Pacific Storm Research Articles

Interaction of an Intense Pacific Low Pressure System with a Strong Arctic Outbreak over British Columbia: Forecast Challenges of the Early December 2007 Storm

The interaction of a warm moist air mass from an intense Pacific low pressure system with a cold air mass from a strong Arctic outbreak during 1–5 December 2007 produced a record number of high-impact weather events across British Columbia, including heavy snow, freezing rain, heavy rain, strong winds and extreme wind chill. The unusual concurrence of these two strong weather systems caused many forecast challenges for both the Canadian numerical weather prediction (NWP) models and meteorologists at the Pacific Storm Prediction Centre (PSPC) of Environment Canada. In this study, the evolution of the weather systems and the observed severe weather events during the 1–5 December 2007 storm are analyzed. Weather forecasts by the NWP models and PSPC meteorologists are compared with the observed high-impact weather events. It is shown that the Canadian NWP models forecast the storm reasonably well. Meteorologists at PSPC further improved the model forecasts by considering various local effects of the complex terrain over British Columbia that the model has difficulty resolving and model biases caused by inadequacies in its boundary layer parameterizations. R ésumé [Traduit par la rédaction] L'interaction d'une masse d'air chaud et humide issue d'une forte dépression dans le Pacifique avec une masse d'air froid générée par une forte coulée polaire entre le 1er et le 5 décembre 2007 a engendré un nombre record d’événements météorologiques à fort impact en Colombie-Britannique : neige abondante, pluie verglaçante, pluie forte, vents forts et indice extrêmement élevé de refroidissement éolien. Pour les modèles canadiens de prévision numérique du temps (NWP) et les météorologues du Centre de prévision des tempêtes du Pacifique (PSPC) d'Environnement Canada, la coïncidence inhabituelle de ces deux systèmes météorologiques forts a posé de nombreux défis. Dans la présente étude, nous analysons l’évolution des systèmes météorologiques et les intenses événements météorologiques observés au cours de la tempête survenue entre le 1er et le 5 décembre 2007. Nous comparons les prévisions de la météo établies à partir des résultats des modèles NWP et par les météorologues du PSPC avec les événements météorologiques à fort impact qui ont été observés. Nous démontrons que les modèles canadiens de NWP, ont prédit raisonnablement bien l’évolution de la tempête. Les météorologues au PSPC ont rajusté à leur tour les prévisions établies par ces modèles en intégrant les divers effets locaux des terrains accidentés en Colombie-Britannique, qui sont difficiles à résoudre par ces modèles, et leurs biais attribuables au manque de précision des paramétrages de la couche limite.

Seasonal Evolutions of Atmospheric Response to Decadal SST Anomalies in the North Pacific Subarctic Frontal Zone: Observations and a Coupled Model Simulation

Abstract Potential impacts of pronounced decadal-scale variations in the North Pacific sea surface temperature (SST) that tend to be confined to the subarctic frontal zone (SAFZ) upon seasonally varying atmospheric states are investigated, by using 48-yr observational data and a 120-yr simulation with an ocean–atmosphere coupled general circulation model (CGCM). SST fields based on in situ observations and the ocean component of the CGCM have horizontal resolutions of 2.0° and 0.5°, respectively, which can reasonably resolve frontal SST gradient across the SAFZ. Both the observations and CGCM simulation provide a consistent picture between SST anomalies in the SAFZ yielded by its decadal-scale meridional displacement and their association with atmospheric anomalies. Correlated with SST anomalies persistent in the SAFZ from fall to winter, a coherent decadal-scale signal in the wintertime atmospheric circulation over the North Pacific starts emerging in November and develops into an equivalent barotropic anomaly pattern similar to the Pacific–North American (PNA) pattern. The PNA-like signal with the weakened (enhanced) surface Aleutian low correlated with positive (negative) SST anomalies in the SAFZ becomes strongest and most robust in January, under the feedback forcing from synoptic-scale disturbances migrating along the Pacific storm track that shifts northward (southward) in accord with the oceanic SAFZ. This PNA-like signal, however, breaks down in February, which is suggestive of a particular sensitivity of that anomaly pattern to subtle differences in the background climatological-mean state. Despite its collapse in February, the PNA-like signal recurs the next January. This subseasonal evolution of the signal suggests that the PNA-like anomaly pattern may develop as a response to the persistent SST anomalies that are maintained mainly through ocean dynamics.

