Pacific North American Pattern Research Articles

Flow dependent ensemble spread in seasonal forecasts of the boreal winter extratropics

Flow‐dependent spread (FDS) is a desirable characteristic of probabilistic forecasts; ensemble spread should represent the expected forecast error. However this is difficult to estimate for seasonal hindcasts as they tend to have a relatively small sample size. Here we use a long (110 year) seasonal hindcast dataset to evaluate FDS in forecasts of boreal winter North Atlantic Oscillation (NAO) and Pacific North American pattern (PNA). A good FDS relationship is found for interannual variations in both the NAO and PNA, with mild underdispersion for negative NAO and PNA events and slight overdispersion for positive NAO. Decadal‐scale variability is seen in forecast errors but not in ensemble spread, which shows little variation on this timescale. Links between forecast errors and tropical heating anomalies are also investigated, though no strong links are found. However, a weak link between strong El Niño warming in the East Pacific and reduced PNA error is suggested.

Navajo Nation, USA, Precipitation Variability from 2002 to 2015

AbstractDue to its semi‐arid climate, the Navajo Nation, situated in the southwestern United States, is sensitive to small changes in precipitation. However, little information on patterns and causes of rainfall variation is available for this sparsely populated region. In order to study stability and variability over time, this study characterized hydroclimatic changes for the Navajo Nation over timescales of months to years based on data from 90 sites from 2002 to 2015. This research will help local water managers identify related precipitation areas within the region, compare Navajo Nation precipitation with climate indices to ascertain larger‐scale atmospheric contributors to precipitation in the Four Corners region, and support future water planning in this understudied region. A vector quantization method, called k‐means clustering, identified five sub‐regions of contrasting precipitation climatology. The regions differed in the timing, magnitude, and relative importance of the winter and summer peaks comprising the bimodal precipitation regime of the area. Correlation examination of spatial and temporal trends of precipitation variability with three climate indices revealed strong winter precipitation relationships to the Pacific North American teleconnection pattern for all regions; summer precipitation teleconnections were weaker and more variable; however, modest correlations with Pacific Decadal Oscillation were observed. Climate field analysis indicates that cold‐season precipitation is enhanced by intensification of the Aleutian Low with a storm trajectory into the southwest United States; warm season precipitation is enhanced by poleward shift of the North American monsoon ridge.

Decadal Relationship between the Stratospheric Arctic Vortex and Pacific Decadal Oscillation

Abstract Using reanalysis datasets and numerical simulations, the relationship between the stratospheric Arctic vortex (SAV) and the Pacific decadal oscillation (PDO) on decadal time scales was investigated. A significant in-phase relationship between the PDO and SAV on decadal time scales during 1950–2014 is found, that is, the North Pacific sea surface temperature (SST) cooling (warming) associated with the positive (negative) PDO phases is closely related to the strengthening (weakening) of the SAV. This decadal relationship between the North Pacific SST and SAV is different from their relationship on subdecadal time scales. Observational and modeling results both demonstrate that the decadal variation in the SAV is strongly affected by the North Pacific SSTs related to the PDO via modifying the upward propagation of planetary wavenumber-1 waves from the troposphere to the stratosphere. The decreased SSTs over the North Pacific tend to result in a deepened Aleutian low along with a strengthened jet stream over the North Pacific, which excites a weakened western Pacific pattern and a strengthened Pacific–North American pattern. These tropospheric circulation anomalies are in accordance with the decreased refractive index (RI) at middle and high latitudes in the northern stratosphere during the positive PDO phase. The increased RI at high latitudes in the upper troposphere impedes the planetary wavenumber-1 wave from propagating into the stratosphere, and in turn strengthens the SAV. The responses of the RI to the PDO are mainly contributions of the changes in the meridional gradient of the zonal-mean potential vorticity via alteration of the baroclinic term .

