Highlight: Cool phase of the PDO is brief. Neutral AMO returns to weak warm phase in 2021.
Fig. 1: The Climate Impact Company analog forecast for 2021 of the Pacific decadal oscillation.
Fig. 2: The Climate Impact Company analog forecast for 2021 of the Atlantic multi-decadal oscillation.
Pacific decadal oscillation discussion: During recent months a cool phase of the Pacific decadal oscillation (-PDO) has developed and strengthened in November (Fig. 1). The moderate -PDO is NOT classic as waters in the northeast Pacific are quite warm but less warm (near normal) near the immediate North America West Coast qualifying the SSTA pattern to represent -PDO conditions. The developing -PDO is no surprise given the presence of La Nina. Normally, -PDO runs parallel to La Nina (about 80% of the time in the 1950-2020 climatology).
The warm SSTA pattern across much of the remainder of the mid-latitude North Pacific basin does not bode well for -PDO continuing (or intensifying). The analog forecast indicates the moderate cool phase is short-lived and weakens (with La Nina) during quarter 1 of 2021 becoming neutral phase for quarter 2 through 4 of 2021.
PDO regimes have a tendency to strongly influence Northwest U.S. climate with the opposite regime in the Southeast States. For instance, the -PDO during winter favors above normal precipitation for the Northwest States while the Southeast is likely dry.
The decaying La Nina likely later in 2021 contributes to the -PDO dissipation by April. The ENSO/PDO regime is not likely to be a great mode of influence on North America climate after the winter season is complete.
Atlantic multi-decadal oscillation discussion: The warm phase of the AMO – a significant contributor to the record-breaking North Atlantic tropical cyclone season – faded to neutral phase in November (Fig. 2). The neutral phase is expected to continue through the first third of 2021 then rewarm to weak warm phase for the 2021 North Atlantic tropical cyclone season. The analog years are split between very warm or neutral AMO index for the 2021 tropical cyclone season therefore forecast confidence is below average. However, most years in the long-term warm cycle of the AMO (since the mid-to-late 1990’s) have featured +AMO regimes for summer/autumn.
The PDO/AMO combination is an influencer on U.S. precipitation and drought risk. Dry conditions in the Midwest U.S./Great Plains, Northeast U.S. and Western States late last warm season were enhanced by the PDO/AMO regime. Right now its too early to project the influence of PDO/AMO on the 2021 warm season climate.