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Why La Nina may be ahead: Flip to negative phase of the Pacific Meridional Mode

Discussion: The Pacific meridional mode (PMM) has flipped to the negative phase. Indicated is cross-equatorial wind blowing from the northern to southern hemisphere. During this process the anomalous warm water present just north of the equatorial region in the East Pacific much of the past 1-2 years and sometimes causing ENSO to look borderline El Nino-like tends to cool off. During well-established La Nina events (such as 2007-09 and 2010-12) the PMM is steadily negative. Forecast models are indicating La Nina risk for later in 2020. The –PMM regime change may be the first surface-based climate diagnostic in support of the possible La Nina.

Fig. 1: The monthly Pacific meridional mode (divided by 10) compared to the Nino34 SSTA for 2018 to present. The consensus of most dynamic and statistical models Nino34 forecast is added. If –PMM persists La Nina is more likely.