Atmospheric teleconnection influence on North American land surface phenology

Short-term forecasts of vegetation activity are currently not well constrained due largely to our lack of understanding of coupled climate-vegetation dynamics mediated by complex interactions between atmospheric teleconnection patterns. Using ecoregion-scale estimates of North American vegetation activity inferred from remote sensing (1982–2015), we examined seasonal and spatial relationships between land surface phenology and the atmospheric components of five teleconnection patterns over the tropical Pacific, north Pacific, and north Atlantic. Using a set of regression experiments, we also tested for interactions among these teleconnection patterns and assessed predictability of vegetation activity solely based on knowledge of atmospheric teleconnection indices. Autumn-to-winter composites of the Southern Oscillation Index (SOI) were strongly correlated with start of growing season timing, especially in the Pacific Northwest. The two leading modes of north Pacific variability (the Pacific-North American, PNA, and West Pacific patterns) were significantly correlated with start of growing season timing across much of southern Canada and the upper Great Lakes. Regression models based on these Pacific teleconnections were skillful predictors of spring phenology across an east-west swath of temperate and boreal North America, between 40°N–60°N. While the North Atlantic Oscillation (NAO) was not strongly correlated with start of growing season timing on its own, we found compelling evidence of widespread NAO-SOI and NAO-PNA interaction effects. These results suggest that knowledge of atmospheric conditions over the Pacific and Atlantic Oceans increases the predictability of North American spring phenology. A more robust consideration of the complexity of the atmospheric circulation system, including interactions across multiple ocean basins, is an important step towards accurate forecasts of vegetation activity.

Human influences superseded climate to disrupt the 20th century fire regime in Jasper National Park, Canada

To enhance understanding of how climate and humans influenced historical fire occurrence in the montane forests of Jasper National Park, we crossdated fire-scar and tree age samples from 172 plots. We tested effects of drought and climatic variation driven by the El Niño-Southern Oscillation (ENSO) and Pacific North American (PNA) pattern on fire occurrence. We also tested whether local droughts were associated with ENSO, PNA, Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation. We used a combination of instrumental and proxy-climate records to test whether climatic variation explained the absence of fire scars in our study area during the 20th century. From 1646 to 1915, 18 fires burned mainly during drier than average years. Drought years, but not fire years, were associated with positive ENSO and PNA indices, corresponding to warmer conditions with reduced snowpacks. Fire frequency varied through time, although no fire scars have formed since 1915. Potential recording trees present at all plots and climate conducive to fire over multiple years provide evidence that human influences superseded climatic variation to explain the lack of fire scars during the 20th century. Fire suppression significantly altered the fire regime after the formation of Jasper National Park, justifying the ongoing mechanical fuel treatments, prescribed and managed wildfires to improve forest resilience to climate change.

Influence of climate oscillations on urban and rural temperature variability in the Kanto region of Japan

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 75:65-80 (2018) - DOI: https://doi.org/10.3354/cr01507 Influence of climate oscillations on urban and rural temperature variability in the Kanto region of Japan M. X. C. Seow1,*, M. Roth2 1Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan 2Department of Geography, National University of Singapore, 117570, Singapore *Corresponding author: xcmarvin@eps.s.u-tokyo.ac.jp ABSTRACT: Climate oscillations of air and sea-surface temperatures are known to influence surface temperature and precipitation variability. Intraseasonal (4 to 12 mo) and interannual (>12 to 120 mo) variability is investigated using monthly mean temperature data from 4 urban and 3 rural weather stations located in the Kanto region of Japan, during the period January 1973-August 2015. Indices of 6 climate oscillations (El Niño-Southern Oscillation, Indian Ocean Dipole, Quasi-Biennial Oscillation, Arctic Oscillation, Pacific North American Pattern and West Pacific Pattern) known to influence Japan’s climate are investigated using wavelet analysis to identify oscillations in temperature data and climate mode indices at various periods. Continuous wavelet transform analysis shows that temperature variability is more pronounced at the intraseasonal than interannual timescale and urbanisation has no influence on temperature variability. Using the wavelet transform coherence analysis, all climate oscillations except El Niño-Southern Oscillation significantly covary with the temperature at certain periods and years, with the West Pacific Pattern having the highest coherence on average across the whole study period and analysed timescales. On average across the study period, using the multiple linear regression technique, only the Arctic Oscillation and West Pacific Pattern are found to significantly contribute to both intraseasonal and interannual temperature variability. KEY WORDS: Climate variability · Air temperature · Urban-rural stations · Wavelet analysis · Japan’s climate Full text in pdf format PreviousNextCite this article as: Seow MXC, Roth M (2018) Influence of climate oscillations on urban and rural temperature variability in the Kanto region of Japan. Clim Res 75:65-80. https://doi.org/10.3354/cr01507 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 75, No. 1. Online publication date: April 18, 2018 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2018 Inter-Research.

