Pacific North America Research Articles

Thin crème brûlée rheological structure for the Eastern California Shear Zone

AbstractSince the occurrence of the 1992 CE Mw 7.3 Landers and 1999 Mw 7.1 Hector Mine earthquakes in the Mojave Desert (California, USA), postseismic deformation following both earthquakes has been intensively studied, and models with a strong crust overlying a low-viscosity mantle asthenosphere have been favored. However, we recently found that the near-field postseismic transients after the two earthquakes have lasted longer than previously thought, which requires a revision of the postseismic modeling. Our new modeling results based on the revised postseismic transients show that: (1) the effective viscosity of the lower crust beneath the Mojave region at the decadal time scale is ∼2 × 1020 Pa·s (transient viscosity ∼2 × 1019 Pa·s), i.e., only ∼5 times that of the underlying mantle asthenosphere, and (2) the transient viscosity of the upper mantle exhibits a time-dependent increase, providing fresh geodetic evidence for frequency-dependent rheology (e.g., Andrade or extended Burgers rheology). The inferred transient rheology for the first year agrees well with that obtained for the July 2019 Mw 6.4 and Mw 7.1 Ridgecrest earthquakes ∼180 km north of the two Mojave events. Our modeling results support a thin crème brûlée model for the Eastern California Shear Zone (part of the Pacific-North America plate boundary) in which both the lower crust and the upper mantle exhibit ductility at decadal time scales.

Predictable Patterns of Wintertime Surface Air Temperature in Northern Hemisphere and Their Predictability Sources in the SEAS5

AbstractBased on 36-yr hindcasts from the fifth-generation seasonal forecast system of the European Centre for Medium-Range Weather Forecasts (SEAS5), the most predictable patterns of the wintertime 2-m air temperature (T2m) in the extratropical Northern Hemisphere are extracted via the maximum signal-to-noise (MSN) empirical orthogonal function (EOF) analysis, and their associated predictability sources are identified. The MSN EOF1 captures the warming trend that amplifies over the Arctic but misses the associated warm Arctic–cold continent pattern. The MSN EOF2 delineates a wavelike T2m pattern over the Pacific–North America region, which is rooted in the tropical forcing of the eastern Pacific-type El Niño–Southern Oscillation (ENSO). The MSN EOF3 shows a wavelike T2m pattern over the Pacific–North America region, which has an approximately 90° phase difference from that associated with MSN EOF2, and a loading center over midlatitude Eurasia. Its sources of predictability include the central Pacific-type ENSO and Eurasian snow cover. The MSN EOF4 reflects T2m variability surrounding the Tibetan Plateau, which is plausibly linked to the remote forcing of the Arctic sea ice. The information on the leading predictable patterns and their sources of predictability is further used to develop a calibration scheme to improve the prediction skill of T2m. The calibrated prediction skill in terms of the anomaly correlation coefficient improves significantly over midlatitude Eurasia in a leave-one-out cross-validation, implying a possible way to improve the wintertime T2m prediction in the SEAS5.

Enhanced mid-to-late winter predictability of the storm track variability in the North Pacific as a contrast with the North Atlantic

The storm tracks are a major driver of regional extreme weather events. Using the daily output of reanalysis and a latest generation ensemble seasonal forecasting system, this study examines the interannual variability and predictability of the boreal winter storm tracks in the North Pacific and North Atlantic. In both basins, the leading mode of storm track variability describes a latitudinal shifting of the climatological storm tracks. The shifting mode is closely connected with the extratropical large-scale teleconnection patterns (i.e. Pacific-North America teleconnection and North Atlantic Oscillation).The main predictability source for the shifting mode of the North Pacific storm tracks are the ENSO-related sea surface temperature anomalies. Assessment of the seasonal prediction skill further shows that the shifting mode of the North Pacific storm tracks is in general better predicted than that of the North Atlantic storm tracks likely due to stronger ENSO effects.Our analyses also find that, through the modulations of ENSO and the subtropical jet, the shifting mode of the North Pacific storm tracks exhibit a mid-to-late winter predictability enhancement. During El Niño phases, the North Pacific subtropical jet shifts equatorward and becomes strongest in mid-to-late winter, which dominates the upper-level flow and guides the storm track most equatorward. We argue that the intensification and equatorward shift of the North Pacific subtropical jet in mid-to-late winter of El Niño years provide the main reason for the increased mid-to-late winter predictability for the storm tracks. The results imply that good representation of the background subtropical jet in models is important for winter climate prediction of storm tracks.

