Pacific North American Research Articles

Spatio-Seasonal Variations in Long-Term Trends of Offshore Wind Speeds Over the Black Sea; an Inter-Comparison of Two Reanalysis Data

Spatio-seasonal variability of long-term trends in mean and 95th percentile wind speeds for the term between 1979 and 2016, over the Black Sea is presented. Our aim is to contribute the existing literature by presenting the inhomogeneous spatial distribution of the long-term trends in both moderate and severe wind speeds on a monthly basis. The analysis is conducted by using two different data; European Centre for Medium-Range Weather Forecasts-ERA-Interim and U.S. National Centers for Environmental Prediction-Climate Forecast System Reanalysis (CFSR) to perform a comparative analysis. The non-parametric Mann–Kendall and Sen’s Slope methods are used to determine the trends and their significance over the Black Sea. CFSR winds presented higher interannual variability than the ERA-Interim. ERA-Interim indicates that annual mean and 95th percentile wind speeds have decreasing trends down to − 0.17%/year and − 0.20%/year in the Sea of Azov, while they have an increasing trend up to 0.35%/year and 0.38%/year in the eastern part, respectively. Results indicate that wind speeds are increasing over 28% ~ 36% of the Black Sea surface area while the wind speeds are decreasing over 2% ~ 4% of the surface area. Pacific North American Oscillation presented an influence almost all over the Black Sea with statistically significant correlation coefficients over 0.5. North Atlantic Oscillation dominates over the southwestern, western and northern Black Sea with inverse correlation coefficients over 0.6. ERA-Interim and CFSR data illustrated a similar distribution pattern over the Black Sea in means of the relation of variations in wind speeds to the teleconnection indices.

Interdecadal change in the principal mode of winter–spring precipitation anomaly over tropical Pacific around the late 1990s

Evidence show that the dominant mode of winter–spring precipitation anomaly over the tropical Pacific experienced a pronounced interdecadal change around 1998. Based on the extended empirical orthogonal function (EEOF) technique, a zonal dipole pattern of precipitation anomaly is dominant during the pre-1998 period. Corresponding to a positive principle component (PC1), positive precipitation anomaly over the equatorial eastern Pacific (EP) decays eastward from winter to spring, which is identified as an eastward-decaying EP mode. During the post-1999 period, there is a zonal triple pattern with enhanced precipitation over the equatorial central Pacific (CP) and reduced precipitation over the western North Pacific and equatorial southeastern Pacific with a positive PC1. This triple pattern of precipitation anomaly maintains with decreasing amplitude from winter to spring, which is called the fixedly-decaying CP mode for short. Such interdecadal shift is controlled by differing sea surface temperature (SST) anomaly pattern. The interdecadal differences of winter–spring wind stress and thermocline over tropical Pacific might result in the interdecadal change in the SST anomaly and then precipitation mode, via modulating physical oceanic feedback processes. In addition, the effect of two distinct precipitation modes on the North Pacific teleconnection and North American surface temperature anomaly was strikingly different. Before 1998, the eastward-decaying EP mode might induce a significant tropical Northern Hemisphere (TNH)-like teleconnection only in winter, leading to a north-to-south pattern of North American temperature anomaly, which is greatly limited in spring. After 1999, a northwest-to-southeast response of North American temperature sustains from winter to spring, resulting from the maintenance of the Pacific-North America (PNA)-like pattern, simulated by the fixedly-decaying CP mode.

Energetics of the Western Hemisphere Circulation Pattern

Abstract The Western Hemisphere (WH) circulation pattern, identified by self-organizing maps cluster analysis, is a low-frequency atmospheric regime that influences the fluctuations of large-scale circulation over the North Pacific–North American–North Atlantic areas. The reanalysis datasets from ECMWF are used to estimate the energetics of the WH pattern in this study. The composite results based on monthly WH events reveal that the kinetic energy (KE) associated with the WH pattern is maintained through the barotropic conversion from the climatological-mean westerlies, mainly in the Atlantic jet exit regions. The KE could also be gained through the barotropic feedback forcing from transient eddies. The corresponding baroclinic conversion of available potential energy (APE) from the climatological-mean state, which contributes most efficiently to the energy maintenance of the WH pattern, is obvious in the middle and lower troposphere, owing to the thermal contrast of the colder continent and warmer ocean over the North America–North Atlantic sector. The baroclinic conversion associated with the heat flux on the climatological temperature gradient is consistent with the southwestward-tilting height anomalies from 850 to 500 hPa. The baroclinic feedback from transient eddies contributes negatively to the energy conversion and destroys the maintenance of the WH pattern.

