The North Pacific Oscillation (NPO) is a climate pattern centered over the northern Pacific Ocean, characterized by a north-south “seesaw” in atmospheric sea-level pressure. The NPO involves a recurring pattern of changes in both atmospheric conditions and ocean surface temperatures. Its behavior is a result of large-scale interactions between the ocean and the atmosphere.
Understanding the NPO Phases
The North Pacific Oscillation alternates between two distinct phases, positive and negative, defined by the behavior of the Aleutian Low pressure system. In its positive phase, the Aleutian Low, a semi-permanent low-pressure center near the Aleutian Islands, becomes deeper and shifts eastward from its typical position. This intensification of the low-pressure system alters wind patterns across the mid-latitudes of the Pacific Ocean.
These atmospheric changes directly influence the ocean’s surface. During a positive phase, a distinct pattern of sea surface temperatures emerges. A large, horseshoe-shaped area of cooler-than-average water develops in the central North Pacific. This is complemented by a ribbon of warmer-than-average water that pools along the west coast of North America.
Conversely, the negative phase represents the opposite conditions. The Aleutian Low becomes weaker and retreats westward. This change in the atmospheric pressure gradient leads to a reversal of the sea surface temperature pattern seen in the positive phase. The horseshoe of cool water in the central Pacific is replaced by warmer waters, while the coastal regions of North America experience a cooling effect.
North American Weather Impacts
The phases of the North Pacific Oscillation significantly influence weather patterns across North America by altering the path of the jet stream. During a positive NPO phase, the strengthened Aleutian Low helps to steer the jet stream in a more northward-arching path over the eastern Pacific. This shift has predictable consequences for regional weather conditions.
For the Pacific Northwest and Western Canada, a positive NPO phase typically leads to warmer and drier winters. The altered jet stream diverts storms that would normally bring moisture to the region farther north. At the same time, the southeastern United States often experiences cooler and wetter conditions. This is because the jet stream dips southward over this part of the continent, bringing colder air and increased storm activity.
The negative phase of the NPO produces the opposite effects. With a weaker Aleutian Low, the jet stream takes a more southerly track as it approaches North America. This brings more frequent storms and moisture to the Pacific Northwest, resulting in cooler and wetter winters. In contrast, the southeastern United States tends to experience warmer and drier conditions, as the southerly dip in the jet stream is less pronounced.
These impacts demonstrate how a climate pattern far out in the Pacific Ocean can have direct and tangible effects on seasonal weather thousands of miles away. The NPO’s control over the jet stream’s trajectory is the primary mechanism through which it delivers these distinct, continent-wide weather patterns. The oscillation’s state provides a valuable indicator for long-range weather forecasting.
Comparing NPO and ENSO
A common point of confusion is the distinction between the North Pacific Oscillation and the El Niño-Southern Oscillation (ENSO). While both are patterns of ocean-atmosphere interaction in the Pacific, they are geographically and mechanistically distinct. The NPO is primarily a mid-latitude phenomenon, centered in the North Pacific Ocean. In contrast, ENSO is an equatorial phenomenon, focused on the tropical Pacific Ocean.
Their driving mechanisms also differ. The NPO is defined by the strength and position of the Aleutian Low pressure system and its influence on the mid-latitude westerlies. ENSO, on the other hand, is driven by changes in trade winds along the equator and the resulting shifts in warm water across the equatorial Pacific.
Although they are separate climate modes, the NPO and ENSO can interact and influence each other’s effects on global weather. For instance, a strong El Niño event can sometimes affect the atmospheric circulation in the North Pacific, which in turn can modify the NPO’s behavior.