PDO is Neutral BUT is the Longterm Cycle Changing to Warm Phase?

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Since the 1990's the Pacific decadal oscillation has mostly been in the cool phase until 2014-16 when one of the strongest warm events on record occurred. During the past 9 months PDO has eased back to neutral phase. Is the lack of any cool phase of PDO months in 4 years suggest the long-term PDO cycle is changing? If so, there are implications for U.S. precipitation.

Is the long-term ENSO/PDO cycle changing? If so, Implications for U.S. Precipitation

Discussion: Yesterday the Mantua Pacific decadal oscillation (PDO) report for April 2018 was +0.11 which is neutral phase. During the past 9 months PDO has been uncharacteristically neutral following a sharp warm event in 2014-2016 and preceded by an equally impressive cool event in 2010-2012. The last time near neutral PDO index over a 9-month period occurred was more than 10 years ago (Fig. 1).

Fig. 1: The 2017-18 near neutral PDO index was last observed in 2006-07.

Interestingly, the descent in the PDO index from one of the strongest positive phase events on record in 2014-2016 not reaching the cool phase implies that the long-term PDO climate may have indeed reversed to the warm cycle (Fig. 2).

Fig. 2: Has long-term PDO reversed to the warm cycle in recent years?

The PDO decadal cycle has a tendency to parallel El Nino southern oscillation (ENSO) events. A flip of the long-term PDO cycle to the warm phase implies above average frequency of El Nino.

During the 1976-1994 warm cycle of PDO the U.S. averaged wetter than normal in the Southwest, South-Central, Midwest and Northeast States (Fig. 3). The West Coast (north of San Francisco) was dry. We may be returning to this much wetter national climate.

Also interesting is the winter 2018-19 precipitation climate outlook issued yesterday by NOAA/CPC indicating a similar wet depiction to the possible ENSO/PDO cylce change in their forecast (Fig. 4).

Fig. 3: U.S. precipitation anomalies during the 1976-1994 warm ENSO/PDO.

Fig. 4: The NOAA/CPC probabilistic precipitation outlook for next winter.