Impact of ENSO on the Atmospheric Variability over the North Atlantic in Late Winter—Role of Transient Eddies

Abstract The dynamical mechanism for the late-winter teleconnection between El Niño–Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) is examined using the output from a 2000-yr integration of a coupled general circulation model (GCM). The coupled model captures many salient features of the observed behavior of both ENSO and NAO, as well as their impact on the surface climate in late winter. Both the observational and model data indicate more occurrences of negative phase of NAO in late winter during El Niño events, and positive NAO in La Niña episodes. The potential role of high-frequency transient eddies in the above teleconnection is diagnosed. During El Niño winters, the intensified transient disturbances along the equatorward-shifted North Pacific storm track extend their influences farther downstream. The eddy-induced negative height tendencies are found to be more coherent and stronger over North Atlantic than that over North Pacific. These negative height tendencies over the North Atlantic are coincident with the southern lobe of NAO, and thus favor more occurrences of negative NAO events. During those El Niño winters with relatively strong SST warming in eastern equatorial Pacific, the eastward extension of eddy activity is reinforced by the enhanced near-surface baroclinicity over the subtropical eastern Pacific. This flow environment supports a stronger linkage between the Pacific and Atlantic storm tracks, and is more conducive to a negative NAO phase. These model results are supported by a parallel analysis of various observational datasets. It is further demonstrated that these transient eddy effects can be reproduced in atmospheric GCM integrations subjected to ENSO-related SST forcing in the tropical Pacific.

Future Change in Extratropical Cyclones Associated with Change in the Upper Troposphere

Abstract Future changes in Northern Hemisphere wintertime storm activity as a consequence of global warming are investigated using the AGCM of Meteorological Research Institute (MRI-AGCM) with horizontal grid sizes of 60 and 20 km. A future (2075–99) climate experiment, in which the change in sea surface temperature (SST) derived from the Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel ensemble mean is added to observed SST, is compared with a present-day (1979–2003) climate experiment. Results of three-member simulations using the 60-km model are presented. A single simulation using the 20-km model is also presented, showing that similar results are obtained. In the future climate experiment, the number of intense cyclones (sea level pressure below 980 hPa) shows a significant increase whereas the number of total cyclones shows a significant decrease, similar to the results obtained from the CMIP3 models themselves. The increase in intense cyclones is seen on the polar side and downstream side of Atlantic and Pacific storm tracks. At the same time, the growth rate of the cyclones increases in areas upstream of these regions. For the regions with the increasing growth rate, a high correlation is seen between the growth rate of the surface cyclones and upper-tropospheric zonal wind at a monthly-mean time scale. Months of high cyclone growth rate with strong zonal wind in these regions become more frequent, and months of low cyclone growth rate with weak zonal wind become less frequent. One of the possibilities that can explain this relationship is changes in the wave-breaking pattern, that is, a decrease in wave breakings in areas of cyclonic shear and an increase in wave breakings in areas of anticyclonic shear. Associated with these changes, rapid cyclone developments are more commonly seen, and weak, long-lived cyclones become less frequent.

Paleometeorology: High resolution Northern Hemisphere wintertime mid-latitude dynamics during the Last Glacial Maximum

