Warm Phase of the Pacific Decadal Oscillation Fading

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After a brief period of moderately intense warm phase of the Pacific decadal oscillation neutral phase has returned. During the past 2 decades the character of PDO has changed...weaker and shorter duration events where as previous patterns had a tendency to lock into warm or cool phase with intensity.

Highlight: Long-standing warm phase eases.

Fig. 1-2: The Pacific SSTA analysis reveals a weak warm anomaly in the northeast Pacific and neutral SSTA north of Hawaii indicating the warm phase of PDO in 2019 is fading. The 30-day change indicates the cooler SSTA trend.

Discussion: What is the Pacific decadal oscillation? Coined Pacific decadal oscillation in 1996 after scientists discovered the influence of a long-term climate pattern in the Pacific profoundly affecting various fisheries particularly salmon the PDO is a decadal pattern of ocean surface temperature variability north of 20N having important influence on North America climate sometimes exceeding the El Nino southern oscillation (ENSO). During the positive phase (warm temperatures off the west coast of North America) the winter season is warm in the West U.S. while the East tends to be cold. During the negative phase (cool temperatures off the west coast of North America) the reverse is true. The positive phase has an El Nino-like precipitation pattern enhancing California/Southwest U.S. winter precipitation while dryness prevails for much of the southern U.S. during the negative phase. Coupled with ENSO (i.e. positive phase with El Nino and negative phase with La Nina) the PDO “effects” are further enhanced. PDO and ENSO commonly parallel each other particularly in stronger phases.

PDO is a multi-decadal oscillation. From the late 1940’s to middle 1970’s the PDO was mostly in the cool phase. From the late 1970’s to middle 1990’s PDO was mostly in the warm phase. A flip to the cool phase was proposed in the late 1990’s, part of the natural decadal cycle change. However since 2000 the PDO has been variable and for the most part long duration intense episodes have been absent. Similarly, only 3 intense ENSO episodes have occurred during the past 20 years.

Proposed is the general warming of the global oceans since late last century is defeating the tendency of PDO to settle in one phase and instead causing high variability and weaker events.

In August 2019 the recent warm phase of PDO may have ended according the Dr. Nate Mantua of the NOAA Southwest Fisheries Science Center. Using the PDO tendencies of the past 20 years to produce an analog forecast of the PDO index through next summer reveals the most likely regime is neutral maintaining the pattern of lacking strong PDO events. The projection of neutral phase also matches the prevailing ENSO outlooks which also indicate lack of La Nina or El Nino into 2020.

A transition to neutral PDO while ENSO is also neutral has a tendency to cause the climate to defeat other regional SSTA patterns as having large influence on climate. An example is the Atlantic multi-decadal oscillation which is in the positive phase but may also weaken due to the high variability in climate favored by neutral PDO/ENSO regimes.

Forecast confidence of climate predictions tends to be lower than average when PDO, ENSO and AMO shift toward or are in the neutral phase and that pattern seems to be emerging in later 2019.

Fig. 3: The PDO analog forecast indicates neutral phase is ahead through northern hemisphere summer 2020. Neutral PDO means increased risk of highly fluctuating climate patterns across North America.

Fig. 4: The PDO index for 2000-2019 indicates variable PDO regimes. On a multi-decadal basis PDO is usually in a persistent cool or warm phase but the past 20 years has been variable possibly caused by the general warmer ocean surface since late last century.

Fig. 5: The long-term PDO cycles including the 1950-1976 cool phase and 1977-1997 warm phase and what was thought to be a return to cool phase in the late 1990’s.