An Update on the Pacific Decadal Oscillation/Atlantic Multidecadal Oscillation

Cool phase of the Pacific decadal oscillation likely to continue although weaker

Warm phase Atlantic multi-decadal oscillation strengthens in 2022

-PDO/+AMO = Increased drought risk for Southwest to Midwest U.S.

+AMO/+TNA support increased strength of hurricanes in 2022

Fig. 1: The Climate Impact Company constructed analog forecast of the Pacific decadal oscillation through 2022.

Discussion: The Pacific decadal oscillation (PDO) shifted into the cool phase in early 2020 remaining in-place into 2022 except stronger during recent months. The 2-year longevity was last observed in 2010-13 when 3+ years of -PDO persisted. Currently, -PDO remains quite strong and implications by the constructed analog forecast is that another 3-year -PDO regime is on the horizon (Fig. 1).

-PDO has a tendency to parallel La Nina. The projection of a weaker -PDO for 2022 implies El Nino is less likely and La Nina could hang on longer than forecast. In the U.S., -PDO climate has a tendency to bring above normal precipitation to the Northwest U.S. and cause dry-to-drought conditions in the Southeast States.

On the West Coast of the U.S., a -PDO regime has a tendency to deliver a semi-permanent upper trough pattern which would lower heat and dry risk. However, given the strength of the warm SSTA in the North Pacific near the Dateline to north of Hawaii and knowns as the “warm blob” the ability for a cooling and/or wet trough to form on the West Coast especially once the warm season arrives is near zero.

In the North Atlantic basin, the SSTA pattern is steady warmer than normal including the deep tropics implying warm phase of the Atlantic multi-decadal oscillation (+AMO) and tropical North Atlantic (+TNA) index is present. Since the mid-to-late 1990’s the +AMO/+TNA pattern is present about 70-75% of the time due the shift into a warmer North Atlantic regime about 25 years ago. The constructed analog forecast reveals the combination of AMO and TNA produce a warm to very warm phase for 2022, warmest during the core of the tropical cyclone season (Fig. 2). Presence of +AMO and +TNA fuels upper ocean heat for abundant and strong tropical cyclones.

Fig. 2: The Climate Impact Company constructed analog forecast of the Atlantic multi-decadal oscillation combining the tropical North Atlantic index.

The ECMWF global SSTA forecast for July 2022 reveals the weak -PDO signature and the moderate-to-strong +AMO/+TNA regime (Fig. 3). Note that despite lingering -PDO the ECMWF indicates onset of a developing El Nino in the eastern equatorial Pacific Ocean. If El Nino were to develop later this year, the -PDO regime would likely dissipate.

A semi-permanent cool pool of SSTA is projected south of Greenland by mid-summer. This feature is semi-permanent over the past 10 years. The cool pool is referred to as the North Atlantic warm hole (NAWH) due to the uniqueness of this cool anomaly located in the generally warmer than normal North Atlantic basin during the past 20+ years.

The NAWH is an important feature. Aloft, there is a tendency for a low-pressure trough to persist during the warm season. This trough forces upstream upper-level high-pressure ridge areas over Europe and the Northeast U.S. which can cause unusual heat and dryness during the summer season for these region(s).

Additionally, the NAWH has slowed the northeast progress of the Gulf Stream forcing warm water to pile east of New England. The persistent warm waters in this region and the attendant sea level rise are causal in accelerated coastal erosion whenever onshore wind persists particularly during storms and tropical cyclones.

Fig. 3: ECMWF global SSTA forecast for July 2022.

Finally, research has shown a relationship on U.S. warm season drought risk based on a combination of the PDO/AMO regime(s). In 2022, the -PDO/+AMO combination is forecast by Climate Impact Company. During -PDO/+AMO there is a tendency for increased risk of drought across the Southwest and West-central U.S. plus Texas and the Missouri/Ohio Valley(s). Drought is already present across the Southwest U.S. into Texas and concern regarding a northward expansion is warranted by the -PDO/+AMO regime during the warm season.

Fig. 4: Relationships between the PDO and AMO regime on warm season drought (red) risk across the U.S.