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08/06/2020, 9:50 am EDT

Pacific Decadal Oscillation Outlook

Highlight: Changeable PDO SSTA pattern averages neutral phase ahead. Fig. 1: The Climate Impact Company analog forecast for the Pacific decadal oscillation through May 2021. Discussion: The Pacific decadal oscillation (PDO) has been mostly neutral since mid-2017 and remains neutral during mid-2020. Coincidentally ENSO has also been mostly neutral during the past 3 years. There is a tendency for PDO and ENSO to run parallel. If PDO flips into the cool or warm phase on average ENSO has also turned warm (El Nino) or cool (La Nina) usually about 3 months ahead of the PDO change. The Climate Impact Company analog forecast is made with below average forecast confidence. The analogs average neutral phase ahead and two analog years are actually in the neutral phase through next May. But there are two outliers indicating a 25% chance of both a cool or warm phase. Analogs and forecast models are indicating a La Nina ahead for the last quarter of 2020. If so, PDO will need to trend toward (at least) a weak cool phase. The current Pacific SSTA identifies why PDO is neutral. Both comparison areas (north of Hawaii and in the northeast Pacific) are very warm. The lack of SSTA differential between the two regions produces neutral PDO. However, this is not a conventional neutral PDO due the extraordinary warmth of the central and northeast Pacific Ocean. Note the 30-day change which suggests PDO is likely to shift cooler/warmer (and average neutral). The northeast Pacific has warmed sharply the past 30 days while waters north of Hawaii have cooled. Implied is a trend toward weak warm phase. However, another warm anomaly is shifting east and located in the mid-latitude West Pacific. The current trend could easily reverse in 1-2 months. The end of 2020 global SSTA forecast by the ECMWF indicates PDO is likely neutral to (very) weak negative phase and that trend is required if La Nina in the eastern equatorial Pacific develops as indicated. Fig. 2-3: The current Pacific Ocean SSTA pattern and the 30-day change. Summary: A very warm central and northeast Pacific Ocean ascribes to neutral Pacific decadal oscillation. Diagnostics reveal PDO is likely to fluctuate the next 1-3 months. However, a trend toward weak negative phase should develop later this year given the La Nina forecast. The Climate Impact Company analog forecast favors neutral phase. The dynamic models favor either neutral phase or very weak cool PDO. Due to the lack of strong PDO character the influence on climate is likely minimal. However, there is a tendency for a ridge of high pressure across and north very warm SSTA regions which would place Alaska in that risk. Fig. 4: The global SSTA forecast by ECMWF for December 2020 and annotations of ENSO, PDO, AMO and IOD phase.