Increased impacts on US West Coast

The Madden–Julian oscillation causes teleconnections that impact mid-latitudes. Now research predicts dramatic eastward shifts of these impacts in the Pacific–North America region as the climate warms, leading to higher winter rainfall variability along the US West Coast and California in particular.

Amplified Madden–Julian oscillation impacts in the Pacific–North America region

The Madden–Julian Oscillation (MJO) is a slow-moving tropical mode that produces a planetary-scale envelope of convective storms. By exciting Rossby waves, the MJO creates teleconnections with far-reaching impacts on extratropical circulation and weather. Although recent studies have investigated the response of the MJO to anthropogenic warming, not much is known about potential changes in its teleconnections. Here, we show that the MJO teleconnection pattern in boreal winter will likely extend further eastward over the North Pacific. This is primarily due to an eastward shift in the exit region of the subtropical jet, to which the teleconnection pattern is anchored, and assisted by an eastward extension of the MJO itself. The eastward-extended teleconnection enables the MJO to have a greater impact downstream on the Northeast Pacific and North American west coast. Over California specifically, the multi-model mean projects a 54% increase in MJO-induced precipitation variability by 2100 under a high-emissions scenario. The Madden–Julian oscillation (MJO) is a band of convection that travels eastward through the tropics and impacts mid-latitude weather via teleconnections. Under climate warming, these teleconnections are predicted to extend eastward in the Pacific–North America region, amplifying MJO impacts there.

Terrestrial exports of dissolved and particulate organic carbon affect nearshore ecosystems of the Pacific coastal temperate rainforest

AbstractWatersheds of the coastal temperate rainforests of Pacific North America export large amounts of organic carbon (OC) to the coastal ocean. While it has been suggested that terrestrially derived organic matter could subsidize marine food webs and affect ocean biogeochemistry along the coastal margin, little work has been done to quantify and characterize OC across the freshwater to marine continuum. We conducted monthly and targeted rainfall event surveys of dissolved and particulate organic carbon (DOC and POC) quantity and quality (δ13C, dissolved organic matter characterization) across a freshwater to marine salinity gradient between Calvert and Hecate Islands, British Columbia, Canada. Freshwater DOC concentrations (9.97 ± 0.25 mg L−1) far exceeded those in marine waters (1.24 ± 0.03 mg L−1), while POC concentrations were similar across all sites (0.23 ± 0.01 mg L−1). δ13C‐DOC and ‐POC in freshwaters were constant, but varied seasonally at the marine stations with freshwater and marine processes. Rainfall events facilitated the rapid export of terrestrial DOC and POC to coastal waters, altering water quality and potentially subsidizing microbial productivity across marine surface waters. On an annual basis, primary production in marine waters (21–42 Gg C) exceeded total freshwater OC contributions (1.8–2.2 Gg C); however, freshwater exports were more important during the autumn and winter months, when rainfall was highest and primary production was limited by shorter days and deep turbulent mixing. Our results highlight the importance of storms for connecting the coastal temperate rainforest with surface coastal waters, especially during the summer when connectivity between the freshwater and marine ecosystems is otherwise low.