The Role of Tropical Heating and Internal Variability in the California Response to the 2015/16 ENSO Event

AbstractEnsemble reforecast experiments are performed for the boreal winters of 2015/16 (an El Niño year) and 2016/17 in order to understand the origin of the highly unexpected low precipitation over Southern California (SoCal) during 2015/16. The specific hypothesis tested is that the December–March anomalies in (i) SoCal precipitation (highly negative) and (ii) the 200-hPa height field in the Pacific–North American (PNA) region (unusual for an El Niño year) were forced by tropical diabatic heating, as estimated from reanalysis fields. Ten-member ensemble reforecasts for November–March for each year were carried out using the Community Earth System Model and the Climate Forecast System, version 2, models. They were repeated with diabatic heating fields added to the temperature tendencies produced by the models over the tropical Indo-Pacific region throughout the reforecasts. These added fields were obtained from an iterative process that greatly reduces the error in the seasonal mean, trend, and parabolic fit of the model heating (compared to reanalysis estimates). The SoCal precipitation and PNA height field ensemble-mean differences between 2015/16 and 2016/17 for the control and added-heating experiments showed a strong El Niño–like response in both models, including high SoCal precipitation anomalies. Using partial least squares regression to relate variations in SoCal precipitation to the PNA height field within the reforecasts, we further confirm that the observed anomalies are due to noise, and do not constitute a response forced by the seasonal-mean tropical diabatic heating in the Indo-Pacific region.

Effects of environmental variability on abundance of commercial marine species in the northern Gulf of California

Studies have shown that environmental variables significantly affect variation in stock abundance of marine populations. The northern Gulf of California (NGC) is a highly productive region of interest due to its fish resources and diversity. Conservation of the marine species inhabiting the region is of public interest. Our study analysed the influence of physical environmental factors on several commercial marine species, using catch per unit effort (CPUE) as a proxy for abundance. Generalized additive models were used to test the significance of selected environmental variables on stock abundance. Deseasonalized cross-correlation analysis was used to examine time-lagged correlations between CPUE and abiotic variables to identify response timings. The results suggest that for most commercial species the sea surface temperature and the long-term climate Pacific Decadal Oscillation index are the predominant predictors for species abundance, followed by the Colorado River discharge. The Multivariate ENSO Index and the Pacific-North American pattern indices also showed specific effects on certain species. The NGC is a highly dynamic region, where species respond to environmental changes according to the characteristics of their life histories.

Modulation of the Northern Winter Stratospheric El Niño–Southern Oscillation Teleconnection by the PDO

Using the CMIP5 multimodel ensemble (MME) historical experiments, the modulation of the stratospheric El Niño–Southern Oscillation (ENSO) teleconnection by the Pacific decadal oscillation (PDO) is investigated in this study. El Niño (La Niña) significantly impacts the extratropical stratosphere mainly during the positive (negative) PDO in the MME. Although the composite tropical ENSO SST intensities are similar during the positive and negative PDO in models, the Pacific–North American (PNA) responses are only significant when the PDO and ENSO are in phase. The local SST anomalies in the North Pacific can constructively (destructively) interfere with the tropical ENSO forcing to influence the extratropical eddy height anomalies when the PDO and ENSO are in (out of) phase. The difference between the positive and negative PDO in El Niño or La Niña winters filters out the tropical SST forcing, permitting the deduction of the extratropical SST contribution to the atmospheric response. The composite shows that the cold (warm) SST anomalies in the central North Pacific associated with the positive (negative) PDO have a similar impact to that of the warm (cold) SST anomalies in the tropical Pacific, exhibiting a positive (negative) PNA-like response, enhancing (weakening) the upward propagation of waves over the western coast of North America. The composite difference between the positive and negative PDO in El Niño or La Niña winters, as well as in eastern Pacific ENSO or central Pacific ENSO winters, presents a highly consistent atmospheric response pattern, which may imply a linear interference of the PDO’s impact with ENSO’s.

