very strong coastal rainfall. This is indicated by the nonlinear relation between the Piura river record at 5 degrees S and the SST difference between the FEP and the western-central equatorial Pacific, a stability proxy. In summary, there are two types of EN events with very strong impacts in the FEP, both apparently associated with nonlinear convective feedbacks but with very different dynamics: the very strong warm ENSO events like 1982-1983 and 1997-1998, and the very strong coastal EN events like 1925.