Bring on weak El Nino: Since the mid-to-late 1990’s the long term cycle of ENSO/Pacific decadal oscillation (PDO) has favored the cool phase while Atlantic multi-decadal oscillation (AMO) has favored the warm phase. Within that 20-year climatology the ENSO regime has produced weak La Nina early in the year on 4 occasions plus 2018 makes five. In each analog year La Nina dissipated quickly in early spring and was followed by weak El Nino by mid-summer according to multivariate ENSO index (Fig. 1). The climatology is too strong to ignore.
Fig. 1: Using MEI the 2018-19 ENSO phase forecast reveals an El Nino.
Is there diagnostics present to support this claim? Yes! A strong Kelvin Wave is moving east into the subsurface equatorial East Pacific (Fig. 2) and should dissipate La Nina in April. This is the second Kelvin Wave of 2018. If a third Kelvin Wave follows later this spring into early summer El Nino will form by July. The set of circumstance required to produce El Nino by mid-summer have started and the MEI analogs strongly support the trend toward El Nino. This change in ENSO philosophy will be applied to all climate forecasts. The U.S. summer outlook will be issued tomorrow.
Fig. 2: Second Kelvin Wave of 2018 moving east to dissipate La Nina.