Association between Solar Variability and Teleconnection Index

In this study, we investigate the associations between the solar variability and teleconnection indices, which influence atmospheric circulation and subsequently, the spatial distribution of the global pressure system. A study of the link between the Sun and a large-scale mode of climate variability, which may indirectly affect the Earth’s climate and weather, is crucial because the feedbacks of solar variability to an autogenic or internal process should be considered with due care. We have calculated the normalized cross-correlations of the total sunspot area, the total sunspot number, and the solar North–South asymmetry with teleconnection indices. We have found that the Southern Oscillation Index (SOI) index is anti-correlated with both solar activity and the solar North–South asymmetry, with a ∼3-year lag. This finding not only agrees with the fact that El Niño episodes are likely to occur around the solar maximum, but also explains why tropical cyclones occurring in the solar maximum periods and in El Niño periods appear similar. Conversely, other teleconnection indices, such as the Arctic Oscillation (AO) index, the Antarctic Oscillation (AAO) index, and the Pacific-North American (PNA) index, are weakly or only slightly correlated with solar activity, which emphasizes that response of terrestrial climate and weather to solar variability are local in space. It is also found that correlations between teleconnection indices and solar activity are as good as correlations resulting from the teleconnection indices themselves.

Thermal evolution of the Northern Cordillera Volcanic Province: implication for heat flow in remnant back-arc regions

ABSTRACT Active and remnant back-arc regions do not follow a typical conductive lithosphere cooling model, but instead have an apparent two-stage cooling, defined by a high heat flow back-arc region during subduction and a second post-subduction heating event that extends elevated heat flow for several 10s million years. Numerical one-stage cooling models have not reproduced observed heat flow anomalies in active subduction zones using physically realistic parameters and require a secondary post-subduction heating mechanism. Here, an extension driven-volcanism model is developed to examine extension driven heating and volcanism as a mechanism to produce a prolonged thermal anomaly within back-arc lithosphere. This model is tested using the recorded thermal evolution of the Northern Cordillera Volcanic Province (NCVP), a Neogene-Quaternary alkaline volcanic province located in the remnant back-arc region of the Pacific-North American subduction zone in British Columbia, Canada. A single steady-state lithosphere geotherm does not intersect all previously published temperature estimates, suggesting previous data record the thermal evolution of the NCVP. Calculated geotherms at equilibrium with the minimum and maximum MELTS temperatures predict an increase in reduced mantle heat flow (Qm ) from 43 to 72 mW/m2 and lithosphere thinning from a depth of 87 to 48 km. The newly developed extension-volcanism model reproduced the calculated pre- and post-volcanism thermal regimes for the NCVP and supports that extension within the remnant back-arc could produce the present heat flow anomaly and volcanism. The model most readily produces volcanism when Qm is ~45–65 mW/m2, a typical range for back-arcs. Back-arc regions are prime locations for limited volcanism because their warmer thermal regime minimizes tectonic stress requirements to produce volcanism. Additionally, two-stage cooling of back-arcs can be explained with a time-dependent extension-volcanism thermal feedback mechanism that is possible because of the subduction driven pre-heating of back-arc regions.

