Positive Pacific/North American Pattern Research Articles

Arctic Oscillation and Pacific-North American pattern dominated-modulation of fire danger and wildfire occurrence

Based on statistical analyses and Arctic Oscillation (AO) and the Pacific-North American pattern (PNA) induced climate anomalies in the 2001–2020 interval, it has been found that these climate modes drastically influence the fire danger (PFIv2) in differing ways across coastal and inland regions. The AO induces higher fire risk in northern Eurasia and central North America, whereas the PNA increases the fire danger across southern Asia and western North America. Moreover, fires have been predominantly identified, up to 70%, during the positive phases of AO and PNA northward of 50°N, in particular over Alaska, Baltic States and eastern Asia. For coincident positive AO and negative PNA days, a large number of fires have been identified over northwestern North America and northern Eurasia. Spectral analyses demonstrate that weather anomalies related to AO and PNA lead fire danger by 10–20 days, and both modes are significantly correlated to PFIv2 over north America and most of Eurasia. Despite some drawbacks related to the fire danger methods currently applied (PFI and FWI), it is demonstrated that the influence of AO and PNA on potential environmental driven-fires can be anticipated, in some locations on almost 90% of days. Fire danger forecasts are urgently needed and the understanding of factors and conditions, which are able to modify the environmental susceptibility to fire development, are crucial for adequate management to reduce the harmful effects of fire. In this sense, our results reveal that a better prediction of the fire season can be achieved by advanced assessment of the PNA and AO behavior, and shed light on the need to investigate the impact of other modes of climate variability upon wildfire frequency and severity.

Acceleration of western Arctic sea ice loss linked to the Pacific North American pattern

Recent rapid Arctic sea-ice reduction has been well documented in observations, reconstructions and model simulations. However, the rate of sea ice loss is highly variable in both time and space. The western Arctic has seen the fastest sea-ice decline, with substantial interannual and decadal variability, but the underlying mechanism remains unclear. Here we demonstrate, through both observations and model simulations, that the Pacific North American (PNA) pattern is an important driver of western Arctic sea-ice variability, accounting for more than 25% of the interannual variance. Our results suggest that the recent persistent positive PNA pattern has led to increased heat and moisture fluxes from local processes and from advection of North Pacific airmasses into the western Arctic. These changes have increased lower-tropospheric temperature, humidity and downwelling longwave radiation in the western Arctic, accelerating sea-ice decline. Our results indicate that the PNA pattern is important for projections of Arctic climate changes, and that greenhouse warming and the resultant persistent positive PNA trend is likely to increase Arctic sea-ice loss.

Variability of Arctic Sea Ice Based on Quantile Regression and the Teleconnection with Large-Scale Climate Patterns

AbstractUnder global warming, Arctic sea ice has declined significantly in recent decades, with years of extremely low sea ice occurring more frequently. Recent studies suggest that teleconnections with large-scale climate patterns could induce the observed extreme sea ice loss. In this study, a probabilistic analysis of Arctic sea ice was conducted using quantile regression analysis with covariates, including time and climate indices. From temporal trends at quantile levels from 0.01 to 0.99, Arctic sea ice shows statistically significant decreases over all quantile levels, although of different magnitudes at different quantiles. At the representative extreme quantile levels of the 5th and 95th percentiles, the Arctic Oscillation (AO), the North Atlantic Oscillation (NAO), and the Pacific–North American pattern (PNA) have more significant influence on Arctic sea ice than El Niño–Southern Oscillation (ENSO), the Pacific decadal oscillation (PDO), and the Atlantic multidecadal oscillation (AMO). Positive AO as well as positive NAO contribute to low winter sea ice, and a positive PNA contributes to low summer Arctic sea ice. If, in addition to these conditions, there is concurrently positive AMO and PDO, the sea ice decrease is amplified. Teleconnections between Arctic sea ice and the climate patterns were demonstrated through a composite analysis of the climate variables. The anomalously strong anticyclonic circulation during the years of positive AO, NAO, and PNA promotes more sea ice export through Fram Strait, resulting in excessive sea ice loss. The probabilistic analyses of the teleconnections between the Arctic sea ice and climate patterns confirm the crucial role that the climate patterns and their combinations play in overall sea ice reduction, but particularly for the low and high quantiles of sea ice concentration.