Hourly winter weather of the Last Glacial Maximum (LGM) is simulated using the Community Climate Model version 3 (CCM3) on a globally resolved T170 (75 km) grid. Results are compared to a longer LGM climatological run with the same boundary conditions and monthly saves. Hourly-scale animations are used to enhance interpretations. The purpose of the study is to explore whether additional insights into ice age conditions can be gleaned by going beyond the standard employment of monthly average model statistics to infer ice age weather and climate. Results for both LGM runs indicate a decrease in North Atlantic and increase in North Pacific cyclogenesis. Storm trajectories react to the mechanical forcing of the Laurentide Ice Sheet, with Pacific storms tracking over middle Alaska and northern Canada, terminating in the Labrador Sea. This result is coincident with other model results in also showing a significant reduction in Greenland wintertime precipitation – a response supported by ice core evidence. Higher-temporal resolution puts in sharper focus the close tracking of Pacific storms along the west coast of North America. This response is consistent with increased poleward heat transport in the LGM climatological run and could help explain “early” glacial warming inferred in this region from proxy climate records. Additional analyses shows a large increase in central Asian surface gustiness that support observational inferences that upper-level winds associated with Asian- Pacific storms transported Asian dust to Greenland during the LGM.

Reply

Penny et al. recently showed that the midwinter suppression in storminess over the western and central Pacific Ocean is due to a reduction in the number and amplitude of “seed” disturbances entering the Pacific storm track from midlatitude Asia. In this reply, the authors strengthen the conclusions that were originally put forth and show that the apparent departure from this behavior presented in a recent comment originates in the commenters having undersampled the full dataset of interannual variability. It is shown that when the Pacific storm track is only weakly “seeded” by an upstream source, as is common during winter and uncommon during fall and spring, it is likely to be weaker than average, and this reduction is highly statistically significant and the amplitude compares well with the midwinter suppression.

Comments on “The Source of the Midwinter Suppression in Storminess over the North Pacific”

In a recent paper, Penny et al. employed feature tracking to investigate why there is a relative minimum in storminess during winter within the Pacific storm track. They concluded that reduced upstream seeding, especially seeding from northern Asia, is the main “source” of the midwinter suppression of the Pacific storm track. Results presented here show that during midwinter months when upstream seeding is as strong as that in spring/fall, the Pacific storm track is not significantly stronger than average and is still much weaker than that in spring/fall, suggesting that the strength of upstream seeding cannot be the primary cause of the midwinter suppression of Pacific storm-track activity.

Interdecadal changes in the storm track activity over the North Pacific and North Atlantic

Analysis of NCEP-NCAR I reanalysis data of 1948–2009 and ECMWF ERA-40 reanalysis data of 1958–2001 reveals several significant interdecadal changes in the storm track activity and mean flow-transient eddy interaction in the extratropics of Northern Hemisphere. First, the most remarkable transition in the North Pacific storm track (PST) and the North Atlantic storm track (AST) activities during the boreal cold season (from November to March) occurred around early-to-mid 1970s with the characteristics of global intensification that has been noticed in previous studies. Second, the PST activity in midwinter underwent decadal change from a weak regime in the early 1980s to a strong regime in the late 1980s. Third, during recent decade, the PST intensity has been enhanced in early spring whereas the AST intensity has been weakened in midwinter. Finally, interdecadal change has been also noted in the relationship between the PST and AST activities and between the storm track activity and climate indices. The variability of storm track activity is well correlated with the Pacific Decadal Oscillation and North Atlantic Oscillation prior to the early 1980s, but this relationship has disappeared afterward and a significant linkage between the PST and AST activity has also been decoupled. For a better understanding of the mid-1970s’ shift in storm track activity and mean flow-transient eddy interaction, further investigation is made by analyzing local barotropic and baroclinic energetics. The intensification of global storm track activity after the mid-1970s is mainly associated with the enhancement of mean meridional temperature gradient resulting in favorable condition for baroclinic eddy growth. Consistent with the change in storm track activity, the baroclinic energy conversion is significantly increased in the North Pacific and North Atlantic. The intensification of the PST and AST activity, in turn, helps to reinforce the changes in the middle-to-upper tropospheric circulation but acts to interfere with the changes in the low-tropospheric temperature field.