Contrastive Influence of ENSO and PNA on Variability and Predictability of North American Winter Precipitation

Abstract In this work, the roles of El Niño–Southern Oscillation (ENSO) in the variability and predictability of the Pacific–North American (PNA) pattern and precipitation in North America in winter are examined. It is noted that statistically about 29% of the variance of PNA is linearly linked to ENSO, while the remaining 71% of the variance of PNA might be explained by other processes, including atmospheric internal dynamics and sea surface temperature variations in the North Pacific. The ENSO impact is mainly meridional from the tropics to the mid–high latitudes, while a major fraction of the non-ENSO variability associated with PNA is confined in the zonal direction from the North Pacific to the North American continent. Such interferential connection on PNA as well as on North American climate variability may reflect a competition between local internal dynamical processes (unpredictable fraction) and remote forcing (predictable fraction). Model responses to observed sea surface temperature and model forecasts confirm that the remote forcing is mainly associated with ENSO and it is the major source of predictability of PNA and winter precipitation in North America.

Late-Holocene floodplain development, land-use, and hydroclimate–flood relationships on the lower Ohio River, US

Floodplain development, land-use, and flooding on the lower Ohio River are investigated with a 3100-year-long sediment archive from Avery Lake, a swale lake on the Black Bottom floodplain in southern Illinois, US. In all, 12 radiocarbon dates show that Avery Lake formed at 1130 BCE (3100 cal. yr BP), almost 3000 years later than previously thought, indicating that the Black Bottom floodplain is younger and more dynamic than previously estimated. Three subsequent periods of extensive land clearance were identified by changes in pollen composition, corresponding to Native American occupations before 1500 CE and the current Euro-American occupation beginning in the 18th century. Sedimentation rates prior to 1820 CE changed independently of land clearance events, suggesting natural as opposed to land-use controls. Comparison with high-resolution paleoclimate data from Martin Lake, IN, indicates that lower Ohio River flooding was frequent when cold-season precipitation originating from the Pacific/Arctic predominated when atmospheric circulation resembled positive Pacific North American (PNA) conditions and the Pacific Decadal Oscillation (PDO) was in a positive mean state (1130 BCE to 350 CE and 1150–1820 CE). Conversely, Ohio River flooding was less frequent when warm-season precipitation from the Gulf of Mexico prevailed during negative PDO- and PNA-like mean states (350 and 1150 CE). This flood dynamic appears to have been fundamentally altered after 1820 CE. We suggest that extensive land clearance in the Ohio River watershed increased runoff and landscape erosion by reducing interception, infiltration, and evapotranspiration, thereby increasing flooding despite a shift to negative PDO- and PNA-like mean states. Predicted increases in average precipitation and extreme rainfall events across the mid-continental US are likely to perpetuate current trends toward more frequent flood events, because anthropogenic modifications have made the landscape less resilient to changing hydroclimatic conditions.

Variability and trends of landfalling atmospheric rivers along the Pacific Coast of northwestern North America

AbstractAtmospheric rivers (ARs), defined as narrow, transient corridors of strong moisture transport in the lower troposphere, are important phenomena for freshwater recharge and water resources, especially along the west coast of North America. This study presents the variability and trends of landfalling ARs (LARs) along the higher (53.5°–60.0°N) and lower (47.0°–53.5°N) latitudes of British Columbia and southeastern Alaska (BCSAK) during the 1948–2016 period. Moreover, we present the synoptic evolution and distribution of LARs in BCSAK during different phases of ocean–atmosphere climate variability using a six‐hourly AR catalogue from the Scripps Institution of Oceanography and reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric Research. During 1948–2016, BCSAK averages 35 ± 5 LARs annually, with the highest frequency during fall (13 ± 2) and lowest during spring (5 ± 2). The frequency of LARs across BCSAK rises during the study period, and the increase between 1979 and 2016 is statistically significant (p < .05). A strong ridge over the Pacific Northwest and BC and a trough over the Gulf of Alaska and the Northeastern Pacific Ocean favours AR landfalls at the higher and lower latitudes, respectively. BCSAK experiences greater numbers of LARs during neutral phases of El Niño/Southern Oscillation, the 2013/2014 Pacific oceanic blob, and during the positive phases of the Pacific Decadal Oscillation and Pacific North American Pattern.