Plausible causes of the interdecadal change of the North Pacific teleconnection pattern in boreal spring around the late 1990s

An interdecadal change of the North Pacific teleconnection pattern in the upper troposphere in boreal spring was noted around the late 1990s. During the period of 1979–1998, a meridional dipole pattern with alternative cyclone and anticyclone to the south and the north of 45° N over North Pacific, was identified as the leading mode by applying the empirical orthogonal function (EOF) technique onto the 300-hPa geopotential height anomaly after removing its zonal average. This pattern is similar to the upper-tropospheric structure of North Pacific Oscillation (NPO)/western Pacific (WP) pattern. During the period of 1999–2018, the meridional dipole is replaced by a northeast–southwest orientated teleconnection with centers of activity over the subtropical central Pacific and northeastern North Pacific, resembling a Pacific-North America (PNA) teleconnection. That is, the upper-level atmospheric teleconnection shifted from a NPO/WP-like pattern to a PNA-like pattern in 1998/1999. For the plausible causes of this interdecadal change, the effect of synoptic high-frequency (HF, 2–8 day) eddy and tropical convection over the central Pacific are emphasized. Before 1998/1999, the NPO/WP-like teleconnection could be ascribed to the vorticity forcing of the HF transient eddy activity over North Pacific between 40° and 60° N, with the role of tropical heating ignorable. After 1998/1999, the spatial distribution of HF transient eddy activity shifted significantly from a north–south pattern to a northeast–southwest pattern, sustaining a PNA-like teleconnection through the transportation of the eddy vorticity fluxes, while the vorticity forcing of the subseasonal low-frequency (LF, 10–90 day) transient eddy activity plays a role in the maintenance of the teleconnection lobe over the subtropical central Pacific. In addition, the tropical convection anomaly centers shifted towards the equatorial central Pacific and Maritime Continent, which can induce a poleward-propagating Rossby wave train, and amplify the PNA-like teleconnection pattern.

The Building Blocks of Northern Hemisphere Wintertime Stationary Waves

AbstractAn intermediate-complexity moist general circulation model is used to investigate the forcing of stationary waves in the Northern Hemisphere boreal winter by land–sea contrast, horizontal heat fluxes in the ocean, and topography. The additivity of the response to these building blocks is investigated. In the Pacific sector, the stationary wave pattern is not simply the linear additive sum of the response to each forcing. In fact, over the northeast Pacific and western North America, the sum of the responses to each forcing is actually opposite to that when all three are imposed simultaneously due to nonlinear interactions among the forcings. The source of the nonlinearity is diagnosed using the zonally anomalous steady-state thermodynamic balance, and it is shown that the background-state temperature field set up by each forcing dictates the stationary wave response to the other forcings. As all three forcings considered here strongly impact the temperature field and its zonal gradients, the nonlinearity and nonadditivity in our experiments can be explained, but only in a diagnostic sense. This nonadditivity extends up to the stratosphere, and also to surface temperature, where the sum of the responses to each forcing differs from the response if all forcings are included simultaneously. Only over western Eurasia is additivity a reasonable (though not perfect) assumption; in this sector land–sea contrast is most important over Europe, while topography is most important over western Asia. In other regions, where nonadditivity is pronounced, the question of which forcing is most important is ill-posed.

The Weakening and Eastward Movement of ENSO Impacts during the Last Glacial Maximum

AbstractThe assumption of a stationary global signal linked to El Niño–Southern Oscillation (ENSO) events is often used in paleo-ENSO proxy data interpretation. This paper attempts to investigate whether the assumption is valid during the last glacial maximum (LGM) over the region 60°S–90°N, 60°E−60°W. Using four models within phase 3 of the Paleoclimate Modeling Intercomparison Project framework that well reproduce ENSO-induced variabilities, differences from the preindustrial period to LGM in the ENSO-related sea surface temperature pattern and its impacts are investigated. Compared to the preindustrial period, the ENSO impacts are revealed to weaken and shift eastward during the LGM. According to multimodel medians, ENSO impacts on precipitation and near-surface air temperature are attenuated over most regions of concern, with percentage changes in both parameters averaging −21% for the whole region; the ENSO-induced Pacific–North America (PNA) teleconnection pattern is weakened, manifested by the 41% diminished center over the North Pacific and the almost vanished activity centers over the continent. Spatially, there is a zonal contraction of 13° for the sea surface warming of ENSO, as well as eastward migration over 10° for the ENSO-induced positive precipitation anomaly center over the tropical Pacific and the PNA teleconnection pattern outside the tropics. The aforementioned changes are linked to the altered climatic background during the LGM, which features a 16° eastward shift for the Pacific Walker circulation rising branch and a weakened waveguide in the midlatitudes. The results suggest that the hypothesis of stationary ENSO impacts should be applied cautiously to the past.