Correction to “Recent and dramatic changes in Pacific storm trajectories as recorded in theδ18O of Bristlecone Pine tree ring cellulose”

Correction to “Recent and dramatic changes in Pacific storm trajectories as recorded in the<i>δ</i>¹⁸O of Bristlecone Pine tree ring cellulose”

A comparison of climatological subseasonal variations in the wintertime storm track activity between the North Pacific and Atlantic: local energetics and moisture effect

Distinct differences of the storm track–jet relationship over the North Pacific and North Atlantic are investigated in terms of barotropic and baroclinic energetics using NCEP-2 reanalysis data for the period of 1979–2008. From fall to midwinter the Pacific storm track (PST) activity weakens following the southward shift of the Pacific jet, whereas the Atlantic storm track (AST) activity remains steady in position and intensifies regardless of the slight southward shift of the Atlantic jet. This study is devoted to seeking for the factors that can contribute to this conspicuous difference between the two storm tracks on climatological subseasonal variation by analyzing eddy properties and local energetics. Different eddy properties over the two oceans lead to different contribution of barotropic energy conversion to the initiation of storm tracks. In the North Atlantic, meridionally elongated eddies gain kinetic energy efficiently from stretching deformation of the mean flow in the jet entrance. On the other hand, the term associated with shearing deformation is important for the initiation of PST. Analysis of baroclinic energetics reveals that the intensification of the AST activity in midwinter is mainly attributed to coincidence between location of maximum poleward and upward eddy heat fluxes and that of the largest meridional temperature gradient over slight upstream of the AST. The relatively large amount of precipitable water and meridional eddy moisture flux along baroclinic energy conversion axis likely provides a more favorable environment for baroclinic eddy growth over the North Atlantic than over the North Pacific. In the meantime, the midwinter minimum of the PST activity is attributable to the southward shift of the Pacific jet stream that leads to discrepancy between core region of poleward and upward heat fluxes and that of meridional thermal gradient. Weakening of eddy-mean flow interaction due to eddy shape and reduction of moist effect are also responsible for the weakening of storm track activities in midwinter when the strongest baroclinicity exists over the North Pacific.

Intraseasonal Modulation of the North Pacific Storm Track by Tropical Convection in Boreal Winter

Abstract The modulation of the North Pacific storm track by tropical convection on intraseasonal time scales (30–90 days) in boreal winter (December–March) is investigated using the NCEP–NCAR reanalysis and NOAA satellite outgoing longwave radiation (OLR) data. Multivariate empirical orthogonal function (MEOF) analysis and case compositing based upon the principal components (PCs) of the EOFs reveal substantial changes in the structure and intensity of the Pacific storm track quantified by vertically (925–200 mb) averaged synoptic eddy kinetic energy (SEKE) during the course of a typical Madden–Julian oscillation (MJO) event. The storm-track response is characterized by an amplitude-varying dipole propagating northeastward as the center of the anomalous tropical convection moves eastward across the eastern Indian Ocean and the western-central Pacific. A diagnosis of the SEKE budget indicates that the storm-track anomaly is induced primarily by changes in the convergence of energy flux, baroclinic conversion, and energy generation due to the interaction between synoptic eddies and intraseasonal flow anomalies. This demonstrates the important roles played by eddy–mean flow interaction and eddy–eddy interaction in the development of the extratropical response to MJO variability. The feedback of synoptic eddy to intraseasonal flow anomalies is pronounced: when the center of the enhanced tropical convection is located over the Maritime Continent (western Pacific), the anomalous synoptic eddy forcing partly drives an upper-tropospheric anticyclonic (cyclonic) and, to its south, a cyclonic (anticyclonic) circulation anomaly over the North Pacific. Associated with the storm-track anomaly, a three-band (dry–wet–dry) anomaly in both precipitable water and surface precipitation propagates poleward over the eastern North Pacific and induces intraseasonal variations in the winter hydroclimate over western North America.

