Use of water vapor transport field for deriving a new ENSO index

A new ENSO index was constructed by using the divergent component of water vapor transport field (Q_D) derived from both satellite observations and reanalysis data. Introducing an effective wind (V_E) to remove the effect of trend in the total precipitable water (W), the divergent component of effective wind (V_E) can be obtained from V_E=Q_D/W, and its associated divergence (∇•V_E) and effective velocity potential (Φ_E) are readily calculated from the obtained V_E and a relationship of (∇•V_E = -∇²Φ_E), respectively. The obtained effective winds (V_E) and the convergence of effective wind (-∇•V_E) are then used for calculating ENSO index that is referred to as the “ENSO effective wind index (EEI)”. The EEI is constructed by using the east-west component of effective wind as averaged over the tropical Pacific Ocean, and compared to three traditional ENSO indices, i.e., the Multivariate ENSO Index (MEI), the Southern Oscillation Index (SOI), and NINO 3.4 index. Correlations between EEI and MEI, SOI, and NINO 3.4 index range from about 0.77, 0.63, and 0.75, respectively. Time-lag correlations showed that the EEI leads other three indices by one to two month. The results suggested that the EEI derived from water vapor flux can be tools for monitoring ENSO, and can also be useful for analyzing past and future ENSO events.