MJO Teleconnections over the PNA Region in Climate Models. Part II: Impacts of the MJO and Basic State

AbstractIn an assessment of 29 global climate models (GCMs), Part I of this study identified biases in boreal winter MJO teleconnections in anomalous 500-hPa geopotential height over the Pacific–North America (PNA) region that are common to many models: an eastward shift, a longer persistence, and a larger amplitude. In Part II, we explore the relationships of the teleconnection metrics developed in Part I with several existing and newly developed MJO and basic state (the mean subtropical westerly jet) metrics. The MJO and basic state diagnostics indicate that the MJO is generally weaker and less coherent and propagates faster in models compared to observations. The mean subtropical jet also exhibits notable biases such as too strong amplitude, excessive eastward extension, or southward shift. The following relationships are found to be robust among the models: 1) models with a faster MJO propagation tend to produce weaker teleconnections; 2) models with a less coherent eastward MJO propagation tend to simulate more persistent MJO teleconnections; 3) models with a stronger westerly jet produce stronger and eastward shifted MJO teleconnections; 4) models with an eastward extended jet produce an eastward shift in MJO teleconnections; and 5) models with a southward shifted jet produce stronger MJO teleconnections. The results are supported by linear baroclinic model experiments. Our results suggest that the larger amplitude and eastward shift biases in GCM MJO teleconnections can be attributed to the biases in the westerly jet, and that the longer persistence bias is likely due to the lack of coherent eastward MJO propagation.

A Nonmodal Instability Perspective of the Declining Northern Midlatitude Synoptic Variability in Boreal Summer

AbstractThe Pacific–North America–North Atlantic sector in general experienced a dryer and warmer climate in summer during the past 40 years. These changes are partly associated with declining midlatitude synoptic variability in boreal summer, especially over the two ocean basins. A nonmodal instability analysis of the boreal summer background flow is conducted for two periods, 1979–94 and 2000–15, to understand dynamical processes potentially responsible for the observed decline of synoptic variability. The synoptic variability associated with fast, nonmodal growth of atmospheric disturbances shows a decline over northern midlatitudes in the later period, in both a barotropic model and a two-level quasigeostrophic model. These results highlight the importance of the changing summer background flow in contributing to the observed changes in synoptic variability. Also discussed are factors likely associated with background flow changes including sea surface temperature and sea ice change.

MJO Teleconnections over the PNA Region in Climate Models. Part I: Performance- and Process-Based Skill Metrics

AbstractWe propose a set of MJO teleconnection diagnostics that enables an objective evaluation of model simulations, a fair model-to-model comparison, and a consistent tracking of model improvement. Various skill metrics are derived from teleconnection diagnostics including five performance-based metrics that characterize the pattern, amplitude, east–west position, persistence, and consistency of MJO teleconnections and additional two process-oriented metrics that are designed to characterize the location and intensity of the anomalous Rossby wave source (RWS). The proposed teleconnection skill metrics are used to compare the characteristics of boreal winter MJO teleconnections (500-hPa geopotential height anomaly) over the Pacific–North America (PNA) region in 29 global climate models (GCMs). The results show that current GCMs generally produce MJO teleconnections that are stronger, more persistent, and extend too far to the east when compared to those observed in reanalysis. In general, models simulate more realistic teleconnection patterns when the MJO is in phases 2–3 or phases 7–8, which are characterized by a dipole convection pattern over the Indian Ocean and western to central Pacific. The higher model skill for phases 2, 7, and 8 may be due to these phases producing more consistent teleconnection patterns between individual MJO events than other phases, although the consistency is lower in most models than observed. Models that simulate realistic RWS patterns better reproduce MJO teleconnection patterns.