Robust Increases in Extreme Pacific North American Events Under Greenhouse Warming

AbstractThe Pacific North American (PNA) pattern is an internal mode of the Northern Hemisphere atmosphere, which strongly affects the hydroclimate and ecosystems of the Pacific‐North American sector. Recent studies have suggested a more positive PNA pattern in response to greenhouse warming, but how extreme PNA events will change remains unclear. Based on results from the Coupled Model Intercomparison Project Phase 5 multimodel ensemble, we find a 50% increase in the frequency of extreme positive PNA events and a 32% increase in extreme negative PNA events in the 21st century relative to the twentieth century. The increased frequency arises from a shift toward a more positive PNA pattern and is enhanced by more frequent ENSO events under greenhouse warming. Our study suggests that a continued increase in greenhouse‐gas emissions is likely to cause increased occurrence of extreme PNA events, thus increasing the risk of extreme weather and climate in North America.

Interannual Variability of North American Winter Temperature Extremes and Its Associated Circulation Anomalies in Observations and CMIP5 Simulations

AbstractThe interannual variability of wintertime North American surface temperature extremes and its generation and maintenance are analyzed in this study. The leading mode of the temperature extreme anomalies, revealed by empirical orthogonal function (EOF) analyses of December–February mean temperature extreme indices over North America, is characterized by an anomalous center of action over western-central Canada. In association with the leading mode of temperature extreme variability, the large-scale atmospheric circulation features an anomalous Pacific–North American (PNA)-like pattern from the preceding fall to winter, which has important implications for seasonal prediction of North American temperature extremes. A positive PNA pattern leads to more warm and fewer cold extremes over western-central Canada. The anomalous circulation over the PNA sector drives thermal advection that contributes to temperature anomalies over North America, as well as a Pacific decadal oscillation (PDO)-like sea surface temperature (SST) anomaly pattern in the midlatitude North Pacific. The PNA-like circulation anomaly tends to be supported by SST warming in the tropical central-eastern Pacific and a positive synoptic-scale eddy vorticity forcing feedback on the large-scale circulation over the PNA sector. The leading extreme mode–associated atmospheric circulation patterns obtained from the observational and reanalysis data, together with the anomalous SST and synoptic eddy activities, are reasonably well simulated in most CMIP5 models and in the multimodel mean. For most models considered, the simulated patterns of atmospheric circulation, SST, and synoptic eddy activities have lower spatial variances than the corresponding observational and reanalysis patterns over the PNA sector, especially over the North Pacific.

Multi-source forcing effects analysis using Liang\u2013Kleeman information flow method and the community atmosphere model (CAM4.0)

To understand the individual influences of the land cover, sea temperature, sea ice and carbon dioxide concentration on the global climate, sensitive experiments using the General Atmospheric Circulation Model 4.0 are designed to compare with the observation in this study. Firstly, through the analysis of Liang–Kleeman information flow method, it is straightforward that the pronounced causal relationships exist from these forcing to air temperature. In numerical experiments, the temperature is influenced by the albedo and atmospheric dynamic process. More detailed, in winter, the changes of each forcing will cause the positive Pacific-North American Pattern (PNA) phase, which makes the North American colder. The negative North Atlantic Oscillation (NAO) phase caused by the changes of CO2 and sea ice induces cold winter over the Europe. This coincides with the extreme cold weather in Europe and North America in 2018. Whereas in summer, all forcings cause positive Arctic Oscillation (AO) phase, resulting in the most northern hemisphere warmer. It is noteworthy that for precipitation, the changes of each forcing increase winds from the sea surface to the land in East Asia, so the precipitable water increases, thus the precipitation overall increases. However, when CO2 changes, the precipitation decreases due to the lack of dynamic conditions in some areas.

Can solar cycle modulate the ENSO effect on the Pacific/North American pattern?

