View source for Pacific–North American teleconnection pattern

{{Short description|Large-scale weather pattern with two modes}}
{{Use mdy dates|date=December 2023}} 

[[File:Pacific–North American teleconnection pattern modes.jpg|thumb|upright=1.4|An example of positive PNA (left) and negative PNA (right), based on anomalies in the [[geopotential height]] of the 500&nbsp;[[pascal (unit)|hPa]] [[pressure level]]|alt=Two plots of geopotential height over the North Pacific and North America]]

The '''Pacific–North American teleconnection pattern''' ('''PNA''') is a large-scale [[climate oscillation|weather pattern]] with two modes, denoted positive and negative, and which relates the [[atmospheric circulation]] pattern over the [[North Pacific Ocean]] with the one over the [[North America]]n continent. It is the second leading [[normal modes|mode]] of natural climate variability in the higher latitudes of the Northern Hemisphere (behind the [[Arctic Oscillation]] or [[North Atlantic Oscillation]]) and can be diagnosed using the arrangement of anomalous [[geopotential height]]s or [[air pressure]]s over the North Pacific and North America.<ref name="CPC" /><ref name="StomachSleeper" /> 

On average, the [[troposphere]] over North America features a [[ridge (meteorology)|ridge]] on the western part of the continent and a [[trough (meteorology)|trough]] over the eastern part of the continent.{{sfn|Rodionov|Assel|2001|p=1519}} The ''positive phase'' of the PNA [[teleconnection]] is identified by anomalously low geopotential heights south of the [[Aleutian Island]]s and over [[Southeastern U.S.]] straddling high geopotential heights over the North Pacific from [[Hawaii]] to the U.S. [[Intermountain West]].<ref name="CPC" /> This represents an amplification of the long-term average conditions.{{sfn|Rodionov|Assel|2001|p=1519}} The ''negative phase'' features the opposite pattern over the same regions, with above-average geopotential heights straddling below-average heights.<ref name="CPC">{{cite web |title=Pacific/North American (PNA) |url=https://www.cpc.ncep.noaa.gov/data/teledoc/pna.shtml |website=Teleconnection Pattern |publisher=Climate Prediction Center |access-date=December 23, 2023 |date=January 10, 2012}}</ref> This represents a damping of the long-term average conditions.{{sfn|Rodionov|Assel|2001|p=1519}}

== Indices ==
The PNA is typically quantified using an index using [[geopotential height]] anomalies at the 500-[[pascal (unit)|hPa]] [[pressure level]], with positive and negative PNA phases based on the [[sign (mathematics)|sign]] of the index. Wallace and Gutzler (1981) expressed the PNA index as the average of [[Normalization (statistics)|normalized]] height anomalies at the four [[centers of action]] most relevant to the PNA,

:<math>\text{PNA}_4 = \frac{1}{4}(z^*(20^{\circ}\text{N}, 160^{\circ}\text{W}) - z^*(45^{\circ}\text{N}, 165^{\circ}\text{W}) + z^*(55^{\circ}\text{N}, 115^{\circ}\text{W}) - z^*(30^{\circ}\text{N}, 85^{\circ}\text{W}))</math>

where <math>z^*</math> describes the normalized 500-hPa height anomaly as a function of location. The [[subtropics|subtropical]] center at (20°N, 160°W) can be excluded, though the difference between the resulting <math>\text{PNA}_3</math> index and the <math>\text{PNA}_4</math> index is small.{{sfn|Rodionov|Assel|2001|p=1519}}

Applying rotated [[principal component analysis]] to the 500-hPa geopotential height anomaly field in the Northern Hemisphere can also provide a quantification of the PNA (<math>\text{PNA}_\text{RPCA}</math>), with the canonical PNA pattern emerging as the second-leading principal component. This methodology is used by the U.S. [[Climate Prediction Center]] to compute its PNA index.{{sfn|Rodionov|Assel|2001|p=1520}}<ref name="NCEI">{{cite web |title=Pacific-North American (PNA) |url=https://www.ncei.noaa.gov/access/monitoring/pna/ |publisher=National Centers for Environmental Information |access-date=27 December 2023}}</ref><ref name="CPCProcedures">{{cite web |title=Teleconnection Pattern Calculation Procedures |url=https://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/history/method.shtml |publisher=Climate Prediction Center |access-date=27 December 2023 |date=December 12, 2005}}</ref>

== Dynamics ==
[[File:Mjo north america rain.png|thumb|Convection over the tropical Pacific and Indian oceans can impact the jet stream over the North Pacific|alt=Illustrations of the jet stream varying in response to convection.]]
Although the PNA is usually defined based on anomalies relative to monthly or seasonal averages, the PNA often varies at weekly timescales. However, as a pattern of internal climate variability, the state of the PNA occasionally changes without a clear and identifiable cause. This reduces the predictability of the PNA and can complicate long-range seasonal weather forecasts. Predictability of the PNA is limited to roughly within 10&nbsp;days.<ref name="StomachSleeper" /> The PNA is associated with changes in the intensity and positioning of the East Asian [[jet stream]]. During the positive phase of the PNA, the East Asian jet intensifies and extends eastward across the North Pacific towards the western U.S. During the negative phase, the jet stream is retracted over East Asia, producing a [[Block (meteorology)|blocking weather pattern]] over the North Pacific.<ref name="CPC" /><ref name="ClimateVariability" /> Some of the energy that drives the PNA originates from the [[barotropic instability]] produced by the jet, potentially exciting [[Rossby wave]]s. Shifts in the jet stream can induce changes in air pressure distributions both near and downstream of the jet.<ref name="StomachSleeper">{{cite web |last1=L'Heureux |first1=Michelle |title=The Pacific-North American Pattern: the stomach sleeper of the atmosphere |url=https://www.climate.gov/news-features/blogs/enso/pacific-north-american-pattern-stomach-sleeper-atmosphere |website=ENSO Blog |publisher=Climate.gov |access-date=December 23, 2023 |date=April 23, 2019}}</ref> 

