The PNA index is a numerical measure of the Pacific-North American teleconnection pattern, one of the most influential modes of atmospheric circulation variability in the Northern Hemisphere. It captures a recurring see-saw of pressure anomalies across four regions stretching from the subtropical Pacific to eastern North America, and it has an outsized effect on winter weather across the continent. Understanding what the index is telling you, and what drives it up or down, is central to making sense of seasonal forecasts, temperature swings, and even wildfire risk.
What the PNA Pattern Actually Describes
The PNA pattern is defined by pressure anomalies at roughly 500 millibars (about five and a half kilometers above sea level) centered on four geographic nodes: the subtropical eastern Pacific near Hawaii, the North Pacific south of the Aleutian Islands, western Canada and Alaska, and the southeastern United States. When pressure is higher than normal at the first and third nodes and lower than normal at the second and fourth, the pattern is in its positive phase. Flip those anomalies and you get the negative phase.
The PNA index itself is derived from the standardized geopotential height anomalies at those four centers of action. A strongly positive index means the pattern’s pressure seesaw is pronounced in one direction; a strongly negative value means the opposite configuration is well established. Values near zero indicate the pattern is weak or absent. The index is published daily by agencies like the U.S. Climate Prediction Center, and it is one of the standard teleconnection indices that meteorologists and climate scientists track alongside better-known patterns like El Niño.
Positive Phase Versus Negative Phase Weather
When the PNA index swings positive, the jet stream over North America takes on a pronounced ridge-trough pattern. A deep trough digs into the eastern half of the continent while a strong ridge builds over western Canada and Alaska. The practical result is that cold Arctic air pours southeastward while warm subtropical air streams northwestward. This drives a sharp temperature contrast across the continent: the southeastern United States tends to get colder-than-normal winters, and the Pacific Northwest and western Canada tend to be milder than usual.
Research using observations and reanalysis datasets has shown that an intensification of positive PNA events during the second half of the twentieth century strengthened exactly this kind of winter temperature contrast. The enhanced positive phase drove cold-air outbreaks into the southeast while funneling warm air into the northwest, increasing the occurrence of opposite-signed temperature extremes in those two regions simultaneously.1Geophysical Research Letters. Recent contrasting winter temperature changes over North America linked to enhanced positive Pacific‐North American pattern
A strongly negative PNA index flattens or even reverses that jet stream configuration. The ridge shifts east and the trough weakens, often resulting in milder conditions across the eastern United States and cooler, stormier weather in western North America. The negative phase also tends to bring more active storm tracks into the Pacific Northwest and can suppress severe cold outbreaks in the Southeast.
How Tropical Convection Sets the Pattern in Motion
The PNA pattern does not originate over North America. It starts thousands of kilometers to the west, in the tropics. Synoptic analysis has shown that the positive PNA phase is initiated by enhanced convection over the western tropical Pacific paired with weakened convection over the tropical Indian Ocean. The negative phase is triggered by the opposite tropical setup. These tropical heating anomalies excite a small-amplitude PNA-like wave about eight to twelve days before the pattern reaches its full strength. That initial wave then grows slowly for roughly five days before synoptic-scale disturbances and stationary eddy advection accelerate the growth dramatically, locking the full PNA pattern into place.2Quarterly Journal of the Royal Meteorological Society. Synoptic analysis of the Pacific–North American teleconnection pattern
This two-stage development matters for forecasting. The slow initial phase gives forecasters a window of a week or more to see the PNA developing, but the rapid acceleration phase is harder to predict and is where forecast models often struggle. The tropical trigger also means that monitoring convection around the Maritime Continent and western Pacific warm pool is one of the most useful early-warning tools for anticipating PNA shifts.
ENSO as the Dominant Driver
El Niño and La Niña are the single strongest interannual influence on the PNA pattern. During El Niño winters, the shift in tropical convection toward the central and eastern Pacific tends to favor the positive PNA phase, strengthening the ridge over western North America and the trough over the east. La Niña years tend to favor the opposite setup, pushing the PNA toward its negative phase.
The relationship is not symmetric, though. Recent research has quantified a growing asymmetry in how El Niño and La Niña events project onto the PNA. As ENSO itself has become more asymmetric in recent decades, the downstream impact on the PNA has been amplified even further, by up to about 82% relative to the tropical asymmetry itself.3Geophysical Research Letters. Amplified Asymmetric Impact of ENSO Events on the Wintertime Pacific‐North American Teleconnection Pattern In plain terms, El Niño tends to push the PNA positive more strongly than La Niña pushes it negative, and this imbalance has been getting worse.