Mechanisms for a remote response to Asian anthropogenic aerosol in boreal winter

Abstract. Asian emissions of anthropogenic aerosols and their precursors have increased rapidly since 1980, with half of the increase since the pre-industrial era occurring in this period. Transient experiments with the HadGEM3-GC2 coupled model were designed to isolate the impact of Asian anthropogenic aerosols on global climate in boreal winter. It is found that this increase has resulted in local circulation changes, which in turn have driven decreases in precipitation over China, alongside an intensification of the offshore monsoon flow. No large temperature changes are seen over China. Over India, the opposite response is found, with decreasing temperatures and increasing precipitation. The dominant feature of the local circulation changes is an increase in low-level convergence, ascent, and precipitation over the Maritime Continent, which forms part of a tropical Pacific-wide La Niña-like response. HadGEM3-GC2 also simulates pronounced far-field responses. A decreased meridional temperature gradient in the North Pacific leads to a positive Pacific–North American circulation pattern, with associated temperature anomalies over the North Pacific and North America. Anomalous northeasterly flow over northeast Europe drives advection of cold air into central and western Europe, causing cooling in this region. An anomalous anticyclonic circulation over the North Atlantic causes drying over western Europe. Using a steady-state primitive equation model, LUMA, we demonstrate that these far-field midlatitude responses arise primarily as a result of Rossby waves generated over China, rather than in the equatorial Pacific.

PRINCIPAL CLIMATE INDICES FOR THE NORTH PACIFIC: NATURE AND HISTORY (A REVIEW)

Climatic indices reflecting the environmental conditions and patterns of their variability in the entire northern Pacific and in its local regions are overviewed. Their physical nature and mechanisms of the processes, their geography and methods of calculation are presented, with citing of the first descriptions. Among a variety of global and regional climatic indices concern ing the North Pacific, the following ones are described: the indices of Arctic Oscillation (AO), El Niño — Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), Aleutian Low Pressure Index (ALPI), Siberian High Index (SHI), North Pacific Index (NPI), Pacific/North American (PNA) Index, and West Pacific Index (WP). AO is a large-scale index of atmospheric circulation reflecting the processes both in the troposphere and stratosphere, where «pumping» of air masses between the high and moderate latitudes occurs continuously. ENSO is also a large-scale index that reflects large-scale interactions in the fields of temperature, atmospheric pressure, wind, and cloudiness over the whole Pacific. Other indices are rather regional, since their influence does not extend far beyond the limits of the domains of their definition. Nevertheless, their role in environmental fluctuations in certain areas could be significant and their influence could be traced throughout the Northern Hemisphere.

Data Mining Climate Variability as an Indicator of U.S. Natural Gas.

Anomalously cold winters with extreme storms strain natural gas (NG) markets due to heightened demand for heating and electricity generation. While extended weather forecasting has become an indicator for NG management, seasonal (2–3 month) prediction could mitigate the impact of extreme winters on the NG market for consumers and industry. Interrelated climate patterns of ocean and atmospheric circulation anomalies exhibit characteristics useful for developing effective seasonal outlooks of NG storage and consumption due to their influence on the persistence and intensity of extreme winter weather in North America. This study explores the connection between the Pacific-North American climate systems and the NG market in the U.S., connecting macro-scale oceanic and atmospheric processes to regional NG storage and consumption. Western Pacific sea surface temperatures and atmospheric pressure patterns describe significant variation in seasonal NG storage and consumption. Prediction of these coupled climate processes is useful for estimating NG storage and consumption; this could facilitate economic adaptation toward extreme winter weather conditions. Understanding the implicated impact of climate variability on NG is a crucial step toward economic adaptation to climate change.

Mechanical Models Suggest Fault Linkage through the Imperial Valley, California, U.S.A.