Storms over the tropical Pacific and Indian oceans may play a role in exciting the positive and negative phases of the PNA by influencing the East Asian jet. Tropical [[atmospheric convection|convection]] can induce a low-amplitude PNA pattern that amplifies to its peak strength after 8–12&nbsp;days. Atmospheric [[Eddy (fluid dynamics)|eddies]] and Rossby waves can further intensify the PNA pattern. Positive PNA is correlated with increased convective activity over western tropical Pacific and reduced convective activity over the tropical Indian Ocean, while negative PNA is correlated with the opposite convective anomalies. The Rossby waves associated with positive PNA tend to track eastward and undergo cyclonic [[breaking wave|wavebreaking]], while those associated with negative PNA tend to track equatorward towards the [[subtropics]] and break anticyclonically; the wavebreaking behavior of the Rossby waves is determined by the [[Zonal and meridional flow|meridional]] gradient of [[potential vorticity]] and the magnitude and orientation of [[wind shear]], which in turn are modulated by variations in the East Asian jet stream. In either case, [[positive feedback]]s associated with the wavebreaking sustain amplified PNA patterns.{{sfn|Franzke|Feldstein|Lee|2011|p=329}}

Other teleconnections can modulate the PNA by modifying the jet stream.<ref name="StomachSleeper" />  The [[El Niño–Southern Oscillation]] (ENSO) impacts the behavior of PNA, with the positive phase of the PNA more commonly associated with [[El Niño]] and the negative phase more commonly associated with [[La Niña]].<ref name="CPC" /> This relationship is most evident at seasonal timescales, making the seasonal PNA more predictable than the monthly PNA. The negative phase is also favored when the [[Madden–Julian oscillation]] (MJO) enhances [[atmospheric convection|convection]] over the [[Indian Ocean]] and [[Maritime Continent]]; the positive phase is favored when the MJO enhances convection closer to the central Pacific. The MJO's influence on the PNA arises from the interaction between the enhanced convection and the Pacific jet stream.<ref name="StomachSleeper" />

== Effects on weather ==
The regional variations in weather associated with the PNA are generally the result of the PNA's influence on the East Asian jet.<ref name="ClimateVariability">{{cite web |last1=Dahlman |first1=Luann |title=Climate Variability: Pacific–North American Pattern |url=https://www.climate.gov/news-features/understanding-climate/climate-variability-pacific-north-american-pattern |website=Understanding Cliamte |publisher=Climate.gov |access-date=December 23, 2023 |date=September 1, 2009}}</ref> The temperature pattern associated with the PNA follows the pattern of anomalous [[ridge (meteorology)|ridging]] and [[trough (meteorology)|troughing]]. The positive phase of the PNA is correlated with above-average temperatures over the [[U.S. Pacific Coast]] and [[Western Canada]]. During the positive phase, an anomalously strong [[high pressure area|ridge of high pressure]] over Canada reduces the frequency of [[cold air outbreak]]s over western [[Northern America]].<ref name="StomachSleeper" /> Below-average temperatures over the [[South Central United States|South-Central]] U.S., Southeastern U.S., and [[U.S. East Coast]] are associated with the positive phase due to the presence of anomalously low pressure.<ref name="CPC" /><ref name="StomachSleeper" /> The influence of the PNA on [[surface temperature]]s over North America is reduced during the summer.<ref name="CPC" /> 

Correlations between precipitation patterns and the PNA are weaker than temperature patterns, but are nonetheless evident.{{sfn|Rodionov|Assel|2001|p=1519}} Anomalously high precipitation over the [[Gulf of Alaska]] and [[Pacific Northwest]] accompany the positive phase, along with below-average precipitation totals over the Pacific Northwest, [[Northern Rocky Mountains]], and [[Ohio River|Ohio]] and [[Tennessee River|Tennessee]] river valleys.<ref name="StomachSleeper" /> The negative PNA phase exhibits the opposite departures from average.<ref name="CPC" />

== See also ==
{{Portal|Weather}}
* [[Teleconnection]]
* [[Arctic dipole anomaly]]
* [[Kuroshio Current]]
* [[North Pacific Gyre]]
* [[Pacific Decadal Oscillation]]
* [[Pineapple Express]]
* [[Walker circulation]]

== References ==
{{Reflist}}

=== Sources ===
{{refbegin}}
* {{cite journal |last1=Franzke |first1=Christian |last2=Feldstein |first2=Steven B. |last3=Lee |first3=Sukyoung |title=Synoptic analysis of the Pacific–North American teleconnection pattern |journal=Quarterly Journal of the Royal Meteorological Society |publisher=Royal Meteorological Society | date=January 2011 |volume=137 |issue=655 |pages=329–346 |doi=10.1002/qj.768 |s2cid=55883064 |bibcode=2011QJRMS.137..329F }}
* {{cite journal |last1=Rodionov |first1=Sergei |last2=Assel |first2=Raymond |title=A new look at the Pacific/North American Index |journal=Geophysical Research Letters |date=15 April 2001 |volume=28 |issue=8 |pages=1519–1522 |doi=10.1029/2000GL012185 |doi-access=free}}
{{refend}}

{{Climate oscillations}}

{{DEFAULTSORT:Pacific-North American teleconnection pattern}}
[[Category:Regional climate effects]]
[[Category:Physical oceanography]]


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