A new subsurface precursor across the spring predictability barrier for the ENSO prediction

Despite decades of research on forecasting the El Ni & ntilde;o-Southern Oscillation (ENSO) in the past decades, skillfully predicting the ENSO across the spring predictability barrier remains challenging. In this study, we utilize the ensemble of four model products to identify a new subsurface precursor consisting of the anomalous equatorial zonal velocity over 180 degrees-150 degrees W and 140-180 m and potential temperature in the western Pacific (over 120 degrees-140 degrees E and 120-160 m), which could predict the ENSO before spring by applying regression analysis. Such a precursor can predict the Ni & ntilde;o 3.4 index in December at a lead time of 13 months with a regressed correlation of 0.79. The anomalous subsurface zonal velocity in the central Pacific favors the eastward migration of the warm water during the development of El Ni & ntilde;o. Further validations with diverse training and application periods indicate that for initialization in November, this new precursor shows high skills in predicting the Ni & ntilde;o 3.4 index at a lead time of 6-18 months over 1993-2016, outperforming counterparts for the other traditional precursors. Our study can help highlight the importance of subsurface processes in the ENSO development, improving our general ability to predict the ENSO, and enabling better preparedness for the implications of its occurrence.