Post–12 Ma deformation in the lower Colorado River corridor, southwestern USA: Implications for diffuse transtension and the Bouse Formation

AbstractStructural evidence presented here documents that deformation was ongoing within the lower Colorado River corridor (southwestern USA) during and after the latest Miocene Epoch, postdating large-magnitude extension and metamorphic core complex formation. Geometric and kinematic data collected on faults in key geologic units constrain the timing of deformation in relation to the age of the Bouse Formation, a unit that records the first arrival and integration of the Colorado River. North-south–striking extensional, NW-SE–striking oblique dextral, NE-SW–striking oblique sinistral, and east-west–striking contractional faults and related structures are observed to deform pre– (>6 Ma), syn– (6–4.8 Ma), and post–Bouse Formation (<4.8 Ma) strata. Fault displacements are typically at the centimeter to meter scale, and locally exhibit 10-m-scale displacements. Bouse Formation basalt carbonate locally exhibits outcrop-scale (tens of meters) syndepositional dips of 30°–90°, draped over and encrusted upon paleotopography, and has a basin-wide vertical distribution of as much as 500 m. We argue that part of this vertical distribution of Bouse Formation deposits represents syn- and post-Bouse deformation that enhanced north-south–trending depocenters due to combined tectonic and isostatic subsidence in a regional fault kinematic framework of east-west diffuse extension within an overall strain field of dextral transtension. Here we (1) characterize post-detachment tectonism within the corridor, (2) show that diffuse tectonism is cumulatively significant and likely modified original elevations of Bouse Formation outcrops, and (3) demonstrate that this tectonism may have played a role in the integration history of the lower Colorado River. We suggest a model whereby intracontinental transtension took place in a several hundred kilometers-wide area inboard of the San Andreas fault within a diffuse Pacific–North America plate margin since the latest Miocene.

North American Winter Dipole: Observed and Simulated Changes in Circulations

In recent years, a pair of large-scale circulation patterns consisting of an anomalous ridge over northwestern North America and trough over northeastern North America was found to accompany extreme winter weather events such as the 2013–2015 California drought and eastern U.S. cold outbreaks. Referred to as the North American winter dipole (NAWD), previous studies have found both a marked natural variability and a warming-induced amplification trend in the NAWD. In this study, we utilized multiple global reanalysis datasets and existing climate model simulations to examine the variability of the winter planetary wave patterns over North America and to better understand how it is likely to change in the future. We compared between pre- and post-1980 periods to identify changes to the circulation variations based on empirical analysis. It was found that the leading pattern of the winter planetary waves has changed, from the Pacific–North America (PNA) mode to a spatially shifted mode such as NAWD. Further, the potential influence of global warming on NAWD was examined using multiple climate model simulations.

Geodynamic evolution of southwestern North America since the Late Eocene

Slab rollback, lithospheric body forces, or evolution of plate boundary conditions are strongly debated as possible lithospheric driving mechanisms for Cenozoic extension in southwestern North America. By incorporating paleo-topography, lithospheric structure, and paleo-boundary conditions, we develop a complete geodynamic model that quantifies lithospheric deviatoric stresses and predicts extension and shear history since Late Eocene. We show that lithospheric body forces together with influence of change-over from subduction to transtensional boundary conditions from Late Eocene to Early Miocene were the primary driving factors controlling direction and magnitude of extensional deviatoric stresses that produced topographic collapse. After paleo-highlands collapsed, influence of Pacific-North America plate motion and associated deformation style along the plate boundary became increasingly important from Middle Miocene to present. Smaller-scale convection stress effects from slab rollback and associated mantle flow played only a minor role. However, slab rollback guided deformation rate through introduction of melts and fluids that impacted rheology.