Genesis, Pathways, and Terminations of Intense Global Water Vapor Transport in Association with Large‐Scale Climate Patterns

AbstractThe CONNected objECT (CONNECT) algorithm is applied to global Integrated Water Vapor Transport data from the NASA's Modern‐Era Retrospective Analysis for Research and Applications – Version 2 reanalysis product for the period of 1980 to 2016. The algorithm generates life‐cycle records in time and space evolving strong vapor transport events. We show five regions, located in the midlatitudes, where events typically exist (off the coast of the southeast United States, eastern China, eastern South America, off the southern tip of South Africa, and in the southeastern Pacific Ocean). Global statistics show distinct genesis and termination regions and global seasonal peak frequency during Northern Hemisphere late fall/winter and Southern Hemisphere winter. In addition, the event frequency and geographical location are shown to be modulated by the Arctic Oscillation, Pacific North American Pattern, and the quasi‐biennial oscillation. Moreover, a positive linear trend in the annual number of objects is reported, increasing by 3.58 objects year‐over‐year.

Relationships of Rainy Season Precipitation and Temperature to Climate Indices in California: Long-Term Variability and Extreme Events

To better understand the change in California’s climate over the past century, the long-term variability and extreme events of precipitation as well as minimum, mean, and maximum temperatures during the rainy season (from November to March) are investigated using observations. Their relationships to 28 rainy season average climate indices with and without time lags are also studied. The precipitation variability is found to be highly correlated with the tropical/Northern Hemisphere pattern (TNH) index at zero time lag with the highest correlation in Northern California and the Sierra and the correlation decreasing southward. This is an important finding because there have been no conclusive studies on the dominant climate modes that modulate precipitation variability in Northern California. It is found that the TNH modulates California precipitation variability through the development of a positive (negative) height anomaly and its associated low-level moisture fluxes over the northeast Pacific Ocean during the positive (negative) TNH phase. Temperature fields, especially minimum temperature, are found to be primarily modulated by the east Pacific/North Pacific pattern, Pacific decadal oscillation, North Pacific pattern, and Pacific–North American pattern at zero time lag via changes in the lower-tropospheric temperature advections. Regression analysis suggests a combination of important climate indices would improve predictability for precipitation and minimum temperature statewide and subregionally compared to the use of a single climate index. While California’s precipitation currently is primarily projected by ENSO, this study suggests that using the combination of the TNH and ENSO indices results in better predictability than using ENSO indices only.

Synoptic Analysis of Cold Air Outbreaks over the California Central Valley

How does extreme cold air reach the California Central Valley (CCV) and most of the U.S. west coast? This question is answered using composite patterns for the 10 coldest cold air outbreaks (CAOs) to reach the CCV during 1979–2013. While unusually cold air over California occurs in all events by design, how it arrives there is complicated and varies. The only other feature present in all events for several days prior to CAO onset is unusually strong surface high pressure in and south of the Gulf of Alaska. This high has low-level cold air on its west side and a deep layer of cold air moving southward on its east side. Cold air aloft flows parallel to the North American west coast and sinks as it approaches the CCV. Farther west, warm advection builds a ridge aloft. The large-scale meteorological pattern (LSMP) is equivalent barotropic. The LSMP’s ridge over Alaska, trough near California, and ridge over the southeastern United States appear in all cases by onset and resemble the Pacific–North American teleconnection pattern. Cross sections show cold air flowing from the continental interior consistent with a strong pressure gradient created by extreme cold in the continental interior. Where and when the interior cold and surface flow occurs varies between events. A geopotential height trough associated with that cold air aloft passes over the CCV before onset fostering sinking behind that is reinforced by the cold air advection below. Although sinking, as a locally defined anomaly, the cold intensifies as it migrates from the polar region to the climatologically warmer CCV.

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.