The ENSO effect on the Pacific/North American pattern (PNA) is well-known robust. Recent studies from observations and model simulations have reported that some important atmospheric circulation systems of extratropics are markedly modulated by the 11-year solar cycle. But less effort has been devoted to revealing the solar influence on the PNA. We thus hypothesize that the instability and uncertainty in the relationship between solar activity and PNA could be due to the ENSO impacts. In this study, solar cycle modulation of the ENSO effect on the PNA has been statistically examined by the observations from NOAA and NCEP/NCAR for the period of 1950–2014. Results indicate that during the high solar activity (HS) years, the PNA has stronger relevance to the ENSO, and the response of tropospheric geopotential height to ENSO variability is broadly similar to the typical positive PNA pattern. However, in the case of low solar activity (LS) years, the correlation between ENSO and PNA decreases relatively and the response has some resemblance to the negative phase of Arctic Oscillation (AO). Also, we find the impacts of solar activity on the middle troposphere are asymmetric during the different solar cycle phases, and the weak PNA-like response to solar activity only presents in the HS years. Closer inspection suggests that the higher solar activity has a much more remarkable modulation on the PNA-like response to the warm ENSO (WE) than that to the cold ENSO (CE), particularly over the Northeast Pacific region. The possible cause of the different responses might be the solar influence on the subtropical westerlies of upper troposphere. When the sea surface temperature (SST) of east-central tropical Pacific is anomalously warm, the upper tropospheric westerlies are significantly modulated by the higher solar activity, resulting in the acceleration and eastward shift of the North Pacific subtropical jet, which favors the propagation of WE signal from the tropical Pacific to the North Pacific, and consequently leading to the development of positive PNA-like pattern during the WE phase. Thus, it seems that the solar cycle can significantly modulate the WE effect on the PNA under the HS background.

Synoptic climatology of lake‐effect snowfall conditions in the eastern Great Lakes region

ABSTRACTThe characteristics of overlying synoptic conditions play an important role in the production of lake‐effect snowfall within the Great Lakes region of North America. With a synoptic climatological approach, the synoptic‐scale patterns conducive to lake‐effect snowfall are identified for the eastern Great Lakes region during the November–March seasons of 1950–2009. Seven synoptic types identified as lake effect are combined with daily snowfall observations to examine the role of individual synoptic types on seasonal snowfall. The types exhibit a wide range of surface and 850 hPa temperatures and winds, sea‐level pressures, snowfall intensities, and inter‐annual frequencies. These factors result in substantial differences in the location and magnitude of seasonal lake‐effect snowfall.Recent research has revealed that snowfall within portions of the Great Lakes region subject to lake‐effect snow has undergone a trend reversal, declining in recent decades. Examining the inter‐annual frequency of the seven lake‐effect synoptic types, a majority of types increased in frequency prior to 1980, before becoming less frequent thereafter. The variability in frequency of lake‐effect synoptic types can partially be explained by relationships to large‐scale atmospheric and oceanic modes of variability. The majority of lake‐effect synoptic types occur in association with an upper‐level trough over the eastern United States. This trough is consistent with the upper‐level pattern associated with a negative phase Arctic Oscillation (AO) and North Atlantic Oscillation (NAO), or positive Pacific/North American pattern (PNA), such that when the AO, NAO, and/or PNA pattern are in those respective phases, there is a significant increase in the frequency of lake‐effect synoptic types.

Pacific North American circulation pattern links external forcing and North American hydroclimatic change over the past millennium

Land and sea surface temperatures, precipitation, and storm tracks in North America and the North Pacific are controlled to a large degree by atmospheric variability associated with the Pacific North American (PNA) pattern. The modern instrumental record indicates a trend toward a positive PNA phase in recent decades, which has led to accelerated warming and snowpack decline in northwestern North America. The brevity of the instrumental record, however, limits our understanding of long-term PNA variability and its directional or cyclic patterns. Here we develop a 937-y-long reconstruction of the winter PNA based on a network of annually resolved tree-ring proxy records across North America. The reconstruction is consistent with previous regional records in suggesting that the recent persistent positive PNA pattern is unprecedented over the past millennium, but documents patterns of decadal-scale variability that contrast with previous reconstructions. Our reconstruction shows that PNA has been strongly and consistently correlated with sea surface temperature variation, solar irradiance, and volcanic forcing over the period of record, and played a significant role in translating these forcings into decadal-to-multidecadal hydroclimate variability over North America. Climate model ensembles show limited power to predict multidecadal variation in PNA over the period of our record, raising questions about their potential to project future hydroclimatic change modulated by this circulation pattern.