The PNA-related atmospheric heating pattern also differs from the ENSO-related heating pattern in important ways. When researchers have isolated the PNA’s own diabatic heating signature after removing the ENSO signal, the dominant heating anomalies show up in the eastern Pacific as a north-south dipole in the midlatitudes and subtropics, while the tropical heating component is surprisingly weak. That contrasts sharply with the strong tropical heating that characterizes ENSO variability directly.4Atmospheric Science Letters. The Pacific–North American pattern associated diabatic heating and its relationship to ENSO The PNA, in other words, has its own internal atmospheric dynamics beyond what ENSO alone can explain.
The Pacific Decadal Oscillation and Other Slower Drivers
ENSO operates on timescales of a few years, but the PNA also responds to slower ocean-atmosphere coupling. During the middle of the twentieth century, the PNA was closely linked not just to ENSO but also to the Pacific Decadal Oscillation through air-sea coupling in the extratropical North Pacific.5Geophysical Research Letters. Multidecadal Variation in the Seasonal Predictability of Winter PNA and Its Sources The PDO operates on decadal timescales, so its influence on the PNA introduces a lower-frequency modulation that can shift the baseline the PNA oscillates around for years or decades at a stretch.
This matters because the seasonal predictability of the PNA is not constant over time. When the PDO and ENSO are both providing strong, coherent forcing, winter PNA forecasts tend to be more skillful. When those drivers are out of phase or weak, the PNA becomes harder to predict months ahead. The practical upshot for anyone watching seasonal forecasts is that PNA predictions issued during a strong El Niño event carry more weight than those issued when the tropical Pacific is near neutral.
Wildfire Risk and the PNA
The PNA’s influence on temperature and precipitation patterns has downstream consequences that go well beyond whether you need a heavier coat. One of the most consequential is wildfire. Statistical analyses of the period from 2001 to 2020 found that the PNA’s positive phase increases fire danger across southern Asia and western North America. In high-latitude regions above 50°N, fires were predominantly identified during the positive phases of both the PNA and the Arctic Oscillation, with up to 70% of fires in places like Alaska occurring during those positive phases.6npj Climate and Atmospheric Science. Arctic Oscillation and Pacific-North American pattern dominated-modulation of fire danger and wildfire occurrence
The mechanism is straightforward. A positive PNA phase brings warmer, drier conditions to western and northern North America, reducing soil moisture and drying out vegetation. In Alaska and northern Canada, where the effect is concentrated, the warm ridge associated with positive PNA events extends the period of fire-favorable weather. For fire managers and land agencies, tracking the PNA index through winter and into spring offers an early signal of whether the coming fire season could be more active than usual.
Why Forecasting the PNA Weeks Ahead Remains Difficult
You might expect that a pattern driven by large-scale tropical forcing would be straightforward to forecast at lead times of a few weeks. It is not. Subseasonal forecasts of PNA behavior face real challenges, and performance varies depending on the background state of the tropical Pacific.
An assessment of subseasonal forecast skill for weekly mean atmospheric variability found that the North Pacific and Canada, which correspond to key PNA centers of action, do show higher predictability than many other areas at three- to four-week lead times. But that skill is unevenly distributed across ENSO phases. PNA predictions perform better during El Niño years at lead times of three to four weeks, while during La Niña years, the effect on forecast skill is much smaller. Models also tend to develop a negative PNA bias during La Niña conditions at those lead times, meaning they systematically underpredict the PNA index.7Geophysical Research Letters. Subseasonal Forecast Skill for Weekly Mean Atmospheric Variability Over the Northern Hemisphere in Winter and Its Relationship to Midlatitude Teleconnections
A case study of this forecasting difficulty is the extreme negative PNA event of February 2018, which multiple extended-range forecast systems failed to capture. Research into why models missed that event identified the East Asian trough, a well-known upstream precursor to PNA development, as the primary bottleneck. Models that did a poor job simulating the East Asian trough produced inaccurate PNA forecasts downstream. Misrepresentation of stratospheric conditions and tropical forcing further compounded the errors. Models that better captured the upstream trough produced markedly more accurate PNA predictions.8Journal of Geophysical Research: Atmospheres. Why Do Extended‐Range Forecasts Underpredict the Extreme Negative Pacific/North American Pattern in February 2018?
