8.3 Internal climate variability and oscillations

Internal Climate Variability and Oscillations

Earth's climate system naturally fluctuates even without outside pushes like volcanic eruptions or changes in solar output. The atmosphere, oceans, land surfaces, and ice sheets constantly interact, producing shifts in temperature and weather patterns that can last months, years, or even decades.

These internal fluctuations matter because they shape weather across entire continents. Oscillations like El Niño and the North Atlantic Oscillation drive changes in rainfall, storm intensity, and regional temperatures worldwide. Understanding them is also essential for separating natural climate swings from the long-term trend of human-caused climate change.

Internal Climate Variability

Concept of internal climate variability

Internal climate variability refers to natural fluctuations that arise within the climate system, without any external forcing like volcanic eruptions or changes in solar radiation. These fluctuations emerge from complex interactions among the atmosphere, ocean, land, and cryosphere (ice sheets, glaciers, and sea ice).

A key reason these fluctuations are so hard to predict is the chaotic nature of the climate system. Tiny differences in starting conditions can cascade into large changes over time, a concept sometimes called the butterfly effect. This means that even with perfect knowledge of physics, exact predictions of climate variability years in advance remain difficult.

Ocean-atmosphere interactions are the primary engine of internal variability. The ocean surface and the atmosphere above it constantly exchange heat and moisture. Wind-driven ocean currents then redistribute that heat across the globe, affecting regional temperatures, precipitation, and storm tracks.

Mechanisms of major climate oscillations

Three oscillations show up frequently in climate science. Each involves a different region and operates on a different timescale.

El Niño-Southern Oscillation (ENSO) is the most well-known. It involves coupled fluctuations in ocean temperatures and atmospheric pressure across the equatorial Pacific.

• El Niño (warm phase): Trade winds weaken, allowing warm water to spread eastward. Sea surface temperatures rise in the eastern Pacific near Peru and Ecuador.
• La Niña (cool phase): Trade winds strengthen, pushing warm water westward. The eastern Pacific cools as cold, deep water wells up to the surface.
• The cycle between El Niño and La Niña is irregular, typically repeating every 2–7 years.

North Atlantic Oscillation (NAO) is driven by fluctuations in the pressure difference between two semi-permanent atmospheric features: the Icelandic Low (a low-pressure zone near Iceland) and the Azores High (a high-pressure zone near the Azores islands).

• Positive NAO: A stronger pressure gradient between these two centers drives more intense and frequent winter storms across the North Atlantic toward Europe and eastern North America.
• Negative NAO: A weaker pressure gradient means fewer and weaker storms crossing the North Atlantic.

Pacific Decadal Oscillation (PDO) operates on much longer timescales, with phases lasting roughly 20–30 years. It involves shifts in sea surface temperature patterns across the North Pacific.

• Positive PDO: Warmer sea surface temperatures along the west coast of North America (Alaska through California) and cooler temperatures in the central North Pacific.
• Negative PDO: The pattern reverses, with cooler coastal waters and warmer central North Pacific temperatures.

Impacts of oscillations on weather

Each oscillation creates a distinct fingerprint of weather effects across different regions.

ENSO impacts:

• El Niño increases rainfall in the eastern Pacific, the southern U.S., and parts of South America, while causing drought in Australia and Southeast Asia.
• La Niña does roughly the opposite: more rainfall in the western Pacific, Australia, and Southeast Asia, with drier conditions in the southern U.S. and parts of South America.

NAO impacts:

• Positive NAO brings milder, wetter winters to northern Europe and the eastern U.S., but colder, drier conditions to southern Europe and the Mediterranean.
• Negative NAO reverses this pattern: colder, drier winters in northern Europe and the eastern U.S., with milder, wetter conditions around the Mediterranean.

PDO impacts:

• Positive PDO increases rainfall in the southern U.S. and northern Mexico but decreases it in the Pacific Northwest (Oregon, Washington) and western Canada.
• Negative PDO flips the pattern, bringing more rain to the Pacific Northwest and western Canada while drying out the southern U.S. and northern Mexico.

Because the PDO operates on multi-decade timescales, it can amplify or mask the effects of ENSO. A positive PDO phase combined with El Niño, for instance, can intensify drought in Australia beyond what El Niño alone would produce.

Variability vs. forced climate change

One of the trickiest problems in climate science is telling internal variability apart from human-caused climate change. Short-term natural fluctuations can temporarily speed up or slow down long-term warming trends, making it harder to detect the forced signal.

For example, a strong La Niña event can cool global average temperatures for a year or two, which might look like warming has "paused" even though the underlying trend continues. This is why climate scientists focus on trends over 30+ years rather than year-to-year changes.

Several factors make this separation difficult:

• Climate models may not fully capture all the complex feedbacks involved in internal variability, adding uncertainty to projections.
• Long-term, continuous observations are needed to statistically separate natural swings from forced trends, but high-quality climate records are limited in many regions, especially the oceans and polar areas.

Despite these challenges, decades of research using multiple independent lines of evidence have confirmed that the warming trend since the mid-20th century cannot be explained by internal variability alone.

Ocean-atmosphere interactions in variability

The ocean is the climate system's largest heat reservoir. It absorbs and stores vastly more heat than the atmosphere, which means it regulates climate on timescales from seasons to centuries. Changes in ocean heat content influence atmospheric circulation and the rate of heat exchange between the ocean surface and the air above it.

Large-scale ocean circulation patterns play a central role. The Atlantic Meridional Overturning Circulation (AMOC), for instance, carries warm surface water northward in the Atlantic and returns cold, dense water southward at depth. Changes in the strength of the AMOC can shift temperature and precipitation patterns across Europe, Africa, and the Americas.

Feedback loops between the ocean and atmosphere can either amplify or dampen variability:

1. The Bjerknes feedback is a classic example of how ocean-atmosphere coupling works during ENSO events. It links trade wind strength to sea surface temperatures in the equatorial Pacific.
2. Positive feedback during El Niño: Weaker trade winds reduce the upwelling of cold deep water in the eastern Pacific, which warms the surface further, which weakens the trade winds even more. The initial warming reinforces itself.
3. Negative feedback during La Niña: Stronger trade winds enhance cold-water upwelling, cooling the surface, which strengthens the trade winds further. The initial cooling reinforces itself.

These feedbacks explain why ENSO events, once they begin, tend to intensify before eventually reversing. The reversal itself involves additional ocean dynamics, including the slow movement of subsurface temperature waves (called Kelvin and Rossby waves) across the Pacific basin.