Rate of floodplain reworking in response to increasing storm‐induced floods, Squamish River, south‐western British Columbia, Canada

AbstractRelations among hydroclimatic and channel planform changes on Squamish River are presented for the period 1956–2007. Squamish River basin occupies 3600 km2 of mountainous terrain in south‐western British Columbia, about 50 km north of Vancouver. The magnitude, volume and duration of extreme floods (Q ≥ 1500 m3/s) exhibit respective temporal increases of 50, 450 and 300%. The increase in extreme floods is attributed to the intensification of late‐season (August–December) Pacific storms that have produced increases in precipitation amounts, intensity and duration of respectively 340, 200 and 200% over the same period. Changes in floodplain‐surface area calculated from the geographic information system (GIS) differencing of sequential large‐scale aerial photographs indicate that the rate of geomorphic change in Squamish River has accelerated during the 1980s to the mid‐1990s. Among four study reaches of varying planform, erosional, depositional and cumulative changes in floodplain surface‐area have rapidly increased. Channel‐change activity after 1980 has increased by a factor of two to six compared with the period prior to 1980. Erosion is currently outpacing deposition in the majority of study reaches. Although channel geometry generally exhibits no uniform pattern of response to the increase in extreme floods, the meandering reaches have straightened over the duration of the study period. The increase in the magnitude and duration of the annual flood appears to be the principal cause of this recent acceleration of channel change. Copyright © 2010 John Wiley &amp; Sons, Ltd.

CCSM3 simulated regional effects of anthropogenic aerosols for two contrasting scenarios: rising Asian emissions and global reduction of aerosols

AbstractThis paper examines the effects of two largely contrasting aerosol emissions scenarios on regional climate using National Center for Atmospheric Research Community Climate System Model version 3: (1) increasing the anthropogenic aerosols over China and India by a factor of three and (2) reducing the global anthropogenic aerosols by a factor of 10. Dynamic footprints of the increased Asian aerosols with monthly variations are obtained from Model for OZone And Related chemical Tracers simulations. Increasing Asian aerosol emissions would result in cooling and reduction of precipitation over China and India, with large warming over the USA and southern Canada in winter and cooling in summer. Additionally, large changes in rainfall rate are identified over the tropical regions. In contrast, reducing the global aerosol emissions by a factor of 10 would significantly warm the atmosphere especially over the polluted land areas of both hemispheres. Increases in rainfall over polluted land areas are also noted. Deepening of the Aleutian low and weakening of the Icelandic low in winter are noted in the 500‐mb geopotential height under both scenarios suggesting a strengthening of the North Pacific storm track and weakening of the North Atlantic Oscillation. The polar regions of winter hemisphere are subject to large changes in the 500‐mb geopotential height. Teleconnection patterns associated with ENSO play important roles in causing large changes in surface air temperature and rainfall far away from the source regions of the altered aerosol concentrations. Copyright © 2009 Royal Meteorological Society

Influence of the East Asian winter monsoon on the storm track activity over the North Pacific

We investigate the relationship between the East Asian winter monsoon (EAWM) and synoptic eddy activity over the North Pacific Ocean. It is well known that the subtropical Pacific jet becomes stronger during strong monsoon periods than during weak monsoons. The storm track is generally expected to be active when the EAWM is strong, owing to the enhanced subtropical jet. Contrary to this expectation, synoptic eddy activity is significantly reduced in strong EAWM years and, conversely, significantly increased during weak years. In addition, we found that EAWM‐induced lower level cooling brings about the reverse relationship: a stronger jet but with weaker storm activities. It is suggested that the local baroclinicity induced by the EAWM modulates the variation of both the jet and North Pacific storm track simultaneously in the interannual time scale. Because major lower level cooling is located in the western parts of both the subtropical jet and the storm track in longitudes and between the two central axes in latitudes, it induces the opposite sign of anomalous local baroclinicity at the two active regions and causes this reverse relationship. The wave seeding from East Asia to the North Pacific storm track is weakened owing to the anomalous lower level cooling in the northwestern Pacific, which also results in reduction of eddy generation and storm track activities.