Seasonal and inter-annual variations of Arctic cyclones and their linkage with Arctic sea ice and atmospheric teleconnections

The seasonal and inter-annual variations of Arctic cyclone are investigated. An automatic cyclone tracking algorithm developed by University of Reading was applied on the basis of European Center for Medium-range Weather Forecasts (ECMWF) ERA-interim mean sea level pressure field with 6 h interval for 34 a period. The maximum number of the Arctic cyclones is counted in winter, and the minimum is in spring not in summer. About 50% of Arctic cyclones in summer generated from south of 70°N, moving into the Arctic. The number of Arctic cyclones has large inter-annual and seasonal variabilities, but no significant linear trend is detected for the period 1979–2012. The spatial distribution and linear trends of the Arctic cyclones track density show that the cyclone activity extent is the widest in summer with significant increasing trend in CRU (central Russia) subregion, and the largest track density is in winter with decreasing trend in the same subregion. The linear regressions between the cyclone track density and large-scale indices for the same period and pre-period sea ice area indices show that Arctic cyclone activities are closely linked to large-scale atmospheric circulations, such as Arctic Oscillation (AO), North Atlantic Oscillation (NAO) and Pacific-North American Pattern (PNA). Moreover, the pre-period sea ice area is significantly associated with the cyclone activities in some regions.

Symmetric and asymmetric components of anomalous tropospheric-mean horizontal fluxes of latent and sensible heat associated with ENSO events of variable magnitude

The El Niño-Southern Oscillation (ENSO) represents the dominant mode of global climate variability and is inherently nonlinear such that the linearity of the atmospheric response remains an area of ongoing research. The phase of North Atlantic Oscillation (NAO) and Pacific North American (PNA) patterns of intra-annual climate variability are favored to be the same as the phase of ENSO resulting in important climate impacts across Europe and North America. Advanced understanding of the symmetry of this response at global scale using monthly composite analyses of anomalous horizontal sensible and latent heat fluxes at various ENSO event magnitudes quantified from ERA-Interim output (January 1979 through June 2016) will advance impact predictability. A linear relationship between ENSO, PNA, and NAO patterns was identified, particularly for strong ENSO events. The nonlinear component indicated general eastward (westward) shifts in anomalous heat fluxes during El Niño (La Niña) events such that the greatest impacts were implied across North America during Decembers and Januarys of strong El Niño and weak La Niña events. Analyses of anomalous latent heat fluxes indicated spatial patterns consistent with more frequent atmospheric river phenomena, especially during Decembers and Januarys of strong El Niño events. This work demonstrates that the symmetric component of anomalous horizontal, tropospheric-mean heat fluxes corresponding to ENSO events are effective for identifying north-south dipoles of anomalous circulations consistent with PNA or NAO patterns and connections between tropical heat source regions and the PNA and NAO regions. This work also demonstrates the asymmetric component identified differences in anomalous circulation position and whether El Niño or La Niña resulted in larger heat flux anomalies. Therefore, this work provides insight into impacts associated with future ENSO events, especially across North America during strong El Niño and weak La Niña.

On the use of Cox regression to examine the temporal clustering of flooding and heavy precipitation across the central United States

The central United States is plagued by frequent catastrophic flooding, such as the flood events of 1993, 2008, 2011, 2013, 2014 and 2016. The goal of this study is to examine whether it is possible to describe the occurrence of flood and heavy precipitation events at the sub-seasonal scale in terms of variations in the climate system. Daily streamflow and precipitation time series over the central United States (defined here to include North Dakota, South Dakota, Nebraska, Kansas, Missouri, Iowa, Minnesota, Wisconsin, Illinois, West Virginia, Kentucky, Ohio, Indiana, and Michigan) are used in this study. We model the occurrence/non-occurrence of a flood and heavy precipitation event over time using regression models based on Cox processes, which can be viewed as a generalization of Poisson processes. Rather than assuming that an event (i.e., flooding or precipitation) occurs independently of the occurrence of the previous one (as in Poisson processes), Cox processes allow us to account for the potential presence of temporal clustering, which manifests itself in an alternation of quiet and active periods. Here we model the occurrence/non-occurrence of flood and heavy precipitation events using two climate indices as time-varying covariates: the Arctic Oscillation (AO) and the Pacific-North American pattern (PNA). We find that AO and/or PNA are important predictors in explaining the temporal clustering in flood occurrences in over 78% of the stream gages we considered. Similar results are obtained when working with heavy precipitation events. Analyses of the sensitivity of the results to different thresholds used to identify events lead to the same conclusions. The findings of this work highlight that variations in the climate system play a critical role in explaining the occurrence of flood and heavy precipitation events at the sub-seasonal scale over the central United States.