Evaluating the Joint Influence of the Madden‐Julian Oscillation and the Stratospheric Polar Vortex on Weather Patterns in the Northern Hemisphere

AbstractSubseasonal‐to‐seasonal (S2S) forecasts of Northern Hemisphere (NH) extratropical winter weather patterns continue to be a challenging venture. Past studies have considered the individual influence of two modes of climate variability—the Madden‐Julian oscillation (MJO) and the state of the stratospheric polar vortex (SPV)—on the NH polar jet stream and associated weather regimes. This study takes a different approach and quantifies the joint influence of the SPV and the MJO on NH S2S winter weather patterns. Using ERA‐Interim, we illustrate that variability associated with the MJO primarily influences tropospheric patterns across the North Pacific and western North America 10 to 14 days later, while SPV variability has a stronger influence on tropospheric patterns over the North Atlantic and Europe for the same lags. Over the rest of North America and into the Arctic, however, constructive and destructive interference between the MJO and the SPV teleconnections yields unique jet stream and temperature patterns that differ from the single‐mode composites. As such, S2S forecasts of temperature and jet stream patterns across much of North America may improve in accuracy by considering both modes. The study also shows that MJO Phases 2 and 3 events influence the resulting tropospheric circulation via primarily a tropospheric pathway. By contrast, MJO Phases 7 and 8 events modify both the tropospheric and stratospheric circulation with potential feedbacks on the tropospheric circulation at longer lags. Hence, the study suggests another benchmark by which to test S2S dynamical prediction systems, including the importance for modeling stratosphere‐troposphere coupling dynamics correctly.

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.

Parallel Comparison of Major Sudden Stratospheric Warming Events in CESM1-WACCM and CESM2-WACCM

After the recent release of the historical runs by community Earth system model version 2–the whole atmosphere community climate model (CESM2-WACCM), the major sudden stratospheric warming (SSW) events in this model and in its previous version (CESM1-WACCM) are compared based on a modern reanalysis (JRA55). Using the World Meteorological Organization (WMO) definition of SSWs and a threshold-based classification method that can describe the polar vortex morphology, SSWs in models and the reanalysis are further classified into two types, vortex displacement SSWs and vortex split SSWs. The general statistical characteristics of the two types of SSW events in the two model versions are evaluated. Both CESM1-WACCM and CESM2-WACCM models are shown to reproduce the SSW frequency successfully, although the circulations differences between vortex displacement SSWs and vortex split SSWs in CESM2-WACCM are smaller than in CESM1-WACCM. Composite polar temperature, geopotential height, wind, and eddy heat flux anomalies in both the two models and the reanalysis show similar evolutions. In addition, positive Pacific–North America and negative Western Pacific patterns in the troposphere preceding vortex displacement and split SSWs are observed in both observations and the models. The strong negative North Atlantic oscillation-like pattern, especially after vortex split SSW onset, is also identified in models. The near-surface cold Eurasia–warm North America pattern before both types of SSW onset, the warm Eurasia–cold North America pattern after displacement SSW onset, and the cold Eurasia–cold North America pattern after split SSW onset are consistently identified in JRA55, CESM1-WACCM, and CESM2-WACCM, although the temperature anomalies after the split SSW onset in CESM2-WACCM are somewhat underestimated.