Observed Patterns of Month-to-Month Storm-Track Variability and Their Relationship to the Background Flow*

Abstract Month-to-month storm-track variability is investigated via EOF analyses performed on ERA-40 monthly-averaged high-pass filtered daily 850-hPa meridional heat flux and the variances of 300-hPa meridional wind and 500-hPa height. The analysis is performed both in hemispheric and sectoral domains of the Northern and Southern Hemispheres. Patterns characterized as “pulsing” and “latitudinal shifting” of the climatological-mean storm tracks emerge as the leading sectoral patterns of variability. Based on the analysis presented, storm-track variability on the spatial scale of the two Northern Hemisphere sectors appears to be largely, but perhaps not completely, independent. Pulsing and latitudinally shifting storm tracks are accompanied by zonal wind anomalies consistent with eddy-forced accelerations and geopotential height anomalies that project strongly on the dominant patterns of geopotential height variability. The North Atlantic Oscillation (NAO)–Northern Hemisphere annular mode (NAM) is associated with a pulsing of the Atlantic storm track and a meridional displacement of the upper-tropospheric jet exit region, whereas the eastern Atlantic (EA) pattern is associated with a latitudinally shifting storm track and an extension or retraction of the upper-tropospheric jet. Analogous patterns of storm-track and upper-tropospheric jet variability are associated with the western Pacific (WP) and Pacific–North America (PNA) patterns. Wave–mean flow relationships shown here are more clearly defined than in previous studies and are shown to extend through the depth of the troposphere. The Southern Hemisphere annular mode (SAM) is associated with a latitudinally shifting storm track over the South Atlantic and Indian Oceans and a pulsing South Pacific storm track. The patterns of storm-track variability are shown to be related to simple distortions of the climatological-mean upper-tropospheric jet.

A Last Saturation Analysis of ENSO Humidity Variability in the Subtropical Pacific

Abstract Water vapor tracers of last saturation were used in an atmospheric tracer transport model to evaluate ENSO variability in the generation of the dry air that defines the subtropical middle troposphere over the North Pacific. Fifteen Northern Hemisphere winters, including El Niño, La Niña, and ENSO-neutral seasons, were evaluated using both Northern Hemisphere and global last saturation water vapor tracer configurations. During El Niño northern winter, the free troposphere over the subtropical North Pacific is both drier and warmer than during La Niña. The probability distributions of the last saturation position for the dry air in the middle troposphere were evaluated over the subtropical North Pacific and were found to be further poleward, at a higher altitude, and more westerly in their components during the warm phase compared to the cold phase. During warm phase (cold phase) northern winter, 57% (49%) of the air at 20°N and 633 hPa over the North Pacific was last saturated poleward of 20°N and above 500 hPa. Coherency was demonstrated between tropical sea surface temperatures, extratropical atmospheric saturation, and subtropical aridity of the middle troposphere. The stronger westerly component of last saturation during the warm phase ties ENSO-variable subtropical aridity to midlatitude westerlies when there is enhanced baroclinicity and an equatorward migration of the Pacific storm track. Humidity reconstructions from the water vapor tracers capture observed ENSO humidity variability and demonstrate that it can be explained in terms of changes in the location of last saturation, and not by changes in the temperature field. This study shows how teleconnections between the tropical ocean and the extratropical upper troposphere can impact the humidity of the middle troposphere of the subtropical dry regions.

The Source of the Midwinter Suppression in Storminess over the North Pacific

Abstract Feature-tracking techniques are employed to investigate why there is a relative minimum in storminess during winter within the Pacific storm track (the midwinter suppression). It is found that the frequency and amplitude of disturbances entering the Pacific storm track from midlatitude Asia are substantially reduced during winter relative to fall and spring and that the magnitude of this reduction is more than sufficient to account for the midwinter supression. Growth rates of individual disturbances are calculated and compared to expectations from linear theory for several regions of interest. Although there are discrepancies between linear expectations and observed growth rates over the Pacific, the growth of disturbances within the Pacific storm track cannot explain why the midwinter suppression exists. Furthermore, it is determined that the development of a wintertime reduction in storminess over midlatitude Asia is consistent with linear expectations, which predict a wintertime minimum in Eady growth rates in this region, mainly because of increased static stability. Several other mechanisms that may contribute to the initiation of the midwinter suppression over midlatitude Asia are discussed, including the interaction between upper-level waves and topography, the behavior of waves upwind of the Tibetan Plateau, and the initiation of lee cyclones.