The Predictors and Forecast Skill of Northern Hemisphere Teleconnection Patterns for Lead Times of 3–4 Weeks

The Pacific–North American pattern (PNA), North Atlantic Oscillation (NAO), and Arctic Oscillation (AO) are three dominant teleconnection patterns known to strongly affect December–February surface weather in the Northern Hemisphere. A partial least squares regression (PLSR) method is adopted in this study to generate wintertime two-week statistical forecasts of these three teleconnection pattern indices for lead times of up to five weeks over the 1980–2013 period. The PLSR approach generates forecasts for the teleconnection pattern indices by maximizing the variance explained by predictor indices determined as linear combinations of predictor fields, which include gridded outgoing longwave radiation (OLR), 300-hPa geopotential height (Z300), and 50-hPa geopotential height (Z50). Overall, the PLSR models yield statistically significant skill at all lead times up to five weeks. In particular, cross-validated correlations between the combined weeks 3–4 PLSR forecasts and verification for the PNA, NAO, and AO indices are 0.34, 0.28, and 0.41, respectively. The PLSR approach also allows the authors to isolate a small number of predictor patterns that help shed light on the sources of prediction skill for each teleconnection pattern. As expected, the results reveal the importance of tropical convection (OLR) for forecast skill in weeks 3–4, but the initial atmospheric flow (Z300) accounts for a substantial fraction of the skill as well. Overall, the results of this study provide promise for improving subseasonal-to-seasonal (S2S) forecasts and the physical understanding of predictability on these time scales.

Interannual climate variability drives regional fires in west central British Columbia, Canada

We investigated the influence of climate variability on forest fire occurrence at eight sites in west central British Columbia, Canada. Forty-six local fire years affecting a single site and 16 moderate fire years affecting two or more sites were identified (1600–1900 A.D.). Existing fire history data were incorporated to identify 17 regionally synchronous fire years (fires that affected ≥3 sites). Interannual and multidecadal relationships between fire occurrence and the Palmer Drought Severity Index (PDSI), El Nino–Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and the Pacific North American (PNA) pattern were examined, in addition to the effects of additive positive phases of ENSO and PDO. We examined multiple reconstructions of ENSO, PDO, and PNA and utilized three methodological approaches to characterize climate-fire relationships. We found that the influence of interannual climate, expressed as PDSI, increasingly synchronized the occurrence of fires when examined from local to regional scales. An association between local fires and positive antecedent moisture conditions suggests moisture-driven fine fuel development and the proximity of some sites to grasslands likely function as key determinants of local-scale fire activity. The relationships between regional fires and ENSO, PDO, and PNA suggest that large-scale patterns of climate variability exert a weak and/or inconsistent influence over fire activity in west central British Columbia between 1700 and 1900 A.D. Although inconsistent among reconstructions of climate patterns, we identified a significant relationship between regional fires and large-scale climate patterns when ENSO and PDO were both in positive phases.

Assessing the climatic and environmental impacts of mid‐tropospheric anticyclones over Alaska

ABSTRACTOngoing climatic and cryospheric changes observed throughout the greater Alaska region are interconnected and often linked to oceanic and atmospheric patterns and processes that operate on varying spatiotemporal scales. To evaluate the long‐term, mid‐tropospheric circulation field across Alaska, and possible connections to climate and environmental change in the Pacific sector of the Arctic, the Alaska Blocking Index (ABI) is developed over the domain (54°–76°N, 125°–180°W) using daily gridded 500 hPa geopotential height fields derived from the ERA‐40 (1958–1978) and ERA‐Interim (1979–2014) data sets, 1958–2014. Climatological characteristics of the seasonal and annual ABI conditions are evaluated and periods of prevalent blocking conditions are identified and subsequently linked to possible local and large‐scale forcing mechanisms. The ABI has exhibited positive trends during all seasons and annually since 1979. Many of the extreme high ABI values occurred since 2000, including the highest annual values in 2013 and 2014. Anomalous blocking patterns in all seasons are associated with diminished snow depth and sea‐ice cover, positive near‐surface air temperature anomalies, and anticyclonic flows of heat and moisture across the domain. The ABI is also shown to differ from the long‐term variability and atmospheric circulation responses associated with phases of the Pacific–North American pattern and Pacific Decadal Oscillation, revealing some notable spatial and temporal disconnects between the region‐centric, high‐latitude blocking flow and some of the predominant modes of sea surface temperature and middle tropospheric circulation variability in the Northern Hemisphere.

