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The Silent Giant of Climate Variability
Picture this: a climate pattern so massive it can flip weather conditions across an entire ocean basin for decades at a time. The PDO is a naturally occurring phenomenon that shifts between warm and cool phases, with each phase lasting around 20-30 years. The cool cycle of the Pacific decadal oscillation has strengthened during recent years and will reach 30 years in duration if continued through 2026. Yet despite its enormous influence on everything from salmon populations to California droughts, most people have never heard of the Pacific Decadal Oscillation. Scientists are now sounding alarm bells as this overlooked climate giant prepares for what could be a dramatic phase shift that will reshape weather patterns from Alaska to California. The current -PDO cycle should be nearing an ending based on climatology. However, a cycle change to the positive phase is likely dependent on cooling of northwest and central North Pacific. The implications could be staggering for millions of people who depend on predictable climate patterns for their livelihoods.
Understanding the Pacific Decadal Oscillation
The “Pacific Decadal Oscillation” (PDO) is a long-lived El Niño-like pattern of Pacific climate variability. The Pacific decadal oscillation (PDO) is a robust, recurring pattern of ocean-atmosphere climate variability centered over the mid-latitude Pacific basin. Think of it as El Niño’s older, more patient cousin – while El Niño events typically last 6 to 18 months, PDO cycles persist for 20 to 30 years. Two main characteristics distinguish PDO from El Niño/Southern Oscillation (ENSO): first, 20th century PDO “events” persisted for 20-to-30 years, while typical ENSO events persisted for 6 to 18 months. During positive phases, the eastern Pacific warms while the central Pacific cools, creating a pattern that looks remarkably similar to El Niño but operates on a completely different timescale. Fisheries scientist Steven Hare coined the term “Pacific Decadal Oscillation” (PDO) in 1996 while researching connections between Alaska salmon production cycles and Pacific climate.
The Current Cool Phase Approaching Its End
The cool cycle of the Pacific decadal oscillation has strengthened during recent years and will reach 30 years in duration if continued through 2026 as forecast by the Climate Impact Company constructed analog prediction. The 30-year duration matches the previous longest cool cycle observed during the middle 1940’s to middle 1970’s. According to recent forecasts, we’re approaching a critical juncture. The Climate Impact Company PDO constructed analog forecast through 2026 indicates the cool phase continues in 2025, weakens during early 2026, and regains strength later next year. What makes this particularly intriguing is that historically, PDO cycles don’t just gradually shift – they can flip dramatically within just a few years. Periods of predominantly negative PDO phase include 1905-1925, 1947-1976 and 1998-2013, with periods of predominantly positive PDO phase in between. These prolonged intervals of mainly one sign give the PDO its name. If the pattern holds true to form, we could be witnessing the end of an era that has shaped North American climate for nearly three decades.
Climate Change Complicating Natural Patterns
The character of -PDO has changed. The influence of “new” PDO has similarities to +PDO climatology in North America whereas the Northwest U.S. is drier and warmer than typical -PDO climate. Modern climate science faces an unprecedented challenge: separating natural variability from human-induced climate change. The warming oceans, accelerating during recent years, has caused the PDO character change. There has been a debate about whether this observed trend is forced or natural (i.e., the Interdecadal Pacific Oscillation; IPO) and this study shows that there are two patterns, one that oscillates along with the IPO, and one that is emerging since the mid-1950s. A groundbreaking 2024 study published in Nature Communications revealed that what we’ve been calling natural PDO variability actually contains a climate change signal that has been emerging since the mid-1950s. The question is made complicated by the warming of the mid-latitude oceans accelerating during recent years. This discovery means that future PDO behavior may not follow historical patterns, making predictions more uncertain than ever.
Revolutionary Changes in Marine Ecosystems
Major changes in northeast Pacific marine ecosystems have been correlated with phase changes in the PDO; warm eras have seen enhanced coastal ocean biological productivity in Alaska and inhibited productivity off the west coast of the contiguous United States, while cold PDO eras have seen the opposite north-south pattern of marine ecosystem productivity. The ocean doesn’t just change temperature during PDO shifts – entire ecosystems reorganize themselves. In these years, a “warm” phase of the climate (indicated by a positive PDO) favored high production for Alaska fish stocks and low production for Washington, Oregon and California stocks. The “cool” phase of the climate had the opposite effect on Alaska and west coast stocks. The PDO has impacts on fisheries, especially on salmon production in Alaska which is enhanced during a warm phase. When the PDO flips to its next warm phase, we can expect Alaska’s fisheries to boom while California and Oregon fishing communities face potential hardships. The NPGO was shown to be particularly important for predicting salmon survival for West Coast salmon. However, climate change is making these traditional relationships less reliable than they once were.
