The Pacific Decadal Oscillation: How it works and why it matters for monsoon patterns

The Pacific Decadal Oscillation (PDO), a long-term climate pattern in the North Pacific, has come into the spotlight following Gujarat's heavy rainfall. This explainer breaks down how the PDO works, how it differs from El Niño, and why scientists monitor it to better understand shifts in monsoon behaviour and other large-scale weather patterns.

Days after Gujarat witnessed unusually intense rainfall, scientists have pointed to a large-scale climate phenomenon known as the Pacific Decadal Oscillation (PDO) as one of the factors that may have amplified the weather system. While no single event can be blamed solely on the PDO, experts say its interaction with the ongoing monsoon and other ocean-atmosphere patterns can influence rainfall distribution across several parts of the world, including India. The PDO is often compared with the better-known El Niño and La Niña because all three involve changes in Pacific Ocean temperatures. However, the PDO operates on a much longer timescale and influences climate over decades rather than months or a few years. Here are five important things you need to know about the Pacific Decadal Oscillation.

1. What exactly is the Pacific Decadal Oscillation?

The Pacific Decadal Oscillation (PDO) is a long-term climate pattern characterised by changes in sea surface temperatures in the North Pacific Ocean, particularly north of 20°N latitude. Unlike El Niño and La Niña, which typically last between nine months and two years, the PDO shifts between warm and cool phases over periods ranging from 10 to 30 years or even longer. Scientists first identified the pattern while studying changes in salmon populations in the North Pacific during the 1990s. In simple terms, the PDO represents large-scale changes in how warm or cold the surface waters of the North Pacific become over extended periods.

2. How does the PDO work?

The PDO has two phases:

Positive (Warm) Phase: The eastern North Pacific becomes warmer than normal, while the western Pacific turns relatively cooler.

Negative (Cool) Phase: The pattern reverses, with cooler waters in the eastern Pacific and warmer waters in the west.

These shifts are driven by complex interactions between the ocean and atmosphere, including wind patterns, ocean currents, atmospheric pressure systems, and the lingering effects of El Niño and La Niña. Unlike ENSO (El Niño-Southern Oscillation), the PDO is not caused by a single process. Instead, scientists describe it as the combined result of several climate mechanisms working together over many years.

3. How is the PDO different from El Niño?

Although both involve the Pacific Ocean, the PDO and El Niño are fundamentally different. The Pacific Decadal Oscillation (PDO) operates mainly in the North Pacific, can last for decades, changes gradually over long periods, and influences long-term climate trends. El Niño develops in the tropical Pacific, usually lasts less than two years, develops and weakens relatively quickly, and primarily affects seasonal weather patterns.

Scientists also note that El Niño can influence the PDO through atmospheric connections, but the two are not the same phenomenon.

4. Why does the PDO matter for India?

The PDO does not directly create storms or heavy rainfall over India. Instead, it alters large-scale atmospheric circulation, which can strengthen or weaken monsoon systems depending on its phase and its interaction with other climate drivers. Research suggests that:

The negative phase of the PDO has often been linked with stronger Indian summer monsoon rainfall.

The positive phase may suppress monsoon activity in some years.

When combined with phenomena such as El Niño, La Niña or the Indian Ocean Dipole (IOD), the PDO can significantly alter rainfall patterns.

This is why meteorologists monitor the PDO while assessing long-term monsoon behaviour and seasonal climate outlooks.

5. Why is the PDO being discussed after Gujarat's heavy rainfall?

Scientists believe Gujarat's recent heavy rainfall cannot be attributed to the PDO alone. Weather extremes usually result from the interaction of several atmospheric and oceanic factors. However, the PDO may have created background oceanic and atmospheric conditions that enhanced moisture transport and strengthened weather systems over the region. Such climate oscillations often increase the likelihood of prolonged wet or dry spells when they coincide with favourable monsoon conditions. Experts caution that the PDO should be viewed as a climate influencer rather than a direct trigger. Local weather systems, monsoon circulation, moisture availability and other global climate patterns together determine the intensity of rainfall over a particular region.

Why scientists closely monitor the PDO: The Pacific Decadal Oscillation influences much more than rainfall. It affects air temperatures, marine ecosystems, fisheries, drought patterns and storm behaviour across the Pacific basin and beyond. Historical records show that major shifts in the PDO have coincided with changes in fish populations, precipitation trends and regional climate patterns. As climate change continues to reshape weather systems, understanding long-term oscillations such as the PDO has become increasingly important. While it does not dictate daily weather, it provides valuable clues about how climate conditions may evolve over the coming years and how they could influence monsoon variability, rainfall extremes and global weather patterns.