AbstractThe Imperial Valley hosts a network of active strike‐slip faults that comprise the southern San Andreas fault (SAF) and San Jacinto fault systems and together accommodate the majority of relative Pacific–North American plate motion in southern California. To understand how these faults partition slip, we model the long‐term mechanics of four alternative fault networks with different degrees of connectivity through the Imperial Valley using faults from the Southern California Earthquake Center Community Fault Model version 5.0 (v.5.0). We evaluate model results against average fault‐slip rates from the Uniform California Earthquake Rupture Model v.3 (UCERF3) and geologic slip‐rate estimates from specific locations. The model results support continuous linkage from the SAF through the Brawley seismic zone to the Imperial and to the Cerro Prieto faults. Connected faults decrease surface strain rates throughout the region and match more slip‐rate data. Only one model reproduces the UCERF3 rate on the Imperial fault, reaching the lower bound of 15 mm/yr. None of the tested models reproduces the UCERF3 preferred rate of 35 mm/yr. In addition, high‐strain energy density rates around the Cerro Prieto fault in all models suggest that the UCERF3 preferred rate of 35 mm/yr may require revision. The Elmore Ranch fault‐slip rate matches the UCERF3 rate only in models with continuous linkage. No long‐term slip‐rate data are available for the El Centro and Dixieland faults, but all models return less than 2 mm/yr on the El Centro fault and 3.5–9.6 mm/yr on the Dixieland fault. This suggests that the Dixieland fault may accommodate a significant portion of plate‐boundary motion.

Role of hydrometeorological factors and solar activity in interannual variability of ice extent in the East Siberian Sea

Interannual changes of the summer ice coverage were investigated, and the role of hydrometeorological factors and solar activity in long-period fluctuations of the ice area in the East Siberian Sea was determined. Multivariate statistical analysis of time series of the ice cover, hydrometeorological elements, and the solar activity (SA), was performed for the period from 1950 to 2012 with regard for the cross-correlations of the analyzed variables that made possible to develop the equations of interannual fluctuations of the ice coverage in the East Siberian Sea in August and September. The equations include the following variables: air temperature in June–August of the current year TVI‑VIII; the atmospheric circulation presented by indices of Arctic oscillation (Arctic Oscillation, AO), Arctic dipole (Arctic Dipole, AD), Pacific North American oscillation (Pacific North American Oscillation, PNA); average annual runoff of river waters into the Laptev and East Siberian seas (RivLES) with a time shift of one and two years; average annual index of the North Atlantic thermal state (AMO) with a time lag of eight years; solar activity SA, presented by the average annual Wolf number with advancing of one year. Diagnostic calculations of the ice area by the obtained equations using the actual values of the indices did show a good agreement between the actual and calculated values in August and September from 1950 to 2012. These equations were used to calculate contribution of each factor to the general dispersion of fluctuations of the ice coverage. The most important factors influencing the ice cover of the Sea in August and September are: the air temperature; the atmospheric circulation, presented by the Arctic Oscillation at the end of winter; and Atlantic waters which are characterized by AMO with a time lag of eight years. The role of other factors, i.e. summer atmospheric circulation, river runoff into the above seas, and 11-year cycle of solar activity were found to be equal to only 5–10% for each. Basing on these estimates, it has been concluded that the obtained statistical equations may be used as the diagnostic models of interannual changes in the ice coverage.

Relationship Between the Pacific‐North American Pattern and the Frequency of Tropical Cyclones Over the Western North Pacific

AbstractThe frequency of tropical cyclones (TCs) over the western North Pacific during June–November has a significant inverse correlation with the concurrent Pacific‐North American (PNA) pattern from 1965 to 2016. During low PNA years, more TCs form north of 12.5°N, with significantly greater TC occurrences from 15 to 20°N, compared to high PNA years. The difference in TC genesis location can be explained by the differences in the genesis potential index derived from the environmental variables in both PNA phases. The PNA influences TC formation primarily by modulating large‐scale dynamic conditions, with thermodynamic conditions playing a lesser role. In low PNA years, low‐level anomalous cyclonic lows over the Philippines and in the subtropical central Pacific provide significant positive relative vorticity anomalies favorable for TC genesis. Additionally, there is also less vertical wind shear to the north of the Philippines due to enhanced winds at low levels and weaker winds at upper levels.

