The Pacific decadal oscillation (PDO) is a strong, repeating pattern of changes in ocean and air temperatures over the middle part of the Pacific Ocean. Scientists notice the PDO by observing warm or cool surface water temperatures in the Pacific Ocean, north of 20°N. Over the last 100 years, the strength of this pattern has changed irregularly over time periods ranging from a few years to several decades. Evidence shows that the pattern has reversed, switching between warm and cool surface waters, around the years 1925, 1947, and 1977. The last two reversals were linked to major changes in salmon populations in the North Pacific Ocean. This climate pattern also influences coastal ocean temperatures and air temperatures over land from Alaska to California.

During a "warm" or "positive" phase, the western part of the Pacific becomes cooler, while the eastern part warms. During a "cool" or "negative" phase, the opposite happens. The Pacific decadal oscillation was named by Steven R. Hare, who discovered it while studying patterns in salmon production in 1997.

The PDO index is calculated using a method that analyzes monthly changes in sea surface temperatures in the North Pacific (north of 20°N) after removing the overall global sea surface temperature trend. This index is a standardized record of temperature changes over time. Scientists have used tree-ring patterns in Baja California to estimate the PDO signal as far back as the year 1661.

Mechanisms

Many studies show that the PDO index is made up of tropical and extra-tropical processes combined. Unlike El Niño–Southern Oscillation (ENSO), the PDO is not a single ocean pattern. Instead, it is the result of several different processes with different causes.

At short time scales, the PDO index is created by adding random changes and ENSO-related changes in the Aleutian Low. At longer time scales, ENSO effects, random atmospheric changes, and shifts in the North Pacific ocean current contribute about equally. Sea surface temperature changes also last from one winter to the next because of a process called the reemergence mechanism.

ENSO can affect weather patterns far from the equatorial Pacific through the "atmospheric bridge." During El Niño events, strong ocean heat transfer happens over unusually warm sea surface temperatures. This creates Rossby waves that move toward the poles and east, then turn back toward the equator. These waves form in specific areas of the North and South Pacific, and the pattern of weather changes happens within 2–6 weeks. ENSO changes affect temperature, humidity, wind, and cloud patterns in the North Pacific, which then change how heat, wind, and water move across the ocean, leading to changes in sea surface temperature, salinity, and mixed layer depth.

The atmospheric bridge works best during winter when the Aleutian Low deepens, creating strong, cold winds over the central Pacific and warm, humid winds along the North American west coast. These changes in heat and wind patterns create cooler sea surface temperatures in the central Pacific and deeper mixed layer depths. The ocean warms from Hawaii to the Bering Sea.

Sea surface temperature patterns in the midlatitudes often repeat from one winter to the next but not during summer. This happens because the mixed layer depth in the North Pacific is much deeper in winter (about 100–200 meters) than in summer. Temperature changes that form in winter and reach the bottom of the mixed layer are hidden under the shallow summer mixed layer when it forms in spring. When the mixed layer deepens again in autumn or early winter, these temperature changes can influence the surface again. This process is called the "reemergence mechanism" and is common in the North Pacific, especially in the west where winter mixed layers are deeper.

Long-term sea surface temperature changes can be caused by random

Impacts

Temperature and precipitation

The patterns and effects of the PDO are similar to those seen during ENSO events. During the positive phase, the Aleutian Low becomes stronger and moves south during winter. Warm and moist air moves along the west coast of North America, causing higher-than-normal temperatures from the Pacific Northwest to Alaska but lower-than-normal temperatures in Mexico and the Southeastern United States. Winter precipitation increases in the Alaska Coast Range, Mexico, and the Southwestern United States but decreases in Canada, Eastern Siberia, and Australia. Research by McCabe et al. found that the PDO, along with the AMO, strongly influences long-term drought patterns in the United States. Droughts occur more often in much of the Northern United States during the positive PDO phase and in the Southwest United States during the negative PDO phase, especially when the PDO is linked to a positive AMO. The Asian Monsoon is also affected, with more rainfall and lower summer temperatures observed over the Indian subcontinent during the negative phase.

Reconstructions and regime shifts

The PDO index has been studied using tree rings and other natural clues that are affected by water, found in western North America and Asia.

Scientists named MacDonald and Case used tree rings from California and Alberta to study the PDO as far back as the year 993. The index shows a cycle that repeats every 50 to 70 years, but it became a major pattern of change only after the year 1800. During the medieval period (993–1300), the PDO remained in a long-lasting negative phase, which matches cooler ocean conditions in the tropical Pacific and long dry periods in the South-West United States.

Several major changes in the PDO are seen in both older studies and modern data. These changes happened in the 20th century and were linked to shifts in ocean temperatures, air pressure, rainfall, and cloud cover:

• 1750: The PDO showed an unusually strong cycle.
• 1924–1925: The PDO shifted to a "warm" phase.
• 1945–1946: The PDO shifted to a "cool" phase. This change had a similar pattern to the one in the 1970s, with the strongest effects near Japan, unlike the 1970s shift, which was strongest near the west coast of the United States.
• 1976–1977: The PDO shifted to a "warm" phase.
• 1988–1989: A weakening in a low-pressure area near the Aleutian Islands and changes in ocean temperatures were observed. This change was linked to patterns in the North Pacific and North Atlantic, not tropical areas.
• 1997–1998: After this time, ocean temperatures and marine life in the North Pacific changed. Along the west coast of the United States, ocean temperatures dropped, and populations of fish like salmon, anchovy, and sardine changed as the PDO shifted to a "cool" phase. However, the pattern of temperature changes was different, showing a north-south temperature shift in the central and western Pacific, similar to a pattern linked to ocean currents in the North Pacific.
• 2014: The PDO shifted from a cool phase to a warm phase, resembling a long and slow El Niño event. This shift contributed to record-high global surface temperatures in 2014.

Predictability

The NOAA Earth System Research Laboratory creates official ENSO forecasts and experimental forecasts using a method called linear inverse modeling (LIM) to predict the PDO. This method assumes the PDO can be divided into two parts: one that follows predictable patterns and another that is influenced by random changes.

Most of the ability to predict the PDO using LIM comes from ENSO and long-term global changes, rather than events in the extra-tropical regions. This predictability is limited to about four seasons. The prediction matches a process called the seasonal footprinting mechanism, where a specific pattern of sea surface temperatures (SST) develops into the mature phase of ENSO 6–10 months later. This later affects the North Pacific Ocean’s SST through the atmospheric bridge.

Improving predictions of the PDO’s changes over decades may involve considering both changes forced by outside factors and those naturally occurring in the Pacific Ocean.

Related patterns

• The interdecadal Pacific oscillation (IPO) is a similar but less spread-out pattern; it affects areas from 50°S to 50°N in the Southern Hemisphere.
• El Niño-Southern Oscillation (ENSO) often starts changes in the Pacific Decadal Oscillation (PDO) patterns.
• Changes in the IPO influence where and how strong ENSO events are. The South Pacific convergence zone moves northeast during El Niño and southwest during La Niña. This same movement happens during positive and negative IPO phases, respectively (Folland et al., 2002).
• Temperature changes in China over long periods are closely connected to changes in the North Atlantic Oscillation (NAO) and North Pacific Oscillation (NPO).
• The strength of the NAO and NPO grew stronger in the 1960s, and patterns of yearly temperature changes shifted from lasting 3–4 years to 8–15 years.
• Sea level rise happens when large areas of water warm and expand, or cool and contract.
• The Great Drought of 1968 in Chile occurred at the same time as a La Niña event and a cold phase of the Pacific Decadal Oscillation from 1965 to 1976.