Dramatic Weather Pattern Shifts Ahead
During the positive phase of the PDO in the northern hemisphere wintertime, the Southern and Eastern US is more likely to experience above normal temperatures, with below temperatures more likely in the West and Northwest. At the same time, the Northern Rockies and the Midwest are likely to be drier than normal, while Texas, the Gulf states, and the East are likely to be wetter than normal. A shift to the warm PDO phase would completely reorganize North American weather patterns. Positive phases of the PDO tend to be associated with periods of more rapid global warming whilst cold PDO events have been linked to severe droughts spanning many years in south western USA, as well as increased rainfall over eastern Australia. Think of it as nature’s thermostat being reset – suddenly, the Pacific Northwest could experience warmer, drier winters while the Southeast becomes wetter. McCabe showed that the PDO along with the AMO strongly influence multi-decadal droughts pattern in the United States, drought frequency is enhanced over much of the Northern United States during the positive PDO phase and over the Southwest United States during the negative PDO phase. Agricultural regions that have adapted to current conditions over the past three decades would need to rapidly adjust to entirely new climate norms.
Agricultural Disruption on a Massive Scale
We find that the effect of ENSO on dry–wet changes varies with the PDO phase. When in phase with the PDO, ENSO-induced dry–wet changes are magnified with respect to the canonical pattern. When out of phase, these dry–wet variations weaken or even disappear. America’s farming communities could face their biggest challenge in decades. Many countries faced a combination of weather extremes, such as drought and heat battering crops at the same time. This is known as a “compound” event. A PDO phase change doesn’t just alter average conditions – it fundamentally changes how extreme weather events behave. The El Niño-induced drying when in phase with the PDO covers approximately 18% of the global land area, which is nearly double the coverage during the out-of-phase condition. Farmers in California’s Central Valley, who have built irrigation systems based on current drought patterns, might suddenly face entirely different water availability. Some of these extremes won’t be able to be avoided, [such as] flooding or entire crop areas being wiped out. So in some areas, we’re going to have to move from smaller coping strategies, adjustment strategies, to transforming. Meanwhile, regions that currently struggle with dry conditions could become agricultural goldmines.
Economic Shockwaves Across Industries
In terms of aggregate salmon production and the ecosystem services salmon provide, asynchrony among the dynamics of individual populations (stocks) within a larger population complex (the portfolio) reduces the overall portfolio variance. The economic implications extend far beyond fishing and farming. Salmon fisheries in 2025 will provide commercial and recreational fishing opportunities along the West Coast from mid-Oregon north to northern Washington. Commercial fishing opportunities will be closed or severely constrained off the coasts of California and southern Oregon for a third year due to anticipated extremely low returns of several California Chinook stocks. Tourism industries built around current climate patterns – ski resorts, beach destinations, wine regions – could see their foundational assumptions crumble overnight. A 2020 study shows that two important indicators for understanding and predicting the effects of climate variability on eastern North Pacific marine ecosystems are less reliable than they were historically. This finding has important implications for fisheries and ecosystem management from Alaska to California. Energy companies that have planned renewable projects based on current wind and solar patterns may need to completely reconsider their strategies. Declines in animal body sizes are widely reported and likely impact ecological interactions and ecosystem services. For harvested species subject to multiple stressors, limited understanding of the causes and consequences of size declines impedes prediction, prevention, and mitigation. The interconnected nature of modern economies means that a shift affecting Pacific fish could ultimately impact grocery stores in Kansas.
Warning Signs Scientists Are Watching
The continuing -PDO is fueled primarily by very warm waters near and west of the Dateline while the ocean surface in the northeast Pacific is less anomalous warm. Consequently, the prevailing climate across western North America is closer to the historic +PDO character (warm and dry bias) versus wet and cool conditions associated with the “old” -PDO. Climate scientists have their eyes glued to specific indicators that could signal an imminent phase change. As of November 2023, the Copernicus Climate Change Service seasonal climate forecast models are predicting the Pacific decadal oscillation to increase over the next 5 months to near neurtral. Ocean temperatures in the North Pacific are being monitored with unprecedented precision, looking for the telltale warming along the North American coast that would signal a flip to the positive phase. Ensemble hindcasts for the 20th century demonstrate a predictive skill in the upper-ocean temperature