Plate boundary localization, slip-rates and rupture segmentation of the Queen Charlotte Fault based on submarine tectonic geomorphology

Linking fault behavior over many earthquake cycles to individual earthquake behavior is a primary goal in tectonic geomorphology, particularly across an entire plate boundary. Here, we examine the 1150-km-long, right-lateral Queen Charlotte-Fairweather fault system using comprehensive multibeam bathymetry data acquired along the Queen Charlotte Fault (QCF) offshore southeastern Alaska and western British Columbia. Fine-scale analysis of tectonic geomorphology allowed us to identify and reconstruct 184 strike-slip piercing points over a 630 km stretch of the QCF. Age constraints from glacial recession and offshore sedimentation patterns yield a consistent slip-rate of ∼50–57 mm/yr since ∼17–12 ka, the fastest rate for a continent-ocean strike-slip fault on Earth. These slip-rates equal or exceed estimates of Pacific-North America (PA-NA) relative motion from global plate reconstructions, indicating that PA-NA motion is highly localized. The QCF cuts the seafloor along a narrow and unusually straight trace for its entire length and multiple fault traces are observed only at local step-overs. The geometry and behavior of the QCF over many earthquake cycles is simple and typical of mature faults with relatively homogeneous stress fields. Since the QCF is the primary PA-NA plate boundary, we used the trace of the QCF to define the small circle path for relative plate motion and computed the associated Euler pole. Predicted along-strike obliquity variations based on the new pole agree with observed tectonic geomorphology and suggest that previous global plate reconstructions overestimated the degree of oblique convergence along the QCF. We also find that subtle, long-wavelength (75–150 km) bends and discrete step-overs appear to define the endpoints of M>7 earthquakes, suggesting that obliquity and resultant fault geometry may control rupture segmentation and asperity development. Lastly, the agreement between predicted obliquity and tectonic geomorphology along the entire length of QCF compelled a reevaluation of regional tectonic models. In the north, the eastern Yakatat Terrane appears to be translating northwest with the Pacific plate, and slip transferred from the QCF to the Fairweather Fault results in ∼20 mm/yr of convergence along the southern St. Elias mountains. In the south, we predict a reduced rate of convergence along the QCF west of Haida Gwaii (∼5–6 mm/yr of shortening, on average) relative to previous studies. Our results support a model for transpression and strike-slip partitioning along the edge of a hot and weak Pacific Plate, leading to crustal thickening and growth of the Queen Charlotte Terrace to the west of Haida Gwaii.

The role of the Atlantic Multidecadal Oscillation precondition in the teleconnection of different El Niño‐Southern Oscillation types and impacts on the 15°N–15°S South American sector precipitation

AbstractThe Eastern Pacific (EP) and Central Pacific (CP) El Niño‐Southern Oscillation (ENSO) types and their impacts on the tropical North Atlantic (TNA) SST variability and 15°N–15°S South American precipitation during the warm and cold phases of the Atlantic Multidecadal Oscillation (WAMO and CAMO) were evaluated during the 1901–2012 period. The results show more frequent ENSO events during the CAMO. The El Niño (EN) (La Niña [LN]) events, regardless of type (EP or CP), during the WAMO (CAMO) were accompanied by a warming (cooling) in the TNA after its mature phases. In these cases, extratropical teleconnection patterns are established through variations in the Pacific/North America (PNA) teleconnection pattern and are accompanied by variations in the Walker circulation. For the EN (LN) in the CAMO (WAMO), the tropical teleconnections occur predominant, through the Walker cell and the zonal inter‐basin gradient, which is intensified due to the SST gradient between the eastern equatorial Pacific (non‐neutral anomalies) and the equatorial Atlantic (neutral anomalies). These circulation pattern changes affect the precipitation patterns in the 15°N–15°S South American sector during December–January–February (D(0)JF(+1)) and March–April–May (MAM(+1)). The EP EN (EPEN) events are associated with the intensification of the negative precipitation anomalies in northeastern Brazil (NEB) during the WAMO and in the central part of the Amazon during the CAMO. In the case of CP EN (CPEN) events, the greatest differences between the AMO phases occur during MAM(+1), with reverse sign anomalies over northwestern South America. In the case of LN events, the largest differences occur in NEB, with reduced rainfall in the WAMO, regardless of type EP or CP. The results presented here highlight the role of low frequency oscillations in defining the teleconnection patterns between tropical Pacific and Atlantic Oceans, not discussed previously.