The Indian Ocean Weakens the ENSO Spring Predictability Barrier: Role of the Indian Ocean Basin and Dipole Modes

Prediction of El Nino-Southern Oscillation (ENSO) is hindered by a spring predictability barrier (SPB). In this paper, we investigate the effects of the Indian Ocean (IO) on the SPB. Using a seasonally varying extended IO-ENSO recharge oscillator model, we find that the SPB is much weakened when IO is coupled with ENSO. To gauge the relative role of the Indian Ocean dipole (IOD) and the Indian Ocean Basin (IOB) modes in weakening ENSO SPB, we develop an empirical dynamical model, the linear inverse model (LIM). By coupling/decoupling the IOB or IOD mode with ENSO, we show that the IOB significantly weakens eastern Pacific and central Pacific ENSO SPBs, while the IOD plays a weaker role. The evolution of the optimum initial structures also illustrates the importance of the IOB in ENSO SPB. Moreover, the IOB strongly influences the forecast skill of La Nina SPB rather than El Nino SPB. This point is also identified through six coupled models from the North American multimodel ensemble. It may be related to the role of the IO in the asymme-try in the duration of El Nino and La Nina. The IOB-induced easterly wind anomalies are conducive to the development of La Nina and thus the prediction of La Nina events, whereas these anomalous easterlies are less important during the de-velopment of El Nino and the related forecast of El Nino events.