On cold spells in North America and storminess in western Europe

AbstractWe discuss the dynamical and statistical links between cold extremes over eastern North America and storminess over western Europe, with a focus on the midlatitude jet stream, the North Atlantic Oscillation (NAO) and the Pacific‐North American Pattern (PNA). The analysis is performed on the European Centre for Medium‐Range Weather Forecasts 20th Century Reanalysis. The large‐scale circulation associated with the cold spells corresponds to advection of cold air from the Arctic region into North America and to a very zonal and intense North Atlantic jet, shifted persistently south of its climatological location. These features of the Atlantic jet are conducive to destructive windstorms and intense precipitation over a large part of southern and continental Europe and the British Isles. The cold spells are preceded by a negative NAO and followed by a positive PNA; however, we interpret the associated circulation anomalies as being distinct from these standard modes of climate variability.

Reconciling reconstructed and simulated features of the winter Pacific/North American pattern in the early 19th century

Abstract. Reconstructions of past climate behavior often describe prominent anomalous periods that are not necessarily captured in climate simulations. Here, we illustrate the contrast between an interdecadal strong positive phase of the winter Pacific/North American pattern (PNA) in the early 19th century that is described by a PNA reconstruction based on tree rings from northwestern North America, and a slight tendency towards negative winter PNA anomalies during the same period in an ensemble of state-of-the-art coupled climate simulations. Additionally, a pseudo-proxy investigation with the same simulation ensemble allows for assessing the robustness of PNA reconstructions using solely geophysical predictors from northwestern North America for the last millennium. The reconstructed early 19th-century positive PNA anomaly emerges as a potentially reliable feature, although the pseudo-reconstructions are subject to a number of sources of uncertainty and deficiencies highlighted especially at multidecadal and centennial timescales. The pseudo-reconstructions demonstrate that the early 19th-century discrepancy between reconstructed and simulated PNA does not stem from the reconstruction process. Instead, reconstructed and simulated features of the early 19th-century PNA can be reconciled by interpreting the reconstructed evolution during this time as an expression of internal climate variability, which is unlikely to be reproduced in its exact temporal occurrence by a small ensemble of climate simulations. However, firm attribution of the reconstructed PNA anomaly is hampered by known limitations and deficiencies of coupled climate models and uncertainties in the early 19th-century external forcing and background climate state.

Eclipse Ice Core Accumulation and Stable Isotope Variability as an Indicator of North Pacific Climate

Abstract The high accumulation rate and negligible amount of melt at Eclipse Icefield (3017 m) in the Saint Elias Range of Yukon, Canada, allows for the preservation of a high-resolution isotopic and glaciochemical records valuable for reconstruction of climatic variables. Each of the three Eclipse ice cores have a well-constrained depth–age scale with dozens of reference horizons over the twentieth century that permits an exceptional level of confidence in the results of the current calibration exercise. Stacked time series of accumulation and stable isotopes were divided into cold and warm seasons and seasons of extreme high and extreme low accumulation and stable isotope values (eight groups). For each group, season-averaged composites of 500-hPa geopotential height grids, and the individual seasons that constitute them, were analyzed to elucidate common anomalous flow patterns. This analysis shows that the most fractionated isotopes and lowest accumulation cold seasons reflect a more zonal height pattern in the North Pacific associated with negative Pacific–North American (PNA) and Pacific decadal oscillation (PDO) indices. Conversely, the least fractionated isotopes and highest accumulation cold seasons are associated with a positive PNA pattern. Although only a maximum of approximately 20% of the total number of accumulation and stable isotope seasons exhibit a relatively consistent relationship with 500-hPa geopotential height patterns, these results support the hypothesis that the most extreme accumulation and extreme isotope cold-season values in the Saint Elias Mountains are related to consistent atmospheric circulation and oceanic sea surface temperature patterns.

Model and Observational Analysis of the Northeast U.S. Regional Climate and Its Relationship to the PNA and NAO Patterns during Early Winter

Abstract The relationship between the large-scale circulation and regional climate of the northeast United States is investigated for early winter using observational data and the State University of New York at Albany regional climate model. Simulated patterns of temperature, precipitation, and atmospheric circulation compare well with observations, despite a cold, dry bias. Ten December runs are analyzed to investigate the impact of the Pacific–North American (PNA) pattern on temperature, precipitation, clouds, and circulation features. During a positive PNA pattern, the simulated and observed eastern U.S. jet shifts to the southeast, coinciding with cold, dry conditions in the Northeast. This shift and intensification of the upper-level jet stream during a positive PNA pattern coincides with a greater frequency of cyclones and anticyclones along a distinct southwest–northeast track. Despite increased cyclone activity, total wintertime precipitation is below normal during a positive PNA pattern because of enhanced stability and subsidence over land, along with lower-atmospheric moisture content. Lower surface air temperatures during a positive PNA pattern result in enhanced simulated cloud cover over the Great Lakes and Atlantic Ocean due to increased thermal contrast and fluxes of sensible and latent heat, and a reduction in clouds over land. Interactions between the PNA and North Atlantic Oscillation (NAO) patterns impact the Northeast winter climate. Observed frontal passages through New York are most abundant during a negative PNA and positive NAO pattern, with a zonal upper-level jet positioned over New York. A positive PNA pattern is frequently characterized by an earlier observed Great Lakes ice season, while the greatest lake-effect snowfall occurs during a positive PNA and negative NAO pattern. The NAO pattern has the largest impact on northeast U.S. temperatures and the eastern U.S. upper-level jet during a positive PNA pattern.