Madden–Julian Oscillation Pacific Teleconnections: The Impact of the Basic State and MJO Representation in General Circulation Models

Teleconnection patterns associated with the Madden–Julian oscillation (MJO) significantly alter extratropical circulations, impacting weather and climate phenomena such as blocking, monsoons, the North Atlantic Oscillation, and the Pacific–North American pattern. However, the MJO has been extremely difficult to simulate in many general circulation models (GCMs), and many GCMs contain large biases in the background flow, presenting challenges to the simulation of MJO teleconnection patterns and associated extratropical impacts. In this study, the database from phase 5 of the Coupled Model Intercomparison Project (CMIP5) is used to assess the impact of model MJO and basic state quality on MJO teleconnection pattern quality, and a simple dry linear baroclinic model is employed to understand the results. Even in GCMs assessed to have good MJOs, large biases in the MJO teleconnection patterns are produced as a result of errors in the zonal extent of the Pacific subtropical jet. The horizontal structure of Indo-Pacific MJO heating in good MJO models is found to have modest impacts on the teleconnection pattern skill, in agreement with previous studies that have demonstrated little sensitivity to the location of tropical heating near the subtropical jet. However, MJO heating east of the date line can alter the teleconnection pathways over North America. Results show that GCMs with poor basic states can have equally low skill in reproducing the MJO teleconnection patterns as GCMs with poor MJO quality, suggesting that both the basic state and the MJO must be well represented in order to reproduce the correct teleconnection patterns.

Northern Hemisphere Stratospheric Pathway of Different El Niño Flavors in Stratosphere-Resolving CMIP5 Models

The Northern Hemisphere (NH) stratospheric signals of eastern Pacific (EP) and central Pacific (CP) El Niño events are investigated in stratosphere-resolving historical simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), together with the role of the stratosphere in driving tropospheric El Niño teleconnections in NH climate. The large number of events in each composite addresses some of the previously reported concerns related to the short observational record. The results shown here highlight the importance of the seasonal evolution of the NH stratospheric signals for understanding the EP and CP surface impacts. CMIP5 models show a significantly warmer and weaker polar vortex during EP El Niño. No significant polar stratospheric response is found during CP El Niño. This is a result of differences in the timing of the intensification of the climatological wavenumber 1 through constructive interference, which occurs earlier in EP than CP events, related to the anomalous enhancement and earlier development of the Pacific–North American pattern in EP events. The northward extension of the Aleutian low and the stronger and eastward location of the high over eastern Canada during EP events are key in explaining the differences in upward wave propagation between the two types of El Niño. The influence of the polar stratosphere in driving tropospheric anomalies in the North Atlantic European region is clearly shown during EP El Niño events, facilitated by the occurrence of stratospheric summer warmings, the frequency of which is significantly higher in this case. In contrast, CMIP5 results do not support a stratospheric pathway for a remote influence of CP events on NH teleconnections.

Characteristics of southern California atmospheric rivers

Atmospheric rivers (ARs) are channels of high water vapor flux that transport moisture from low to higher latitudes on synoptic timescales. In areas of topographical variability, ARs may lead to high-intensity precipitation due to orographic forcing. ARs landfalling along North America’s west coast are linked to extreme events including those leading to flooding and landslides. In southern California (SCA), proper AR forecasting is important for regional water resources as well as hazard mitigation and as the area’s annual precipitation totals occur from relatively few storms per season, any changes to storm frequency and/or intensity may have dramatic consequences. Yet, as most regional AR studies focus on the Pacific Northwest, there is little information about SCA ARs. We develop an algorithm to identify ARs landfalling on North America’s west coast between 1979 and 2013 within total precipitable water reanalysis fields. ARs are then categorized according to landfall region. To determine and differentiate the characteristics and spatial distributions of ARs affecting these areas, we examine lag composites of various atmospheric variables for each landfall region. SCA ARs differ from ARs landfalling farther north in the days prior to landfall with the position and amplitude of a trough offshore from the Asian continent and ridge over Alaska, as well as the displacement and eastward extension of the jet core that potentially guides AR moisture southwards. The relationships between AR landfalls and the El Nino/Southern Oscillation (ENSO), the Madden-Julian Oscillation (MJO), and the Pacific/North American Teleconnection Pattern (PNA) are also investigated.

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.