Strong Extratropical Impact on Observed ENSO

Previous studies have indicated that the extratropics can influence ENSO via specific processes. However, it is still unclear to what extent ENSO is influenced by the extratropics in observation. Now we assess this issue by applying the regional data assimilation (RDA) approach in an advanced model, the GFDL CM2.1. Our study confirms a strong ex-tratropical impact on observed ENSO. Quantitatively, the extratropical atmospheric variability poleward of 20 degrees explains 56% of the observed variance of ENSO and greatly influences ;67% of observed El Nifio events during 1969-2008. This extratropical impact is still significant even as far as poleward of 30 degrees. Furthermore, the impact from the southern extra -tropics is slightly stronger than that from the northern extratropics, partly caused by the Pacific ITCZ location north of the equator and different mixed-layer depth along the northern Pacific meridional mode (NPMM) and the southern Pacific meridional mode (SPMM). Our study further shows that all of three super El Nifio events, those in 1972/73, 1982/83, and 1997/98, are influenced greatly by both hemispheric extratropics, with NPMM and SPMM interfering constructively, while most weak and moderate El Nifio events are triggered by only one hemispheric extratropics, with NPMM and SPMM interfering destructively. Besides the extratropical Pacific influence on ENSO via NPMM/SPMM, the extratropics also has a potential impact on ENSO by influencing other tropical oceans and then by interbasin interactions.