Temporal stability in bristlecone pine tree-ring stable oxygen isotope chronologies over the last two centuries

The absolutely dated bristlecone pine ( Pinus longaeva) tree-ring chronology spans almost 9000 years, offering great potential for inferring past environmental change. Existing ring width chronologies have been widely used to produce some of the most influential millennial length temperature reconstructions for the Northern Hemisphere. A recently published δ 18O record from two bristlecone pine trees growing at Methuselah Walk in the White Mountains of California showed a dramatic decrease in δ 18O between AD 1850 and 1920 ( c. 13‰), interpreted as indicating a major shift in Pacific storm tracks over the past 300 years. Here we present new bristlecone pine δ18O time series from 15 trees at three White Mountains sites, including two series from Methuselah Walk. Whilst occasional high interannual variability is observed in our δ 18O series, none of our chronologies exhibit an equivalent pronounced or sustained twentieth-century decrease, suggesting the earlier results are anomalous and may require palaeoclimatic re-interpretation.

Characteristics and Variability of Storm Tracks in the North Pacific, Bering Sea, and Alaska*

Abstract The North Pacific and Bering Sea regions represent loci of cyclogenesis and storm track activity. In this paper climatological properties of extratropical storms in the North Pacific/Bering Sea are presented based upon aggregate statistics of individual storm tracks calculated by means of a feature-tracking algorithm run using NCEP–NCAR reanalysis data from 1948/49 to 2008, provided by the NOAA/Earth System Research Laboratory and the Cooperative Institute for Research in Environmental Sciences, Climate Diagnostics Center. Storm identification is based on the 850-hPa relative vorticity field (ζ) instead of the often-used mean sea level pressure; ζ is a prognostic field, a good indicator of synoptic-scale dynamics, and is directly related to the wind speed. Emphasis extends beyond winter to provide detailed consideration of all seasons. Results show that the interseasonal variability is not as large during the spring and autumn seasons. Most of the storm variables—genesis, intensity, track density—exhibited a maxima pattern that was oriented along a zonal axis. From season to season this axis underwent a north–south shift and, in some cases, a rotation to the northeast. This was determined to be a result of zonal heating variations and midtropospheric moisture patterns. Barotropic processes have an influence in shaping the downstream end of storm tracks and, together with the blocking influence of the coastal orography of northwest North America, result in high lysis concentrations, effectively making the Gulf of Alaska the “graveyard” of Pacific storms. Summer storms tended to be longest in duration. Temporal trends tended to be weak over the study area. SST did not emerge as a major cyclogenesis control in the Gulf of Alaska.

Water Vapor Fluxes and Orographic Precipitation over Northern California Associated with a Landfalling Atmospheric River

Abstract Atmospheric rivers accompanying Pacific storm systems play an important role in supplying moisture to the West Coast. Heavy precipitation associated with these systems falls not only along the west-facing slopes of the Coastal Range but also along the windward slopes of the interior Sierra Mountains. Simulations of the 29–31 December 2005 storm in northern California using the Weather Research and Forecasting (WRF) model were able to realistically resolve the structure and strength of the water vapor fluxes over ocean and land. The cross-barrier, southwesterly water vapor fluxes, peaking near 700 kg m−1 s−1 at the coast, dominated the airmass transformation over the northern California mountain complex. However, there was also significant northward water vapor flux along the base of the Sierras. The combination of a narrow, short-lived water vapor source from the atmospheric river, the gap in terrain facilitating flow around the coastal mountains, and the occurrence of a strong barrier jet at the base of the Sierras all contributed to the northward along-barrier water vapor fluxes within the storm. The coincident timing of the maximum water vapor flux into the central valley with the period when the barrier jet was well developed yielded up valley fluxes &amp;gt;300 kg m−1 s−1 for several hours. For the 29–31 December 2005 Pacific storm, the flow around the coastal terrain and up valley replenished about a quarter of the depleted water vapor lost over the coastal mountains.