Atmospheric circulation influences on seasonal precipitation patterns in Alaska during the latter 20th century

A set of long, nearly complete daily precipitation series for Alaska spanning the latter half of the 20th century has been analyzed for seasonal relationships between variations in mean, heavy, and extreme precipitation and large‐scale atmospheric circulation variations at interannual, decadal, and secular timescales. Relationships with four candidate predictors (the Pacific North American (PNA), Arctic Oscillation (AO), Pacific Decadal Oscillation (PDO), and Nino3 indices) are used for insights into possible large‐scale climate forcing. Winter precipitation (mean and extreme) variability and trends along the south coast and interior of Alaska appear to be closely related to variations in the PNA pattern over this timeframe, while El Nino/Southern Oscillation (ENSO) influences, through the Nino3 index, appear to be significant along the south coast alone. Along the south coast the PNA and ENSO exert opposing influences on extreme (and mean) precipitation. Within interior Alaska the positive PNA pattern tends to suppress precipitation owing to orographic factors. Summer variations appear more closely related to the influence of the AO and PDO. The north slope region of Alaska appears to be too far removed from the influences of any of the examined predictors for any clear relationship to be evident.

An intercomparison between low-frequency variability indices

Possible connections between spatial patterns, of limited regional extent and identified in teleconnectionpatterns and in blocking climatology studies, with hemispheric planetary-wave activitymodes defined by the wave amplitude index (WAI) are investigated. The WAI probabilitydensity function (PDF) for the northern extratropics winter fields is estimated and the sensitivityof the WAI distribution to the presence of low-frequency variability modes is evaluated bystratifying the available dataset according to the sign of blocking and teleconnection indices.It is found that low-frequency variability modes affect both the mean and the variance of thewave amplitude index. Both the positive phase of the North Atlantic Oscillation (NAO) andthe negative phase of the Pacific North American pattern (PNA) are associated with anenhanced frequency of very large amplitude planetary waves. Furthermore, distributions characterisedby a maximum corresponding to high WAI values also exhibit a large variance. NegativeNAO and positive PNA influence the mean and the variance of WAI PDF in the oppositesense. Similar results are found when the blocking index is considered. WAI PDFs relative tohighly blocked months are broader with a secondary maximum corresponding to very highWAI values.

On the modification of the high- and low-frequency eddies associated with the PNA anomaly: an observational study

A 24-year NMC data set was used to study the interannual fluctuations in the extratropical atmospheric flow and the interactions between transients of different frequencies. Significant differences were found in the transient activities between winters with positive and negative Pacific/North American (PNA) seasonal anomalies. During the winters with an enhanced positive PNA pattern, the eddy activity is reduced over the North Pacific. The reduction of eddy activity is dominated by the low-frequency transients (periods from 10 days to a season). The occurrence of persistent anomalies in this region is also less frequent. The high-frequency baroclinic waves are shifted south-eastward of their normal position in the Pacific. The weak low-frequency activity over the North Pacific during positive PNA winters is linked to two processes. Firstly, the large-scale seasonal mean flow makes a smaller contribution to the low-frequency height variance. Secondly, the strong seasonal-mean Aleutian low tends to keep the baroclinic synoptic-scale eddies moving along its southern side, causing only a weak interaction with the low-frequency eddies over the North Pacific, and thus a smaller synoptic-scale eddy forcing of the low-frequency flow. The low-frequency activity is closely associated with the atmospheric natural variability which causes error growth in medium- and extended-range numerical predictions. Our result suggests that during positive PNA winters a better predictive skill for medium- and extended-range numerical predictions may be possible.