The lesson for anyone interpreting seasonal or subseasonal forecasts is that PNA predictions are most reliable during El Niño winters and become increasingly uncertain under La Niña or neutral conditions. And even during El Niño, the rapid-growth phase of PNA development can still surprise models if the upstream dynamics are not well captured.
The PNA Through Deep Time
The PNA pattern is not unique to the modern climate. Climate modeling studies have explored whether PNA-like circulation modes existed tens of millions of years ago, and the answer is yes, but with important differences. Simulations of Cenozoic climates show that a PNA-like pattern existed throughout much of the past 70 million years, but its strength and spatial pathways varied with global temperature.
During the warm climates of 70 to 40 million years ago, the PNA was weaker than today and followed distorted pathways. The reason ties to basic atmospheric dynamics: warmer climates had weaker temperature gradients between the tropics and the poles, which weakened the extratropical jet stream and altered the waveguide that planetary-scale stationary waves follow. After roughly 40 million years ago, as Earth cooled into its current icehouse climate, the PNA pattern strengthened and its spatial structure converged toward what we observe today.9Global and Planetary Change. The Pacific-North American teleconnection in the Cenozoic
This finding has implications for thinking about how the PNA might respond to future warming. If weaker pole-to-equator temperature gradients produced a weaker PNA in the distant past, then significant greenhouse warming could, over long timescales, dampen the pattern. But climate change is also expected to alter tropical convection and sea surface temperature patterns in complex ways, so the net effect on the PNA remains an open question.
A Millennium of PNA Behavior From Tree Rings
For a more recent window into PNA variability, researchers have used networks of tree-ring records across North America to reconstruct the winter PNA over the past 937 years. Tree rings record moisture and temperature signals that correlate with the large-scale circulation patterns the PNA drives, making them a useful proxy.
The reconstruction is consistent with regional paleoclimate records in suggesting that the persistent positive PNA pattern seen in recent decades is unprecedented over the past millennium. It also documents patterns of decadal-scale variability that contrast with some previous, shorter reconstructions, revealing that the PNA has fluctuated substantially on timescales of decades to centuries even before industrialization.10PubMed Central. Pacific North American circulation pattern links external forcing and North American hydroclimatic change over the past millennium
The finding that the current positive PNA trend is historically unusual adds context to the enhanced winter temperature contrasts and increased wildfire activity already discussed. Whether this trend reflects natural multidecadal variability, a response to anthropogenic forcing, or some combination is still debated, but the tree-ring evidence makes it clear that what North America is experiencing now is not just a normal oscillation of the pattern. It is something the continent has not seen in at least a thousand years.
How to Read the PNA Index in Practice
If you are trying to use the PNA index to inform your own understanding of upcoming weather, the U.S. Climate Prediction Center publishes daily and weekly PNA index values derived from observed 500-millibar height fields. Positive values above about +1.0 indicate a well-established positive phase; negative values below about -1.0 indicate the negative phase. Values in between suggest the pattern is weak or transitioning.
A few practical pointers help with interpretation. First, the PNA is predominantly a winter pattern. Its influence on North American weather is strongest from roughly November through March and weakens considerably in summer, when the jet stream retreats northward and the tropical forcing that drives the PNA changes character. Second, the PNA rarely acts alone. It interacts with the North Atlantic Oscillation, the Arctic Oscillation, and regional blocking patterns in ways that can amplify, dampen, or redirect its effects. A positive PNA paired with a negative NAO, for instance, can produce a very different temperature outcome over eastern North America than a positive PNA in isolation.
Third, while the PNA is one of the best-understood teleconnection patterns, its predictability at lead times beyond two weeks depends heavily on the state of ENSO and the stratosphere. During a strong El Niño winter, a sustained positive PNA is a reasonable bet, and seasonal forecasts reflecting that tend to verify well. During neutral or La Niña conditions, the PNA becomes more volatile and less predictable, so you should treat any individual weekly value with more caution.
Finally, the PNA index is a hemispheric-scale diagnostic. It describes what the atmosphere is doing on scales of thousands of kilometers. Your local weather on any given day reflects the PNA only as one ingredient among many, filtered through regional topography, moisture sources, and smaller-scale weather systems. The index is most useful for understanding the statistical tendency of weather over a region and a season, not for predicting whether it will snow on Tuesday.