Pacific-North American pattern

Pacific-North American pattern
Name	Pacific–North American pattern
Abbreviation	PNA
Type	Atmospheric teleconnection
Region	North Pacific, North America
Components	Aleutian Low, Gulf of Alaska ridge, North American trough

Pacific-North American pattern is a dominant atmospheric teleconnection linking circulation anomalies across the North Pacific Ocean, Aleutian Islands, Gulf of Alaska, and North America. It modulates seasonal-to-interannual variability in temperature, precipitation, and storm tracks affecting Alaska, the Pacific Northwest, California, the Canadian Prairies, and the Northeastern United States. The pattern interacts with other modes such as the El Niño–Southern Oscillation, the Arctic Oscillation, and the North Atlantic Oscillation to influence extreme events including Blizzard of 1978, Dust Bowl, and notable droughts.

Definition and Overview

The Pacific–North American signal is defined as a leading empirical orthogonal function in upper-tropospheric geopotential height and surface pressure that links anomalies near the Aleutian Low with ridging near the Gulf of Alaska and troughing over the western United States and eastern Canada. Early studies used data from the National Centers for Environmental Prediction, the National Oceanic and Atmospheric Administration, and the University of Washington to extract the mode, and landmark papers appeared in journals associated with the American Meteorological Society and Royal Meteorological Society. The index is commonly calculated from 500 hPa geopotential height or 700 hPa anomalies over sectors bounded by the International Date Line, the Bering Sea, the Rocky Mountains, and the Great Lakes.

Meteorological Mechanisms

Dynamics underlying the pattern involve Rossby wave trains emanating from tropical convection anomalies tied to the El Niño–Southern Oscillation and baroclinic instability over the North Pacific Current. Upper-level jet stream variations associated with the Subtropical Jet Stream and the Polar Jet Stream modulate the propagation of quasi-stationary waves, while stratosphere–troposphere coupling via the Stratospheric Polar Vortex can alter persistence. Interaction with oceanic boundary forcing from the California Current and sea surface temperature anomalies influences baroclinicity and storm track genesis linked to cyclogenesis events studied in cases like Hurricane Sandy extratropical transition and Great Storm of 1987.

Variability and Phases

The pattern exhibits canonical positive and negative phases characterized by opposite-signed anomalies: the positive phase features a deepened Aleutian Low and downstream ridging over the Gulf of Alaska with troughing across the North American Rockies, whereas the negative phase shows weakened Aleutian anomalies and altered trough–ridge configuration. Phases vary with seasonal modulation by the Pacific Decadal Oscillation, interannual forcing from El Niño and La Niña events, and low-frequency shifts linked to the Atlantic Multidecadal Oscillation and Arctic sea ice extent changes following episodes like the North American Drought of 2012.

Climate Impacts and Teleconnections

Through its influence on the jet stream and storm tracks, the pattern affects winter temperature and precipitation over the Pacific Northwest, California, the Great Plains, and the Northeastern United States, altering conditions that contribute to events such as the 2013–2014 North American cold wave and multiyear droughts that impacted California water resources. Teleconnections link PNA variability to snowpack in the Sierra Nevada, river discharge in the Columbia River, and wildfire risk affecting regions managed by the U.S. Forest Service and provinces like British Columbia. Socioeconomic impacts translate to sectors including agriculture in the Midwestern United States, energy demand in the Northeast Corridor, and transportation disruptions exemplified by closures during the Great Blizzard of 1978.

Observational History and Research Methods

Identification of the pattern arose from empirical orthogonal function analyses applied to reanalysis datasets from NOAA, ECMWF, and the NCEP/NCAR reanalysis projects, with foundational work by researchers affiliated with the Scripps Institution of Oceanography, Lamont–Doherty Earth Observatory, and the University Corporation for Atmospheric Research. Observational methods combine radiosonde networks maintained by national meteorological services such as Environment and Climate Change Canada, satellite remote sensing from platforms like NOAA-19 and ERS-2, and in situ ocean measurements from programs including Argo and TAO/TRITON. Paleoclimate reconstructions using tree rings from the Mojave Desert and sediment records from the Gulf of Mexico have been used to place PNA variability into a centennial context alongside analyses of instrumental datasets archived by the National Climatic Data Center.

Modeling and Predictability

Numerical studies employ atmospheric general circulation models from centers such as the Hadley Centre, Geophysical Fluid Dynamics Laboratory, and Canadian Centre for Climate Modelling and Analysis, as well as coupled models participating in the Coupled Model Intercomparison Project to assess PNA response to greenhouse gas forcing and altered sea surface temperatures. Seasonal prediction systems run by the European Centre for Medium-Range Weather Forecasts and NOAA Climate Prediction Center exploit initialization of ocean–atmosphere conditions to forecast phase tendencies, while ensemble experiments examine sensitivity to tropical forcing, stratospheric perturbations, and snow cover anomalies over Siberia. Predictability is constrained by model biases in mean state and storm track representation, leading to ongoing model development efforts within institutions like the National Center for Atmospheric Research and the Defense Meteorological Satellite Program community.

Category:Atmospheric teleconnections