Relative roles of El Niño-induced extratropical and tropical forcing in generating Tropical North Atlantic (TNA) SST anomaly

During El Nino decaying spring, Tropical North Atlantic (TNA) region displays a significant warm SST anomaly (SSTA). The relative role of El Nino-induced extratropical and tropical forcing in generating TNA SSTA is investigated through both an observational analysis and idealized numerical model simulation. The maximum warming tendency in TNA occurred in El Nino mature winter and was mainly caused by enhanced surface evaporation, which was primarily affected by reduced trade wind. A partial regression analysis was employed to reveal the relative roles of El Nino induced extratropical forcing through Pacific North American pattern (PNA) and tropical forcing via remote Gill response. It is found that the extratropical forcing contributes around 62–66% in inducing a southwesterly anomaly over TNA region while the tropical forcing contributes approximately 34–38%. Idealized numerical model (ECHAM4) experiments were further carried out. This model could successfully reproduce PNA pattern and tropical Gill response by adding an El Nino forcing. Then two sensitivity experiments were designed to assess the relative contribution of the extratropical and tropical forcing. (1) An experiment was designed in which a zone (15°–26°N, 90°E–85°W) was given a strong Newtonian-type damping to prevent the Rossby wave propagating to the mid-latitudes, which greatly suppresses PNA induced mid-latitude forcing effect. (2) Tropical forcing is attempted to be suppressed in the model by adding a positive diabatic heating rate anomaly over the tropical Atlantic region where the El Nino forcing tends to induce negative heating anomaly. The numerical model results show that the southwesterly anomaly over TNA region is attributed to extratropical forcing and tropical forcing with 58–68% and 32–42% contribution respectively, which is consistent with the observational partial regression analysis.

The changing relationship between ENSO and its extratropical response patterns

El Niño-Southern Oscillation (ENSO) can influence Northern Hemisphere seasonal conditions through its interaction with the Pacific-North American pattern (PNA) and the Tropical Northern Hemisphere pattern (TNH). Possibly due to Earth’s changing climate, the variability of ENSO, as well as the zonal location of its sea-surface temperature (SST) anomaly, is changing. Along with this, the strength and location of the jet, in which these atmospheric patterns are embedded, are changing. Using a simple tracking algorithm we create a continuous time series for the zonal location of ENSO’s SST anomaly, and show that the relationship between ENSO and the PNA is linearly sensitive to this location, while its relationship with the TNH is not. ENSO’s relationship with both the TNH and PNA is shown to be strongly influenced by the position of the Pacific jet stream. The ENSO-TNH relationship is found to be linked to phase changes of the Atlantic Multidecadal Oscillation, and the resulting changes in SST and jet speed.

A Climatology of Extratropical Cyclones Leading to Extreme Weather Events over Central and Eastern North America

Abstract Cool-season extreme weather events (EWEs) (i.e., high-impact weather events that are societally disruptive, geographically widespread, exceptionally prolonged, and climatologically infrequent) are typically associated with strong extratropical cyclones (ECs). The opportunity to investigate the genesis locations, tracks, and frequencies of ECs leading to EWEs over central and eastern North America and compare them to those of ordinary ECs forming over and traversing the same region motivates this study. ECs leading to EWEs are separated from ordinary ECs according to the magnitude, areal extent, and duration of their 925-hPa standardized wind speed anomalies in the 0.5° NCEP CFSR dataset. This separation allows for the construction of an October–March 1979–2016 climatology of ECs leading to EWEs over central and eastern North America. The climatology of ECs leading to EWEs over central and eastern North America reveals that these ECs typically form in the lee of the Rocky Mountains, over the south-central United States, and along the east coast of North America at latitudes equatorward of the typical genesis locations of ordinary ECs. ECs leading to EWEs exhibit equatorward-shifted tracks relative to ordinary ECs, likely associated with an equatorward shift in the position of the subtropical or polar-front jet. ECs leading to EWEs form most frequently in November and March, when the seasonal alignment of baroclinic and diabatic forcings is maximized. Similar to ordinary ECs, the genesis locations, tracks, and frequencies of ECs leading to EWEs are partially determined by the states of the Pacific–North American pattern and North Atlantic Oscillation.